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What Is Quantum Physics Science for Sleep · Night Science Stories

0:01
Speaker 1: One small thing before we begin. If you regularly fall
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Speaker 1: the description. Right then, get comfortable. Phone down. Worries temporarily postponed.
0:59
Speaker 2: Good evening and welcome to The Sleepy Scientist. It's very
1:02
Speaker 2: nice to have you here. Tonight's subject has spent more
1:05
Speaker 2: than a century making perfectly sensible people say things like,
1:09
Speaker 2: surely that cannot be right. And unfortunately for common sense,
1:14
Speaker 2: the experiments keep suggesting that it is. But before we
1:18
Speaker 2: get into any of that, let's settle in. Hopefully whatever
1:22
Speaker 2: filled your day is beginning to feel a little further
1:24
Speaker 2: away now. Perhaps the lights are already off, Perhaps you're
1:28
Speaker 2: curled up on the sofa telling yourself you'll go to
1:31
Speaker 2: bed in a minute. Or perhaps you're awake at one
1:33
Speaker 2: of those hours when the rest of the world seems
1:35
Speaker 2: to have gone unusually still. Wherever it is, I'd love
1:40
Speaker 2: to hear about it. If you'd like to, leave a
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Speaker 2: comment and tell me where you're listening from tonight. Unlike
1:46
Speaker 2: the particles we're about to meet, you're allowed to say
1:49
Speaker 2: exactly where you are. It's always rather lovely seeing these
1:53
Speaker 2: episodes find their way into bedrooms and living rooms all
1:56
Speaker 2: over the world, If you enjoy spending these slower evenings
1:59
Speaker 2: exploring science together, a like and a subscribe are always appreciated.
2:05
Speaker 2: In quantum physics, we can only ever talk about the
2:08
Speaker 2: probability of finding something in a particular place, and a
2:11
Speaker 2: subscribe nudges the odds of finding you here again nicely
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Speaker 2: in our favour. And if you've listened before, thank you
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Speaker 2: for coming back. It means a great deal that you
2:21
Speaker 2: choose to spend some of your quiet time here. Now,
2:25
Speaker 2: about that stubborn corner of physics. At the scale of
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Speaker 2: atoms and the particles inside them, nature stops behaving quite
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Speaker 2: like the world we experience every day. Particles can produce
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Speaker 2: patterns we normally associate with waves. Physical properties are described
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Speaker 2: through probabilities. Objects can occupy quantum superpositions. And the act
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Speaker 2: of measuring a system becomes part of the physics itself.
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Speaker 2: None of this means the universe has abandoned its rules.
2:55
Speaker 2: It means the rules are simply stranger than the ones
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Speaker 2: our everyday intuition prepared us for. And those strange rules
3:02
Speaker 2: aren't confined to distant laboratories. They help explain why atoms
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Speaker 2: are stable, why chemistry works, why the sun shines, and
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Speaker 2: why computers, lasers and modern electronics can exist at all.
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Speaker 2: So get comfortable, let your eyes grow heavy whenever they
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Speaker 2: want to, and we'll start where all the strangeness starts,
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Speaker 2: with a surprisingly simple question. What is quantum physics, and
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Speaker 2: why does the universe seem to behave so differently once
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Speaker 2: we look closely enough? Quantum physics begins in a place
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Speaker 2: that is almost impossible to picture, not because it is
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Speaker 2: far away, but because it is extraordinarily small. It is
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Speaker 2: the part of physics we use when matter and light
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Speaker 2: stop behaving in the familiar ways that our everyday experience
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Speaker 2: has taught us to expect. At the scale of atoms,
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Speaker 2: electrons and other subatomic particles, The comfortable rules that describe footballs,
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Speaker 2: falling apples and orbiting planets are no longer enough. Nature
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Speaker 2: follows a different framework, one that can seem deeply strange
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Speaker 2: at first, but which has been tested with remarkable precision.
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Speaker 2: And despite its reputation, quantum physics is not some obscure
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Speaker 2: corner of science that only matters to people working in laboratories.
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Speaker 2: It is underneath almost everything around you, the atoms in
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Speaker 2: the air, chemistry inside your body, the light reaching your eyes,
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Speaker 2: the electrical behaviour of the device playing this recording, and
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Speaker 2: even the solidity of whatever you happen to be resting
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Speaker 2: on are all ultimately governed by quantum physics. The word
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Speaker 2: quantum itself comes from the Latin word for an amount.
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Speaker 2: In physics, it usually refers to the idea that certain
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Speaker 2: physical quantities can occur in discrete amounts. One useful way
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Speaker 2: to imagine this is to compare a staircase with a ramp.
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Speaker 2: On a perfectly smooth ramp, you can stand at any
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Speaker 2: height you like. You might move upwards by a metre,
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Speaker 2: half a metre, a centimetre, or some arbitrarily tiny fraction
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Speaker 2: in between. There is no requirement to occupy one particular height.
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Speaker 2: A staircase works differently. You can stand on the first step,
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Speaker 2: the second step, or the third step, but you cannot
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Speaker 2: comfortably stand halfway between two steps without doing something rather awkward.
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Speaker 2: Some quantum systems behave more like that staircase. Certain quantities,
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Speaker 2: such as the allowed energy levels of an electron bound
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Speaker 2: inside an atom, come in particular permitted values rather than
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Speaker 2: forming one completely continuous range. This does not mean that
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Speaker 2: everything in quantum physics comes chopped into tiny pieces. Position,
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Speaker 2: for example, is not simply divided into a universal grid
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Speaker 2: of microscopic squares in ordinary quantum mechanics. Nor does established
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Speaker 2: quantum theory tell us that space itself must consist of
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Speaker 2: tiny indivisible blocks. Quantization applies in particular ways to particular
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Speaker 2: physical systems. The important idea is simply that nature does
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Speaker 2: not always allow every imaginable value that classical intuition might suggest.
6:11
Speaker 2: To understand where these effects become important, it helps to
6:15
Speaker 2: shrink our perspective. A human body is made from organs.
6:19
Speaker 2: Those organs are made from tissues, and those tissues are
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Speaker 2: made from cells. A typical human cell might be several
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Speaker 2: tens of millionths of a metre across, although cells vary
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Speaker 2: enormously in size. Inside those cells are molecules. Molecules are
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Speaker 2: arrangements of atoms held together by electromagnetic interactions and quantum
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Speaker 2: mechanical rules. Water, for example, consists of molecules built from
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Speaker 2: hydrogen and oxygen atoms. The proteins in your body are
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Speaker 2: enormous molecular structures assembled from long chains of smaller components.
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Speaker 2: Shrink further and eventually you arrive at the atom. A
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Speaker 2: typical atom is roughly one ten billionth of a metre across.
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Speaker 2: That is such a small distance that ordinary comparisons begin
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Speaker 2: to struggle. If an atom were somehow enlarged until it
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Speaker 2: was the size of a large sports stadium, Its nucleus
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Speaker 2: would be something closer to a small object near the
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Speaker 2: centre than a miniature version of the stadium itself. The
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Speaker 2: nucleus is typically tens of thousands of times smaller than
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Speaker 2: the atom around it. Inside atomic nuclei are protons and neutrons.
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Speaker 2: Protons and neutrons are themselves made from more fundamental particles
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Speaker 2: called quarks, bound together through the strong interaction. Outside the
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Speaker 2: nucleus are electrons. An electron... as far as present experiments
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Speaker 2: can determine, behaves like an elementary particle. We have not
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Speaker 2: discovered smaller internal components inside it, and already the familiar
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Speaker 2: pictures begin becoming unreliable. An electron inside an atom is
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Speaker 2: not simply a tiny ball circling the nucleus in the
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Speaker 2: way Earth circles the Sun. That image can be useful
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Speaker 2: when first meeting atomic physics, but the modern description is
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Speaker 2: more subtle. Electrons occupy quantum states and their possible positions,
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Speaker 2: energies and other properties are described using quantum mechanics. This
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Speaker 2: brings us to two phrases that are often used almost interchangeably.
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Speaker 2: Quantum physics is the broad area of physics concerned with
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Speaker 2: quantum phenomena. Quantum mechanics is the mathematical framework developed to
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Speaker 2: describe the behaviour of quantum systems. Quantum physics therefore includes
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Speaker 2: quantum mechanics, but the broader subject also stretches into areas
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Speaker 2: such as quantum field theory, quantum optics, quantum information science
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Speaker 2: and quantum chemistry. For the moment, the distinction does not
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Speaker 2: need to trouble us very much. The central point is
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Speaker 2: that quantum mechanics gives physicists a set of rules for
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Speaker 2: calculating what microscopic systems can do and what results experiments
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Speaker 2: are likely to produce. Those rules work astonishingly well. They
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Speaker 2: explain why atoms have stable structures instead of immediately collapsing.
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Speaker 2: They explain why different chemical elements behave differently. They describe
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Speaker 2: how molecules form, why some materials conduct electricity while others
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Speaker 2: resist it, and why certain substances absorb or emit particular
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Speaker 2: colours of light. Even something as ordinary as colour depends
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Speaker 2: upon quantum physics. When light strikes matter, electrons within atoms
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Speaker 2: and molecules can interact with particular ranges of electromagnetic radiation.
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Speaker 2: Different materials absorb, reflect and transmit different wavelengths of light,
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Speaker 2: contributing to the colours we eventually perceive. Solidity also has
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Speaker 2: a quantum foundation. When you place your hand against a table,
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Speaker 2: the atoms in your hand do not simply behave like
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Speaker 2: tiny billiard balls colliding with equally tiny billiard balls in
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Speaker 2: the wood. Electromagnetic interaction between electron distributions matter enormously, along
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Speaker 2: with the quantum mechanical behaviour of electrons and the Pauli
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Speaker 2: exclusion principle. The apparently simple fact that your hand does
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Speaker 2: not pass straight through the table is therefore connected to
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Speaker 2: rules that operate far beneath the scale we can see.
10:20
Speaker 2: temperature has a microscopic story as well. The thermal behaviour
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Speaker 2: of matter involves the energies and motions of enormous numbers
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Speaker 2: of particles, and at sufficiently small scales or low temperatures,
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Speaker 2: quantum effects become essential to describing those energies correctly. Electricity
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Speaker 2: depends on quantum physics too. The behaviour of electrons inside
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Speaker 2: solids gives rise to the electronic properties of metals, insulators
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Speaker 2: and semiconductors. Without understanding those quantum rules, the modern transistor
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Speaker 2: would be difficult to explain, and without transistors the world
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Speaker 2: of computers, phones and digital electronics would look very different indeed.
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Speaker 2: So the strange thing about quantum physics is not that
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Speaker 2: it belongs to some hidden world disconnected from ordinary life.
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Speaker 2: It is that ordinary life is made from it. The
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Speaker 2: difficulty comes from the fact that our intuition was not
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Speaker 2: built at these scales. Human beings evolved among objects large
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Speaker 2: enough to see and touch. A stone tends to be
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Speaker 2: in one place. A thrown ball follows a recognisable path.
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Speaker 2: A cup remains on the table unless something moves it.
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Speaker 2: These experiences teach us a deeply classical picture of reality.
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Speaker 2: At atomic scales, that picture stops being sufficient. Quantum systems
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Speaker 2: can behave in ways associated with both particles and waves.
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Speaker 2: Their measurable properties are described through probability. Different possibilities can
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Speaker 2: combine in superpositions. Certain pairs of quantities cannot simultaneously be
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Speaker 2: assigned arbitrarily precise values, and interactions that count as measurements
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Speaker 2: raise questions about what it even means for a physical
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Speaker 2: property to have a definite value. None of this makes
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Speaker 2: quantum physics vague. Quite the opposite. It is among the
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Speaker 2: most successful predictive frameworks ever constructed. The oddness lies in
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Speaker 2: the gap between what the mathematics predicts and what human
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Speaker 2: intuition expects. And perhaps the most useful thing to remember
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Speaker 2: from the beginning is that there is no separate quantum
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Speaker 2: world hiding somewhere beneath our own. There is only one
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Speaker 2: physical world. Every chair, every cloud, every star, every living
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Speaker 2: cell and every particle of dust is built from systems
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Speaker 2: that obey quantum rules. most of the time, when enormous
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Speaker 2: numbers of those systems interact with one another and with
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Speaker 2: their surroundings, their collective behaviour looks reassuringly classical. But underneath
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Speaker 2: that familiar appearance, the quantum rules never went away. For
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Speaker 2: a long time, physics appeared to be settling into something
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Speaker 2: remarkably neat. The foundations laid by Isaac Newton had transformed
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Speaker 2: the study of motion. His laws described how forces change
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Speaker 2: the motion of objects. while his law of universal gravitation
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Speaker 2: connected falling apples, orbiting planets and the motion of the
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Speaker 2: moon within the same physical framework. That was an extraordinary achievement.
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Speaker 2: A stone falling from a hand and a planet travelling
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Speaker 2: around the sun seemed, at first glance, to belong to
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Speaker 2: completely different worlds. Newton showed that the same basic rules
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Speaker 2: could describe both. Classical mechanics became so successful that it
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Speaker 2: encouraged a powerful picture of nature. If you knew where
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Speaker 2: an object was, how fast it was moving, in which
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Speaker 2: direction it was travelling, and what forces were acting upon it,
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Speaker 2: then in principle you could calculate what would happen next.
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Speaker 2: The future became something physics could predict. A cannonball fired
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Speaker 2: from a hill follows a trajectory that can be calculated.
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Speaker 2: A pendulum swings according to predictable rules. Planets trace paths
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Speaker 2: around the Sun that can be forecast years in advance.
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Speaker 2: The machinery of nature looked almost clock-like. This did not
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Speaker 2: mean every practical calculation was easy. A system containing many
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Speaker 2: interacting objects can become extraordinarily complicated. But the underlying assumption
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Speaker 2: remained reassuringly simple. Objects had definite properties. They occupied definite positions.
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Speaker 2: They moved with definite velocities. And if you possessed enough information,
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Speaker 2: the equations told you how the system would evolve. By
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Speaker 2: the 19th century, another enormous piece of physics had fallen
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Speaker 2: into place. Electricity and magnetism had once seemed like separate
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Speaker 2: collections of curious effects. Electric charges could attract and repel
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Speaker 2: one another. Magnets possessed north and south poles. Electric currents
15:00
Speaker 2: could influence compass needles. Moving magnets could produce electric currents. Gradually,
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Speaker 2: physicists realised that these phenomena were deeply connected. One of
15:11
Speaker 2: the people who brought the story together was James Clerk Maxwell.
15:16
Speaker 2: During the 19th century, Maxwell developed a mathematical description showing
15:20
Speaker 2: that electric and magnetic fields are two parts of a
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Speaker 2: unified electromagnetic theory. His equations described how electric fields can
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Speaker 2: produce magnetic effects. how changing magnetic fields can produce electric effects,
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Speaker 2: and how disturbances in these fields can travel through space.
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Speaker 2: One of the most beautiful consequences was that these electromagnetic
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Speaker 2: waves travelled at the known speed of light. Light, it seemed,
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Speaker 2: was an electromagnetic wave. This was another astonishing unification. The
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Speaker 2: glow of a candle, the colours of a rainbow, the
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Speaker 2: force around a magnet and the current in a wire–
16:00
Speaker 2: all belonged within the same broad physical theory. By the
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Speaker 2: end of the 19th century, classical physics had therefore become
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Speaker 2: extraordinarily powerful. Newtonian mechanics described motion. Maxwell's electromagnetism described electricity,
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Speaker 2: magnetism and light. Thermodynamics described heat and energy on large scales.
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Speaker 2: It was not unreasonable for some physicists to wonder whether
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Speaker 2: the deepest work was nearly finished. Perhaps nature had revealed
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Speaker 2: most of her basic rules. Perhaps the remaining task was
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Speaker 2: largely to improve measurements, refine calculations and fill in a
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Speaker 2: few awkward details. The awkward details, unfortunately, had other plans.
16:43
Speaker 2: One of them appeared whenever physicists studied the light emitted
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Speaker 2: by hot objects. Heat a piece of metal, and it
16:50
Speaker 2: begins by glowing dull red. Increase the temperature and the
16:54
Speaker 2: colour changes, eventually moving toward orange-yellow. and then a brilliant white.
17:00
Speaker 2: The exact distribution of emitted radiation depends strongly on temperature.
17:05
Speaker 2: Physicists studied an idealised object called a blackbody, meaning a
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Speaker 2: perfect absorber and emitter of electromagnetic radiation. A blackbody does
17:15
Speaker 2: not have to be literally black when it is hot.
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Speaker 2: The name refers to the idealised way it absorbs incoming radiation.
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Speaker 2: Experiments showed a very particular pattern in the intensity of
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Speaker 2: radiation across different wavelengths. Classical theory struggled badly with it.
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Speaker 2: At long wavelengths, parts of the classical prediction worked reasonably well.
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Speaker 2: At shorter wavelengths, however, the equations began producing a disastrous result.
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Speaker 2: They suggested that a hot object should emit more and
17:47
Speaker 2: more energy as the wavelength became shorter. with the predicted
17:51
Speaker 2: energy eventually increasing without limit. This failure later became known
17:57
Speaker 2: as the ultraviolet catastrophe. Nature thankfully does not behave that way.
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Speaker 2: If it did, ordinary hot objects would radiate absurd amounts
18:06
Speaker 2: of high-frequency energy. The observed spectrum instead rises to a
18:11
Speaker 2: peak and then falls again. The classical calculation was not
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Speaker 2: merely slightly inaccurate. It was fundamentally wrong in exactly the
18:20
Speaker 2: region where experimental measurements were becoming increasingly reliable. Blackbody radiation
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Speaker 2: was not the only problem. Another appeared in the photoelectric effect.
18:31
Speaker 2: When light shines on certain materials, it can eject electrons
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Speaker 2: from their surfaces. Classical wave physics might suggest that sufficiently
18:40
Speaker 2: intense light should eventually provide enough energy to remove electrons,
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Speaker 2: regardless of the light's frequency. But experiments revealed a more
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Speaker 2: peculiar pattern. Below a certain frequency, increasing the intensity did
18:55
Speaker 2: not produce the expected effect. Above that threshold, electrons could
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Speaker 2: be released. Something about the frequency of the light mattered
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Speaker 2: in a way classical theory did not comfortably explain. Atoms
19:08
Speaker 2: created further trouble. When gases are heated or electrically excited,
19:13
Speaker 2: they do not simply produce every possible colour of light. Instead,
19:18
Speaker 2: Individual elements emit particular spectral lines. Hydrogen, for example, produces
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Speaker 2: a characteristic set of wavelengths. It is almost as though
19:28
Speaker 2: atoms possess preferred ways of gaining and losing energy. That
19:33
Speaker 2: was difficult to reconcile with a completely continuous classical picture.
19:38
Speaker 2: Then there was the atom itself. By the early 20th century,
19:42
Speaker 2: experiments were revealing that atoms contain positive and negative charge.
19:48
Speaker 2: Eventually the nuclear model would place positive charge inside a
19:51
Speaker 2: tiny nucleus, with electrons associated with the surrounding atom. But
19:57
Speaker 2: a serious classical problem immediately appears if an electron is
20:00
Speaker 2: imagined simply orbiting the nucleus, like a planet around the sun.
20:05
Speaker 2: An electron carries electric charge. According to classical electrodynamics, an
20:11
Speaker 2: accelerating electric charge should radiate electromagnetic energy. An orbiting electron
20:17
Speaker 2: is constantly accelerating because its direction is continually changing. So
20:22
Speaker 2: if the atom behaved like a miniature classical solar system,
20:25
Speaker 2: the electron should steadily lose energy. Its orbit should shrink.
20:30
Speaker 2: It should spiral inward toward the nucleus. The atom should collapse.
20:35
Speaker 2: Classical calculations suggest that this would happen extremely quickly. Yet
20:40
Speaker 2: ordinary matter plainly does not collapse in this way. The
20:43
Speaker 2: atoms in a table remain atoms. The atoms in your
20:46
Speaker 2: body remain intact. The oxygen in the air does not
20:49
Speaker 2: continually destroy itself because its electrons have fallen into their nuclei.
20:55
Speaker 2: Something was missing. And this is important, because quantum physics
20:59
Speaker 2: was not invented simply because a group of physicists wanted
21:02
Speaker 2: a stranger universe. The opposite was true. The established theories
21:07
Speaker 2: were elegant, successful and deeply respected. Scientists had every reason
21:13
Speaker 2: to preserve them where they worked. But experiments kept exposing
21:17
Speaker 2: situations in which the old rules could not produce the
21:20
Speaker 2: right answers. Blackbody radiation would not cooperate. The photoelectric effect
21:26
Speaker 2: would not cooperate. Atomic spectra would not cooperate. Stable atoms
21:31
Speaker 2: themselves would not cooperate. the cracks were becoming difficult to ignore.
21:37
Speaker 2: Around the beginning of the 20th century, physicists found themselves
21:41
Speaker 2: in the uncomfortable position of possessing some of the most
21:44
Speaker 2: successful theories ever created, while simultaneously watching those theories fail
21:50
Speaker 2: in carefully controlled experiments. And the first major step beyond
21:54
Speaker 2: the old framework arrived in a surprisingly modest form. Max
21:58
Speaker 2: Planck was studying blackbody radiation. He was not trying to
22:03
Speaker 2: overthrow classical physics. He was trying to make the equations
22:06
Speaker 2: agree with what experiments were actually showing. To do that,
22:11
Speaker 2: he introduced an assumption that energy was not exchanged in
22:14
Speaker 2: every imaginable amount. Instead, it came in discrete portions, a
22:20
Speaker 2: small mathematical adjustment, at least in appearance. But once that
22:26
Speaker 2: idea was taken seriously, physics would never quite look classical again.
22:31
Speaker 2: At the very beginning of the 20th century, Max Planck
22:34
Speaker 2: was trying to solve what seemed, at first, like a
22:38
Speaker 2: fairly specialised problem. He was studying the light emitted by
22:42
Speaker 2: hot objects. Not flames themselves, exactly, and not the complicated
22:47
Speaker 2: chemistry of burning material, but something more fundamental. Physicists wanted
22:53
Speaker 2: to understand how an idealised object would emit electromagnetic radiation,
22:58
Speaker 2: purely because of its temperature. This ideal object became known
23:03
Speaker 2: as a blackbody. A perfect blackbody would absorb all electromagnetic
23:08
Speaker 2: radiation falling upon it, regardless of wavelength, and when heated
23:13
Speaker 2: it would emit radiation with a spectrum determined by its temperature.
23:19
Speaker 2: Real objects can approximate this behaviour surprisingly well. A small
23:24
Speaker 2: opening in a heated cavity, for example, can behave very
23:28
Speaker 2: much like an ideal blackbody. because radiation entering the opening
23:32
Speaker 2: is repeatedly absorbed and re-emitted inside. By the late 19th century,
23:37
Speaker 2: physicists had measured blackbody radiation in considerable detail. They knew
23:42
Speaker 2: that a hot object did not emit equal amounts of
23:45
Speaker 2: radiation at every wavelength. Instead, the spectrum rose to a
23:50
Speaker 2: peak and then declined. As the temperature increased, the peak
23:54
Speaker 2: shifted towards shorter wavelengths. This is why an object can
23:58
Speaker 2: begin glowing red as it heats up then become orange, yellowish,
24:02
Speaker 2: and eventually appear white as its temperature rises. The experimental
24:07
Speaker 2: pattern was clear. The theoretical explanation was not. Classical physics
24:13
Speaker 2: could describe part of the spectrum reasonably well, especially at
24:17
Speaker 2: longer wavelengths. But when physicists applied the same classical ideas
24:23
Speaker 2: at very short wavelengths, something went badly wrong. The equations
24:28
Speaker 2: predicted that the amount of emitted energy should keep increasing
24:31
Speaker 2: as the wavelength became shorter. Eventually, the predicted total energy
24:36
Speaker 2: became infinite. This absurd result became known as the ultraviolet catastrophe. Fortunately,
24:43
Speaker 2: ovens do not produce infinite amounts of ultraviolet radiation every
24:47
Speaker 2: time someone warms a pie. The experiments clearly showed that
24:51
Speaker 2: the radiation fell away at short wavelengths. So the problem
24:55
Speaker 2: was not with nature. it was with the theory being
24:58
Speaker 2: used to describe it. Max Planck set out to find
25:02
Speaker 2: a formula that matched the measured spectrum. In 1900 he
25:07
Speaker 2: arrived at an expression that worked remarkably well, but getting
25:11
Speaker 2: there required him to introduce an assumption that seemed deeply
25:14
Speaker 2: unnatural from the perspective of classical physics. Planck proposed that
25:19
Speaker 2: the microscopic systems exchanging energy electromagnetic radiation could not exchange
25:25
Speaker 2: absolutely arbitrary amounts of energy. Instead, the exchange occurred in
25:30
Speaker 2: discrete portions. Those portions became known as quanta. The amount
25:36
Speaker 2: of energy in each quantum depended on the frequency of
25:38
Speaker 2: the radiation. Higher frequency radiation corresponded to larger energy quanta.
25:44
Speaker 2: Lower frequency radiation corresponded to smaller ones. The relationship involved
25:49
Speaker 2: a new fundamental constant, now known as Planck's constant. Its
25:54
Speaker 2: value is extraordinarily small. In ordinary units, it is roughly 6.6
25:59
Speaker 2: times 10 to the power of minus 34 joule seconds.
26:04
Speaker 2: That tiny number is one of the reasons quantum effects
26:07
Speaker 2: can be so difficult to notice in daily life. The
26:10
Speaker 2: essential relationship can be expressed very simply. The energy of
26:15
Speaker 2: a quantum equals Planck's constant multiplied by the frequency. The
26:19
Speaker 2: mathematics is compact. The physical implication was enormous. Classical physics
26:25
Speaker 2: generally treated energy as something that could vary smoothly. Imagine
26:30
Speaker 2: a ramp. You can stand at any height along it.
26:33
Speaker 2: There is no built-in requirement that you occupy one height
26:37
Speaker 2: rather than another. Quantization is more like a staircase. On
26:41
Speaker 2: a staircase, there are specific steps available. You can stand
26:45
Speaker 2: on one step or another. but the structure does not
26:48
Speaker 2: offer every intermediate height as an equally valid place to stand.
26:53
Speaker 2: In Planck's model, energy exchange behaved in something like this
26:57
Speaker 2: discrete way. It came in permitted portions rather than every
27:01
Speaker 2: imaginable amount. The staircase comparison is helpful, but it should
27:06
Speaker 2: not be pushed too far. Quantum physics does not claim
27:10
Speaker 2: that every quantity in nature is always broken into identical
27:13
Speaker 2: little pieces. Different physical systems have different allowed states, and
27:19
Speaker 2: some quantities can still vary continuously. Quantization is a specific
27:24
Speaker 2: feature of particular systems and observables, not a universal statement
27:29
Speaker 2: that reality is made from tiny cubes. Planck himself did
27:33
Speaker 2: not initially present his idea as a declaration that nature
27:37
Speaker 2: must fundamentally consist of discrete chunks. He was deeply rooted
27:42
Speaker 2: in classical physics and for some time regarded quantization as
27:46
Speaker 2: something required by the calculation rather than as the beginning
27:50
Speaker 2: of an entirely new picture of reality. That hesitation was understandable.
27:56
Speaker 2: Planck was trying to explain a radiation spectrum. He was
28:00
Speaker 2: not deliberately attempting to dismantle the foundations of 19th century physics.
28:05
Speaker 2: Yet the constant that now bears his name was quietly
28:09
Speaker 2: telling physicists that classical assumptions could not simply be extended
28:12
Speaker 2: without limit into the microscopic world. Planck's constant also gives
28:17
Speaker 2: a useful sense of why everyday life appears so thoroughly non-quantum.
28:22
Speaker 2: For a football, a car, a falling mug, or a
28:26
Speaker 2: planet moving around a star, the actions and energies involved
28:29
Speaker 2: are enormous compared with Planck's constant. The discreteness associated with
28:35
Speaker 2: quantum effects becomes so fine Relative to these macroscopic scales,
28:40
Speaker 2: the classical physics provides an extraordinarily accurate approximation. It is
28:45
Speaker 2: a little like looking at a digital photograph from far
28:47
Speaker 2: enough away. The image appears smooth, move very close, and
28:53
Speaker 2: the individual pixels become noticeable. The comparison is imperfect, because
28:58
Speaker 2: quantum physics is not literally telling us that the universe
29:01
Speaker 2: is a giant pixelated image, but it captures something useful.
29:06
Speaker 2: Effects that are obvious at very small scales can become
29:09
Speaker 2: practically invisible when enormous numbers of particles are involved. There
29:13
Speaker 2: is another important distinction here. The word quantum sometimes encourages
29:18
Speaker 2: the impression that all of nature must ultimately consist of
29:22
Speaker 2: indivisible chunks. That conclusion goes too far. Quantum mechanics certainly
29:27
Speaker 2: tells us that many physical quantities have discrete allowed values
29:32
Speaker 2: in particular systems. but it does not establish that space
29:35
Speaker 2: itself is divided into smallest possible units. It does not
29:39
Speaker 2: currently tell us that time necessarily advances in indivisible ticks either.
29:45
Speaker 2: Whether space-time itself has a fundamentally quantum structure belongs to
29:49
Speaker 2: the unresolved problem of quantum gravity. Ideas such as loop
29:54
Speaker 2: quantum gravity and other approaches explore possibilities of this kind
29:59
Speaker 2: but there is no experimentally confirmed theory showing that ordinary
30:02
Speaker 2: space is made from tiny fixed cells. The safest conclusion
30:07
Speaker 2: is also the most interesting one. Planck discovered that the
30:10
Speaker 2: classical assumption of perfectly continuous energy exchange could not explain
30:15
Speaker 2: what experiments were showing. By allowing energy to be exchanged
30:19
Speaker 2: in discrete quanta, the observed blackbody spectrum suddenly made sense.
30:24
Speaker 2: What looked like a mathematical adjustment turned out to be
30:27
Speaker 2: the first clear sign of a much deeper change. Planck
30:31
Speaker 2: had opened the door, and once physicists began taking the
30:34
Speaker 2: quantum seriously, it became increasingly difficult to close it again.
30:39
Speaker 2: Only a few years after Max Planck introduced the idea
30:42
Speaker 2: of energy quanta, Albert Einstein took that strange possibility much further.
30:48
Speaker 2: In 1905, Einstein published an explanation of the photoelectric effect
30:53
Speaker 2: that treated the quantization of light as something physically meaningful
30:57
Speaker 2: rather than merely a convenient feature of a calculation. The
31:02
Speaker 2: photoelectric effect appears when light shines onto certain materials and
31:06
Speaker 2: causes electrons to be ejected from their surfaces. At first,
31:11
Speaker 2: this does not sound especially mysterious. Light carries energy. Electrons
31:17
Speaker 2: can absorb energy. give an electron enough energy, and perhaps
31:21
Speaker 2: it can escape from the material. The difficulty was in
31:24
Speaker 2: the details. Classical physics described light as an electromagnetic wave.
31:29
Speaker 2: In that picture, making a light wave more intense means
31:32
Speaker 2: increasing the amount of energy being delivered, so it seemed
31:36
Speaker 2: reasonable to expect that sufficiently intense light should eventually eject
31:40
Speaker 2: electrons from a material, even if the light had a
31:43
Speaker 2: relatively low frequency. Experiments did not behave that way. For
31:48
Speaker 2: a given material, there was a threshold frequency. If the
31:52
Speaker 2: light fell below that frequency, electrons were not ejected no
31:56
Speaker 2: matter how intense the light became. You could make the
31:59
Speaker 2: light brighter and brighter, but if its frequency was too low,
32:04
Speaker 2: the electrons stubbornly remained where they were. Above the threshold frequency, however,
32:09
Speaker 2: electrons could be released. Even more strikingly, Increasing the frequency
32:15
Speaker 2: of the light increased the maximum kinetic energy of the
32:18
Speaker 2: emitted electrons. Increasing the intensity mainly increased how many electrons
32:23
Speaker 2: could be released, provided the frequency was already high enough.
32:27
Speaker 2: This pattern was difficult to reconcile with a simple classical
32:30
Speaker 2: wave picture. Einstein proposed something far more radical. He suggested
32:36
Speaker 2: that light itself behaved during these interactions, as though its
32:40
Speaker 2: energy were concentrated into discrete quanta. Each individual quantum carried
32:46
Speaker 2: an amount of energy, determined by the frequency of the light.
32:50
Speaker 2: The same relationship introduced by Planck now took on a
32:54
Speaker 2: much more physical interpretation. The energy carried by one light
32:58
Speaker 2: quantum equals Planck's constant, multiplied by the frequency of the light.
33:04
Speaker 2: Higher frequency meant more energy per quantum, lower frequency meant less.
33:09
Speaker 2: Imagine each electron in a material needing a certain minimum
33:13
Speaker 2: amount of energy in order to escape. In modern language,
33:17
Speaker 2: that minimum is connected to the work function of the material.
33:20
Speaker 2: If one incoming light quantum does not carry enough energy,
33:24
Speaker 2: then simply sending more identical low energy quanta does not
33:28
Speaker 2: normally allow one electron to collect those separate portions gradually
33:32
Speaker 2: in the basic photoelectric process. Instead, Individual electrons interact with
33:38
Speaker 2: individual quanta of light. If the energy carried by one
33:42
Speaker 2: quantum exceeds the necessary threshold, an electron can escape. Any
33:47
Speaker 2: energy remaining after overcoming the binding requirement appears as kinetic
33:51
Speaker 2: energy of the emitted electron. This immediately explains why frequency
33:55
Speaker 2: matters so much. Low frequency light consists of lower energy quanta.
34:01
Speaker 2: Making that light more intense means delivering more of those quanta,
34:05
Speaker 2: but each one still carries too little energy if the
34:07
Speaker 2: frequency remains below the threshold. High-frequency light carries more energy
34:13
Speaker 2: per quantum. Once that individual energy is sufficient, electrons can
34:17
Speaker 2: be released. Brighter light at the same frequency then generally
34:22
Speaker 2: means more quanta arriving during a given time, so more
34:25
Speaker 2: electrons have opportunities to absorb them. This distinction between intensity
34:30
Speaker 2: and frequency is one of the simplest ways to see
34:33
Speaker 2: why Einstein's proposal was so powerful. Intensity mainly changes the
34:38
Speaker 2: number of light quanta arriving. Frequency changes the energy associated
34:43
Speaker 2: with each quantum. Today we call those quanta photons. The
34:47
Speaker 2: word photon was introduced by the American chemist Gilbert Newton
34:51
Speaker 2: Lewis in 1926. Lewis's own ideas about these like quanta
34:56
Speaker 2: were not identical to the modern concept that eventually became
35:00
Speaker 2: established in quantum physics, but the name survived and acquired
35:05
Speaker 2: its present meaning. A photon should not be imagined too
35:10
Speaker 2: literally as a tiny glowing billiard ball racing through space.
35:15
Speaker 2: That picture creates almost as many problems as it solves.
35:19
Speaker 2: A photon is a quantum excitation of the electromagnetic field,
35:24
Speaker 2: and its behaviour cannot be captured completely by either the
35:28
Speaker 2: classical idea of a particle or the classical idea of
35:31
Speaker 2: a wave. But in interactions such as the photoelectric effect,
35:36
Speaker 2: light clearly exchanges energy in discrete amounts. Einstein's explanation therefore
35:42
Speaker 2: pushed Planck's idea into uncomfortable territory. Planck had introduced quantisation
35:48
Speaker 2: while studying how matter exchanged energy with radiation. Einstein argued
35:54
Speaker 2: that the radiation itself displayed an irreducibly quantist character. This
35:59
Speaker 2: was not immediately popular. There was already overwhelming evidence that
36:03
Speaker 2: light behaved like a wave. Long before Einstein, experiments had
36:08
Speaker 2: demonstrated interference, in which overlapping light waves could reinforce or
36:14
Speaker 2: cancel one another. Diffraction showed light spreading after passing through
36:19
Speaker 2: narrow openings or around obstacles. These were classic wave phenomena.
36:25
Speaker 2: The electromagnetic theory of James Clerk Maxwell had also successfully
36:29
Speaker 2: described light as a propagating electromagnetic wave. So physics now
36:35
Speaker 2: faced an awkward situation. Light behaved like a wave, and
36:39
Speaker 2: light behaved in ways that seemed particle-like. The natural temptation
36:44
Speaker 2: is to imagine that light must secretly switch between the two.
36:48
Speaker 2: Perhaps it decides to be a wave during one experiment
36:51
Speaker 2: and becomes a particle during another. That is not the
36:55
Speaker 2: modern quantum picture. A photon is not secretly a tiny
37:00
Speaker 2: classical particle at one moment and a classical wave at another. Instead,
37:06
Speaker 2: the quantum description contains features that can produce outcomes resembling
37:11
Speaker 2: both categories depending on how the experiment is arranged. The
37:15
Speaker 2: old categories are useful approximations, but neither one by itself
37:19
Speaker 2: provides a complete picture. This became known as wave-particle duality.
37:25
Speaker 2: The phrase is helpful, although even it can sound more
37:28
Speaker 2: straightforward than the underlying physics really is. Quantum objects are
37:33
Speaker 2: not simply half wave and half particle. They are quantum objects.
37:38
Speaker 2: The wave and particle language comes from attempts to describe
37:41
Speaker 2: their behaviour using concepts inherited from the much larger world
37:46
Speaker 2: we experience directly. Einstein's work on the photoelectric effect helped
37:52
Speaker 2: demonstrate that quantization was not merely a mathematical trick used
37:57
Speaker 2: to rescue one awkward equation. Discrete exchanges of energy could
38:02
Speaker 2: be observed in real physical interactions between light and matter.
38:07
Speaker 2: Experiments carried out over the following years strengthened the case.
38:11
Speaker 2: and Einstein's explanation of the photoelectric effect became one of
38:15
Speaker 2: the achievements recognised when he received the Nobel Prize in Physics.
38:20
Speaker 2: There is a pleasant historical irony here. Einstein would later
38:24
Speaker 2: become one of the most famous critics of the interpretation
38:27
Speaker 2: of quantum mechanics. Yet, he was also one of the
38:31
Speaker 2: people responsible for pushing physics decisively into the quantum world.
38:36
Speaker 2: By taking Planck's quanta seriously … He helped reveal something
38:40
Speaker 2: that classical physics had never been designed to describe. Light
38:45
Speaker 2: could spread and interfere in a way associated with waves.
38:49
Speaker 2: It could also exchange energy in distinct, localised quanta. Nature
38:55
Speaker 2: had not chosen one side of the old wave versus
38:57
Speaker 2: particle argument. It had shown that the argument itself was
39:01
Speaker 2: too limited. For a while, the atom presented physicists with
39:06
Speaker 2: an awkward contradiction. Experiments were making it increasingly clear that
39:11
Speaker 2: atoms had internal structure, but every simple classical picture of
39:16
Speaker 2: that structure seemed to create a new problem. One of
39:20
Speaker 2: the most important clues came from experiments carried out by
39:24
Speaker 2: Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford.
39:29
Speaker 2: They fired positively charged alpha particles at extremely thin sheets
39:34
Speaker 2: of gold foil and watched how those particles scattered. Most
39:39
Speaker 2: passed through with only small deflections. That suggested that most
39:43
Speaker 2: of the volume of an atom was relatively open. but
39:46
Speaker 2: a small number of alpha particles were deflected through very
39:50
Speaker 2: large angles. A few even came back toward the direction
39:53
Speaker 2: they had arrived from. Rutherford realised that this could only
39:57
Speaker 2: make sense if most of the positive charge and almost
40:01
Speaker 2: all of the atom's mass were concentrated into a very
40:04
Speaker 2: small region at the centre. The atomic nucleus had entered physics.
40:09
Speaker 2: Compared with the atom as a whole, the nucleus is tiny.
40:13
Speaker 2: A typical atom is around one ten-billionth of a metre across.
40:18
Speaker 2: An atomic nucleus is commonly tens of thousands of times smaller.
40:24
Speaker 2: And yet nearly all of the atom's mass sits inside
40:27
Speaker 2: that minuscule central region. It was an elegant discovery. It
40:32
Speaker 2: was also the beginning of another problem. The simplest picture
40:36
Speaker 2: now imagined electrons moving around the positively charged nucleus, rather
40:42
Speaker 2: like planets around the Sun. The electrical attraction between the
40:46
Speaker 2: negatively charged electrons and the positively charged nucleus could, at
40:51
Speaker 2: first glance, seem rather like gravity holding planets in orbit.
40:55
Speaker 2: But the resemblance did not survive careful examination. An orbiting
41:00
Speaker 2: electron is an accelerating electric charge. In classical electromagnetism, an
41:06
Speaker 2: accelerating charge radiates electromagnetic energy. So an electron travelling around
41:11
Speaker 2: a nucleus should continually lose energy. As it lost energy,
41:15
Speaker 2: its orbit should become smaller, it should spiral inward, and
41:19
Speaker 2: eventually it should fall into the nucleus, the atom should collapse.
41:24
Speaker 2: That is plainly not what happens. Ordinary matter remains stable.
41:29
Speaker 2: The atoms in a wall do not gradually destroy themselves.
41:32
Speaker 2: The atoms in your body do not survive for a
41:34
Speaker 2: fraction of a second and then disappear into their own nuclei.
41:39
Speaker 2: Something in the classical picture was wrong. In 1913, Niels
41:44
Speaker 2: Bohr proposed a model that deliberately broke with some classical expectations.
41:50
Speaker 2: Bohr suggested that electrons in atoms could occupy only certain
41:53
Speaker 2: permitted states. They were not allowed to possess any arbitrary energy. Instead,
41:59
Speaker 2: only particular energy levels were available. An electron could remain
42:04
Speaker 2: in one of these stationary states without continuously radiating energy away.
42:09
Speaker 2: It could then move between allowed levels by absorbing or
42:12
Speaker 2: emitting a particular amount of energy. This was where the
42:17
Speaker 2: earlier quantum ideas of Max Planck and Albert Einstein began
42:21
Speaker 2: to connect directly with atomic structure. If an electron moved
42:26
Speaker 2: from a higher energy level to a lower one, the
42:28
Speaker 2: atom could emit a quantum of electromagnetic radiation. The energy
42:33
Speaker 2: of that light corresponded to the difference between the two
42:36
Speaker 2: atomic energy levels. If an electron absorbed the correct amount
42:41
Speaker 2: of energy, it could move upward to a higher permitted state.
42:45
Speaker 2: The atom, in other words, behaved less like an ordinary
42:48
Speaker 2: mechanical system with a smooth range of possible energies and
42:52
Speaker 2: more like a system with distinct allowed steps. The staircase
42:57
Speaker 2: image becomes useful again. A classical orbit could, in principle,
43:02
Speaker 2: have almost any energy. Bohr's electron was restricted to particular levels.
43:08
Speaker 2: It could occupy one permitted step or another but not
43:11
Speaker 2: simply settle at any arbitrary point in between. The hydrogen
43:16
Speaker 2: atom provided the clearest test. Hydrogen is the simplest chemical
43:21
Speaker 2: element consisting of a nucleus containing one proton with one
43:25
Speaker 2: electron associated with it in the neutral atom. When excited
43:30
Speaker 2: hydrogen gas emits light it does not produce a perfectly
43:34
Speaker 2: continuous rainbow. Instead It produces specific spectral lines. Certain wavelengths
43:42
Speaker 2: appear strongly. Others do not. These lines had been measured
43:47
Speaker 2: before Bohr's model, and mathematicians and physicists had already found
43:51
Speaker 2: numerical patterns in them. Bohr's theory gave those patterns a
43:55
Speaker 2: physical interpretation. The spectral lines could be understood as the
44:01
Speaker 2: result of electrons changing between specific allowed energy levels. Each
44:05
Speaker 2: transition corresponded to a particular energy difference. That energy appeared
44:11
Speaker 2: as light of a particular frequency. The spectrum of hydrogen
44:15
Speaker 2: was therefore not random decoration. It was evidence that atomic
44:19
Speaker 2: energy had structure. Bohr's model was a major success. It
44:24
Speaker 2: explained important features of hydrogen and showed clearly that classical
44:28
Speaker 2: mechanics could not simply be applied unchanged inside the atom.
44:33
Speaker 2: but the Bohr atom was not the final description. The
44:37
Speaker 2: familiar image of electrons travelling around a nucleus in neat
44:41
Speaker 2: circular paths, like miniature planets, remains useful as a historical
44:46
Speaker 2: stepping stone. It is simple to draw. It gives an
44:50
Speaker 2: immediate sense of a small nucleus surrounded by electrons. But
44:54
Speaker 2: modern quantum mechanics does not describe electrons as tiny balls
44:59
Speaker 2: following definite little tracks around the nucleus. Instead, electrons occupy
45:05
Speaker 2: quantum states. In atoms, these states are described by orbitals.
45:10
Speaker 2: An orbital does not represent a physical path. It describes
45:14
Speaker 2: a quantum state with a particular distribution of possible measurement
45:18
Speaker 2: outcomes for the electron's position, along with other defined quantum properties.
45:24
Speaker 2: That distinction matters. A planet can be pictured at a
45:27
Speaker 2: definite point in its orbit, with a definite velocity. An
45:32
Speaker 2: electron in an atom is not described in the same way.
45:35
Speaker 2: The classical idea of a precise microscopic trajectory begins to
45:40
Speaker 2: lose its usefulness. And once atomic states are understood quantum mechanically,
45:45
Speaker 2: much more of chemistry begins to fall into place. Electrons
45:49
Speaker 2: occupy organised sets of energy states around atomic nuclei. The
45:53
Speaker 2: arrangement of those electrons strongly influences how an element behaves chemically.
45:59
Speaker 2: Atoms with different numbers of protons have different nuclear charges,
46:03
Speaker 2: and their electrons settle into quantum states according to rules
46:07
Speaker 2: that include energy quantisation and, later in the Fuller theory,
46:12
Speaker 2: principles connected with electron spin and the Pauli exclusion principle.
46:18
Speaker 2: These patterns help create the structure of the periodic table.
46:22
Speaker 2: Elements within the same column often have similar chemical behaviour
46:26
Speaker 2: because of similarities in their outer electron arrangements. The reason
46:31
Speaker 2: sodium reacts very differently from neon is not merely that
46:35
Speaker 2: one atom contains a few more particles. Their electrons occupy
46:39
Speaker 2: different quantum configurations. Chemical bonding also depends upon these quantum states.
46:45
Speaker 2: When atoms approach one another, their electron distributions interact. Electrons
46:51
Speaker 2: can occupy molecular states extending across more than one nucleus,
46:55
Speaker 2: helping form bonds. The shapes of molecules, the behaviour of
47:00
Speaker 2: materials and the reactions that make chemistry possible are all
47:04
Speaker 2: connected to the allowed quantum states of electrons. This is
47:08
Speaker 2: why the atom became such an important turning point. Planck
47:12
Speaker 2: had introduced discrete energy exchange. Einstein had shown that light
47:17
Speaker 2: itself displayed quantized behaviour. Bohr then brought quantisation directly into
47:22
Speaker 2: the internal structure of matter. But even Bohr's model still
47:26
Speaker 2: relied on a mixture of old and new ideas. Electrons
47:30
Speaker 2: were still imagined in something resembling classical orbits, even though
47:34
Speaker 2: those orbits were restricted by quantum rules. That picture could
47:38
Speaker 2: explain hydrogen surprisingly well, but it struggled with more complicated
47:43
Speaker 2: atoms and could not provide a complete account of the
47:46
Speaker 2: deeper behaviour being uncovered. The atom was telling physicists that
47:50
Speaker 2: a more fundamental theory was needed, not merely classical physics
47:54
Speaker 2: with a few quantum restrictions added on top. Something genuinely new.
48:00
Speaker 2: During the 1920s that new framework began to appear. The
48:04
Speaker 2: electron would no longer be treated simply as a tiny
48:07
Speaker 2: object following a hidden path around the nucleus. Matter itself
48:12
Speaker 2: would begin to acquire wave-like properties. and the comfortable boundary
48:16
Speaker 2: between particle and wave would become increasingly difficult to maintain.
48:21
Speaker 2: Once light had begun behaving like both a wave and
48:24
Speaker 2: something particle-like, an obvious question was waiting nearby. If a
48:29
Speaker 2: thing traditionally described as a wave could reveal particle-like behaviour,
48:34
Speaker 2: could something traditionally described as a particle do the opposite?
48:39
Speaker 2: In 1924, the French physicist Louis de Broglie proposed that
48:43
Speaker 2: it could, de Broglie's idea was remarkably bold. Electrons, and
48:49
Speaker 2: in principle all material particles, should possess a wavelength associated
48:53
Speaker 2: with their motion. Matter itself could display wave-like behaviour. The
48:58
Speaker 2: proposal grew partly from the strange symmetry that had emerged
49:02
Speaker 2: in the physics of light. For centuries, arguments had moved
49:06
Speaker 2: back and forth over whether light should be understood as
49:08
Speaker 2: particles or waves. By the early 20th century, experiments demanded
49:14
Speaker 2: aspects of both descriptions. de Broglie wondered whether nature might
49:18
Speaker 2: be more symmetrical than anyone had assumed. Perhaps wave-particle duality
49:23
Speaker 2: did not belong only to light. Perhaps matter had been
49:27
Speaker 2: hiding a wave-like side as well. The wavelength associated with
49:31
Speaker 2: a material particle became known as the de Broglie wavelength.
49:36
Speaker 2: Its basic relationship is straightforward. The wavelength of a particle
49:41
Speaker 2: equals Planck's constant divided by the particle's momentum. Momentum is
49:46
Speaker 2: related to how much motion an object carries, depending on
49:49
Speaker 2: its mass and velocity. The important point is that greater
49:54
Speaker 2: momentum corresponds to a shorter wavelength. Smaller momentum corresponds to
49:59
Speaker 2: a longer wavelength. For something as light as an electron,
50:03
Speaker 2: these wavelengths can be large enough on atomic scales to
50:07
Speaker 2: produce measurable effects. For something like a tennis ball, the
50:11
Speaker 2: associated wavelength is unimaginably tiny. A moving tennis ball technically
50:17
Speaker 2: has a de Broglie wavelength too, but Planck's constant is
50:21
Speaker 2: so small and the ball's momentum so enormous by comparison
50:26
Speaker 2: that its wavelength becomes far too small for ordinary wave
50:30
Speaker 2: behaviour to be noticeable. This helps explain why nobody has
50:35
Speaker 2: ever watched a football diffract around a goalpost. At everyday scales,
50:40
Speaker 2: the wave-like character of matter is effectively hidden. For electrons, however,
50:45
Speaker 2: it is not. A few years after de Broglie proposed
50:49
Speaker 2: his idea, Clinton-Davison and Lester Germa produced some of the
50:53
Speaker 2: clearest experimental evidence that electron waves were real features of nature.
50:58
Speaker 2: They directed a beam of electrons toward a crystal of nickel.
51:02
Speaker 2: A crystal has atoms arranged in an ordered repeating pattern.
51:06
Speaker 2: That regular structure means waves scattering from different layers can
51:10
Speaker 2: reinforce one another at some angles and cancel one another
51:14
Speaker 2: at others. This is the sort of behaviour familiar from diffraction.
51:19
Speaker 2: When Davison and Germer measured where the scattered electrons appeared,
51:23
Speaker 2: they found pronounced patterns at particular angles. The electrons were
51:28
Speaker 2: producing diffraction. That was deeply significant. If electrons behaved only
51:34
Speaker 2: as tiny classical projectiles, Rather like microscopic pellets bouncing from atoms,
51:40
Speaker 2: the observed pattern would not naturally appear. A purely classical
51:44
Speaker 2: collection of particles could certainly scatter in different directions, but
51:49
Speaker 2: it would not create the same characteristic interference structure associated
51:53
Speaker 2: with a wavelength. The measured wavelength agreed with the relationship
51:57
Speaker 2: de Broglie had proposed. Matter really did possess wave-like behaviour.
52:02
Speaker 2: The electron had not stopped being capable of producing localised impacts.
52:07
Speaker 2: An electron detector could still register an electron at a
52:10
Speaker 2: particular location, yet the distribution of many such events could
52:14
Speaker 2: reveal interference and diffraction. Once again, nature refused to fit
52:20
Speaker 2: comfortably into either of the old categories. Around the same period,
52:25
Speaker 2: physicists were constructing an entirely new mathematical framework capable of
52:30
Speaker 2: describing this behaviour. One of the central figures was Erwin Schrödinger.
52:36
Speaker 2: In 1926, Schrödinger developed what became known as wave mechanics.
52:42
Speaker 2: At the heart of his approach was the Schrödinger equation.
52:45
Speaker 2: The Schrödinger equation plays a role in quantum mechanics rather
52:48
Speaker 2: like equations of motion do in classical physics. Given an
52:53
Speaker 2: appropriate description of a quantum system, it determines how the
52:56
Speaker 2: quantum state evolves with time. Its solutions include mathematical objects
53:01
Speaker 2: known as wave functions. At first glance, the name wave
53:05
Speaker 2: function seems to suggest that the electron has simply become
53:08
Speaker 2: a conventional wave. But that interpretation quickly becomes misleading. A
53:14
Speaker 2: water wave consists of water moving. A sound wave involves
53:18
Speaker 2: variations in pressure travelling through a medium. A classical electromagnetic
53:23
Speaker 2: wave involves changing electric and magnetic fields. A quantum wave
53:28
Speaker 2: function is something different. It is part of the mathematical
53:32
Speaker 2: description of the quantum state. Its meaning became much clearer
53:36
Speaker 2: through the work of Max Born. Born proposed that the
53:40
Speaker 2: wave function should be connected to probability. More precisely, the
53:45
Speaker 2: squared magnitude of the wave function determines probabilities for possible
53:49
Speaker 2: results of measurements. This is known as the Born rule.
53:54
Speaker 2: Suppose an electron is described by a wave function spread
53:57
Speaker 2: across a region of space. The wavefunction does not normally
54:01
Speaker 2: mean that the electron is a little blob of ordinary
54:03
Speaker 2: material physically smeared through that entire region. Instead, the quantum
54:09
Speaker 2: state allows physicists to calculate the probabilities of detecting the
54:13
Speaker 2: electron in different places. Where the squared magnitude of the
54:17
Speaker 2: wavefunction is larger, a position measurement is more likely to
54:21
Speaker 2: find the electron. Where it is smaller, detection is less likely.
54:27
Speaker 2: where the relevant amplitude is zero, that particular outcome will
54:30
Speaker 2: not occur under the conditions being described, probability was no
54:35
Speaker 2: longer merely something introduced because scientists lacked information. It had
54:39
Speaker 2: become built directly into the standard mathematical machinery of quantum mechanics.
54:45
Speaker 2: And Schrödinger was not the only physicist discovering this new structure.
54:50
Speaker 2: Shortly before wave mechanics appeared, Werner Heisenberg had developed a
54:54
Speaker 2: different formulation of quantum mechanics known as matrix mechanics. Its
54:59
Speaker 2: mathematics looked very different from Schrödinger' s wave-based approach. Heisenberg focused
55:05
Speaker 2: closely on quantities that could be connected with observations, such
55:10
Speaker 2: as the frequencies and intensities associated with atomic transitions, rather
55:15
Speaker 2: than trying to preserve familiar classical pictures of electron orbits.
55:20
Speaker 2: The resulting mathematical objects were arrays of numbers known as matrices.
55:24
Speaker 2: To physicists encountering them at the time, the two approaches
55:28
Speaker 2: could appear almost unrelated. One involved waves and differential equations.
55:34
Speaker 2: The other involved matrices and algebraic rules that did not
55:37
Speaker 2: resemble classical mechanics very much at all. Yet it was
55:41
Speaker 2: soon demonstrated that wave mechanics and matrix mechanics were mathematically
55:46
Speaker 2: equivalent descriptions of the same underlying quantum theory. They were
55:51
Speaker 2: different languages for the same physics. This was an important
55:54
Speaker 2: sign that quantum mechanics was becoming something much deeper than
55:59
Speaker 2: a collection of clever fixes for individual experiments. Planck had
56:03
Speaker 2: needed quanta for thermal radiation. Einstein had needed light quanta
56:08
Speaker 2: for the photoelectric effect. Bohr had needed discrete energy levels
56:13
Speaker 2: for the atom. de Broglie had associated wavelengths with matter.
56:18
Speaker 2: Now Schrödinger, Heisenberg, Born and others were revealing a coherent
56:23
Speaker 2: mathematical framework behind these strange results. But the framework demanded
56:28
Speaker 2: a change in how physical objects themselves were imagined. An
56:33
Speaker 2: electron is not adequately described as a tiny classical marble.
56:37
Speaker 2: Neither is it simply a miniature water wave. Calling it
56:41
Speaker 2: a particle captures some aspects of what experiments reveal. Calling
56:45
Speaker 2: it a wave captures others. Neither classical picture is complete.
56:49
Speaker 2: The more accurate statement is simply that an electron is
56:52
Speaker 2: a quantum object. Its state can evolve according to wave-like mathematics.
56:58
Speaker 2: Different parts of that state can interfere. Yet when an
57:01
Speaker 2: electron is detected, the result can appear as a localised event.
57:05
Speaker 2: Quantum mechanics does not require the electron to switch secretly
57:09
Speaker 2: between being one familiar thing and another. It tells us
57:12
Speaker 2: that the familiar categories were never fundamental enough in the
57:15
Speaker 2: first place. The wavefunction became one of the central tools
57:19
Speaker 2: for describing this unfamiliar reality. It contains the amplitudes associated
57:23
Speaker 2: with different possible outcomes, and evolves according to quantum mechanical rules.
57:29
Speaker 2: From it, physicists can calculate measurable probabilities, and those calculations
57:33
Speaker 2: agree with experiments to extraordinary precision. The word probability can
57:38
Speaker 2: make all of this sound imprecise. It is anything but.
57:42
Speaker 2: Quantum mechanics can predict the statistical pattern produced by vast
57:47
Speaker 2: numbers of repeated experiments with remarkable accuracy. What it generally
57:52
Speaker 2: does not provide is the familiar classical story in which
57:55
Speaker 2: every particle travels along one perfectly defined hidden path, while
58:00
Speaker 2: carrying a complete set of definite properties at every moment.
58:04
Speaker 2: Matter had acquired a wavelength. Particles could interfere. Wave functions
58:10
Speaker 2: could describe possibilities. And probability had moved from the edge
58:14
Speaker 2: of physics to the foundations. The smallest pieces of the
58:17
Speaker 2: familiar world were beginning to look much less like tiny
58:20
Speaker 2: versions of familiar objects. They required a language of their own.
58:26
Speaker 2: One of the clearest ways to see the difference between
58:28
Speaker 2: classical physics and quantum physics begins with an experiment that
58:32
Speaker 2: is almost absurdly simple. You need a source, a barrier
58:36
Speaker 2: containing two narrow openings, and a screen placed behind them.
58:40
Speaker 2: That is essentially all. In the early 19th century Thomas
58:45
Speaker 2: Young used a version of this arrangement to investigate the
58:47
Speaker 2: nature of light. At the time physicists were still debating
58:51
Speaker 2: whether light should be understood mainly as a stream of
58:54
Speaker 2: particles or as a wave. Young's experiment produced a result
58:58
Speaker 2: that strongly supported the wave picture. Imagine first that light
59:02
Speaker 2: behaves like a water wave. When a wave encounters two
59:06
Speaker 2: narrow openings each opening can act like a new source
59:09
Speaker 2: of spread waves. Those two wave patterns overlap on the
59:13
Speaker 2: far side. Where the peaks of one wave line up
59:16
Speaker 2: with the peaks of another they reinforce each other. Where
59:19
Speaker 2: a peak meets a trough they can partially or almost
59:22
Speaker 2: completely cancel. This process is called interference. On a screen
59:28
Speaker 2: Interference produces a pattern of alternating bright and dark regions.
59:33
Speaker 2: Bright regions correspond to places where the wave amplitudes reinforce
59:36
Speaker 2: one another. Dark regions correspond to places where they cancel.
59:41
Speaker 2: That pattern is extremely characteristic of waves. Now imagine a
59:45
Speaker 2: very different experiment. Instead of waves, suppose you fire ordinary
59:49
Speaker 2: classical particles toward the two openings. Perhaps tiny pellets. Some
59:54
Speaker 2: pellets pass through the left opening. Others pass through the right.
59:59
Speaker 2: Behind the barrier you would expect most of them to
1:00:01
Speaker 2: land in two broad regions aligned with the two openings.
1:00:05
Speaker 2: There might be some spreading because the particles are not
1:00:07
Speaker 2: launched perfectly, but nothing resembling the alternating pattern of bright
1:00:11
Speaker 2: and dark interference bands should naturally appear. That gives us
1:00:15
Speaker 2: two very different expectations. Classical waves produce interference. Classical particles
1:00:21
Speaker 2: produce two broad concentrations. Quantum experiments refuse to choose so
1:00:26
Speaker 2: neatly between them. Light can be sent through a double
1:00:30
Speaker 2: slit at such low intensity that individual photons are detected
1:00:34
Speaker 2: one at a time. Each detection appears as a localised
1:00:37
Speaker 2: event on the screen. One photon arrives here, another arrives
1:00:42
Speaker 2: somewhere else. At first, the pattern looks almost random, but
1:00:46
Speaker 2: keep repeating the experiment. After enough individual detections have accumulated,
1:00:51
Speaker 2: something remarkable appears. An interference pattern. The photons arrive as
1:00:57
Speaker 2: individual events. Yet the distribution of those events follows a
1:01:01
Speaker 2: pattern associated with waves. It becomes even stranger with matter.
1:01:06
Speaker 2: Electrons can be sent through a double slit arrangement one
1:01:09
Speaker 2: at a time. Each electron is eventually detected at one
1:01:12
Speaker 2: particular location. No detector reports half an electron arriving on
1:01:17
Speaker 2: one side and another half arriving somewhere else. The detections
1:01:21
Speaker 2: remain localised. But as more and more electrons are recorded,
1:01:25
Speaker 2: the same broad interference structure gradually emerges. Bright regions build
1:01:29
Speaker 2: up where electrons are detected more often. Dark regions remain
1:01:33
Speaker 2: where detections are much less likely. The pattern reflects interference,
1:01:38
Speaker 2: and this happens even when the electrons travel through the
1:01:40
Speaker 2: apparatus one at a time. There does not need to
1:01:43
Speaker 2: be a crowd of electrons travelling together and somehow interfering
1:01:46
Speaker 2: with one another. The quantum state associated with each individual
1:01:51
Speaker 2: electron evolves in a way that includes the alternatives provided
1:01:55
Speaker 2: by the two slits. This is where ordinary language begins
1:01:58
Speaker 2: to struggle. It is tempting to ask which slip the
1:02:01
Speaker 2: electron really went through. That sounds like a perfectly reasonable question.
1:02:06
Speaker 2: For a classical particle, it would be. A marble must
1:02:09
Speaker 2: pass through the left opening or the right opening. But
1:02:12
Speaker 2: quantum mechanics does not always permit us to assign that
1:02:15
Speaker 2: same classical story while preserving the observed interference. When the
1:02:20
Speaker 2: experiment is arranged so that no reliable which-path information exists,
1:02:24
Speaker 2: The quantum state contains amplitudes associated with both alternatives. Those
1:02:29
Speaker 2: amplitudes can interfere. The probability of detecting the electron at
1:02:34
Speaker 2: a particular place depends on how those amplitudes combine. In
1:02:38
Speaker 2: that sense, the interference pattern does not arise because the
1:02:41
Speaker 2: electron secretly chose one slit, and we simply fail to
1:02:45
Speaker 2: notice which one. If it were merely an ordinary hidden
1:02:48
Speaker 2: choice of left or right, the interference pattern would not
1:02:51
Speaker 2: follow naturally. The two alternatives must remain coherent. Coherence means
1:02:57
Speaker 2: that the relative phase between different parts of the quantum
1:02:59
Speaker 2: state remains well-defined enough for interference to occur. That phase
1:03:05
Speaker 2: relationship is crucial. If the relationship is disturbed strongly enough,
1:03:10
Speaker 2: the interference disappears, and this is exactly what happens when
1:03:14
Speaker 2: an experiment is modified to reveal which slit the particle
1:03:17
Speaker 2: passed through. Suppose a detector is placed near the opening
1:03:21
Speaker 2: so that the path becomes distinguishable. Now the experimental arrangement
1:03:25
Speaker 2: is different. The particle interacts with the measuring device, or
1:03:29
Speaker 2: with some part of the environment carrying path information. Once reliable,
1:03:34
Speaker 2: which path information becomes physically available, the original interference pattern
1:03:38
Speaker 2: is lost. Instead, the distribution begins to resemble the sum
1:03:43
Speaker 2: of two non-interfering alternatives. This does not require a human
1:03:47
Speaker 2: being to stare at the apparatus. It does not require consciousness.
1:03:52
Speaker 2: The crucial issue is physical interaction and the availability of
1:03:56
Speaker 2: distinguishing information. A detector can register a result automatically in
1:04:01
Speaker 2: an empty laboratory. An environmental interaction can also destroy coherence
1:04:06
Speaker 2: even if nobody ever reads the record. The physics does
1:04:10
Speaker 2: not wait for a person to notice. This point matters
1:04:13
Speaker 2: because the double-slit experiment is often described in a way
1:04:16
Speaker 2: that makes it sound as though reality somehow changes merely
1:04:20
Speaker 2: because a human looks at it. That is an attractive story.
1:04:23
Speaker 2: It is also too simplistic. What changes the experimental behaviour
1:04:28
Speaker 2: is the physical arrangement and the interactions that establish or
1:04:32
Speaker 2: destroy coherence between alternatives. Niels Bohr used the idea of
1:04:37
Speaker 2: complementarity to express one aspect of this problem. Different experimental
1:04:42
Speaker 2: arrangements can reveal different features of a quantum system. An
1:04:45
Speaker 2: arrangement that preserves interference reveals wave-like behaviour. An arrangement that
1:04:50
Speaker 2: provides definite which-path information removes that interference. The two descriptions
1:04:55
Speaker 2: are not simply different camera angles on one hidden classical process.
1:04:59
Speaker 2: They correspond to physically different experimental conditions. This is why
1:05:04
Speaker 2: the double-slit experiment became such a central example in quantum physics.
1:05:09
Speaker 2: It forces the old categories into direct conflict. The electron
1:05:13
Speaker 2: is not behaving like a classical particle following one definite
1:05:16
Speaker 2: little path that merely happens to be unknown. But neither
1:05:20
Speaker 2: is it simply an ordinary classical wave spread through space.
1:05:24
Speaker 2: A classical wave can divide continuously and deposit energy across
1:05:28
Speaker 2: a broad region. An electron is detected as a discrete
1:05:32
Speaker 2: localised event. Quantum mechanics contains both aspects in one framework.
1:05:37
Speaker 2: the quantum state evolves through the apparatus. Its amplitudes associated
1:05:42
Speaker 2: with different possible paths can interfere. Then a measurement produces
1:05:46
Speaker 2: a definite recorded outcome. One electron appears in one place.
1:05:50
Speaker 2: Repeat the experiment many times, and the distribution of those
1:05:53
Speaker 2: individual outcomes reveals the underlying interference. The same general behaviour
1:05:58
Speaker 2: has been observed not only with photons and electrons, but
1:06:02
Speaker 2: with neutrons, atoms, and increasingly large molecules under carefully controlled conditions.
1:06:08
Speaker 2: As the systems become larger, maintaining coherence becomes progressively more
1:06:12
Speaker 2: difficult because interaction with the environment becomes harder to avoid.
1:06:17
Speaker 2: But the underlying lesson remains the same. The experiment does
1:06:20
Speaker 2: not demonstrate that a tiny classical particle literally breaks into
1:06:24
Speaker 2: two miniature copies and then joins itself together again. That
1:06:28
Speaker 2: picture would create more confusion than clarity. What the experiment
1:06:33
Speaker 2: requires is a quantum description in which the state includes
1:06:37
Speaker 2: amplitudes associated with both paths when coherence is maintained. Those
1:06:42
Speaker 2: amplitudes influence the probabilities of later outcomes. Once the paths
1:06:46
Speaker 2: become distinguishable, that interference is removed. There is no classical
1:06:51
Speaker 2: object in everyday experience that behaves in quite this way.
1:06:55
Speaker 2: That is precisely why the experiment remained so useful. It
1:06:59
Speaker 2: strips quantum physics down to something almost embarrassingly simple. A source,
1:07:05
Speaker 2: two openings, a screen, and a result that refuses to
1:07:09
Speaker 2: fit inside the categories we normally use to understand the world.
1:07:13
Speaker 2: The double slit does not merely tell us that small
1:07:17
Speaker 2: things are strange. It tells us something more specific. The
1:07:22
Speaker 2: classical ideas of definite trajectories, independent alternatives and ordinary probabilities
1:07:29
Speaker 2: cannot simply be carried unchanged into the microscopic world. At
1:07:34
Speaker 2: that scale, possibilities can interfere. And the pattern produced by
1:07:39
Speaker 2: those possibilities is every bit as real and measurable as
1:07:42
Speaker 2: the final dots appearing one by one on the screen.
1:07:46
Speaker 2: One of the deepest changes introduced by quantum physics is
1:07:50
Speaker 2: that it changes what we mean by a prediction. In
1:07:53
Speaker 2: classical mechanics, prediction is usually about trajectories, If you know
1:07:58
Speaker 2: where an object is, how fast it is moving, and
1:08:01
Speaker 2: what forces are acting upon it, then Newtonian physics allows you,
1:08:05
Speaker 2: at least in principle, to calculate where that object will
1:08:09
Speaker 2: be later. A planet follows an orbit. A pendulum swings
1:08:14
Speaker 2: through a predictable path. A ball thrown through the air
1:08:18
Speaker 2: traces a trajectory that can be calculated. There may be
1:08:21
Speaker 2: practical difficulties. Measurements are never infinitely precise. and complicated systems
1:08:27
Speaker 2: can become extremely sensitive to small differences in their starting conditions.
1:08:33
Speaker 2: But the classical framework itself is deterministic. The future follows
1:08:37
Speaker 2: from the present. Quantum mechanics is different. A quantum state
1:08:42
Speaker 2: evolves according to precise mathematical rules. But when we ask
1:08:46
Speaker 2: what result a measurement will produce, the theory generally gives
1:08:50
Speaker 2: us probabilities. It tells us what outcomes are possible, and
1:08:54
Speaker 2: how likely each one is to occur. That sounds, at first,
1:08:59
Speaker 2: like a retreat from certainty. It is tempting to think
1:09:03
Speaker 2: that perhaps physicists simply do not know enough. Maybe the
1:09:07
Speaker 2: particle already has a perfectly definite answer hidden somewhere, and
1:09:12
Speaker 2: the probabilities only reflect our ignorance. Sometimes probabilities do mean
1:09:17
Speaker 2: exactly that. If a coin is hidden beneath a cup,
1:09:21
Speaker 2: there is already a definite result even if you have
1:09:23
Speaker 2: not looked. The uncertainty belongs to you. The coin itself
1:09:28
Speaker 2: is not undecided. Quantum probability is more subtle. In standard
1:09:33
Speaker 2: quantum mechanics, the state of a system is represented by
1:09:37
Speaker 2: a wave function, or, more generally, by a quantum state.
1:09:42
Speaker 2: That state does not merely assign ordinary classical probabilities to
1:09:46
Speaker 2: a collection of hidden outcomes. Instead, it contains probability amplitudes,
1:09:52
Speaker 2: and amplitudes behave differently from ordinary probabilities. This distinction is crucial.
1:09:59
Speaker 2: Ordinary probabilities simply add, if there are two completely separate
1:10:03
Speaker 2: ways for something to happen, and there is no interference
1:10:06
Speaker 2: between them, you can add their probabilities. Quantum amplitudes can
1:10:11
Speaker 2: combine first. Depending on their relative phase, they can reinforce
1:10:17
Speaker 2: one another, or partially cancel. Only after those amplitudes have
1:10:21
Speaker 2: been combined do we calculate the probability of a particular result.
1:10:25
Speaker 2: This is exactly why the double-slit experiment produces interference. There
1:10:30
Speaker 2: is an amplitude associated with one path and another amplitude
1:10:35
Speaker 2: associated with the other. When the alternatives remain coherent, those
1:10:39
Speaker 2: amplitudes combine. At some positions on the screen, they reinforce.
1:10:44
Speaker 2: At others, they cancel. The resulting pattern is not obtained
1:10:49
Speaker 2: by simply adding two ordinary probabilities. Quantum amplitudes carry more
1:10:54
Speaker 2: structure than that. Max Born recognised how this mathematical structure
1:10:59
Speaker 2: should be connected to experiment. The rule that bears his
1:11:02
Speaker 2: name is called the Born rule. In its simplest form,
1:11:06
Speaker 2: it tells us that the probability associated with a particular
1:11:09
Speaker 2: outcome is obtained from the squared magnitude of the relevant
1:11:13
Speaker 2: quantum amplitude. For a particle described by a wavefunction in space,
1:11:19
Speaker 2: the squared magnitude of that wavefunction gives a probability density
1:11:24
Speaker 2: for detecting the particle at different positions. Imagine an electron
1:11:28
Speaker 2: whose wavefunction spreads across a region. Quantum mechanics does not
1:11:33
Speaker 2: normally say that the electron is a tiny classical object,
1:11:36
Speaker 2: definitely sitting at one hidden point inside that region, while
1:11:40
Speaker 2: we merely fail to know where it is. The wavefunction
1:11:45
Speaker 2: provides amplitudes associated with possible position measurements. Where the squared
1:11:50
Speaker 2: magnitude of the wavefunction is larger, a detection is more likely.
1:11:55
Speaker 2: Where it is smaller, a detection is less likely. If
1:11:59
Speaker 2: the relevant probability is zero, that result will not occur.
1:12:03
Speaker 2: Perform the measurement once and the electron appears in one
1:12:07
Speaker 2: definite place. Prepare the same quantum state again and repeat
1:12:12
Speaker 2: the experiment and the electron might appear somewhere else. Repeat
1:12:16
Speaker 2: it many times, and a pattern emerges. That pattern follows
1:12:21
Speaker 2: the probability distribution predicted by the wave function. This is
1:12:25
Speaker 2: where quantum mechanics becomes both strange and remarkably precise. One
1:12:32
Speaker 2: individual result may be unpredictable. The statistical distribution of many
1:12:37
Speaker 2: results can be predicted with extraordinary accuracy. That is not
1:12:42
Speaker 2: a contradiction. It is one of the defining features of
1:12:45
Speaker 2: the theory. Suppose quantum mechanics predicts that a particular detector
1:12:51
Speaker 2: has a certain probability of clicking. The theory may not
1:12:54
Speaker 2: tell you which individual trial will produce the click, but
1:12:58
Speaker 2: if the experiment is repeated thousands or millions of times
1:13:01
Speaker 2: under the same conditions, the overall frequency of outcomes can
1:13:05
Speaker 2: agree extremely closely with the quantum prediction. The uncertainty is
1:13:10
Speaker 2: in the individual event, The statistical law can still be precise.
1:13:14
Speaker 2: There is another subtle point here. The evolution of an
1:13:18
Speaker 2: isolated quantum state is itself deterministic. The Schrodinger equation tells
1:13:24
Speaker 2: us how the wavefunction changes with time. Once the initial
1:13:27
Speaker 2: quantum state and the relevant physical conditions are specified, the
1:13:32
Speaker 2: later wavefunction follows according to a definite mathematical rule. So
1:13:36
Speaker 2: quantum mechanics contains two very different kinds of behaviour. The
1:13:40
Speaker 2: evolution of the wavefunction is deterministic. The outcomes of measurements
1:13:45
Speaker 2: are probabilistic. This contrast lies near the heart of the
1:13:48
Speaker 2: measurement problem. Before measurement, the quantum state may evolve smoothly
1:13:53
Speaker 2: according to the Schrödinger equation. When a measurement is made,
1:13:57
Speaker 2: the theory gives probabilities for the possible recorded outcomes, and
1:14:02
Speaker 2: standard textbook quantum mechanics then updates the state to reflect
1:14:06
Speaker 2: the result obtained. This combination has worked extraordinarily well, but
1:14:11
Speaker 2: it has also generated some of the deepest arguments about
1:14:14
Speaker 2: what quantum mechanics means. It is important, though, not to
1:14:19
Speaker 2: confuse unpredictability with imprecision. Quantum theory is not saying that
1:14:24
Speaker 2: anything can happen. The probabilities are tightly constrained. The mathematics
1:14:29
Speaker 2: determines exactly which outcomes are allowed and with what relative probabilities.
1:14:35
Speaker 2: In many experiments, those predictions have been tested to exceptional precision.
1:14:40
Speaker 2: A particularly intuitive example comes from radioactive decay. Consider a
1:14:45
Speaker 2: collection of unstable or atomic nuclei. Quantum mechanics can describe
1:14:49
Speaker 2: the probability that a nucleus will decay during a given
1:14:52
Speaker 2: interval of time. For an individual nucleus, however, there is
1:14:57
Speaker 2: generally no experimentally accessible countdown telling us the exact moment
1:15:02
Speaker 2: at which it will decay. one nucleus may decay almost immediately.
1:15:06
Speaker 2: Another identical nucleus prepared in the same kind of state
1:15:11
Speaker 2: may survive much longer. What can be predicted extremely well
1:15:16
Speaker 2: is the behaviour of a large population. If you begin
1:15:19
Speaker 2: with an enormous collection of radioactive nuclei, the number remaining
1:15:24
Speaker 2: decreases according to a highly regular statistical law. The half-life
1:15:29
Speaker 2: tells us the time over which half of a large
1:15:31
Speaker 2: initial population is expected to decay. For a particular nucleus,
1:15:36
Speaker 2: the half-life is not a scheduled appointment. It does not
1:15:40
Speaker 2: mean the nucleus reaches a certain age and then decides
1:15:43
Speaker 2: its time has come. It is a statement about probability.
1:15:47
Speaker 2: The chance of decay produces a predictable pattern across many nuclei.
1:15:53
Speaker 2: This is one reason probability in quantum physics should not
1:15:56
Speaker 2: be treated as an unfortunate flaw in the theory. The
1:16:00
Speaker 2: probabilistic structure is precisely what makes the correct predictions. It
1:16:05
Speaker 2: is built into the way quantum states connect with measurements,
1:16:08
Speaker 2: and experiments repeatedly confirm those statistical patterns. There have, of course,
1:16:14
Speaker 2: been long arguments over whether a deeper deterministic description might
1:16:18
Speaker 2: exist underneath quantum mechanics. Albert Einstein famously disliked the idea
1:16:23
Speaker 2: that fundamental physics should end with irreducible probabilities. Other physicists
1:16:29
Speaker 2: wondered whether quantum mechanics might be incomplete. and whether additional
1:16:34
Speaker 2: hidden variables could restore a more classical form of certainty.
1:16:38
Speaker 2: Later work, particularly John Stuart Bell's theorem and the experiments
1:16:42
Speaker 2: inspired by it, would place profound restrictions on what certain
1:16:46
Speaker 2: hidden variable theories could look like. But even without entering
1:16:50
Speaker 2: those deeper debates, the practical lesson is clear. Quantum mechanics
1:16:55
Speaker 2: does not predict the microscopic world in the same way
1:16:59
Speaker 2: Newtonian mechanics predict a planetary orbit. It predicts distributions of
1:17:04
Speaker 2: possible outcomes. Those distributions are not vague guesses. They emerge
1:17:09
Speaker 2: from precisely defined amplitudes and the Born rule. And when
1:17:13
Speaker 2: experiments are repeated enough times, the results settle into those
1:17:17
Speaker 2: predicted distributions with striking reliability. So there is a peculiar
1:17:22
Speaker 2: balance at the centre of quantum physics. An individual event
1:17:26
Speaker 2: can resist exact prediction. A million events can produce an
1:17:30
Speaker 2: exquisitely precise pattern. The microscopic future may contain alternatives. The
1:17:37
Speaker 2: mathematics governing their probabilities can still be exact. Quantum physics
1:17:42
Speaker 2: therefore replaced one kind of certainty with something more unfamiliar.
1:17:46
Speaker 2: Not a universe without rules. A universe in which the
1:17:49
Speaker 2: rules themselves tell us the chances. Quantum superposition is one
1:17:54
Speaker 2: of those ideas that sounds impossible. mainly because the words
1:17:58
Speaker 2: we use to describe it come from a world where
1:18:00
Speaker 2: nothing seems to behave that way. In quantum mechanics, superposition
1:18:05
Speaker 2: means that a valid quantum state can be formed from
1:18:08
Speaker 2: a combination of other possible quantum states. That definition is
1:18:12
Speaker 2: precise but not immediately friendly. So imagine a system with
1:18:16
Speaker 2: two possible states. An electron, for example, has a quantum
1:18:20
Speaker 2: property called spin. If we choose a particular direction in
1:18:25
Speaker 2: which to measure that spin, might obtain one of two
1:18:28
Speaker 2: possible outcomes. Quantum mechanics allows the electron to be prepared
1:18:32
Speaker 2: in a state that is not simply one outcome or
1:18:35
Speaker 2: the other, beforehand in the ordinary classical sense. Instead its
1:18:39
Speaker 2: quantum state can be a superposition of both possibilities. That
1:18:43
Speaker 2: does not mean the electron is literally behaving like a
1:18:46
Speaker 2: tiny spinning ball pointing in two directions at one Electron
1:18:52
Speaker 2: spin is itself a genuinely quantum property and does not
1:18:56
Speaker 2: correspond perfectly to an ordinary object rotating in space. The
1:19:00
Speaker 2: important point is that the state contains amplitudes associated with
1:19:04
Speaker 2: more than one possible measurement outcome. Those amplitudes can combine
1:19:09
Speaker 2: they can interfere. And that interference is what makes a
1:19:13
Speaker 2: superposition physically different from simple ignorance. Consider something much more familiar.
1:19:19
Speaker 2: Suppose there is a coin hidden beneath a cup. It
1:19:22
Speaker 2: is either heads or tails, you simply do not know which.
1:19:26
Speaker 2: You might describe your uncertainty by saying there is a
1:19:28
Speaker 2: chance of heads and a chance of tails, but the
1:19:31
Speaker 2: coin itself already has one definite orientation. That is called
1:19:36
Speaker 2: a classical mixture. The uncertainty belongs to your knowledge. A
1:19:40
Speaker 2: coherent quantum superposition is different. The alternatives are contained within
1:19:45
Speaker 2: one quantum state, and their amplitudes possess a relationship known
1:19:49
Speaker 2: as relative phase. That relative phase can influence what happens later.
1:19:54
Speaker 2: If the different parts of the state are brought back
1:19:57
Speaker 2: together under suitable conditions, they can reinforce or cancel one another.
1:20:02
Speaker 2: A hidden, classical coin cannot do that. Its unknown state
1:20:07
Speaker 2: does not interfere with an alternative version of itself. This
1:20:11
Speaker 2: interference is what allows physicists to distinguish a genuine coherent
1:20:16
Speaker 2: superposition from an ordinary mixture of unknown possibilities. The double-slit
1:20:22
Speaker 2: experiment gives a particularly clear example. When an electron travels
1:20:27
Speaker 2: toward two open slits and no reliable which-path information exists
1:20:31
Speaker 2: … The quantum state includes amplitudes associated with the different paths.
1:20:37
Speaker 2: Those amplitudes remain coherent, they eventually overlap, and when they do,
1:20:43
Speaker 2: their relative phase determines whether they reinforce or cancel at
1:20:46
Speaker 2: different positions on the detector. That is why an interference
1:20:50
Speaker 2: pattern can emerge. If the electron were simply passing through
1:20:54
Speaker 2: either the left slit or the right slit while we
1:20:57
Speaker 2: remained ignorant of which one, we would expect the final
1:21:00
Speaker 2: distribution to look like an ordinary mixture of two alternatives.
1:21:05
Speaker 2: We would not obtain the same interference pattern. The quantum
1:21:08
Speaker 2: state behaves differently because the alternatives belong to one coherent superposition.
1:21:14
Speaker 2: A similar idea appears in devices called interferometers. A quantum
1:21:19
Speaker 2: system can be placed into a superposition of two paths.
1:21:23
Speaker 2: Those paths can separate physically, travel through different regions and
1:21:27
Speaker 2: then be recombined. Changing the phase along one path can
1:21:31
Speaker 2: alter the probabilities measured later, even though an individual detection
1:21:35
Speaker 2: still produces one definite result. This makes superposition measurable. It
1:21:41
Speaker 2: is not merely philosophical language added after the mathematics. It
1:21:45
Speaker 2: has observable consequences. The word superposition can, however, become misleading
1:21:51
Speaker 2: when it's translated too quickly into everyday language. You will
1:21:55
Speaker 2: often hear that a quantum object can be in two
1:21:57
Speaker 2: places at once. There is some truth behind that phrase
1:22:01
Speaker 2: when a spatial quantum state contains coherent amplitudes associated with
1:22:05
Speaker 2: separated regions. But it should not be taken to mean
1:22:08
Speaker 2: that a tiny classical ball has somehow duplicated itself and
1:22:13
Speaker 2: is secretly sitting in two ordinary places in exactly the
1:22:17
Speaker 2: same way that two separate marbles might. The quantum state
1:22:21
Speaker 2: is the more fundamental description. Our everyday language is trying
1:22:25
Speaker 2: to catch up with it. The difficulty becomes especially obvious
1:22:29
Speaker 2: when we imagine extending superposition to much larger objects. In 1935,
1:22:35
Speaker 2: Erwin Schrödinger proposed one of the most famous thought experiments
1:22:40
Speaker 2: in physics. It involved a cat. Schrödinger imagined a sealed
1:22:44
Speaker 2: box containing a cat, together with a quantum event whose
1:22:48
Speaker 2: outcome would determine whether a mechanism released poison. If the
1:22:52
Speaker 2: quantum system were placed into a superposition of two possibilities,
1:22:56
Speaker 2: and if the entire apparatus were treated quantum mechanically without
1:22:59
Speaker 2: any collapse, then the mathematics appear to lead toward a
1:23:03
Speaker 2: combined state involving both a living cat and a dead cat.
1:23:08
Speaker 2: Schrödinger was not proposing this as a practical experiment. He
1:23:12
Speaker 2: was not suggesting that someone should place an unfortunate animal
1:23:16
Speaker 2: inside a laboratory box to test quantum theory. The example
1:23:20
Speaker 2: was deliberately exaggerated. Its purpose was to expose a conceptual problem,
1:23:26
Speaker 2: Quantum mechanics allows superpositions very naturally at microscopic scales. Yet
1:23:32
Speaker 2: the world around us appears to contain definite macroscopic outcomes.
1:23:37
Speaker 2: We see living cats or dead cats. We do not
1:23:40
Speaker 2: ordinarily encounter coherent combinations of both. So where does the
1:23:44
Speaker 2: transition occur? That question leads directly into the measurement problem.
1:23:49
Speaker 2: But it also highlights something important about superposition itself. There
1:23:54
Speaker 2: is no simple rule in standard quantum mechanics saying that
1:23:58
Speaker 2: superposition stops working once an object becomes larger than some
1:24:02
Speaker 2: particular size. In principle, quantum mechanics applies to atoms, molecules,
1:24:07
Speaker 2: measuring devices and everything else. In practice, however, large objects
1:24:12
Speaker 2: interact constantly with their surroundings. Photons strike them. Air molecules
1:24:17
Speaker 2: collide with them. Thermal motion disturbs them. those interactions rapidly
1:24:22
Speaker 2: destroy the delicate phase relationships required for observable interference. This
1:24:28
Speaker 2: process is called decoherence, and it is one major reason
1:24:32
Speaker 2: macroscopic superpositions are extremely difficult to maintain. Even so, physicists
1:24:38
Speaker 2: have created and controlled superpositions in an impressive variety of systems.
1:24:43
Speaker 2: Single photons can be placed into superpositions of different paths
1:24:46
Speaker 2: or polarisation states. Individual atoms can be prepared in superpositions
1:24:52
Speaker 2: of energy states. Molecules can produce interference patterns under carefully
1:24:56
Speaker 2: controlled conditions. Superconducting circuits containing enormous numbers of particles can
1:25:02
Speaker 2: behave collectively as quantum systems and be engineered into superpositions
1:25:07
Speaker 2: of distinct electrical states. These systems are among the building
1:25:11
Speaker 2: blocks used in quantum computing. A quantum bit, often called
1:25:16
Speaker 2: a qubit, can exist in a coherent superposition of two
1:25:19
Speaker 2: basis states. Unlike an ordinary classical bit, which is simply
1:25:24
Speaker 2: zero or one at a particular moment, a qubit can
1:25:27
Speaker 2: be prepared in a combination of both. That does not
1:25:31
Speaker 2: mean a quantum computer simply tries every answer at once
1:25:34
Speaker 2: and magically chooses the correct one. The real power comes
1:25:39
Speaker 2: from controlling amplitudes and relative phases so that useful possibilities
1:25:43
Speaker 2: are reinforced while others are suppressed through interference. Once again,
1:25:48
Speaker 2: phase is doing the important work. A superposition is not
1:25:52
Speaker 2: just a list of possibilities. It is a structured quantum state.
1:25:57
Speaker 2: The components have amplitudes. Those amplitudes can carry relative phases,
1:26:02
Speaker 2: and those phase relationships influence future measurement probabilities. Remove the coherence,
1:26:08
Speaker 2: and the superposition begins to behave more like an ordinary
1:26:11
Speaker 2: statistical mixture. Preserve it, and interference remains possible. This is
1:26:17
Speaker 2: why quantum laboratories devote so much effort to isolation. Cooling
1:26:21
Speaker 2: systems to extremely low temperatures can reduce unwanted thermal interactions.
1:26:27
Speaker 2: Vacuum chambers can remove stray gas molecules. Electromagnetic shielding can
1:26:32
Speaker 2: reduce environmental disturbances. Even then, maintaining coherence can be difficult.
1:26:38
Speaker 2: Nature is rather good at allowing a quantum system to
1:26:41
Speaker 2: become entangled with everything around it. And yet, when isolation succeeds,
1:26:46
Speaker 2: Superposition is not subtle. Its consequences can be measured directly.
1:26:52
Speaker 2: Interference fringes shift. Detector probabilities change. Quantum circuits behave differently.
1:26:59
Speaker 2: Experiments agree with the mathematical predictions. So superposition is not
1:27:04
Speaker 2: a poetic way of saying that physicists are unsure what's happening.
1:27:08
Speaker 2: It is part of the structure of quantum mechanics itself.
1:27:12
Speaker 2: A quantum state can genuinely contain coherent alternatives whose relationship
1:27:17
Speaker 2: affects measurable outcomes. And then comes the part that remains
1:27:21
Speaker 2: so unsettling. When we finally make a measurement, we do
1:27:26
Speaker 2: not normally record several outcomes at once. A detector clicks
1:27:30
Speaker 2: in one place. An electron is found with one spin result.
1:27:35
Speaker 2: A photon appears at one detector rather than another. The
1:27:39
Speaker 2: superposition can contain several coherent possibilities, yet the record we
1:27:43
Speaker 2: obtain is definite. That contrast between the richness of the
1:27:47
Speaker 2: quantum state and the singularity of the measured outcome lies
1:27:51
Speaker 2: at the heart of quantum mechanics. The theory tells us
1:27:55
Speaker 2: extraordinarily well how to calculate the probabilities, it tells us
1:28:00
Speaker 2: how superpositions evolve, it tells us how their amplitudes interfere,
1:28:05
Speaker 2: and it tells us what results experiments should produce. What
1:28:08
Speaker 2: remains less settled is why one definite outcome is the
1:28:12
Speaker 2: one we actually experience. That question is where the mathematics
1:28:16
Speaker 2: of superposition begins to meet the deeper problem of measurement.
1:28:20
Speaker 2: One of the most persistent misunderstandings in quantum physics begins
1:28:23
Speaker 2: with a very ordinary word. Observation. It sounds as though
1:28:28
Speaker 2: a quantum system behaves one way while nobody is watching,
1:28:32
Speaker 2: and then somehow changes its behaviour when a human being
1:28:35
Speaker 2: looks at it. That picture is far too simple. In physics,
1:28:39
Speaker 2: a measurement is fundamentally a physical interaction. A quantum system
1:28:44
Speaker 2: interacts with something else, such as a detector, a measuring device,
1:28:49
Speaker 2: or the surrounding environment, and information about the original system
1:28:53
Speaker 2: becomes correlated with the state of that larger system. No
1:28:57
Speaker 2: conscious observer is required, A detector can register an electron
1:29:02
Speaker 2: in an empty laboratory. A photon can strike a sensor
1:29:06
Speaker 2: while everyone is asleep. A radioactive nucleus can decay without
1:29:10
Speaker 2: waiting patiently for someone to check on it. The problem
1:29:14
Speaker 2: is not that reality needs an audience. The problem is
1:29:18
Speaker 2: what quantum mechanics says about the transition from several possible
1:29:22
Speaker 2: outcomes to one definite recorded result. Before measurement, an isolated
1:29:29
Speaker 2: quantum state evolves according to the Schrodinger equation. That evolution
1:29:33
Speaker 2: is smooth and deterministic. If the initial quantum state is
1:29:38
Speaker 2: known and the relevant physical conditions are specified, the equation
1:29:42
Speaker 2: determines how the state changes with time. There is no
1:29:47
Speaker 2: random jump built into that ordinary evolution. A superposition remains
1:29:52
Speaker 2: a superposition. Its amplitudes change in a precisely calculable way.
1:29:57
Speaker 2: Then a measurement occurs, and suddenly the description seems to change.
1:30:03
Speaker 2: Suppose an electron is prepared in a superposition of two
1:30:06
Speaker 2: possible spin outcomes. A measuring device is arranged to detect
1:30:10
Speaker 2: which result appears. Quantum mechanics assigns probabilities to the possible outcomes.
1:30:16
Speaker 2: But when the measurement is actually recorded, the apparatus does
1:30:20
Speaker 2: not usually display a hazy blend of both results. It
1:30:23
Speaker 2: gives one. Perhaps the detector corresponding to one spin outcome
1:30:28
Speaker 2: registers the electron. The other does not. The final laboratory
1:30:32
Speaker 2: record is definite. Traditional textbook quantum mechanics deals with this
1:30:38
Speaker 2: by introducing the idea of wave function collapse. Before measurement,
1:30:43
Speaker 2: the wave function may contain amplitudes associated with several possible outcomes.
1:30:48
Speaker 2: After the measurement produces one result, the wavefunction is updated
1:30:53
Speaker 2: to the state corresponding to that outcome. The other possibilities
1:30:56
Speaker 2: no longer appear in the same way within the description
1:30:59
Speaker 2: of the measured system. As a calculating rule, this works
1:31:03
Speaker 2: extremely well. The awkward question is what the collapse actually represents.
1:31:08
Speaker 2: Is it a real physical process? Does the wavefunction genuinely
1:31:12
Speaker 2: undergo some sudden change? Is collapse merely a rule for
1:31:16
Speaker 2: updating our description after new information becomes available? Or does
1:31:21
Speaker 2: something else entirely happen underneath the mathematics? Physicists do not
1:31:26
Speaker 2: all agree. This disagreement is one reason the measurement problem
1:31:30
Speaker 2: remains so important. The mathematics tells us very clearly how
1:31:34
Speaker 2: to calculate probabilities. The experiments tell us very clearly what
1:31:39
Speaker 2: results are recorded. The difficulty lies in connecting those two
1:31:43
Speaker 2: parts into one universally accepted picture of what reality itself
1:31:48
Speaker 2: is doing. Historically, Many discussions of this problem became associated
1:31:53
Speaker 2: with what is called the Copenhagen Interpretation. The name is
1:31:57
Speaker 2: linked especially with Niels Bohr and Werner Heisenberg, but it
1:32:02
Speaker 2: is better to think of the Copenhagen Interpretation as a
1:32:05
Speaker 2: family of related historical attitudes, rather than one perfectly defined
1:32:10
Speaker 2: doctrine accepted in exactly the same form by everyone involved. Broadly,
1:32:17
Speaker 2: Copenhagen-style approaches emphasised the distinction between the quantum system and
1:32:23
Speaker 2: the classical measuring arrangement used to describe experimental outcomes. They
1:32:28
Speaker 2: also resisted the demand that quantum theory should always provide
1:32:31
Speaker 2: a familiar classical picture of what exists between measurements. The theory,
1:32:37
Speaker 2: in this view, is deeply tied to the experimental questions
1:32:41
Speaker 2: we ask and the measurable outcomes we obtain. But that
1:32:45
Speaker 2: immediately creates another problem. Where exactly does the quantum system
1:32:50
Speaker 2: end and the measuring apparatus begin? A detector is made
1:32:54
Speaker 2: from atoms. Those atoms are made from electrons and nuclei.
1:32:59
Speaker 2: Those particles obey quantum mechanics. So if quantum theory applies
1:33:03
Speaker 2: to electrons, atoms and molecules, why should it suddenly stop
1:33:08
Speaker 2: applying when enough of them have been assembled into a
1:33:11
Speaker 2: measuring device? This question is sometimes described as the problem
1:33:15
Speaker 2: of the quantum classical boundary. Imagine a photon travelling toward
1:33:20
Speaker 2: a detector. The photon may be described by a quantum
1:33:23
Speaker 2: state containing different possible paths. It then interacts with the detector.
1:33:29
Speaker 2: Perhaps one possible path would trigger the left side of
1:33:31
Speaker 2: the device, while another would trigger the right side. If
1:33:36
Speaker 2: the detector is treated quantum mechanically too, then the mathematics
1:33:40
Speaker 2: can produce an entangled state. One part of that combined
1:33:44
Speaker 2: state contains the photon correlated with the left-hand detector response.
1:33:49
Speaker 2: Another contains the photon correlated with a right-hand response. Now
1:33:53
Speaker 2: the measuring device itself has become part of the quantum description,
1:33:57
Speaker 2: and the problem has not disappeared. It has simply become larger.
1:34:03
Speaker 2: Instead of asking why the photon produced one outcome, We
1:34:06
Speaker 2: are now asking why the combined photon and detector system
1:34:10
Speaker 2: gives one definite macroscopic record. This is where decoherence becomes
1:34:16
Speaker 2: extremely important. The foundations of decoherence were developed through work
1:34:20
Speaker 2: by physicists, including Heinz-Dieter See, and later expanded significantly by
1:34:26
Speaker 2: researchers including Wojciech Zurek. The basic idea is that realistic
1:34:32
Speaker 2: quantum systems almost never remain perfectly isolated. A measuring device
1:34:37
Speaker 2: interacts with its surroundings. Air molecules collide with it. Photons
1:34:43
Speaker 2: scatter from it. Thermal vibrations occur inside it. Information about
1:34:48
Speaker 2: the system spreads rapidly into the environment. Those interactions entangle
1:34:53
Speaker 2: the system with enormous numbers of environmental degrees of freedom.
1:34:59
Speaker 2: As that happens, the relative phases between components of a
1:35:02
Speaker 2: macroscopic superposition become effectively inaccessible. The different alternatives stop producing
1:35:09
Speaker 2: observable interference with one another. This process is called decoherence.
1:35:14
Speaker 2: It happens extraordinarily quickly for everyday objects. A microscopic quantum
1:35:20
Speaker 2: system can sometimes remain coherent long enough for interference to
1:35:23
Speaker 2: be observed. A macroscopic measuring apparatus surrounded by air, light
1:35:28
Speaker 2: and thermal motion becomes entangled with its environment so rapidly
1:35:34
Speaker 2: that interference between macroscopically distinct outcomes becomes effectively impossible to observe. Decoherence, therefore,
1:35:43
Speaker 2: helps explain why the world around us appears classical. It
1:35:47
Speaker 2: explains why a measuring device does not normally display obvious
1:35:51
Speaker 2: interference between one pointer position and another. It explains why
1:35:56
Speaker 2: macroscopic alternatives behave, for practical purposes, as though they have
1:36:01
Speaker 2: become separate. But decoherence does not automatically solve every version
1:36:06
Speaker 2: of the measurement problem. This distinction is important. Decoherence explains
1:36:12
Speaker 2: how interference between alternatives becomes suppressed through environmental interaction. It
1:36:18
Speaker 2: explains why certain states become extremely stable, and why classical
1:36:23
Speaker 2: looking outcomes emerge so naturally. What it does not, by itself,
1:36:28
Speaker 2: necessarily explain is why one particular outcome is the one
1:36:32
Speaker 2: that appears in a single observation. The mathematics can describe
1:36:36
Speaker 2: a larger entangled state containing effectively non-interfering alternatives. The laboratory
1:36:43
Speaker 2: record we experience is still one definite result. Different interpretations
1:36:48
Speaker 2: of quantum mechanics respond to that remaining question in different ways.
1:36:52
Speaker 2: Some retain a fundamental collapse. Some remove collapse from the
1:36:56
Speaker 2: theory entirely. Some introduce additional variables. Others modify the dynamics
1:37:02
Speaker 2: of quantum mechanics. The experimental predictions of these approaches often
1:37:07
Speaker 2: overlap in the regimes we can currently test, which is
1:37:10
Speaker 2: one reason the debate has lasted so long. And through
1:37:14
Speaker 2: all of this, consciousness does not need to be assigned
1:37:17
Speaker 2: any special physical power. A measurement is not magical because
1:37:21
Speaker 2: someone notices it. What matters is interaction, correlation, information, and
1:37:28
Speaker 2: the loss of coherence between alternatives. Human beings eventually read
1:37:32
Speaker 2: the instrument, of course, but the detector does not wait
1:37:36
Speaker 2: politely for us to arrive. This leaves quantum physics in
1:37:40
Speaker 2: an unusual position. There is overwhelming agreement about how to
1:37:45
Speaker 2: use the theory. Physicists can calculate probabilities, design experiments, build lasers,
1:37:52
Speaker 2: control individual atoms and engineer quantum computers using the same
1:37:56
Speaker 2: mathematical framework. The predictions work. The disagreement concerns what that
1:38:02
Speaker 2: successful mathematics is telling us about reality beneath the observations.
1:38:07
Speaker 2: A quantum state can evolve smoothly. It can contain several
1:38:11
Speaker 2: coherent alternatives. A measuring interaction can entangle those alternatives with
1:38:17
Speaker 2: a much larger apparatus. Decoherence can suppress interference between them,
1:38:22
Speaker 2: astonishing efficiency. And yet the world of actual records still
1:38:27
Speaker 2: appears definite. One detector clicks. One pointer moves. One result
1:38:33
Speaker 2: is written into the data. Quantum mechanics tells us with
1:38:37
Speaker 2: extraordinary precision how likely those outcomes are. The deeper question
1:38:41
Speaker 2: is why probability becomes experience as one particular event. That
1:38:46
Speaker 2: is the measurement problem. not a failure of quantum physics
1:38:50
Speaker 2: to predict experiments, but a reminder that predicting what we
1:38:54
Speaker 2: observe and explaining what reality is doing may not be
1:38:58
Speaker 2: quite the same thing. One of the most famous ideas
1:39:02
Speaker 2: in quantum physics is also one of the easiest to misunderstand.
1:39:06
Speaker 2: The uncertainty principle is often described as though the problem
1:39:10
Speaker 2: is simply that particles are very small and our measuring
1:39:13
Speaker 2: equipment is not quite good enough. Perhaps, the story goes,
1:39:18
Speaker 2: An electron really does have a perfectly precise position and momentum,
1:39:22
Speaker 2: but whenever we try to measure one of them, we
1:39:24
Speaker 2: accidentally disturb the other. That is not what the Uncertainty
1:39:29
Speaker 2: Principle fundamentally says. Werner Heisenberg introduced the principle in 1927,
1:39:35
Speaker 2: as quantum mechanics was rapidly taking its modern form. Its
1:39:40
Speaker 2: meaning is built into the structure of quantum states themselves.
1:39:44
Speaker 2: Certain pairs of physical quantities cannot simultaneously possess arbitrarily narrow
1:39:49
Speaker 2: distributions within the same quantum state. The most familiar example
1:39:53
Speaker 2: is position and momentum. Position tells us where something is.
1:39:58
Speaker 2: Momentum describes its motion, and depends upon both mass and velocity.
1:40:03
Speaker 2: In classical physics, there is no fundamental reason an object
1:40:06
Speaker 2: cannot possess an exact position and an exact momentum at
1:40:11
Speaker 2: the same time. A planet can be at a particular
1:40:14
Speaker 2: location while travelling with a particular momentum. A ball can
1:40:19
Speaker 2: leave your hand from one position with a definite speed
1:40:22
Speaker 2: and direction. Any uncertainty usually comes from imperfect measurements. Quantum
1:40:28
Speaker 2: mechanics changes this. If the position of a quantum particle
1:40:32
Speaker 2: is represented very sharply, its momentum must have a wider
1:40:36
Speaker 2: spread of possible values. If its momentum is represented extremely sharply,
1:40:42
Speaker 2: Its position becomes more spread out. The two uncertainties are connected.
1:40:47
Speaker 2: Improving one indefinitely forces the other to grow. This is
1:40:51
Speaker 2: not simply an unfortunate engineering problem. Building a better microscope
1:40:56
Speaker 2: does not remove it. Waiting for more advanced technology does
1:41:00
Speaker 2: not remove it. The relationship belongs to the quantum state itself.
1:41:04
Speaker 2: One of the gentlest ways to understand why comes from
1:41:07
Speaker 2: thinking about waves. Imagine a perfectly regular wave extending through space.
1:41:13
Speaker 2: It has one clear wavelength. Because wavelength and momentum are
1:41:17
Speaker 2: connected through the de Broglie relationship, a quantum state with
1:41:21
Speaker 2: one very precisely defined wavelength can correspond to a sharply
1:41:26
Speaker 2: defined momentum. But there is a problem. A perfectly extended
1:41:31
Speaker 2: wave is not localised anywhere. It stretches across space. there
1:41:36
Speaker 2: is no narrow region that can be identified as its
1:41:38
Speaker 2: particular position. Now imagine trying to create something more localised.
1:41:44
Speaker 2: To produce a compact wave packet, you need to combine
1:41:47
Speaker 2: waves with different wavelengths. Those waves overlap in such a
1:41:51
Speaker 2: way that they reinforce one another strongly within a limited region,
1:41:55
Speaker 2: while cancelling more effectively elsewhere. The result can be concentrated
1:42:00
Speaker 2: into a smaller area. But because several wavelengths are now present,
1:42:05
Speaker 2: several corresponding momenta are present as well. The position has
1:42:10
Speaker 2: become more definite. The momentum has become less definite. Make
1:42:15
Speaker 2: the packet narrower still and an even wider range of
1:42:18
Speaker 2: wavelengths is required. That produces an even wider spread in momentum.
1:42:23
Speaker 2: So the uncertainty principle is closely connected to the wave-like
1:42:27
Speaker 2: structure of quantum mechanics. Localization comes at a price. A
1:42:32
Speaker 2: quantum state cannot be arbitrarily narrow in both position and
1:42:35
Speaker 2: momentum at once. The mathematical relationship can be stated without
1:42:40
Speaker 2: becoming buried in algebra. The uncertainty in position multiplied by
1:42:44
Speaker 2: the uncertainty in momentum cannot be smaller than a fundamental
1:42:48
Speaker 2: lower limit determined by Planck's reduced constant. Planck's reduced constant
1:42:53
Speaker 2: is simply Planck's constant divided by 2 times pi. Its
1:42:58
Speaker 2: appearance is another reminder that this uncertainty is specifically quantum mechanical.
1:43:04
Speaker 2: The same tiny constant that appeared at the birth of
1:43:07
Speaker 2: quantum theory sets the scale of this limitation. For everyday objects,
1:43:12
Speaker 2: that scale is so small that the uncertainty is usually
1:43:16
Speaker 2: completely negligible. A tennis ball can have a position and
1:43:19
Speaker 2: momentum sufficiently well defined that ordinary classical mechanics works beautifully. Technically,
1:43:27
Speaker 2: quantum uncertainty still applies. Practically, you will not lose track
1:43:31
Speaker 2: of the tennis ball because Heisenberg has become involved. For
1:43:35
Speaker 2: electrons and atoms, however, the effect becomes fundamental. There is
1:43:39
Speaker 2: another common misconception worth removing. The uncertainty principle does not
1:43:44
Speaker 2: say that everything in quantum physics is permanently fuzzy. Quantum
1:43:49
Speaker 2: mechanics can describe some quantities extremely precisely. A system can
1:43:53
Speaker 2: be prepared in a state with a sharply defined value
1:43:57
Speaker 2: of a particular observable. An electron can, for example, occupy
1:44:01
Speaker 2: a state with a precisely specified energy under appropriate conditions.
1:44:06
Speaker 2: The problem concerns particular pairs of observables whose mathematical operators
1:44:12
Speaker 2: do not commute. That phrase sounds technical, but the underlying
1:44:16
Speaker 2: point is manageable. In quantum mechanics, physical quantities are represented
1:44:21
Speaker 2: mathematically by operators. For some pairs, the order in which
1:44:25
Speaker 2: those operators are applied matters. When that happens, the structure
1:44:29
Speaker 2: of the theory produces an uncertainty relationship between the corresponding observables.
1:44:35
Speaker 2: Position and momentum are the most famous pair but they
1:44:38
Speaker 2: are not the only example. Different components of angular momentum
1:44:42
Speaker 2: also have uncertainty relationships. Quantum uncertainty is therefore not one
1:44:47
Speaker 2: general cloud of ignorance hanging over everything. It has a
1:44:50
Speaker 2: precise mathematical structure. and that structure has physical consequences. One
1:44:57
Speaker 2: of those consequences appears in the atom. Imagine trying to
1:45:01
Speaker 2: force an electron into an extremely tiny region right on
1:45:04
Speaker 2: top of an atomic nucleus. If the electron's position became
1:45:09
Speaker 2: extraordinarily well confined, the uncertainty principle tells us that its
1:45:13
Speaker 2: momentum would have to acquire a very large spread. Large
1:45:18
Speaker 2: momentum corresponds to substantial kinetic energy. so squeezing the electron
1:45:23
Speaker 2: into an arbitrarily tiny volume would not simply produce a
1:45:27
Speaker 2: neat motionless particle resting inside the nucleus. The quantum state
1:45:32
Speaker 2: would involve very large momentum components and therefore a large
1:45:37
Speaker 2: energetic cost. This contributes to the stability and characteristic size
1:45:42
Speaker 2: of atoms. Atomic stability has several ingredients, including the quantum
1:45:47
Speaker 2: mechanics of bound states and the electromagnetic attraction between negatively
1:45:52
Speaker 2: charged electrons and positively charged nuclei. But the uncertainty principle
1:45:58
Speaker 2: gives a useful intuitive picture of why electrons cannot simply
1:46:02
Speaker 2: be compressed indefinitely into the smallest possible region. Confinement increases
1:46:08
Speaker 2: the momentum spread. That increases the kinetic energy associated with
1:46:12
Speaker 2: the state. The lowest energy arrangement therefore involves a balance.
1:46:17
Speaker 2: The electron remains bound to the nucleus, but its quantum
1:46:20
Speaker 2: state extends across a finite region around it. This is
1:46:24
Speaker 2: part of why the ground state of hydrogen has a
1:46:27
Speaker 2: non-zero size rather than collapsing into a point. The same
1:46:32
Speaker 2: underlying principle becomes important elsewhere in physics. Confinement of quantum
1:46:37
Speaker 2: particles influences the behaviour of matter under extreme conditions. In
1:46:43
Speaker 2: white dwarf stars, for example, quantum mechanics and the Pauli
1:46:47
Speaker 2: exclusion principle produce electron degeneracy pressure, resisting gravitational compression. The
1:46:54
Speaker 2: uncertainty principle provides part of the intuitive background for why
1:46:58
Speaker 2: tightly confining particles leads to increasingly large momenta, although the
1:47:03
Speaker 2: complete explanation of degeneracy pressure requires the statistics of identical
1:47:08
Speaker 2: fermions as well. It is also important to distinguish Heisenberg's
1:47:13
Speaker 2: principle from the simple act of disturbing something during measurement.
1:47:18
Speaker 2: Suppose you tried to locate a tiny classical object by
1:47:21
Speaker 2: bouncing light from it. The light could disturb the object.
1:47:26
Speaker 2: That would certainly create a practical measurement problem. Heisenberg himself
1:47:30
Speaker 2: discussed ideas involving such measurement disturbances, and the famous thought
1:47:35
Speaker 2: experiment involving a gamma-ray microscope helped build intuition around the subject.
1:47:41
Speaker 2: But the modern mathematical uncertainty principle goes deeper. Even before
1:47:46
Speaker 2: anyone performs a measurement, The quantum state can possess intrinsic
1:47:50
Speaker 2: spreads in position and momentum. No clumsy observer needs to interfere.
1:47:57
Speaker 2: The limitation is already there. This reveals another way in
1:48:00
Speaker 2: which quantum mechanics departs from the classical picture. Classical intuition
1:48:05
Speaker 2: encourages us to imagine that every object must secretly possess
1:48:09
Speaker 2: a complete catalogue of definite properties. A precise position. A
1:48:14
Speaker 2: precise momentum. a definite value for every relevant quantity. Perhaps
1:48:20
Speaker 2: we simply have not measured them yet. Quantum mechanics does
1:48:23
Speaker 2: not generally permit that picture to be carried over unchanged.
1:48:28
Speaker 2: The quantum state itself determines which quantities can be sharply
1:48:32
Speaker 2: defined together and which cannot. That does not make nature chaotic.
1:48:37
Speaker 2: It gives nature a different structure. The uncertainty principle is
1:48:43
Speaker 2: therefore not really a statement about the limits of human knowledge.
1:48:47
Speaker 2: It is a statement about the kinds of quantum states
1:48:49
Speaker 2: that can physically exist. A particle can be strongly localised.
1:48:55
Speaker 2: It can have very sharply defined momentum. But quantum mechanics
1:48:59
Speaker 2: does not allow both descriptions to become arbitrarily exact at
1:49:03
Speaker 2: the same time. The uncertainty is not hiding behind our instruments.
1:49:07
Speaker 2: It is already written into the rules. Quantum tunnelling sounds
1:49:12
Speaker 2: like the sort of thing physics should forbid. Imagine rolling
1:49:15
Speaker 2: a ball toward a hill. If the ball has enough energy,
1:49:18
Speaker 2: it reaches the top and continues over the other side.
1:49:21
Speaker 2: If it does not, it slows down, stops, and rolls back.
1:49:26
Speaker 2: Classical physics gives us no reason to expect the ball
1:49:29
Speaker 2: to suddenly appear beyond the hill without ever having had
1:49:32
Speaker 2: enough energy to climb it. At quantum scales, however …
1:49:37
Speaker 2: Particles are not described simply as tiny classical objects following
1:49:41
Speaker 2: one definite path. They are described by quantum states and
1:49:45
Speaker 2: wave functions, and those wave functions can extend into regions
1:49:49
Speaker 2: that classical physics would call forbidden. Suppose a particle approaches
1:49:54
Speaker 2: a potential barrier. The barrier might represent a region where,
1:49:58
Speaker 2: according to classical mechanics, the particle does not have enough
1:50:02
Speaker 2: energy to enter. A classical particle would simply be reflected.
1:50:06
Speaker 2: A quantum wave function behaves differently. As it reaches the barrier,
1:50:11
Speaker 2: the wave function does not necessarily drop instantly to zero. Instead,
1:50:16
Speaker 2: it can extend into the barrier. Usually its amplitude decreases
1:50:20
Speaker 2: rapidly as it penetrates farther into that classically forbidden region.
1:50:25
Speaker 2: But if the barrier is thin enough, the wave function
1:50:28
Speaker 2: can retain a non-zero amplitude on the far side. That
1:50:32
Speaker 2: means there is a calculable probability that a later measurement
1:50:36
Speaker 2: will find the particle beyond the barrier, this is quantum tunnelling.
1:50:40
Speaker 2: The particle has not smashed through the barrier in the
1:50:43
Speaker 2: ordinary sense, nor has it climbed over it. The quantum
1:50:47
Speaker 2: state simply extends through a region that a classical trajectory
1:50:51
Speaker 2: could never cross. One of the most important points is
1:50:55
Speaker 2: what tunnelling does not require. You will sometimes hear that
1:50:59
Speaker 2: a quantum particle briefly borrows energy. In order to get
1:51:03
Speaker 2: through the barrier, then gives that energy back afterwards. That
1:51:07
Speaker 2: explanation is catchy. It is also misleading. Quantum tunnelling does
1:51:12
Speaker 2: not require energy conservation to be violated. The particle does
1:51:16
Speaker 2: not need to acquire a temporary secret supply of energy.
1:51:20
Speaker 2: If the particle begins with a particular total energy, that
1:51:23
Speaker 2: energy can remain conserved throughout the tunnelling process. The strange
1:51:28
Speaker 2: part is not that the particle has somehow gained enough
1:51:31
Speaker 2: energy to overcome the barrier. The strange part is that
1:51:35
Speaker 2: quantum mechanics does not demand that a particle cross the
1:51:38
Speaker 2: barrier through an ordinary classical trajectory in the first place.
1:51:42
Speaker 2: The wave function extends where the classical particle could not.
1:51:46
Speaker 2: The size of the tunnelling probability depends strongly on the
1:51:49
Speaker 2: barrier itself. A thin barrier is easier to tunnel through
1:51:54
Speaker 2: than a thick one. A lower barrier generally gives a
1:51:58
Speaker 2: greater tunnelling probability than a higher one. The mass of
1:52:01
Speaker 2: the particle matters too. Lighter particles tend to have much
1:52:05
Speaker 2: greater tunnelling probabilities than heavier ones under comparable conditions. That
1:52:10
Speaker 2: is one reason tunnelling is especially important for electrons and
1:52:14
Speaker 2: other very light particles. For a large everyday object, such
1:52:18
Speaker 2: as a tennis ball or a person, the probability of
1:52:21
Speaker 2: tunnelling through an ordinary wall is so fantastically small that
1:52:25
Speaker 2: it can safely be ignored. Quantum mechanics does not literally
1:52:30
Speaker 2: make it impossible in every abstract mathematical sense. but you
1:52:33
Speaker 2: should probably continue using doors. At atomic scales, tunnelling is
1:52:39
Speaker 2: not rare at all. It is part of real physical processes.
1:52:44
Speaker 2: One of the earliest major applications came from nuclear physics.
1:52:48
Speaker 2: Some unstable atomic nuclei undergo alpha decay. In alpha decay,
1:52:53
Speaker 2: the nucleus emits an alpha particle, which consists of two
1:52:57
Speaker 2: protons and two neutrons bound together. The puzzle was that
1:53:01
Speaker 2: an alpha particle inside the nucleus appeared to be trapped
1:53:04
Speaker 2: behind a potential barrier. The strong nuclear interaction helps bind
1:53:09
Speaker 2: nuclear particles at very short ranges, while the positively charged
1:53:14
Speaker 2: alpha particle also experiences electrical repulsion from the positively charged
1:53:19
Speaker 2: daughter nucleus once it moves outward. Classically, an alpha particle
1:53:23
Speaker 2: without enough energy to overcome the barrier should remain trapped.
1:53:27
Speaker 2: Yet alpha decay clearly happens. In the late 1920s, George
1:53:31
Speaker 2: Gamow applied the emerging quantum theory of tunnelling to the problem.
1:53:36
Speaker 2: The alpha particle's wave function could extend through the nuclear barrier.
1:53:40
Speaker 2: There was therefore a small but non-zero probability that the
1:53:43
Speaker 2: particle would appear outside the nucleus. Once beyond the barrier,
1:53:48
Speaker 2: the electrical repulsion could carry it away. This provided a
1:53:52
Speaker 2: remarkably successful explanation of alpha decay. It also demonstrated something
1:53:58
Speaker 2: important about quantum probabilities, The probability of tunnelling might be
1:54:02
Speaker 2: small during any particular brief interval, but given enough time
1:54:06
Speaker 2: and enough unstable nuclei, the process becomes measurable and predictable statistically.
1:54:12
Speaker 2: Different nuclei have different barrier properties, helping explain why radioactive
1:54:17
Speaker 2: half-lives can vary enormously. Tunnelling later became not only something
1:54:23
Speaker 2: physicists observed, but something they deliberately used. One of the
1:54:28
Speaker 2: most striking examples is the scanning tunnelling microscope. This instrument
1:54:33
Speaker 2: allows scientists to investigate the surfaces of materials at extraordinarily
1:54:38
Speaker 2: small scales. A very sharp conducting tip is brought extremely
1:54:42
Speaker 2: close to a conducting or semi-conducting surface. The tip does
1:54:47
Speaker 2: not need to make ordinary physical contact with the surface. Instead,
1:54:51
Speaker 2: electrons can tunnel across the tiny gap between them. The
1:54:55
Speaker 2: tunnelling current is extraordinarily sensitive to distance. Move the tip
1:55:00
Speaker 2: only a tiny amount closer to the surface and the
1:55:03
Speaker 2: current can change dramatically. By carefully moving the tip across
1:55:08
Speaker 2: material while monitoring this current, researchers can reconstruct information about
1:55:13
Speaker 2: the surface on atomic scales. The effect is almost beautifully ironic.
1:55:19
Speaker 2: A phenomenon that sounds as though particles are somehow cheating
1:55:22
Speaker 2: their way through impossible barriers became a precision tool for
1:55:27
Speaker 2: examining individual atoms. Quantum tunnelling is also deeply important in electronics.
1:55:33
Speaker 2: Electrons can tunnel through very thin insulating layers between conducting regions.
1:55:39
Speaker 2: Engineers can deliberately exploit this behaviour in tunnel junctions and
1:55:43
Speaker 2: other devices. In some forms of memory technology, tunnelling helps
1:55:48
Speaker 2: move electrons through insulating barriers so that electrical charge can
1:55:53
Speaker 2: be stored or removed. But tunnelling can also become a nuisance.
1:55:58
Speaker 2: As electronic components become smaller, insulating barriers inside them can
1:56:03
Speaker 2: become only a few nanometres thick. At that scale, electrons
1:56:08
Speaker 2: become increasingly capable of tunnelling through regions that engineers would
1:56:12
Speaker 2: prefer them not to cross. This can create leakage currents.
1:56:16
Speaker 2: So one of the limits confronting the continuing miniaturisation of
1:56:20
Speaker 2: electronics is not merely whether humans can manufacture sufficiently small components.
1:56:26
Speaker 2: The quantum behaviour of electrons themselves becomes harder to ignore.
1:56:30
Speaker 2: Tunnelling also reaches far beyond laboratories and computer chips. It
1:56:35
Speaker 2: plays an important role inside stars. The centre of the
1:56:39
Speaker 2: Sun is extraordinarily hot, at roughly 15 million degrees. That
1:56:44
Speaker 2: sounds more than sufficient for almost anything, but protons present
1:56:48
Speaker 2: a special problem. Each proton carries positive electric charge. When
1:56:54
Speaker 2: two protons approach one another, they repel through the electromagnetic interaction.
1:56:59
Speaker 2: For nuclear fusion to occur, they must come close enough
1:57:02
Speaker 2: for the strong nuclear interaction to become important. Classically, many
1:57:07
Speaker 2: protons in the solar core do not possess enough kinetic
1:57:10
Speaker 2: energy to overcome the full electrical barrier between them directly.
1:57:15
Speaker 2: That barrier is known as the Coulomb barrier. If classical
1:57:19
Speaker 2: mechanics were the entire story, fusion under solar conditions would
1:57:24
Speaker 2: be vastly less likely. Quantum mechanics changes the calculation. The
1:57:29
Speaker 2: quantum wave functions associated with the protons can penetrate the
1:57:33
Speaker 2: Coulomb barrier. There is a small probability that two protons
1:57:37
Speaker 2: can tunnel sufficiently close together for nuclear processes to become possible.
1:57:43
Speaker 2: Even then, the first step of the main proton-proton chain
1:57:46
Speaker 2: is extraordinarily slow. Two protons must not only approach one another,
1:57:52
Speaker 2: but one must be transformed into a neutron through the
1:57:54
Speaker 2: weak nuclear interaction so that a deuterium nucleus can form.
1:57:59
Speaker 2: That weak interaction step is one reason the Sun burns
1:58:02
Speaker 2: so slowly, and from our point of view, slowly is excellent.
1:58:07
Speaker 2: If fusion proceeded much more rapidly, the Sun would not
1:58:11
Speaker 2: provide billions of years of relatively steady energy. Quantum tunnelling
1:58:16
Speaker 2: therefore contributes to an extraordinary balance. The core of the
1:58:20
Speaker 2: Sun is hot and dense enough for fusion to occur,
1:58:23
Speaker 2: but not so overwhelmingly efficient that the fuel disappears quickly.
1:58:28
Speaker 2: Tunnelling helps make reactions possible at temperatures far below what
1:58:32
Speaker 2: a simple classical picture would suggest was required. The sunlight
1:58:36
Speaker 2: reaching Earth is therefore connected in part to particles doing
1:58:40
Speaker 2: something that an ordinary classical trajectory would forbid. Every second,
1:58:46
Speaker 2: immense numbers of fusion reactions occur inside the Sun. Energy
1:58:50
Speaker 2: eventually works its way outward and escapes into space as radiation.
1:58:56
Speaker 2: Some of that light crosses roughly 150 million kilometres and
1:58:59
Speaker 2: reaches Earth. It warms the ground. It drives weather. It
1:59:04
Speaker 2: powers photosynthesis. And all of it depends, somewhere deep in
1:59:09
Speaker 2: the chain of events, on quantum physics. Tunnelling is not
1:59:12
Speaker 2: a loophole in the laws of nature. It is one
1:59:15
Speaker 2: of the laws. The barrier is only impossible from the
1:59:18
Speaker 2: classical point of view. Quantum mechanics never promised that particles
1:59:22
Speaker 2: would behave like miniature balls trying to climb over hills.
1:59:26
Speaker 2: Their wavefunctions can extend into forbidden regions, their amplitudes can
1:59:30
Speaker 2: survive to the other side, and sometimes, when a measurement
1:59:34
Speaker 2: is made, that is exactly where the particle is found.
1:59:38
Speaker 2: What sounds like one of the strangest tricks in quantum
1:59:41
Speaker 2: physics turns out to be involved in radioactive decay, microscopy, electronics,
1:59:47
Speaker 2: and the steady light of the sun. The supposedly impossible
1:59:52
Speaker 2: barrier was never quite as absolute as classical intuition made
1:59:55
Speaker 2: it seem. Quantum entanglement is one of the most famous
1:59:59
Speaker 2: ideas in modern physics, partly because it seems to violate
2:00:03
Speaker 2: one of the most deeply rooted expectations we have about
2:00:06
Speaker 2: the universe. The expectation is simple. If two objects are
2:00:11
Speaker 2: far apart, then whatever happens to one should not instantly
2:00:15
Speaker 2: determine what can be said about the other. In ordinary life,
2:00:19
Speaker 2: that seems perfectly reasonable. Two coins placed on opposite sides
2:00:23
Speaker 2: of a room can be completely independent. Two clocks can
2:00:26
Speaker 2: tick without affecting one another. Two particles separated by a
2:00:31
Speaker 2: great distance might be expected to carry their own individual properties,
2:00:35
Speaker 2: whether or not anyone has measured them yet. Quantum mechanics
2:00:39
Speaker 2: allow something stranger. Two or more quantum systems can be
2:00:42
Speaker 2: prepared in a single joint state that cannot be separated
2:00:46
Speaker 2: into independent descriptions of each part. That situation is called entanglement.
2:00:52
Speaker 2: The individual systems still occupy different places, They can be
2:00:55
Speaker 2: carried into different laboratories. They can even be separated by
2:00:59
Speaker 2: enormous distances, but their quantum description remains shared. A simple
2:01:04
Speaker 2: example involves spin. Imagine two particles prepared in an entangled
2:01:09
Speaker 2: state so that when their spins are measured along the
2:01:12
Speaker 2: same chosen direction, the results are correlated. If one particle
2:01:16
Speaker 2: is found with one result, the other may be found
2:01:19
Speaker 2: with the opposite result. At first, that does not seem
2:01:23
Speaker 2: especially remarkable. A classical trick could produce correlations too. Place
2:01:28
Speaker 2: one red glove in one box and one blue glove
2:01:31
Speaker 2: in another. Then send the boxes to opposite sides of
2:01:34
Speaker 2: the world. Open one box and discover the red glove.
2:01:38
Speaker 2: You immediately know the other contains the blue glove. Nothing
2:01:42
Speaker 2: mysterious has travelled between the boxes. The colours were decided
2:01:45
Speaker 2: from the beginning. Quantum entanglement is more demanding than that.
2:01:50
Speaker 2: Physicists can choose between different measurement settings. For example, the
2:01:54
Speaker 2: spin of each particle can be measured along different directions,
2:01:58
Speaker 2: or the polarisation of photons can be tested at different angles.
2:02:03
Speaker 2: Quantum mechanics predict correlations between the results that depend on
2:02:07
Speaker 2: those measurement choices, and under suitable conditions, those correlations are
2:02:11
Speaker 2: stronger than can be explained by a broad class of
2:02:14
Speaker 2: theories in which the particle simply left the source, carrying
2:02:19
Speaker 2: predetermined local instructions for every possible measurement. This possibility troubled
2:02:24
Speaker 2: Albert Einstein deeply. In 1935, Einstein worked with Boris Podolsky
2:02:31
Speaker 2: and Nathan Rosen on an argument that became one of
2:02:34
Speaker 2: the most important challenges to quantum mechanics. Their paper asked
2:02:39
Speaker 2: whether the theory provided a complete description of physical reality.
2:02:43
Speaker 2: The basic concern was that quantum mechanics seemed to allow
2:02:46
Speaker 2: correlations between distant systems that were difficult to reconcile with locality.
2:02:52
Speaker 2: Locality in this context is the idea that influences should
2:02:55
Speaker 2: not propagate instantaneously across arbitrary distances. Einstein was also famously
2:03:01
Speaker 2: uncomfortable with the probabilistic character of quantum mechanics. His objection
2:03:06
Speaker 2: is commonly summarised by the phrase, God does not play dice.
2:03:11
Speaker 2: The precise wording appeared in correspondence rather than as a
2:03:14
Speaker 2: formal scientific principle. But it captured his broader discomfort with
2:03:19
Speaker 2: the idea that fundamental physical events might not possess definite
2:03:22
Speaker 2: outcomes before measurement. Einstein did not deny that quantum mechanics
2:03:27
Speaker 2: produced successful predictions. He questioned whether the theory might be incomplete.
2:03:33
Speaker 2: Perhaps hidden variables existed beneath the visible mathematics. Perhaps the
2:03:37
Speaker 2: particles carried additional information that quantum mechanics did not describe.
2:03:43
Speaker 2: If those hidden variables determined the outcomes locally, then the
2:03:46
Speaker 2: strange correlations might have an ordinary explanation after all. For decades,
2:03:52
Speaker 2: this debate appeared largely philosophical. Quantum mechanics worked. Its critics
2:03:58
Speaker 2: wondered whether a deeper theory might sit underneath it. Then,
2:04:03
Speaker 2: in the 1960s, the physicist John Stuart Bell discovered a
2:04:07
Speaker 2: way to turn that philosophical disagreement into an experimental question.
2:04:13
Speaker 2: Bell derived a set of statistical limits now known as
2:04:16
Speaker 2: Bell inequalities. These inequalities apply to a broad class of
2:04:21
Speaker 2: local hidden variable theories. If measurement outcomes were determined by
2:04:26
Speaker 2: local pre-existing properties, of the kind Einstein hoped might exist,
2:04:31
Speaker 2: then correlations between distant measurements could not exceed certain limits.
2:04:36
Speaker 2: Quantum mechanics predicted that entangled particles could violate those limits.
2:04:41
Speaker 2: This was an extraordinary development. The argument was no longer
2:04:45
Speaker 2: simply about preferred interpretations of a mathematical theory. Experiments could
2:04:50
Speaker 2: decide whether nature obeyed the constraints imposed by local hidden
2:04:54
Speaker 2: variable models. From the 1970s onward, increasingly sophisticated Bell tests
2:05:00
Speaker 2: were performed. Among the most influential were experiments carried out
2:05:05
Speaker 2: in the early 1980s by Alain Aspect and his collaborators
2:05:09
Speaker 2: in France. They used entangled photons and measured their polarisations
2:05:14
Speaker 2: under different experimental settings. The observed correlations violated Bell inequalities
2:05:20
Speaker 2: in the way quantum mechanics predicted. Later experiments became far
2:05:24
Speaker 2: more precise. Researchers improved detector efficiencies. They increased the distances
2:05:30
Speaker 2: between measurement stations. They changed measurement settings rapidly enough to
2:05:34
Speaker 2: reduce the possibility that ordinary slower-than-light communication could coordinate the results.
2:05:40
Speaker 2: By the 21st century, several major experiments had closed combinations
2:05:45
Speaker 2: of the principal loopholes that had affected earlier tests. The
2:05:49
Speaker 2: broad result has remained consistent. Quantum predictions survive, Bell inequalities
2:05:55
Speaker 2: are violated, and broad classes of local hidden variable explanations fail.
2:06:01
Speaker 2: That does not mean one specific interpretation of quantum mechanics
2:06:04
Speaker 2: has been proven correct. Bell's theorem does not tell us
2:06:08
Speaker 2: that the Copenhagen interpretation must be true, or that many
2:06:11
Speaker 2: worlds must be true, or that some other philosophical picture
2:06:15
Speaker 2: is one by default. What it tells us is more precise.
2:06:20
Speaker 2: Nature cannot be explained by a simple theory in which
2:06:23
Speaker 2: measurement outcomes are determined entirely by local hidden variables carrying
2:06:28
Speaker 2: pre-existing instructions of the kind Bell's inequalities constrain. Something in
2:06:34
Speaker 2: our classical expectations has to give. The word non-local is
2:06:38
Speaker 2: often used here, but it needs care. Entanglement produces correlations
2:06:43
Speaker 2: between distant measurements that cannot be reproduced by the relevant
2:06:47
Speaker 2: local hidden variable models. But entanglement does not allow us
2:06:51
Speaker 2: to send a controllable message faster than light. Suppose two
2:06:55
Speaker 2: observers share an entangled pair. One measures their particle. The
2:07:00
Speaker 2: outcome appears random. They cannot choose what result they obtain.
2:07:04
Speaker 2: The other observer also receives a result that appears random
2:07:07
Speaker 2: when viewed locally. Only later, when the two observers compare
2:07:11
Speaker 2: their records through ordinary communication, can the correlations become visible.
2:07:16
Speaker 2: That comparison cannot travel faster than light, so relativity is
2:07:21
Speaker 2: not casually broken by entanglement. There is no useful faster-than-light
2:07:25
Speaker 2: telephone hidden inside the experiment. The distance between the particles
2:07:30
Speaker 2: is also not the main factor, controlling whether the entanglement survives.
2:07:35
Speaker 2: In principle, entangled correlations can persist across very large separations.
2:07:41
Speaker 2: What matters is whether the quantum state remains sufficiently isolated
2:07:45
Speaker 2: from interactions that destroy coherence. An unwanted collision with an atom,
2:07:50
Speaker 2: a stray photon, thermal noise or other environmental disturbances can
2:07:54
Speaker 2: entangle the system with its surroundings and degrade the original correlations.
2:07:59
Speaker 2: This is decoherence appearing again. Distance itself does not simply
2:08:04
Speaker 2: weaken entanglement in the way a fading radio signal becomes
2:08:08
Speaker 2: weaker with range. Environmental interaction is often the greater practical problem.
2:08:13
Speaker 2: Experiments have demonstrated entanglement over very large distances. including satellite-based
2:08:19
Speaker 2: tests involving photons distributed between ground stations, separated by hundreds
2:08:25
Speaker 2: or even more than a thousand kilometres. These experiments are
2:08:29
Speaker 2: not merely demonstrations of quantum strangeness. They're also part of
2:08:33
Speaker 2: a growing field called quantum information science. Entanglement can be
2:08:38
Speaker 2: treated as a physical resource. In quantum cryptography, quantum correlations
2:08:43
Speaker 2: can help reveal whether a communication channel has been disturbed
2:08:47
Speaker 2: by an eavesdropper. In quantum teleportation, the state of one
2:08:51
Speaker 2: quantum system can be transferred to another distant system, using
2:08:55
Speaker 2: shared entanglement together with ordinary classical communication. Despite the name,
2:09:01
Speaker 2: the particle itself does not disappear from one location and
2:09:05
Speaker 2: magically reappear somewhere else. What is transferred is the quantum state,
2:09:10
Speaker 2: and because classical information is required, teleportation still cannot transmit
2:09:15
Speaker 2: useful information faster than light. Quantum computing also makes extensive
2:09:21
Speaker 2: use of entanglement. Multiple quantum bits can become correlated in
2:09:25
Speaker 2: ways that have no direct classical equivalent. A carefully designed
2:09:29
Speaker 2: quantum algorithm can manipulate these correlations, together with superposition and interference,
2:09:35
Speaker 2: to perform certain calculations in fundamentally different ways from conventional computers.
2:09:42
Speaker 2: But the deepest importance of entanglement is not technological, it
2:09:46
Speaker 2: is conceptual. Entangled systems force us to abandon the idea
2:09:51
Speaker 2: that every part of a composite quantum system must always
2:09:54
Speaker 2: possess its own completely independent description. Sometimes the most complete
2:09:59
Speaker 2: description belongs to the pair. The individual pieces cannot be
2:10:03
Speaker 2: fully understood as separate objects carrying their own little sets
2:10:07
Speaker 2: of predetermined instructions. That is what made entanglement so uncomfortable
2:10:12
Speaker 2: to Einstein, and it is what continues to make it
2:10:15
Speaker 2: so fascinating now. Quantum mechanics tells us that physical systems
2:10:21
Speaker 2: can be linked through a shared state in a way
2:10:23
Speaker 2: that survives separation, violates classical expectations about local hidden properties,
2:10:29
Speaker 2: and yet still refuses to provide a usable shortcut around
2:10:33
Speaker 2: the speed of light. The particles can be kilometres apart,
2:10:37
Speaker 2: they can be measured in different laboratories, their outcomes can
2:10:40
Speaker 2: look random on their own. And yet when the results
2:10:44
Speaker 2: are brought back together and compared, The correlations reveal that
2:10:47
Speaker 2: the two systems were never completely independent in the first place.
2:10:51
Speaker 2: There is an obvious question hiding behind almost every discussion
2:10:55
Speaker 2: of quantum physics. If the rules are really this strange,
2:10:59
Speaker 2: why does ordinary life look so normal? Tables do not
2:11:02
Speaker 2: appear to be in several places at once. People do
2:11:05
Speaker 2: not produce interference patterns as they walk through doorways. Planets
2:11:09
Speaker 2: do not seem to exist in visible superpositions of different orbits.
2:11:14
Speaker 2: A tennis ball travels through the air in a way
2:11:16
Speaker 2: that Isaac Newton would recognise perfectly well. And yet all
2:11:20
Speaker 2: of these things are built from atoms, electrons and other
2:11:23
Speaker 2: quantum systems. There is no separate set of particles reserved
2:11:27
Speaker 2: for the everyday world. A table is made from quantum matter.
2:11:31
Speaker 2: So is a planet. So are you. The difference is
2:11:36
Speaker 2: not that quantum mechanics suddenly stops applying once something becomes
2:11:39
Speaker 2: large enough. The difference is what happens when enormous numbers
2:11:43
Speaker 2: of quantum particles interact with one another and with everything
2:11:47
Speaker 2: around them. One of the most important ideas here is
2:11:50
Speaker 2: environmental decoherence. A perfectly isolated quantum system can preserve the
2:11:56
Speaker 2: delicate phase relationships required for interference, but ordinary objects are
2:12:00
Speaker 2: almost never isolated. Consider something as simple as a grain
2:12:05
Speaker 2: of dust floating in a room. Photons are constantly bouncing
2:12:09
Speaker 2: from it. molecules collide with it. The atoms inside it
2:12:13
Speaker 2: vibrate because of its temperature. Electromagnetic interactions connect its particles
2:12:19
Speaker 2: with the surrounding environment. Every one of those interactions provides
2:12:23
Speaker 2: another opportunity for information about the grain of dust to
2:12:26
Speaker 2: spread outward. If the object began in a coherent superposition
2:12:31
Speaker 2: of two different positions, then photons scattering from one position
2:12:36
Speaker 2: would carry slightly different information from photons scattering from the other.
2:12:40
Speaker 2: Air molecules would do the same. The surrounding environment would
2:12:44
Speaker 2: gradually become correlated with the different parts of the quantum state.
2:12:49
Speaker 2: In practice, that process can happen extremely quickly. The relative
2:12:54
Speaker 2: phase information needed for the two alternatives to interfere becomes
2:12:58
Speaker 2: dispersed into enormous numbers of environmental degrees of freedom. The
2:13:03
Speaker 2: different parts of the state then cease to produce observable
2:13:06
Speaker 2: interference with one another. That is decoherence. Nothing is needed
2:13:11
Speaker 2: to switch quantum mechanics off. No hidden boundary has been
2:13:15
Speaker 2: crossed where the laws of physics suddenly change. The system
2:13:19
Speaker 2: is still quantum mechanical. It has simply become so thoroughly
2:13:22
Speaker 2: entangled with its environment that the interference associated with the
2:13:26
Speaker 2: original superposition becomes effectively inaccessible. This is one reason large
2:13:32
Speaker 2: objects appear classical. The larger the system, the harder it
2:13:36
Speaker 2: usually becomes to isolate. A single atom can be held
2:13:40
Speaker 2: in a carefully controlled vacuum chamber. A photon can travel
2:13:44
Speaker 2: through a well-shielded experimental apparatus. A small molecule can sometimes
2:13:49
Speaker 2: remain coherent long enough to reveal interference. But an ordinary
2:13:53
Speaker 2: object contains an extraordinary number of particles. It interacts with
2:13:58
Speaker 2: countless photons, gas molecules and internal vibrations every moment. Keeping
2:14:03
Speaker 2: all of those interactions under control becomes extraordinarily difficult. Temperature
2:14:08
Speaker 2: makes the challenge even greater. Warm objects contain internal thermal motion.
2:14:14
Speaker 2: Their atoms and molecules are continually moving, vibrating and interacting.
2:14:20
Speaker 2: That internal complexity creates further routes for quantum information to spread.
2:14:25
Speaker 2: cooling systems to very low temperatures is therefore one of
2:14:29
Speaker 2: the most common techniques used when physicists want to preserve
2:14:33
Speaker 2: quantum coherence for longer periods. Quantum computers, superconducting circuits and
2:14:39
Speaker 2: other delicate quantum devices are often operated under carefully controlled
2:14:43
Speaker 2: conditions partly for this reason. The colder and more isolated
2:14:48
Speaker 2: the system, the easier it can become to preserve the
2:14:50
Speaker 2: phase relationships that make recognisably quantum behaviour visible. For a
2:14:55
Speaker 2: warm everyday object, decoherence can occur on timescales so short
2:15:00
Speaker 2: that any coherent superposition of macroscopically distinct states becomes practically
2:15:05
Speaker 2: impossible to observe. This helps explain why the classical world
2:15:10
Speaker 2: appears so stable. A chair stays where it is. A
2:15:14
Speaker 2: falling cup follows a recognisable path. A person does not
2:15:18
Speaker 2: visibly interfere with another possible path through the room. The
2:15:22
Speaker 2: quantum effects have not vanished. They're buried beneath enormous complexity
2:15:27
Speaker 2: and relentless environmental interaction. Classical physics then emerges as an
2:15:32
Speaker 2: extraordinarily good approximation. This happens elsewhere in science too. Newtonian
2:15:38
Speaker 2: mechanics is not the deepest available description of gravity or motion,
2:15:43
Speaker 2: but it works beautifully for many ordinary situations. You do
2:15:46
Speaker 2: not need general relativity to calculate how long a drop
2:15:50
Speaker 2: set of keys takes to hit the floor. Likewise, You
2:15:53
Speaker 2: do not need a full quantum calculation to predict the
2:15:57
Speaker 2: trajectory of a tennis ball. The deeper theory is still there.
2:16:01
Speaker 2: The simpler theory works because the conditions make the more
2:16:05
Speaker 2: subtle effects negligible. Take the tennis ball. Like every material object,
2:16:10
Speaker 2: it has a de Broglie wavelength associated with its momentum.
2:16:14
Speaker 2: In principle, that means wave-like behaviour belongs to its quantum description.
2:16:19
Speaker 2: In practice, the wavelength is unbelievably small. Because Planck's constant
2:16:25
Speaker 2: is tiny, and the momentum of a tennis ball is
2:16:28
Speaker 2: enormous compared with that of an electron, the resulting wavelength
2:16:32
Speaker 2: is so minuscule that any ordinary interference effect becomes completely unobservable,
2:16:37
Speaker 2: and that is before considering the environment. The ball is
2:16:41
Speaker 2: surrounded by air. Light scatters from it constantly. Its internal
2:16:47
Speaker 2: molecules are vibrating. It is exchanging thermal radiation with its surroundings.
2:16:52
Speaker 2: any carefully prepared spatial superposition would decohere with astonishing speed.
2:17:00
Speaker 2: For all practical purposes, Newtonian mechanics is overwhelmingly the sensible description.
2:17:05
Speaker 2: That does not mean large objects are fundamentally classical. It
2:17:09
Speaker 2: means their quantum behaviour is extraordinarily difficult to expose. Experiments
2:17:15
Speaker 2: have shown this very clearly. Quantum interference is not restricted
2:17:19
Speaker 2: to electrons and photons. Physicists have observed interference with atoms.
2:17:24
Speaker 2: They have also demonstrated it with increasingly large molecules containing
2:17:28
Speaker 2: many atoms. Under carefully controlled conditions, molecular beams can pass
2:17:33
Speaker 2: through interferometers and produce patterns that reveal coherent quantum behaviour.
2:17:38
Speaker 2: The molecules involved are vastly more complicated than the individual electrons.
2:17:43
Speaker 2: They contain many nuclei and many electrons, yet the centre
2:17:47
Speaker 2: of mass motion of the entire molecule can still disappear.
2:17:50
Speaker 2: wave-like interference. This is important because it shows that there
2:17:54
Speaker 2: is no simple rule saying quantum mechanics belongs only to
2:17:58
Speaker 2: elementary particles. Larger systems can behave quantum mechanically too. The
2:18:04
Speaker 2: challenge is preserving coherence. As the system becomes more complex,
2:18:09
Speaker 2: more routes appear through which information can leak into the environment. Eventually,
2:18:15
Speaker 2: maintaining interference becomes so difficult that the system looks classical
2:18:19
Speaker 2: almost immediately. There is, however, an important distinction to keep
2:18:23
Speaker 2: in mind. Decoherence explains a great deal about why we
2:18:27
Speaker 2: do not observe obvious macroscopic interference. It explains why certain
2:18:32
Speaker 2: alternatives become effectively unable to interfere once the environment has
2:18:36
Speaker 2: become correlated with them. It explains why stable classical-looking states
2:18:40
Speaker 2: emerge so naturally. But it does not automatically settle every
2:18:45
Speaker 2: question about measurement. The deeper issue of why we experience
2:18:49
Speaker 2: one definite outcome rather than a visible collection of alternatives
2:18:54
Speaker 2: remains an interpretational problem. Different interpretations of quantum mechanics describe
2:18:59
Speaker 2: the transition differently. So the emergence of the classical world
2:19:03
Speaker 2: and the measurement problem are closely connected but they are
2:19:06
Speaker 2: not exactly the same question. The practical puzzle is much clearer.
2:19:12
Speaker 2: Why does a table not behave like a single electron
2:19:15
Speaker 2: in a double-slit experiment? Because a table is an enormous, warm,
2:19:20
Speaker 2: interacting collection of quantum particles that is constantly becoming entangled
2:19:25
Speaker 2: with its environment. Its coherent alternatives are suppressed almost immediately.
2:19:31
Speaker 2: Its de Broglie wavelength is effectively negligible at ordinary scales.
2:19:36
Speaker 2: And classical physics becomes an exceptionally accurate approximation to what
2:19:40
Speaker 2: we observe. That ordinary appearance can make quantum physics seem remote.
2:19:45
Speaker 2: But the everyday world is not sitting above quantum mechanics,
2:19:48
Speaker 2: as though it belonged to a separate layer of reality.
2:19:52
Speaker 2: Its atoms remain quantum. Its electrons remain quantum. The forces
2:19:58
Speaker 2: holding materials together arise from quantum rules. The stability of
2:20:02
Speaker 2: matter depends upon quantum mechanics. The classical world is what
2:20:07
Speaker 2: those quantum systems look like when vast numbers of them interact,
2:20:11
Speaker 2: decohere and behave collectively. So the reason you do not
2:20:15
Speaker 2: see quantum strangeness every time you look across the room
2:20:19
Speaker 2: is not because the quantum world has ended. It is
2:20:22
Speaker 2: because you are looking at far too much of it
2:20:24
Speaker 2: at once. Quantum physics can sound like a theory designed
2:20:28
Speaker 2: for distant laboratories, delicate experiments and particles that behave badly
2:20:33
Speaker 2: when anyone tries to describe them in ordinary language. But
2:20:37
Speaker 2: much of modern life depends on it. The devices around
2:20:41
Speaker 2: you do not merely survive despite quantum physics. Many of
2:20:45
Speaker 2: them work because engineers learned how to control quantum behaviour
2:20:49
Speaker 2: with extraordinary precision. One of the clearest examples is the semiconductor.
2:20:55
Speaker 2: A semiconductor is a material whose electrical behaviour lies between
2:20:59
Speaker 2: that of a good conductor and a good insulator, and
2:21:02
Speaker 2: whose properties can be carefully manipulated. Silicon is the most
2:21:07
Speaker 2: famous example. Inside a solid material, electrons do not simply
2:21:12
Speaker 2: move around as though each atom were completely independent. The
2:21:16
Speaker 2: quantum states associated with enormous numbers of atoms combine. Instead
2:21:21
Speaker 2: of thinking only in terms of individual atomic energy levels,
2:21:25
Speaker 2: Physicists describe ranges of allowed energies called energy bands. Between
2:21:30
Speaker 2: those bands can be regions where electrons are not normally
2:21:32
Speaker 2: allowed to possess energies. These are called bandgaps. The details
2:21:37
Speaker 2: of those energy bands determine whether a material behaves like
2:21:40
Speaker 2: a conductor, an insulator or a semiconductor. In a conductor,
2:21:45
Speaker 2: electrons can move relatively easily through available states. In an insulator,
2:21:51
Speaker 2: A large band gap makes ordinary electrical conduction much more difficult.
2:21:56
Speaker 2: A semiconductor sits in a particularly useful middle ground. Its
2:22:00
Speaker 2: electronic behaviour can be altered through temperature, electric fields and
2:22:05
Speaker 2: the deliberate introduction of small amounts of other elements. This
2:22:10
Speaker 2: process is called doping. By carefully changing the concentration and
2:22:15
Speaker 2: type of charge carriers inside a semiconductor, Engineers can create
2:22:20
Speaker 2: regions with different electrical properties. Bring those regions together and
2:22:24
Speaker 2: the result can form the basis of devices such as
2:22:27
Speaker 2: diodes and transistors. A transistor is essentially a controllable electrical
2:22:33
Speaker 2: switch or amplifier. The modern transistor can be extraordinarily small
2:22:38
Speaker 2: and billions can be packed onto a single integrated circuit.
2:22:43
Speaker 2: Each one helps control the flow of electrical current. Together,
2:22:47
Speaker 2: They perform the logical operations behind digital computing. Every time
2:22:52
Speaker 2: a phone opens an application, a computer runs calculation, or
2:22:57
Speaker 2: a digital camera processes an image. Enormous numbers of transistors
2:23:01
Speaker 2: are changing electrical states at remarkable speed. The entire modern
2:23:06
Speaker 2: information economy rests, quite literally, on quantum behaviour inside solids.
2:23:12
Speaker 2: Without quantum mechanics, the energy bands of semiconductors would make
2:23:16
Speaker 2: little sense. And without those energy bands, the transistor would
2:23:20
Speaker 2: be very difficult to understand. Another familiar technology based on
2:23:25
Speaker 2: semiconductor physics is the light-emitting diode. In certain semiconductors, electrical
2:23:32
Speaker 2: current brings electrons and positively behaving charge vacancies, known as holes,
2:23:38
Speaker 2: into regions where they can recombine. When an electron drops
2:23:43
Speaker 2: into a lower energy state associated with a hole, The
2:23:47
Speaker 2: energy difference can be released as a photon. The energy
2:23:51
Speaker 2: of that photon determines its frequency and therefore its colour.
2:23:55
Speaker 2: By choosing semiconductor materials with different band gaps, engineers can
2:24:00
Speaker 2: create light emitting diodes that produce different wavelengths of light.
2:24:05
Speaker 2: The small lights on electronic devices, modern displays and many
2:24:10
Speaker 2: household lamps therefore depend upon carefully engineered quantum transitions. A
2:24:16
Speaker 2: room illuminated by light-emitting diodes is, in a very practical sense,
2:24:22
Speaker 2: being lit by quantum mechanics. Lasers offer another example. The
2:24:27
Speaker 2: word laser refers to light amplification by stimulated emission of radiation,
2:24:32
Speaker 2: but the mechanism is more interesting than the name suggests.
2:24:36
Speaker 2: Albert Einstein described stimulated emission in work published in 1917.
2:24:42
Speaker 2: Imagine an atom or another quantum system in an excited state.
2:24:48
Speaker 2: It contains more energy than its lowest available state. Ordinarily,
2:24:52
Speaker 2: it may eventually release that excess energy by emitting a photon.
2:24:56
Speaker 2: But if a photon with the right energy interacts with
2:24:59
Speaker 2: the excited system, it can stimulate the emission of another photon.
2:25:04
Speaker 2: The newly emitted photon has a precise relationship with the
2:25:08
Speaker 2: incoming one. Under suitable conditions, repeated stimulated emission can create
2:25:13
Speaker 2: large numbers of photons with highly organised properties. This produces
2:25:18
Speaker 2: the coherent light associated with a laser. Laser light can
2:25:22
Speaker 2: be extremely narrow in wavelength, highly directional and strongly phase-related.
2:25:28
Speaker 2: Those qualities make lasers useful in fibre-optic communication, barcode scanners, manufacturing,
2:25:36
Speaker 2: scientific instruments. surgery and countless other applications. Again, something that
2:25:42
Speaker 2: once belonged to the most abstract discussions of quantum energy
2:25:46
Speaker 2: levels became part of everyday technology. Medicine provides another striking example.
2:25:53
Speaker 2: Magnetic resonance imaging relies on the quantum property called spin.
2:25:58
Speaker 2: Atomic nuclei with suitable spin behave as though they possess
2:26:02
Speaker 2: tiny magnetic moments. When placed in a strong magnetic field,
2:26:07
Speaker 2: These nuclear spins interact with that field and occupy different
2:26:11
Speaker 2: energy configurations. Radiofrequency electromagnetic pulses can then disturb the spin system.
2:26:19
Speaker 2: As the nuclei return toward equilibrium, they produce signals that
2:26:23
Speaker 2: can be detected and processed. The most important nucleus in
2:26:27
Speaker 2: ordinary magnetic resonance imaging is hydrogen. Largely because the human
2:26:31
Speaker 2: body contains so much water and fat, The resulting signals
2:26:36
Speaker 2: can be used to construct detailed images of internal tissues.
2:26:41
Speaker 2: Magnetic resonance imaging does not rely on ionising radiation in
2:26:45
Speaker 2: the way an X-ray or computed tomography scan does. Instead,
2:26:50
Speaker 2: it uses strong magnetic fields, radiofrequency energy and the quantum
2:26:55
Speaker 2: behaviour of nuclear spin. The machine may look like an
2:26:59
Speaker 2: enormous piece of hospital engineering, At its foundation sits quantum mechanics.
2:27:06
Speaker 2: Atomic clocks provide a quieter but equally remarkable example. Atoms
2:27:11
Speaker 2: possess precisely defined quantum energy levels. Transitions between those levels
2:27:17
Speaker 2: correspond to extremely stable frequencies. Atomic clocks exploit these transitions
2:27:23
Speaker 2: as natural references. Rather than relying on the swing of
2:27:28
Speaker 2: a pendulum or the vibration of a mechanical component, an
2:27:31
Speaker 2: atomic clock locks its timing to a frequency associated with
2:27:36
Speaker 2: a particular atomic transition. Cesium atomic clocks clocks are especially
2:27:41
Speaker 2: important because the modern definition of the second is based
2:27:45
Speaker 2: on a transition in cesium-133 atoms. The precision is extraordinary.
2:27:53
Speaker 2: Atomic clocks support satellite navigation, telecommunications, scientific measurements and the
2:27:59
Speaker 2: synchronisation of modern networks. A device in your pocket can
2:28:04
Speaker 2: determine its position, partly because clocks far above Earth are
2:28:08
Speaker 2: using quantum transitions to keep time with immense accuracy. Solar
2:28:13
Speaker 2: cells depend upon quantum physics as well. When photons strike
2:28:17
Speaker 2: suitable semiconductor materials, they can transfer energy to electrons. If
2:28:23
Speaker 2: the photon energy is sufficient relative to the semiconductor band gap,
2:28:27
Speaker 2: Electrons can be promoted into higher energy states and contribute
2:28:32
Speaker 2: to an electrical current. This is one reason different semiconductor
2:28:36
Speaker 2: materials absorb different parts of the solar spectrum. Digital camera
2:28:41
Speaker 2: sensors use related quantum processes. Incoming photons create electrical signals
2:28:47
Speaker 2: inside semiconductor pixels. Those signals are then converted into the
2:28:52
Speaker 2: image you see. So a photograph begins as countless microscopic
2:28:57
Speaker 2: interactions between light and quantum states in a solid. Electron
2:29:02
Speaker 2: microscopes use another quantum property. Because electrons have de Broglie wavelengths,
2:29:08
Speaker 2: accelerating them to high momenta can produce wavelengths much shorter
2:29:13
Speaker 2: than those of visible light. Shorter wavelengths allow much finer
2:29:18
Speaker 2: spatial resolution. This is why electron microscopes can reveal structures
2:29:22
Speaker 2: far smaller than ordinary optical microscopes can resolve. Superconducting devices
2:29:28
Speaker 2: take quantum behaviour into yet another regime. In a superconductor,
2:29:33
Speaker 2: electrical resistance can fall to effectively zero below a critical temperature.
2:29:39
Speaker 2: The phenomenon depends on collective quantum behaviour involving electrons, and
2:29:44
Speaker 2: in conventional superconductors, those electrons form correlated pairs called Cooper pairs.
2:29:51
Speaker 2: Superconducting circuits are now used in technologies ranging from powerful
2:29:55
Speaker 2: magnets to some designs of quantum computers. And beneath all
2:30:00
Speaker 2: these technologies lies something even more familiar. Chemistry itself is
2:30:05
Speaker 2: quantum mechanical. Chemical bonds form because electrons occupy quantum states
2:30:11
Speaker 2: shared across atoms and molecules. The shapes of molecules depend
2:30:16
Speaker 2: upon the allowed arrangements of electrons. The strength of a bond,
2:30:21
Speaker 2: the colour of a pigment, the smell of a molecule
2:30:24
Speaker 2: and the way a medicine interacts with proteins all ultimately
2:30:27
Speaker 2: depend on quantum structure. Water behaves as it does because
2:30:32
Speaker 2: of the electronic structure of hydrogen and oxygen and the
2:30:36
Speaker 2: geometry of the water molecule. Proteins fold because of many
2:30:40
Speaker 2: interacting chemical forces whose foundations are quantum mechanical. The molecules
2:30:46
Speaker 2: carrying genetic information exist because electrons form stable bonds in
2:30:50
Speaker 2: particular configurations. Living systems do not float above quantum physics.
2:30:57
Speaker 2: They are built from chemistry, and chemistry is built from
2:31:00
Speaker 2: quantum mechanics. This is perhaps the most useful way to
2:31:04
Speaker 2: bring quantum physics back down to Earth. Its importance is
2:31:08
Speaker 2: not confined to strange experiments involving individual particles. Quantum mechanics
2:31:14
Speaker 2: explains why materials conduct electricity, why some substances emit light,
2:31:20
Speaker 2: why atoms form molecules, and why electronic devices can be
2:31:25
Speaker 2: engineered at microscopic scales. Modern technology works partly because physicists
2:31:31
Speaker 2: and engineers learned how to use those rules deliberately. They
2:31:34
Speaker 2: learned how to shape band gaps, how to control electron flow,
2:31:39
Speaker 2: how to stimulate photon emission. how to manipulate nuclear spin,
2:31:44
Speaker 2: how to measure atomic transitions, how to turn microscopic quantum
2:31:48
Speaker 2: behaviour into reliable macroscopic machines. The strange rules never disappeared
2:31:55
Speaker 2: when they left the laboratory. They became the laboratory equipment,
2:31:59
Speaker 2: the computer, the phone, the medical scanner and the light
2:32:03
Speaker 2: in the room. The modern world is not merely compatible
2:32:07
Speaker 2: with quantum physics. A surprising amount of it was built
2:32:10
Speaker 2: by learning how to make quantum physics useful. By the
2:32:14
Speaker 2: middle of the 20th century, quantum mechanics had already transformed physics.
2:32:20
Speaker 2: It explained atoms. It explained chemical bonding. It described electrons,
2:32:25
Speaker 2: photons and the strange rules governing microscopic systems with extraordinary success.
2:32:32
Speaker 2: But physicists were not finished. The next step was to
2:32:35
Speaker 2: combine quantum mechanics with the idea of fields. A field
2:32:40
Speaker 2: is something that has a value throughout space and time.
2:32:43
Speaker 2: The electromagnetic field is a familiar example. Electric and magnetic
2:32:48
Speaker 2: effects are described not as invisible ropes stretching between objects,
2:32:53
Speaker 2: but as properties of a field existing throughout space. Quantum
2:32:58
Speaker 2: field theory takes that idea and applies quantum principles to
2:33:02
Speaker 2: the fields themselves. In this framework, particles are understood as
2:33:07
Speaker 2: excitations of underlying quantum fields. An electron is not simply
2:33:12
Speaker 2: a tiny solid object moving through empty space. It is
2:33:16
Speaker 2: an excitation of the electron field. A photon is an
2:33:20
Speaker 2: excitation of the electromagnetic field. Other particles correspond to other
2:33:25
Speaker 2: quantum fields. This does not make particles unreal. It changes
2:33:30
Speaker 2: what physicists mean by a particle at the deepest level.
2:33:33
Speaker 2: a particle becomes a discrete quantum excitation carried by a
2:33:38
Speaker 2: field that extends through space. Quantum field theory proved necessary
2:33:43
Speaker 2: because ordinary non-relativistic quantum mechanics could not fully describe situations
2:33:48
Speaker 2: in which particles are created or destroyed. At sufficiently high energies,
2:33:53
Speaker 2: particles can appear from interactions and disappear into other forms.
2:33:58
Speaker 2: A framework in which the number of particles is permanently
2:34:01
Speaker 2: fixed is therefore not enough Quantum fields provide a much
2:34:05
Speaker 2: more flexible description. Their excitations can be created, absorbed and
2:34:11
Speaker 2: transformed while respecting the conservation laws governing the interaction. One
2:34:16
Speaker 2: of the great successes of this approach is quantum electrodynamics.
2:34:21
Speaker 2: Quantum electrodynamics is the quantum field theory describing electromagnetic interactions
2:34:26
Speaker 2: between electrically charged particles and photons. It brought together quantum mechanics,
2:34:32
Speaker 2: special relativity and electromagnetism within one extraordinarily powerful framework. The
2:34:39
Speaker 2: modern form of the theory emerged through the work of
2:34:41
Speaker 2: several physicists, particularly Richard Feynman, Julian Schwinger and Shinichiro Tomonaga.
2:34:48
Speaker 2: Each developed methods for handling the difficult calculations involved in
2:34:52
Speaker 2: quantum electromagnetic interactions. Their approaches looked different, but they proved
2:34:58
Speaker 2: to be mathematically equivalent. Richard Feynman introduced a particularly intuitive
2:35:04
Speaker 2: way of organising calculations using what became known as Feynman diagrams.
2:35:10
Speaker 2: These diagrams show possible interaction processes in a compact visual form.
2:35:15
Speaker 2: They are useful tools for keeping track of mathematical terms,
2:35:18
Speaker 2: although they should not always be interpreted as literal little
2:35:21
Speaker 2: pictures of exactly what particles physically do between observations. quantum
2:35:26
Speaker 2: electrodynamics has achieved some of the most precise agreements between
2:35:30
Speaker 2: theory and experiment in the history of science. One famous
2:35:34
Speaker 2: example involves the magnetic properties of the electron. Quantum electrodynamics
2:35:40
Speaker 2: predicts tiny corrections to the electron's magnetic behaviour arising from
2:35:45
Speaker 2: quantum fluctuations and interactions with the electromagnetic field. The theoretical
2:35:51
Speaker 2: predictions and experimental measurements agree to an astonishing number of
2:35:56
Speaker 2: decimal places. This is worth pausing over. Quantum physics often
2:36:01
Speaker 2: sounds uncertain, because it deals in probabilities, but the theory
2:36:06
Speaker 2: itself can produce extraordinarily precise numerical predictions. Probability does not
2:36:12
Speaker 2: mean vagueness. The laws determine the statistical structure with remarkable accuracy.
2:36:20
Speaker 2: Quantum field theory eventually became the foundation of the standard
2:36:23
Speaker 2: model of particle physics. The standard model describes the known
2:36:28
Speaker 2: elementary particles and three of the four fundamental interactions. The
2:36:33
Speaker 2: electromagnetic interaction is described through quantum electrodynamics. The weak nuclear interaction,
2:36:39
Speaker 2: responsible for processes such as certain forms of radioactive decay,
2:36:44
Speaker 2: is incorporated with electromagnetism into the electroweak theory. The strong
2:36:49
Speaker 2: nuclear interaction which binds quarks together inside protons and neutrons
2:36:54
Speaker 2: is described by quantum chromodynamics. Together, these quantum field theories
2:36:59
Speaker 2: account for an enormous range of experimental results. The standard
2:37:03
Speaker 2: model includes quarks, electrons, neutrinos, photons, gluons, the particles carrying
2:37:10
Speaker 2: the weak interaction, and the Higgs field and its associated
2:37:14
Speaker 2: Higgs bosons. When the Higgs boson was observed at the
2:37:17
Speaker 2: European Organisation for Nuclear Research in 2012, it provided another
2:37:22
Speaker 2: dramatic confirmation of a structure that had been predicted decades earlier.
2:37:27
Speaker 2: The theory is not complete, but within the territory it covers,
2:37:31
Speaker 2: its success is difficult to overstate. There is, however, one
2:37:35
Speaker 2: enormous piece of fundamental physics that does not fit comfortably
2:37:38
Speaker 2: into the same framework. Gravity. Albert Einstein's general theory of
2:37:43
Speaker 2: relativity describes gravity as the curvature of spacetime produced by
2:37:48
Speaker 2: energy and matter. It works exceptionally well for planets, stars,
2:37:52
Speaker 2: galaxies and the large-scale universe. Quantum mechanics works exceptionally well
2:37:58
Speaker 2: for microscopic systems. But when both theories become important at
2:38:02
Speaker 2: the same time, physicists do not yet possess one experimentally
2:38:07
Speaker 2: confirmed framework that combines them completely. This becomes especially important
2:38:12
Speaker 2: in extreme environments. Near the classical singularity predicted inside a
2:38:17
Speaker 2: black hole, Quantum effects and strong gravity should both matter.
2:38:23
Speaker 2: The same is true when physicists attempt to describe the
2:38:26
Speaker 2: very earliest moments of cosmic history. A theory of quantum
2:38:30
Speaker 2: gravity would need to reconcile these two enormously successful descriptions
2:38:34
Speaker 2: of nature. Several approaches have been proposed. String theory is one.
2:38:40
Speaker 2: Loop quantum gravity is another. There are additional programs exploring
2:38:44
Speaker 2: different possibilities. None has yet received the kind of decisive
2:38:48
Speaker 2: experimental confirmation enjoyed by established quantum mechanics, quantum field theory
2:38:54
Speaker 2: or general relativity. So even the modern quantum framework has limits.
2:38:59
Speaker 2: And there is another sense in which the theory remains unfinished.
2:39:03
Speaker 2: Physicists know extremely well how to use quantum mechanics. They
2:39:07
Speaker 2: do not all agree on what it means. This difference
2:39:10
Speaker 2: is important. There is very little serious disagreement about whether
2:39:14
Speaker 2: quantum mechanics successfully predicts experiments. Its success is overwhelming. The
2:39:20
Speaker 2: arguments concern interpretation. What does the wave function represent? Is
2:39:25
Speaker 2: it a real physical object? Is it information about a system?
2:39:30
Speaker 2: What exactly occurs during measurement? Why do observers record definite outcomes?
2:39:36
Speaker 2: Does wave function collapse physically happen? Or does it only
2:39:40
Speaker 2: appear to happen from one perspective? There is no single
2:39:44
Speaker 2: universally accepted answer. The Copenhagen family of interpretations is historically
2:39:49
Speaker 2: associated with physicists including Niels Bohr and Werner Heisenberg. These
2:39:54
Speaker 2: approaches generally emphasise measurement outcomes and the limits of applying
2:39:58
Speaker 2: ordinary classical descriptions to quantum systems. But even the phrase
2:40:03
Speaker 2: Copenhagen interpretation covers several different views rather than one perfectly
2:40:08
Speaker 2: unified doctrine. Another approach emerged from Hugh Everett III in
2:40:13
Speaker 2: the 1950s. Everett proposed what he called the relative state formulation.
2:40:19
Speaker 2: In this picture, the wave function never fundamentally collapses. Instead,
2:40:24
Speaker 2: all components of the quantum state continue evolving according to
2:40:28
Speaker 2: the ordinary quantum equations. The approach later became strongly associated
2:40:33
Speaker 2: with what is commonly called the many-worlds interpretation. The name
2:40:37
Speaker 2: can create dramatic images of universes popping into existence every
2:40:42
Speaker 2: few seconds. But the underlying proposal is more technical. It
2:40:47
Speaker 2: treats the universal quantum state and its continuous evolution as fundamental,
2:40:52
Speaker 2: with different effectively decahed branches corresponding to different outcomes. David
2:40:57
Speaker 2: Bohm developed another interpretation. Bohmian mechanics, often called pilot wave theory,
2:41:04
Speaker 2: introduces definite particle positions guided by a wave function. It
2:41:09
Speaker 2: produces the usual quantum predictions in many standard situations while
2:41:13
Speaker 2: giving particles definite trajectories. But it does so using an
2:41:18
Speaker 2: explicitly non-local structure, meaning distant parts of an entangled system
2:41:23
Speaker 2: can be connected in a way that does not resemble
2:41:26
Speaker 2: classical locality. Other approaches alter the theory more directly. Objective
2:41:32
Speaker 2: collapse models propose that wavefunction collapse is a real physical process.
2:41:36
Speaker 2: Instead of requiring a vaguely defined measurement, these theories suggest
2:41:41
Speaker 2: that superpositions spontaneously collapse under particular physical conditions. Different models
2:41:47
Speaker 2: propose different mechanisms and scales. Because they modify standard quantum mechanics,
2:41:53
Speaker 2: some objective collapse theories can, at least in principle, lead
2:41:57
Speaker 2: to experimentally distinguishable predictions. These approaches tell very different stories
2:42:03
Speaker 2: about reality. Some involve fundamental collapse, some avoid collapse, some
2:42:08
Speaker 2: introduce definite particle trajectories. Some treat the wavefunction as the
2:42:13
Speaker 2: central physical object, and yet many of them reproduce the
2:42:16
Speaker 2: same predictions for the experiments already familiar to quantum physics.
2:42:21
Speaker 2: This creates an unusual situation. The mathematics can tell us
2:42:25
Speaker 2: very precisely what we should observe while leaving room for
2:42:28
Speaker 2: profound disagreement over what exists underneath the observations. Albert Einstein
2:42:34
Speaker 2: spent much of his later career worrying about exactly this problem.
2:42:38
Speaker 2: He accepted the successes of quantum mechanics, but doubted that
2:42:42
Speaker 2: probability represented the deepest possible description of nature. He hoped
2:42:47
Speaker 2: a more complete theory might restore something closer to definite
2:42:50
Speaker 2: underlying reality. Bell's theorem changed the terms of that debate.
2:42:56
Speaker 2: John Stuart Bell showed that broad classes of local hidden
2:42:59
Speaker 2: variable theories cannot reproduce all the predictions of quantum mechanics.
2:43:05
Speaker 2: Experiments violating Bell inequalities have repeatedly supported the quantum correlations.
2:43:10
Speaker 2: That does not prove one interpretation correct. But it tells
2:43:14
Speaker 2: us that any deeper theory reproducing those correlations cannot simply
2:43:18
Speaker 2: return the comfortable classical picture Einstein originally hoped might exist.
2:43:25
Speaker 2: And this brings us back to the simplest possible answer
2:43:28
Speaker 2: to what quantum physics actually is. Quantum physics is the
2:43:33
Speaker 2: framework we use when the assumptions of classical physics are
2:43:36
Speaker 2: no longer sufficient. It describes a world where energy can
2:43:40
Speaker 2: be quantised, where particles can display interference, where quantum states
2:43:45
Speaker 2: evolve through amplitudes, where probabilities are fundamental to experimental predictions,
2:43:52
Speaker 2: where superposition and entanglement cannot be replaced by ordinary classical ignorance,
2:43:57
Speaker 2: where particles are understood, in modern theory, as excitations of
2:44:01
Speaker 2: quantum fields, and where measurements produce definite records even though
2:44:06
Speaker 2: the meaning of that transition remains debated. The remarkable thing
2:44:11
Speaker 2: is not that physicists understand nothing about this world, They
2:44:15
Speaker 2: understand enough to predict it with astonishing precision. The remarkable
2:44:19
Speaker 2: thing is that the predictive success came before a universally
2:44:23
Speaker 2: agreed intuitive picture. Quantum theory works. Again and again, experiments
2:44:29
Speaker 2: say that it works. But when we ask what the
2:44:32
Speaker 2: mathematical objects in the theory tell us reality actually is,
2:44:36
Speaker 2: the certainty becomes softer. There may be no final classical
2:44:40
Speaker 2: picture waiting underneath. Perhaps the mistake is expecting the universe
2:44:44
Speaker 2: to behave according to categories, developed from falling stones, moving
2:44:49
Speaker 2: planets and objects sitting peacefully on tables. Quantum physics does
2:44:54
Speaker 2: not promise to preserve those intuitions. It promises something more useful,
2:44:59
Speaker 2: a set of rules that agrees with nature. And so far,
2:45:03
Speaker 2: nature has been remarkably unwilling to disagree. Quantum physics began
2:45:07
Speaker 2: by asking what happens when we look closely enough. not
2:45:11
Speaker 2: simply at smaller objects, but at the scales where matter,
2:45:14
Speaker 2: light and energy stop behaving, according to the comfortable rules
2:45:18
Speaker 2: of everyday life. At those scales, the classical picture becomes incomplete.
2:45:24
Speaker 2: And yet the strange thing is that the world around
2:45:27
Speaker 2: us is built from exactly those quantum ingredients. Atoms are
2:45:31
Speaker 2: quantum systems. Light is quantum mechanical. Chemical bonds are quantum mechanical.
2:45:37
Speaker 2: The electrons inside every material around you follow quantum rules.
2:45:42
Speaker 2: The familiar world was never separate from quantum physics. It
2:45:45
Speaker 2: was quantum all along. That became clear only gradually. Near
2:45:50
Speaker 2: the beginning of the 20th century, classical physics was extraordinarily successful,
2:45:56
Speaker 2: but several experiments refused to fit. Blackbody radiation produced the
2:46:01
Speaker 2: wrong predictions. The photoelectric effect behaved differently from what a
2:46:05
Speaker 2: simple wave description suggested. Atoms emitted discrete spectral lines, and
2:46:11
Speaker 2: classical electromagnetism appeared to make stable atoms impossible. Max Planck
2:46:16
Speaker 2: introduced the idea that energy could be exchanged in discrete quanta.
2:46:21
Speaker 2: Albert Einstein pushed that idea further by showing that light
2:46:24
Speaker 2: itself behaved as though its energy arrived in discrete packets.
2:46:29
Speaker 2: Niels Bohr used quantist energy levels to explain key features
2:46:33
Speaker 2: of the hydrogen atom. Then, during the 1920s, physicists such
2:46:38
Speaker 2: as Louis de Broglie, Werner Heisenberg, Erwin Schrödinger and Max
2:46:42
Speaker 2: Born helped build a much deeper framework. Matter could behave
2:46:47
Speaker 2: like a wave, quantum states could evolve according to wave equations,
2:46:52
Speaker 2: and probability became part of the structure of the theory itself.
2:46:56
Speaker 2: Wave-particle duality was one of the first clues that the
2:46:59
Speaker 2: old categories were too limited. A photon is not secretly
2:47:03
Speaker 2: an ordinary wave on Monday, and a tiny classical particle
2:47:06
Speaker 2: on Tuesday. An electron does not quietly choose between being
2:47:11
Speaker 2: one or the other. Quantum objects are described by quantum states.
2:47:16
Speaker 2: Depending on how an experiment is arranged, those states can
2:47:20
Speaker 2: produce effects associated with waves, such as interference, and effects
2:47:25
Speaker 2: associated with particles, such as localised detections. The double slit
2:47:30
Speaker 2: experiment captures that strange combination beautifully. A photon or electron
2:47:36
Speaker 2: can be sent through an apparatus one particle at a time.
2:47:40
Speaker 2: Each detection appears in one definite location. Yet after many detections,
2:47:45
Speaker 2: an interference pattern emerges. The outcome cannot be explained simply
2:47:50
Speaker 2: by imagining each particle following one hidden classical path. The
2:47:54
Speaker 2: quantum amplitudes associated with different alternatives can interfere before the
2:47:59
Speaker 2: final probabilities are calculated. That brings us to the wave
2:48:03
Speaker 2: function and the Born rule. The wavefunction encodes the amplitudes
2:48:08
Speaker 2: associated with possible outcomes. The Born rule connects those amplitudes
2:48:13
Speaker 2: to measurable probabilities. A quantum state can exist in a
2:48:17
Speaker 2: superposition of different possibilities, and those possibilities can interfere as
2:48:21
Speaker 2: long as coherence is maintained. Then a measurement produces one
2:48:26
Speaker 2: recorded result. A detector clicks. A spin measurement gives one outcome.
2:48:31
Speaker 2: A photon arrives in one place. The predictions are clear.
2:48:35
Speaker 2: the interpretation is less settled. Physicists agree overwhelmingly on the
2:48:40
Speaker 2: experimental success of quantum mechanics, but they still debate what
2:48:44
Speaker 2: the wavefunction represents, and what exactly happens during measurement. The
2:48:50
Speaker 2: uncertainty principle adds another layer. Werner Heisenberg showed that certain
2:48:55
Speaker 2: pairs of observables, such as position and momentum, cannot both
2:48:59
Speaker 2: be made arbitrarily sharp within the same quantum state. This
2:49:03
Speaker 2: is not merely a problem with imperfect equipment. It is
2:49:07
Speaker 2: built into the mathematical structure of quantum mechanics. A tightly
2:49:11
Speaker 2: localised quantum state requires a wider spread of momenta. A
2:49:15
Speaker 2: sharply defined momentum corresponds to a more spread out position.
2:49:20
Speaker 2: The uncertainty belongs to the state itself. Quantum tunnelling once
2:49:24
Speaker 2: sounded equally impossible. A classical particle without enough energy to
2:49:28
Speaker 2: cross a barrier should simply bounce back. A quantum wave
2:49:32
Speaker 2: function can extend through the barrier and retain a non-zero
2:49:35
Speaker 2: amplitude on the other side. That effect helps explain radioactive
2:49:39
Speaker 2: alpha decay. It is used in scanning tunnelling microscopes. It
2:49:44
Speaker 2: matters in modern electronics. And inside the Sun, tunnelling helps
2:49:48
Speaker 2: protons get close enough for nuclear fusion to become possible.
2:49:52
Speaker 2: The light warming Earth therefore depends in part on a
2:49:55
Speaker 2: process that makes no sense in a purely classical picture.
2:49:59
Speaker 2: Entanglement is another reminder that the quantum world does not
2:50:03
Speaker 2: organise itself according to ordinary intuition. Two systems can be
2:50:07
Speaker 2: described by one joint quantum state, even when separated by
2:50:11
Speaker 2: large distances. Their measurement outcomes can show correlations stronger than
2:50:16
Speaker 2: certain local hidden variable theories allow. Bell tests have confirmed
2:50:21
Speaker 2: those quantum predictions again and again, and yet entanglement still
2:50:25
Speaker 2: cannot be used to send controllable information faster than light.
2:50:30
Speaker 2: The universe remains strange without becoming careless. So why does
2:50:34
Speaker 2: none of this look obvious in everyday life? Because large
2:50:37
Speaker 2: objects contain enormous numbers of quantum particles interacting constantly with
2:50:42
Speaker 2: one another and with their surroundings. Photons scatter from them.
2:50:47
Speaker 2: Air molecules collide with them. Thermal motion disturbs them. Quantum
2:50:52
Speaker 2: information spreads into the environment. Through decoherence, delicate interference between
2:50:58
Speaker 2: macroscopically distinct alternatives becomes effectively impossible to observe. Classical behaviour
2:51:05
Speaker 2: emerges as an extraordinarily accurate approximation. The quantum rules do
2:51:09
Speaker 2: not switch off. They simply become hidden beneath complexity. And
2:51:14
Speaker 2: once physicists learned how to control those rules, they became technology.
2:51:19
Speaker 2: Semiconductor band structures made modern transistors possible. Transistors made modern
2:51:24
Speaker 2: computers and phones possible. Quantum transitions power light-emitting diodes and lasers.
2:51:30
Speaker 2: Nuclear spin underlies magnetic resonance imaging. Atomic transitions provide the
2:51:36
Speaker 2: extraordinary precision of atomic clocks. Quantum physics explains chemistry, materials
2:51:42
Speaker 2: and the stability of matter itself. So the strange theory
2:51:46
Speaker 2: developed to understand awkward laboratory experiments eventually became part of
2:51:50
Speaker 2: the foundation of modern civilisation. There are still major unanswered questions.
2:51:56
Speaker 2: physicists do not agree on one final interpretation of quantum mechanics.
2:52:01
Speaker 2: The measurement problem remains open, quantum gravity remains unresolved, and
2:52:06
Speaker 2: no experimentally confirmed framework yet combines quantum theory and gravity
2:52:11
Speaker 2: in a completely satisfactory way. But these open questions do
2:52:17
Speaker 2: not erase what has already been achieved. Quantum mechanics remains
2:52:21
Speaker 2: one of the most successful theories ever created. Its predictions
2:52:25
Speaker 2: have survived some of the most precise experiments in science.
2:52:29
Speaker 2: Its principles have been built into technologies used every day.
2:52:32
Speaker 2: Its mathematics explains why atoms, molecules and materials behave as
2:52:37
Speaker 2: they do. Perhaps the deepest source of discomfort is not
2:52:41
Speaker 2: that quantum physics is incoherent. It is that human intuition
2:52:46
Speaker 2: developed in a very different environment. We evolved among large,
2:52:50
Speaker 2: warm objects. We learn to understand falling stones, moving animals,
2:52:54
Speaker 2: flowing water and solid ground. We did not evolve to
2:52:58
Speaker 2: picture coherent superpositions, quantum amplitudes or entangled particles. There is
2:53:04
Speaker 2: no reason nature should arrange itself according to instincts shaped
2:53:08
Speaker 2: at human scales. Quantum physics feels strange because we are
2:53:12
Speaker 2: familiar with the surface of reality. The deeper rules were
2:53:16
Speaker 2: never required to feel familiar, and that may be the
2:53:19
Speaker 2: most remarkable thing of all. The theory can describe nature
2:53:23
Speaker 2: with extraordinary accuracy. It can explain the matter around us.
2:53:28
Speaker 2: It can power technology. It can predict experiments to astonishing precision.
2:53:33
Speaker 2: And even after all of that, it can still leave
2:53:35
Speaker 2: us asking what reality actually is. Not because quantum physics
2:53:40
Speaker 2: has failed, but because the universe, when examined closely enough,
2:53:45
Speaker 2: turned out to be far less obliged to resemble common
2:53:48
Speaker 2: sense than we ever expected.