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Quantum Field Theory What Is the Universe Actually Made Of · Night Science Stories

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Speaker 2: the Sleepy Scientist part of your evenings. Now look around
2:03
Speaker 2: you for a moment. or if your eyes are already closed,
2:06
Speaker 2: just imagine it. The walls of your room, the air
2:10
Speaker 2: you're breathing, the blanket keeping you warm, your own hands.
2:15
Speaker 2: We tend to picture all of it as built in
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Speaker 2: the end from unimaginably tiny particles, little specks of stuff
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Speaker 2: bumping about. It's a perfectly sensible picture. But according to
2:26
Speaker 2: one of the most successful theories in all of physics,
2:29
Speaker 2: the truth is a good deal stranger. Think of a
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Speaker 2: wave going round a packed football stadium. It sweeps all
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Speaker 2: the way round, clear as anything, and yet nobody actually
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Speaker 2: leaves their seat. The wave isn't a thing in itself.
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Speaker 2: It's just the crowd, moving. Modern physics suggests that particles
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Speaker 2: may be rather like that. Not tiny objects in their
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Speaker 2: own right, but ripples passing through something invisible that fills
2:56
Speaker 2: all of space, even the places that seem completely empty.
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Speaker 2: Physicists call this quantum field theory. The name sounds a
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Speaker 2: bit forbidding, but the idea at its heart is surprisingly beautiful. So,
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Speaker 2: make yourself comfortable, let the evening settle around you, and
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Speaker 2: we'll gently explore what physics has uncovered about the deepest
3:18
Speaker 2: foundations of reality. Shall we begin? If everything we can
3:23
Speaker 2: see and touch is really just ripples in invisible fields,
3:28
Speaker 2: then what exactly are we all made of? A stone
3:31
Speaker 2: resting in your hand seems like a very straightforward sort
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Speaker 2: of thing. It has a weight, a shape, and a
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Speaker 2: surface that feels solid beneath your fingers. You can put
3:41
Speaker 2: it on a table, leave it there for a hundred years,
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Speaker 2: and reasonably expect it to remain much the same. The
3:47
Speaker 2: world is full of objects like this. Mountains, oceans, trees, planets,
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Speaker 2: and even the stars scattered across the night sky appear
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Speaker 2: to be made from physical substances occupying particular regions of space.
4:00
Speaker 2: and for most of human history that was a perfectly
4:03
Speaker 2: satisfactory way to understand matter. But if we could examine
4:07
Speaker 2: that stone at smaller and smaller scales, something rather unexpected
4:12
Speaker 2: would begin to happen. The familiar properties of solidity, texture
4:16
Speaker 2: and shape would gradually give way to a world that
4:19
Speaker 2: behaves very differently from the one we experience. At the
4:23
Speaker 2: beginning of the 19th century, the English chemist John Dalton
4:27
Speaker 2: developed an atomic theory of matter. He proposed that chemical
4:31
Speaker 2: elements consisted of tiny particles called atoms, and that different
4:35
Speaker 2: elements contained different kinds of atoms. His model was not
4:39
Speaker 2: entirely correct, but it established an important foundation for modern chemistry.
4:45
Speaker 2: Matter was not continuous. It had an underlying microscopic structure.
4:49
Speaker 2: Later discoveries revealed that atoms themselves were not the smallest
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Speaker 2: possible constituents of matter. In 1911, Ernest Rutherford proposed the
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Speaker 2: nuclear model of the atom, following experiments in which alpha
5:03
Speaker 2: particles were directed towards extremely thin sheets of gold. Most
5:08
Speaker 2: passed through with little deflection, but a small proportion scattered
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Speaker 2: through surprisingly large angles. The results suggested that nearly all
5:17
Speaker 2: the atom's positive charge and most of its mass were
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Speaker 2: concentrated in a remarkably small central nucleus. Two years later,
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Speaker 2: Niels Bohr introduced a model incorporating quantist electron energy levels,
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Speaker 2: helping explain certain features of the light emitted by hydrogen.
5:35
Speaker 2: It was an important step towards quantum mechanics, although the
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Speaker 2: modern description of atomic electrons would eventually become considerably more sophisticated. Today,
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Speaker 2: we understand that atoms contain electrons surrounding nuclei, composed of
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Speaker 2: protons and in most cases neutrons. Those protons and neutrons
5:55
Speaker 2: contain smaller constituents called quarks, bound together through interactions involving
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Speaker 2: particles known as gluons. An ordinary atom is approximately one
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Speaker 2: ten billionth of a metre across, while its nucleus is
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Speaker 2: typically tens of thousands of times smaller. The difference is extraordinary.
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Speaker 2: Yet describing an atom as mostly empty space can be
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Speaker 2: misleading if it suggests that its electrons are tiny planets
6:22
Speaker 2: travelling through a vast, completely vacant interior. Electrons occupy quantum states.
6:29
Speaker 2: Their positions and momenta are described through probability distributions, rather
6:34
Speaker 2: than the precise trajectories followed by familiar objects. And the
6:38
Speaker 2: solidity of matter does not come from miniature solid surfaces
6:42
Speaker 2: pressing directly against one another. It emerges largely from electromagnetic
6:47
Speaker 2: interactions and the quantum rules governing electrons. including the pearly
6:51
Speaker 2: exclusion principle. When your hand rests against a table, the
6:56
Speaker 2: atoms of your hand do not simply collide like microscopic
6:59
Speaker 2: billiard balls. The electromagnetic interactions between their charged constituents, together
7:05
Speaker 2: with the quantum behaviour of electrons, resist the compression and
7:09
Speaker 2: rearrangement of matter. What feels like an ordinary solid surface
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Speaker 2: is therefore the large-scale consequence of an enormous number of
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Speaker 2: microscopic interactions. But modern physics has uncovered something stranger still.
7:24
Speaker 2: Even electrons and quarks, which are treated as elementary particles
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Speaker 2: in our present understanding, are not necessarily best imagined as
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Speaker 2: individual little objects. According to quantum field theory, the fundamental
7:37
Speaker 2: description begins with fields. A field is something whose properties
7:42
Speaker 2: can be described at different locations in space and time.
7:45
Speaker 2: We already encounter classical examples in everyday physics. A magnet
7:50
Speaker 2: produces a magnetic field around it. Earth has a gravitational field,
7:55
Speaker 2: and electromagnetic waves travel across enormous distances through space. Quantum
8:02
Speaker 2: fields are different from these familiar classical descriptions because their
8:06
Speaker 2: states and excitations obey the principles of quantum mechanics. In
8:11
Speaker 2: quantum field theory, an electron can be understood as an
8:14
Speaker 2: excitation of the electron field. The quantum particle associated with
8:20
Speaker 2: light is an excitation of the electromagnetic field. Quarks are
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Speaker 2: associated with their own quantum fields, while gluons are excitations
8:29
Speaker 2: of the fields involved in the strong interaction. The word
8:32
Speaker 2: excitation is important. It does not mean that a tiny
8:36
Speaker 2: object is physically shaking inside an invisible substance. Instead, it
8:40
Speaker 2: describes a quantum state of a field that possesses properties
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Speaker 2: we recognise as those of a particle. A comparison with
8:48
Speaker 2: ripples on a pond can be helpful, provided we remember
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Speaker 2: its limitations. A ripple is not a separate material added
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Speaker 2: to the water. It is a particular pattern of disturbance
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Speaker 2: in something already present. Similarly, a particle can be described
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Speaker 2: as an excitation of a quantum field. But unlike water,
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Speaker 2: a quantum field is not known to consist of some
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Speaker 2: more ordinary substance beneath its surface. There is also a
9:13
Speaker 2: remarkable consequence to this description. The electron field is not
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Speaker 2: confined to places where electrons happen to be found. It
9:21
Speaker 2: is described throughout space, including regions where no electrons are present.
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Speaker 2: The same general principle applies to the other fundamental fields.
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Speaker 2: A region containing no detectable particles does not therefore become
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Speaker 2: a region from which the quantum fields themselves have disappeared.
9:38
Speaker 2: This is one reason the concept of empty space becomes
9:42
Speaker 2: so interesting in modern physics. The standard model of particle
9:47
Speaker 2: physics brings many of these ideas together. It describes the
9:51
Speaker 2: known elementary particles and their electromagnetic, weak and strong interactions
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Speaker 2: through quantum fields. Its predictions have been tested with extraordinary precision,
10:02
Speaker 2: making it one of the most successful theoretical frameworks ever developed.
10:06
Speaker 2: Yet the standard model does not provide a complete quantum
10:10
Speaker 2: description of gravity, nor does it explain every fundamental mystery
10:14
Speaker 2: in physics. And although quantum fields are treated as fundamental
10:19
Speaker 2: within this framework, that does not establish that they must
10:23
Speaker 2: be the final ingredients of reality. There may be deeper
10:26
Speaker 2: structures that remain undiscovered. For now, however, the picture is
10:31
Speaker 2: extraordinary enough. The stuff resting in your hand, the light
10:36
Speaker 2: arriving from distant stars, and the atoms throughout your body
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Speaker 2: can all be understood through the behaviour of quantum fields.
10:44
Speaker 2: The apparently solid universe is not quite the collection of
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Speaker 2: miniature objects that everyday intuition suggests. Its astonishing variety emerges
10:52
Speaker 2: from quantum states, excitations and interactions governed by remarkably consistent
10:58
Speaker 2: physical laws, and beneath the familiar appearance of ordinary matter
11:02
Speaker 2: lies a description of nature that is stranger, more elegant,
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Speaker 2: and considerably less solid than it first appears. Imagine placing
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Speaker 2: a small compass on a wooden table, far away from
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Speaker 2: anything that might disturb it. Its needle settles into a
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Speaker 2: particular direction, responding to the magnetic field of Earth. Now
11:23
Speaker 2: bring an ordinary magnet towards the compass. Long before the
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Speaker 2: magnet touches anything, the needle begins to turn. There is
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Speaker 2: no visible connection between the two objects. Nothing appears to
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Speaker 2: be stretching across the space separating them, yet something about
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Speaker 2: that space is influencing the compass. This seemingly simple observation
11:43
Speaker 2: points towards one of the most important ideas in physics,
11:47
Speaker 2: the concept of a field. In physics, a field describes
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Speaker 2: a physical quantity that has a value at different locations
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Speaker 2: in space and time. A familiar example is Earth's gravitational field.
12:02
Speaker 2: Near the surface of our planet, an object released from
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Speaker 2: rest accelerates downwards at approximately 9.8 metres per second squared.
12:12
Speaker 2: provided we neglect air resistance. That acceleration tells us something
12:16
Speaker 2: about the gravitational field at that location. Move higher above
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Speaker 2: Earth and the gravitational field gradually becomes weaker. Travel sufficiently
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Speaker 2: far away and Earth's gravitational influence becomes extremely small, although
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Speaker 2: it does not suddenly disappear at some particular boundary. The
12:35
Speaker 2: gravitational field is not confined to the ground or even
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Speaker 2: to the atmosphere. It extends into the space surrounding our planet.
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Speaker 2: A magnetic field behaves differently, but the underlying idea is similar.
12:49
Speaker 2: A magnet occupies a definite region of space. You can
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Speaker 2: hold it, measure its dimensions, and place it inside a drawer.
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Speaker 2: Its magnetic field, however, is not restricted to the material
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Speaker 2: from which the magnet is made. The field extends outside
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Speaker 2: the object, with a strength and direction that vary from
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Speaker 2: place to place, And importantly, a field does not need
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Speaker 2: to be equally strong everywhere. It can be enormous in
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Speaker 2: one region and almost imperceptibly weak somewhere else. This distinction
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Speaker 2: between an object and the field surrounding it would eventually
13:24
Speaker 2: transform the foundations of physics. During the 19th century, the
13:29
Speaker 2: English scientist Michael Faraday conducted a remarkable series of experiments
13:34
Speaker 2: involving electricity and magnetism. Faraday was particularly interested in the
13:39
Speaker 2: relationship between electrical currents, magnetic effects, and the space surrounding
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Speaker 2: the objects producing them. In 1831, he demonstrated electromagnetic induction,
13:51
Speaker 2: showing that changing magnetic conditions could produce an electrical current
13:55
Speaker 2: in a conducting circuit. This discovery would eventually become fundamental
14:00
Speaker 2: to electrical generators and transformers, but Faraday's contribution went beyond
14:05
Speaker 2: discovering useful electrical effects. He developed a powerful way of
14:09
Speaker 2: thinking about forces. Rather than imagining that one object somehow
14:14
Speaker 2: reached across an empty distance to influence another, Faraday pictured
14:18
Speaker 2: the surrounding space as possessing a physical field. He represented
14:23
Speaker 2: magnetic fields using lines of force. You may have seen
14:27
Speaker 2: a familiar demonstration involving iron filings. scattered over a sheet
14:32
Speaker 2: of paper placed above magnet. The filings arrange themselves into
14:36
Speaker 2: curved patterns, revealing the direction of the magnetic field. Those
14:41
Speaker 2: patterns can look rather like invisible threads connecting the magnet's poles,
14:46
Speaker 2: but the threads are not actually there. Magnetic field lines
14:50
Speaker 2: are a way of representing the field. Their direction indicates
14:54
Speaker 2: the direction of the magnetic field, and their relative density
14:57
Speaker 2: in a carefully constructed diagram indicates its strength. They are
15:01
Speaker 2: not physical strings, and the field does not consist of
15:05
Speaker 2: separate little tubes. Nevertheless, Faraday's picture provided an extraordinarily useful
15:11
Speaker 2: way of understanding electromagnetic behaviour. The next great development came
15:16
Speaker 2: from the Scottish physicist James Clerk Maxwell. During the 1860s,
15:20
Speaker 2: Maxwell brought together the known laws of electricity and magnetism
15:24
Speaker 2: into a unified mathematical description. His work established that electric
15:29
Speaker 2: and magnetic fields are deeply connected An electric field can
15:33
Speaker 2: arise from electric charges, while magnetic fields are associated with
15:37
Speaker 2: electric currents and changing electric fields. Changing magnetic fields can
15:42
Speaker 2: also produce electric fields. Together, these relationships allow electromagnetic disturbances
15:47
Speaker 2: to propagate through space. Maxwell discovered that the equations describing
15:52
Speaker 2: these disturbances predicted a particular propagation speed, and that speed
15:57
Speaker 2: was astonishingly close to the already measured speed of light.
16:02
Speaker 2: approximately 300 million metres per second. The implication was extraordinary.
16:08
Speaker 2: Light itself was an electromagnetic wave. The sunlight warming Earth's surface,
16:14
Speaker 2: the faint glow of a distant star, and the colours
16:17
Speaker 2: reflected from an ordinary flower were all manifestations of electromagnetic fields.
16:23
Speaker 2: And unlike water waves, which require water, or sound waves,
16:28
Speaker 2: which require a material medium, Electromagnetic waves can travel through
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Speaker 2: a vacuum. They do not need air, liquid or an
16:36
Speaker 2: invisible mechanical substance carrying them along. The electromagnetic field itself
16:42
Speaker 2: supports the propagating disturbance. This helps explain how sunlight can
16:47
Speaker 2: cross approximately 150 million kilometres of space before arriving at Earth.
16:53
Speaker 2: The journey takes a little over eight minutes. The light
16:56
Speaker 2: is not travelling through an enormous ocean of invisible material,
17:00
Speaker 2: It is a propagating electromagnetic disturbance. At the beginning of
17:05
Speaker 2: the 20th century, Albert Einstein carried this understanding further. In 1905,
17:11
Speaker 2: Einstein published his special theory of relativity, establishing a new
17:16
Speaker 2: relationship between space and time. One of its central principles
17:20
Speaker 2: is that light in a vacuum travels at the same
17:22
Speaker 2: speed for all inertial observers, regardless of their relative motion.
17:27
Speaker 2: That speed is not merely a property of light. It
17:31
Speaker 2: represents the fundamental limit governing the transmission of information and
17:35
Speaker 2: causal influences through space-time. Relativity also revealed that electric and
17:40
Speaker 2: magnetic fields are connected in a particularly profound way. What
17:45
Speaker 2: one observer describes as a combination of electric and magnetic
17:48
Speaker 2: effects may be divided differently by another observer, moving relative
17:53
Speaker 2: to the first. The fields are components of a unified
17:56
Speaker 2: electromagnetic description. Yet all of these developments still belonged largely
18:02
Speaker 2: to the world of classical physics. Classical fields are described
18:06
Speaker 2: through quantities that can vary continuously. They can carry energy,
18:10
Speaker 2: possess momentum and produce measurable effects. Quantum physics introduced another
18:16
Speaker 2: layer of behaviour. When electromagnetic radiation interacts with matter, Its
18:22
Speaker 2: energy is exchanged in discrete quantum amounts. The electromagnetic field
18:27
Speaker 2: can therefore be described through quantum states, with photons representing
18:32
Speaker 2: its particle-like excitations. A quantum field is not simply a
18:36
Speaker 2: classical field with a few additional ripples. Its mathematical description
18:41
Speaker 2: incorporates quantum uncertainty, superposition, and the rules governing the creation
18:46
Speaker 2: and detection of particles. This distinction would prove essential to
18:50
Speaker 2: understanding the microscopic universe. Faraday's invisible fields had begun as
18:55
Speaker 2: a way of describing forces between familiar objects. Maxwell demonstrated
19:00
Speaker 2: that fields could carry travelling waves. Einstein revealed their relationship
19:05
Speaker 2: to the structure of spacetime, and quantum physics transformed the
19:09
Speaker 2: description of fields themselves. What began with a compass needle
19:13
Speaker 2: turning beside an ordinary magnet had become something considerably more profound.
19:18
Speaker 2: the space surrounding an object was no longer merely an
19:21
Speaker 2: empty distance separating one piece of matter from another. Fields
19:25
Speaker 2: could exist throughout that space, carry energy across it, and
19:29
Speaker 2: produce observable physical effects, and in the quantum description of nature,
19:34
Speaker 2: those fields would become central to understanding the particles from
19:37
Speaker 2: which the familiar world appears to be constructed. Imagine rolling
19:42
Speaker 2: a small marble across a smooth wooden table. It begins
19:45
Speaker 2: at one position, follows a particular path, and eventually comes
19:49
Speaker 2: to rest somewhere else. If you knew its initial speed, direction,
19:55
Speaker 2: and the forces acting upon it, you could calculate where
19:58
Speaker 2: it would travel. For centuries, physicists expected the smallest pieces
20:03
Speaker 2: of matter to behave in much the same way. An
20:06
Speaker 2: electron might be extraordinarily tiny, but surely it would still
20:10
Speaker 2: possess a definite position and follow some kind of recognisable trajectory.
20:16
Speaker 2: As scientists began investigating the microscopic world, however, this familiar
20:21
Speaker 2: picture became increasingly difficult to maintain. The first major clues
20:26
Speaker 2: appeared in the behaviour of light. Around 1900, the German
20:31
Speaker 2: physicist Max Planck was investigating black-body radiation. the electromagnetic radiation
20:39
Speaker 2: emitted by objects according to their temperature. Classical physics struggled
20:43
Speaker 2: to explain the observed distribution of energy across different frequencies.
20:48
Speaker 2: Planck discovered that the experimental results could be reproduced by
20:53
Speaker 2: assuming that energy was exchanged in discrete amounts, with each
20:57
Speaker 2: amount proportional to the radiation's frequency. The energy of one
21:01
Speaker 2: quantum equals Planck's constant multiplied by the frequency. This introduced
21:06
Speaker 2: a remarkable departure from classical expectations. Energy exchange could occur
21:12
Speaker 2: in distinct quantum amounts rather than in arbitrarily small portions.
21:17
Speaker 2: Five years later, Albert Einstein developed an explanation of the
21:21
Speaker 2: photoelectric effect, in which light striking certain materials causes electrons
21:26
Speaker 2: to be emitted. Experiments revealed something unexpected. Increasing the brightness
21:32
Speaker 2: of low-frequency light did not necessarily release electrons. Below a
21:37
Speaker 2: particular frequency, no electrons were emitted, regardless of how intense
21:42
Speaker 2: the illumination became. Einstein explained this by treating light as
21:47
Speaker 2: transferring energy in individual quanta. Each quantum carried an amount
21:52
Speaker 2: of energy determined by its frequency. If that energy exceeded
21:57
Speaker 2: the energy required to release an electron from the material,
22:00
Speaker 2: emission could occur. Greater brightness meant more light quanta arriving,
22:05
Speaker 2: but not more energy in each individual quantum. Light, which
22:10
Speaker 2: had been understood so successfully through Maxwell's electromagnetic waves, was
22:14
Speaker 2: displaying unmistakably particle-like behaviour, and the mystery was about to
22:19
Speaker 2: become even stranger. In 1924, The French physicist Louis de
22:25
Speaker 2: Broglie proposed that the relationship between waves and particles might
22:30
Speaker 2: operate in both directions. If light could exhibit particle-like properties,
22:36
Speaker 2: perhaps material particles could exhibit wave-like properties. De Broglie suggested
22:42
Speaker 2: that a particle's momentum determines an associated wavelength. The wavelength
22:47
Speaker 2: equals Planck's constant divided by the particle's momentum. For an
22:51
Speaker 2: ordinary moving object, this wavelength is unimaginably small. A rolling
22:57
Speaker 2: marble does not produce a noticeable quantum interference pattern because
23:01
Speaker 2: its associated wavelength is vastly smaller than anything we could
23:05
Speaker 2: conveniently observe. For electrons, however, the wavelength can be comparable
23:10
Speaker 2: to the spacing between atoms in a crystal. And that
23:14
Speaker 2: creates an opportunity for an extraordinary experiment. In 1927, Clinton
23:20
Speaker 2: Davison and Lester Germa reported electron scattering measurements involving a
23:26
Speaker 2: nickel crystal. The electrons emerged preferentially in directions that could
23:30
Speaker 2: be explained through wave diffraction. Their behaviour closely resembled the
23:35
Speaker 2: diffraction of waves encountering a regularly arranged structure. Electrons were
23:40
Speaker 2: not behaving exclusively like tiny classical projectiles. They possessed measurable
23:46
Speaker 2: wave-like properties. Perhaps the clearest illustration of this unusual behaviour
23:51
Speaker 2: comes from the double-slit experiment. Imagine a barrier containing two
23:56
Speaker 2: narrow openings, with a detector positioned behind it. When ordinary
24:02
Speaker 2: particles are fired through the openings, we might expect two
24:05
Speaker 2: broad regions where they accumulate, but waves passing through both
24:10
Speaker 2: openings can interfere. Where their contributions reinforce one another, the
24:15
Speaker 2: resulting intensity increases. Where they cancel, it decreases. The result
24:20
Speaker 2: is a sequence of alternating bright and dark regions. Now
24:25
Speaker 2: consider electrons travelling towards the openings. Under suitable experimental conditions,
24:31
Speaker 2: the distribution of detected electrons develops an interference pattern. Even
24:36
Speaker 2: more remarkably, the electrons can be sent through the apparatus individually,
24:40
Speaker 2: with sufficient separation that only one electron is present at
24:44
Speaker 2: a time. Each detection appears as a localised event. A
24:48
Speaker 2: single electron is recorded at one particular position. Yet after
24:52
Speaker 2: many electrons have been detected, their accumulated positions reveal the
24:57
Speaker 2: familiar interference pattern. The individual outcomes appear particle-like, while the
25:03
Speaker 2: overall distribution reflects wave-like behaviour. And if an experiment obtains
25:08
Speaker 2: reliable information about which opening each electron passes through, The
25:13
Speaker 2: interference between the alternatives is lost. This does not require
25:17
Speaker 2: a conscious observer watching the apparatus. The physical interactions involved
25:22
Speaker 2: in obtaining path information change the quantum conditions responsible for interference.
25:29
Speaker 2: Understanding these experiments required a fundamentally different mathematical description. One
25:35
Speaker 2: of the central concepts of quantum mechanics is the wave function.
25:39
Speaker 2: A wave function describes a quantum state. and contains information
25:44
Speaker 2: used to calculate the probabilities of possible measurement outcomes. It
25:48
Speaker 2: is not an ordinary water wave, or a disturbance travelling
25:52
Speaker 2: through air. Its mathematical values are probability amplitudes, which can
25:57
Speaker 2: interfere with one another. The German physicist Max Born established
26:02
Speaker 2: the probabilistic interpretation of the wavefunction in 1926. According to
26:08
Speaker 2: Born's rule, The probability density for detecting a particle at
26:12
Speaker 2: a particular location is determined by the squared magnitude of
26:16
Speaker 2: its wave function. This means that quantum mechanics does not
26:21
Speaker 2: generally predict one definite location for an individual detection. Instead,
26:27
Speaker 2: it predicts a probability distribution that can be tested through
26:31
Speaker 2: repeated experiments. During the same period, Werner Heisenberg developed the
26:36
Speaker 2: uncertainty principle. This established a fundamental limitation on how precisely
26:42
Speaker 2: certain pairs of physical quantities can be specified simultaneously. For
26:47
Speaker 2: position and momentum, the relationship is commonly expressed as follows.
26:51
Speaker 2: The uncertainty in position multiplied by the uncertainty in momentum
26:56
Speaker 2: must be at least half of the reduced Planck constant.
27:00
Speaker 2: This is not simply a problem caused by imperfect laboratory equipment.
27:04
Speaker 2: It reflects the mathematical structure of quantum states themselves. A
27:09
Speaker 2: state with a very narrow position distribution necessarily possesses a
27:13
Speaker 2: broader range of possible momentum values. The familiar classical picture
27:18
Speaker 2: of a particle, possessing an exactly defined position and momentum
27:23
Speaker 2: at every instant, therefore cannot be carried directly into ordinary
27:27
Speaker 2: quantum mechanics. Erwin Schrödinger provided another essential part of this description.
27:33
Speaker 2: In 1926, he introduced the wave equation that bears his name,
27:38
Speaker 2: describing how quantum states evolve. The Schrödinger equation helped explain
27:43
Speaker 2: why electrons bound within atoms occupy particular allowed energy states.
27:49
Speaker 2: Rather than imagining electrons as miniature planets travelling around a nucleus,
27:53
Speaker 2: quantum mechanics describes atomic electrons through states with distinctive spatial
27:59
Speaker 2: probability distributions and energies. These states help determine atomic structure,
28:04
Speaker 2: chemical bonding and the properties of ordinary materials. And yet
28:09
Speaker 2: the word particle remains useful. Electrons have definite intrinsic properties,
28:15
Speaker 2: including mass, electric charge and spin. They produce localised detections
28:21
Speaker 2: and transfer measurable amounts of energy and momentum. The mistake
28:25
Speaker 2: would be to assume that these observations require electrons to
28:28
Speaker 2: behave like microscopic versions of everyday objects. Quantum behaviour is
28:33
Speaker 2: neither an ordinary wave disguised as a particle, nor necessarily
28:37
Speaker 2: a classical particle concealing some familiar trajectory. It requires its
28:42
Speaker 2: own physical description. And once particles can no longer be
28:46
Speaker 2: understood simply as tiny permanent objects following definite paths, the
28:51
Speaker 2: foundations of matter begin to look very different. The wave-like behaviour,
28:56
Speaker 2: the localised detections, and the remarkable consistency of particle properties
29:02
Speaker 2: all point towards a description in which quantum principles are
29:05
Speaker 2: built into the fundamental physical entities themselves. Quantum field theory
29:10
Speaker 2: carries that idea further. It treats quantum behaviour not as
29:14
Speaker 2: an unusual addition to an otherwise classical world, but as
29:18
Speaker 2: part of the underlying framework through which particles and their
29:21
Speaker 2: interactions are described. The ordinary marble rolling across a table
29:26
Speaker 2: still follows a perfectly understandable path. But the electrons within
29:31
Speaker 2: that marble belong to a world governed by remarkably different rules.
29:36
Speaker 2: And it is those rules, operating quietly beneath the familiar
29:39
Speaker 2: appearance of matter, that help make the solid, dependable world
29:44
Speaker 2: around us possible. For a while, quantum mechanics appeared to
29:48
Speaker 2: have answered some of the most difficult questions in physics.
29:51
Speaker 2: It could explain why atoms remained stable, why electrons occupied
29:55
Speaker 2: particular energy levels, and why different chemical elements behaved in
30:00
Speaker 2: such remarkably consistent ways. The microscopic world was certainly stranger
30:05
Speaker 2: than anyone had expected, but scientists were beginning to understand
30:09
Speaker 2: its rules. There was, however, a rather important difficulty. Much
30:15
Speaker 2: of the early mathematical description assumed that the number of
30:18
Speaker 2: particles in a system remained unchanged. An atom might contain
30:22
Speaker 2: a particular number of electrons. Those electrons could change their energies.
30:27
Speaker 2: interact with one another or occupy different quantum states. But
30:32
Speaker 2: the underlying calculation generally began with a fixed collection of particles.
30:37
Speaker 2: For many ordinary atomic problems, this worked extremely well. The
30:40
Speaker 2: difficulty appeared when physicists considered processes in which particles could
30:45
Speaker 2: actually be created or destroyed, and nature, as it turned out,
30:49
Speaker 2: was perfectly capable of doing both. To understand why this
30:54
Speaker 2: presented such a problem, we need to consider another great
30:57
Speaker 2: achievement of 20th century physics. In 1905, Albert Einstein introduced
31:03
Speaker 2: his special theory of relativity. Relativity established that space and
31:08
Speaker 2: time are connected, and that the speed of light in
31:11
Speaker 2: a vacuum represents a fundamental limit on the transmission of
31:15
Speaker 2: information and causal influences. It also revealed a profound relationship
31:20
Speaker 2: between mass and energy. Energy equals mass multiplied by the
31:25
Speaker 2: speed of light squared. This familiar equation expresses the rest
31:31
Speaker 2: energy associated with mass. More generally, the total energy of
31:36
Speaker 2: a particle also depends on its momentum. The distinction becomes
31:40
Speaker 2: particularly important when particles move at speeds approaching the speed
31:44
Speaker 2: of light. Under sufficiently energetic conditions, interactions can produce entirely
31:49
Speaker 2: new particles. Imagine two particles approaching one another inside a
31:54
Speaker 2: powerful accelerator. They collide, transferring enormous amounts of energy within
31:59
Speaker 2: an extraordinarily small region. Afterwards, detectors may record several particles
32:04
Speaker 2: that were not present before the collision. These additional particles
32:08
Speaker 2: have not appeared through magic, and the process does not
32:12
Speaker 2: violate the conservation of energy. Some of the available energy
32:16
Speaker 2: has been converted into the rest mass and motion of
32:19
Speaker 2: newly produced particles. The total energy, momentum, electric charge and
32:25
Speaker 2: other relevant conserved quantities must still be accounted for. But
32:29
Speaker 2: the number of particles has changed. A theory that assumes
32:32
Speaker 2: a permanently fixed collection of particles cannot adequately describe every
32:37
Speaker 2: such event. One of the most important breakthroughs came from
32:41
Speaker 2: the English theoretical physicist Paul Dirac. In 1928, Dirac developed
32:47
Speaker 2: an equation describing the electron that was compatible with With
32:50
Speaker 2: both quantum mechanics and special relativity, his equation successfully incorporated
32:56
Speaker 2: the electron's intrinsic spin and provided a relativistic description of
33:00
Speaker 2: its behaviour. But it also contained a puzzle. Alongside the
33:05
Speaker 2: familiar positive energy solutions, the mathematics produced negative energy solutions
33:10
Speaker 2: that were difficult to interpret using the conventional picture of
33:14
Speaker 2: individual particles. Dirac eventually developed an interpretation that suggested the
33:20
Speaker 2: existence of a previously unknown counterpart to the electron. This
33:24
Speaker 2: hypothetical particle would possess the electron's mass but carry the
33:28
Speaker 2: opposite electric charge. It was an extraordinary prediction. And in 1932,
33:34
Speaker 2: the American physicist Carl Anderson discovered experimental evidence of precisely
33:39
Speaker 2: such a particle while studying cosmic rays. Using a cloud chamber,
33:44
Speaker 2: Anderson observed the curved track of a positively charged particle,
33:48
Speaker 2: whose behaviour was consistent with an electron's mass. The positron
33:53
Speaker 2: had been discovered. It was the first experimentally identified antiparticle.
33:58
Speaker 2: The discovery demonstrated that the familiar collection of particles was
34:02
Speaker 2: not the complete picture. Matter possessed antimatter counterparts, and interactions
34:08
Speaker 2: could produce or annihilate particle-antiparticle pairs under suitable conditions. but
34:14
Speaker 2: understanding such processes consistently required something more general than ordinary
34:19
Speaker 2: single-particle quantum mechanics. The foundations of that description had already
34:24
Speaker 2: been developing through the work of several physicists. Pasquale Jordan
34:29
Speaker 2: made important contributions to the quantisation of fields, including work
34:34
Speaker 2: undertaken with Max Born and Werner Heisenberg. Paul Dirac developed
34:39
Speaker 2: a quantum treatment of electromagnetic radiation in 1927. allowing the
34:44
Speaker 2: emission and absorption of light quanta to be described mathematically.
34:49
Speaker 2: Werner Heisenberg and Wolfgang Pauli subsequently helped develop a more
34:53
Speaker 2: systematic relativistic theory of quantist fields. These developments gradually established
34:59
Speaker 2: the foundations of quantum field theory. The central change was
35:04
Speaker 2: surprisingly elegant. Rather than beginning with a particular number of particles,
35:09
Speaker 2: physicists could begin with fields whose states obeyed quantum mechanical rules.
35:14
Speaker 2: In a classical description, a field might support waves with
35:18
Speaker 2: continuously varying amplitudes. When the field is quantized, its possible
35:23
Speaker 2: excitations follow quantum principles. For a simple free field, the
35:27
Speaker 2: different patterns of excitation can be described using mathematical structures
35:32
Speaker 2: related to quantum harmonic oscillators. These excitations are associated with
35:37
Speaker 2: the particles detected in experiments. An electromagnetic field can possess
35:42
Speaker 2: states containing different numbers of photons. An electron field can
35:47
Speaker 2: describe electrons and positrons. And interactions between fields can change
35:51
Speaker 2: the number of particles present. The underlying fields remain part
35:55
Speaker 2: of the description even when particular particle excitations disappear. This
36:00
Speaker 2: provides a natural mathematical framework for particle creation and annihilation.
36:06
Speaker 2: It also helps resolve the apparent conflict between quantum mechanics
36:10
Speaker 2: and relativity. In a relativistic quantum field theory, the laws
36:15
Speaker 2: governing fields must respect the causal structure of spacetime. Events
36:20
Speaker 2: separated by a space-like interval cannot influence one another through
36:24
Speaker 2: signals travelling faster than light. A mathematical requirement known as
36:29
Speaker 2: microcausality helps enforce this principle. In simple terms, appropriate local
36:35
Speaker 2: observables associated with space-like separated regions must be compatible in
36:41
Speaker 2: a way that prevents their measurements from providing faster-than-light communication.
36:46
Speaker 2: This does not eliminate quantum entanglement or its unusual correlations.
36:51
Speaker 2: It means those correlations cannot be used to send controllable
36:54
Speaker 2: information beyond the relativistic speed limit. The resulting framework brought
36:59
Speaker 2: together ideas that had initially seemed difficult to reconcile. Quantum
37:04
Speaker 2: mechanics describe probabilities, superposition, and microscopic energy states. Special relativity
37:11
Speaker 2: established the relationship between space, time, energy, and momentum. Quantum
37:17
Speaker 2: field theory provided a framework in which both sets of
37:21
Speaker 2: principles could operate together, including processes involving changing particle numbers.
37:27
Speaker 2: The implications were enormous. Electrons were no longer understood exclusively
37:32
Speaker 2: as permanent little objects travelling through space. They could instead
37:36
Speaker 2: be described as excitations of an underlying quantum field. Photons
37:41
Speaker 2: could be emitted or absorbed through interactions involving the electromagnetic field.
37:47
Speaker 2: and the remarkable events observed in high-energy collisions became understandable
37:52
Speaker 2: as transformations involving the states of interacting fields. This did
37:57
Speaker 2: not make the microscopic universe any less peculiar. If anything,
38:03
Speaker 2: it revealed that the familiar image of matter as a
38:05
Speaker 2: collection of tiny independent objects had been concealing a much
38:09
Speaker 2: deeper structure. The fundamental description was becoming less about counting
38:14
Speaker 2: the little things occupying space. and more about understanding the
38:18
Speaker 2: quantum fields present throughout it. Particles could appear, disappear, and transform.
38:25
Speaker 2: But beneath those changing events, quantum field theory provided a
38:29
Speaker 2: remarkably consistent description of the physical processes taking place, and
38:34
Speaker 2: what began as an attempt to reconcile two successful theories
38:38
Speaker 2: had transformed our understanding of what a particle actually is.
38:43
Speaker 2: The word field is familiar enough in physics. but when
38:46
Speaker 2: we place the word quantum in front of it, something
38:49
Speaker 2: rather unusual happens. We are no longer describing a simple
38:53
Speaker 2: influence surrounding an object, or a familiar quantity that varies
38:57
Speaker 2: from one place to another. We are describing something that
39:01
Speaker 2: lies at the very foundations of modern particle physics. And
39:05
Speaker 2: although quantum fields are extraordinarily successful at explaining the behaviour
39:10
Speaker 2: of matter, imagining what one actually looks like is surprisingly difficult.
39:15
Speaker 2: Consider an ordinary magnetic field. At any particular location physicists
39:21
Speaker 2: can describe its strength and direction. Move to another position
39:25
Speaker 2: and those values may change. A classical field is therefore
39:29
Speaker 2: something that can be assigned appropriate mathematical values throughout a
39:33
Speaker 2: region of space and time. Quantum fields retain this connection
39:37
Speaker 2: with spacetime, but their mathematical description is fundamentally different. Rather
39:43
Speaker 2: than simply assigning ordinary numerical values to every location, Quantum
39:48
Speaker 2: field theory introduces mathematical objects called field operators. These operators
39:53
Speaker 2: act upon quantum states. A quantum state describes the physical
39:58
Speaker 2: possibilities associated with a system, including the probabilities of different
40:03
Speaker 2: measurement outcomes. The distinction may sound rather technical, but it
40:08
Speaker 2: is important. A quantum field is not merely an invisible
40:12
Speaker 2: substance with a particular amount of something sitting at every point.
40:16
Speaker 2: Its description involves quantum states, operators, and the relationships between
40:22
Speaker 2: possible observations. Strictly speaking, quantum fields are generally treated as
40:27
Speaker 2: operator-valued distributions, meaning that their mathematical definition is more subtle
40:32
Speaker 2: than assigning an ordinary operator independently to every exact point. Fortunately,
40:39
Speaker 2: understanding the central idea does not require working through the mathematics.
40:44
Speaker 2: A quantum field provides the underlying structure within which particular
40:48
Speaker 2: quantum excitations can exist, and those excitations are associated with
40:53
Speaker 2: what we ordinarily call particles. Imagine a very long musical
40:58
Speaker 2: string stretched between two supports. When the string is undisturbed,
41:02
Speaker 2: it remains present even though no visible vibration is travelling
41:06
Speaker 2: along it. If the string is disturbed, it can vibrate
41:10
Speaker 2: in different patterns. Some patterns have higher frequencies, while others
41:15
Speaker 2: have lower frequencies. The string remains the same physical object,
41:20
Speaker 2: but its possible vibrations can be remarkably varied. This offers
41:24
Speaker 2: a limited analogy for a quantum field. The underlying field
41:28
Speaker 2: is not created each time a particle appears. Instead, what
41:33
Speaker 2: changes is the quantum state of that field. An excited
41:37
Speaker 2: state can contain particle-like excitations, while the lowest energy state
41:42
Speaker 2: is known as the vacuum state. But the analogy must
41:45
Speaker 2: be treated carefully. A quantum field is not literally a
41:49
Speaker 2: vibrating string. It is not an invisible fabric, a liquid,
41:54
Speaker 2: or an ocean of mysterious material filling every corner of
41:58
Speaker 2: the universe. We have no established evidence that ordinary quantum
42:02
Speaker 2: fields consist of some more familiar microscopic substance. Their behaviour
42:07
Speaker 2: is described through mathematical principles that differ considerably from the
42:11
Speaker 2: mechanics of everyday materials. There is another distinction worth making.
42:17
Speaker 2: The field itself, the state of that field, and a
42:22
Speaker 2: particle associated with the field are not three names for
42:25
Speaker 2: exactly the same thing. The field is part of the
42:29
Speaker 2: underlying theoretical description. The quantum state specifies the physical condition
42:34
Speaker 2: being described, and a particle is associated with particular excitations
42:40
Speaker 2: of that field. especially in circumstances where particle states can
42:44
Speaker 2: be clearly identified. For example, the electron field can exist
42:49
Speaker 2: in a state containing no electrons or positrons. It can
42:52
Speaker 2: also occupy states containing one electron, several electrons or combinations
42:57
Speaker 2: of electrons and positrons. The underlying field remains part of
43:01
Speaker 2: the description throughout. This is an important departure from the
43:05
Speaker 2: familiar idea that a particle must be a permanent little
43:08
Speaker 2: object with its own independent substance. Quantum field theory describes
43:12
Speaker 2: several different fundamental fields. The electron field is associated with
43:17
Speaker 2: electrons and their antiparticles, positrons. The electromagnetic field is associated
43:23
Speaker 2: with photons, the particles of light. Quark fields describe the
43:27
Speaker 2: different varieties of quarks found within particles, such as protons
43:31
Speaker 2: and neutrons. And the Higgs field possesses a particularly unusual property.
43:36
Speaker 2: because its vacuum state has a non-zero field expectation value
43:40
Speaker 2: associated with the mechanism through which many elementary particles acquire mass.
43:45
Speaker 2: These fields are not all interchangeable. They possess different properties
43:49
Speaker 2: and participate in different interactions. Mass is one important distinction.
43:54
Speaker 2: An electron possesses a non-zero rest mass, while a photon
43:58
Speaker 2: has no rest mass. Electric charge is another. Electrons carry
44:03
Speaker 2: negative electric charge, While photons are electrically neutral, spin also matters.
44:09
Speaker 2: An electron has spin one half, while a photon has
44:12
Speaker 2: spin one. Quantum spin is an intrinsic property, not evidence
44:17
Speaker 2: that the particles are tiny solid balls rotating around their
44:20
Speaker 2: own axes. These differences influence how the associated excitations behave,
44:26
Speaker 2: which interactions they participate in, and the quantum statistical rules
44:31
Speaker 2: they obey. In this way, the diversity of elementary particles
44:35
Speaker 2: reflects differences in the properties of quantum fields. But perhaps
44:39
Speaker 2: the strangest idea is that these fields are described throughout space.
44:44
Speaker 2: The electron field is not restricted to the interior of atoms.
44:47
Speaker 2: It is not confined to planets, stars, or regions containing
44:52
Speaker 2: detectable electrons. In the usual formulation of quantum field theory,
44:57
Speaker 2: the electron field is defined across spacetime, including regions in
45:01
Speaker 2: which no electron is present. Of course, this does not
45:04
Speaker 2: mean that electrons are constantly being detected everywhere. The existence
45:09
Speaker 2: of a field within the theoretical description is different from
45:12
Speaker 2: the presence of a particle excitation. An empty region of
45:16
Speaker 2: space can contain no detectable electrons, while the electron field
45:21
Speaker 2: remains part of its quantum description. There is also an
45:24
Speaker 2: important relationship between quantum fields and spacetime itself. Ordinary relativistic
45:30
Speaker 2: quantum field theory describes fields within spacetime, respecting the principles
45:35
Speaker 2: of special relativity and the causal relationships between events. It
45:40
Speaker 2: does not require space to be a material ocean through
45:43
Speaker 2: which particles swim, nor does it establish that spacetime and
45:48
Speaker 2: quantum fields are ultimately the same thing. The deeper relationship
45:52
Speaker 2: between quantum physics, gravity and the structure of spacetime remains
45:56
Speaker 2: an important subject of scientific investigation. And this brings us
46:00
Speaker 2: to a subtle question. Are quantum fields actually real, physical things,
46:05
Speaker 2: or are they exceptionally successful mathematical descriptions of nature? Physicists
46:11
Speaker 2: can calculate their observable consequences with extraordinary precision. Experiments repeatedly
46:17
Speaker 2: confirm predictions derived from quantum field theory. Yet the philosophical
46:22
Speaker 2: question of whether fields should be considered the ultimate substances
46:25
Speaker 2: of reality is not settled simply by the success of
46:29
Speaker 2: those predictions. If future theory might reveal an underlying structure
46:33
Speaker 2: that our present understanding does not yet describe, for now
46:37
Speaker 2: quantum fields provide one of the most powerful descriptions of
46:40
Speaker 2: the physical world ever developed. They allow us to understand
46:44
Speaker 2: particles not as isolated miniature objects, but through the states
46:48
Speaker 2: and excitations of underlying fields, and the familiar world of
46:53
Speaker 2: solid matter, travelling light, and interacting particles become something rather extraordinary.
47:00
Speaker 2: A universe whose visible complexity can arise from the quantum
47:04
Speaker 2: behaviour of fields that we cannot directly see, but whose
47:07
Speaker 2: effects are present in almost everything we observe. There is
47:11
Speaker 2: something rather curious about the light coming from an ordinary
47:14
Speaker 2: bedside lamp. To our eyes, it appears as a continuous glow.
47:19
Speaker 2: The light spreads across the room, illuminates nearby objects, and
47:23
Speaker 2: gradually becomes weaker as we move farther away. Nothing about
47:27
Speaker 2: this familiar experience suggests that light has anything particularly unusual
47:32
Speaker 2: about its smallest ingredients. And yet, according to quantum field theory,
47:37
Speaker 2: that gentle illumination can be described in terms of individual
47:41
Speaker 2: excitations of the electromagnetic field. We call those excitations photons.
47:48
Speaker 2: A photon is not a miniature glowing ball travelling through space,
47:52
Speaker 2: It is a quantum of excitation associated with the electromagnetic field,
47:57
Speaker 2: possessing definite properties that become apparent when light interacts with matter.
48:02
Speaker 2: This raises an interesting question. How can something as continuous
48:06
Speaker 2: and widespread as a field produce something that behaves like
48:10
Speaker 2: an individual particle? The answer begins with the difference between
48:14
Speaker 2: classical waves and quantum excitations. Imagine a long string stretched
48:19
Speaker 2: tightly between two supports. If you disturb the string, it
48:23
Speaker 2: begins to vibrate. You can make those vibrations larger or
48:27
Speaker 2: smaller by supplying different amounts of energy. In classical physics,
48:31
Speaker 2: the amplitude can vary continuously. There is no fundamental requirement
48:36
Speaker 2: that the string must receive energy in particular indivisible portions.
48:41
Speaker 2: A classical electromagnetic wave behaves similarly in this respect. Its
48:46
Speaker 2: amplitude can vary smoothly, and the energy carried by the
48:50
Speaker 2: wave depends on the strength of the electromagnetic disturbance. Quantum
48:55
Speaker 2: field theory introduces a different set of rules. The electromagnetic
48:59
Speaker 2: field can still support wave-like behaviour, but its quantum excitations
49:04
Speaker 2: are organised according to discrete energy levels. For a particular
49:08
Speaker 2: field mode with a definite frequency, the energy associated with
49:12
Speaker 2: each additional photon is determined by that frequency. The relationship
49:16
Speaker 2: was introduced by Max Planck during his investigation of thermal radiation.
49:21
Speaker 2: The energy of a photon equals Planck's constant multiplied by
49:25
Speaker 2: its frequency. Planck's constant is an extraordinarily small quantity, approximately 6.63
49:32
Speaker 2: times 10 to the power of minus 34 joule seconds.
49:38
Speaker 2: Its tiny value helps explain why quantum behaviour is not
49:41
Speaker 2: always obvious in everyday experiences. A photon of visible light
49:46
Speaker 2: carries only a few electron volts of energy, an exceedingly
49:49
Speaker 2: small amount by ordinary standards. But the energy of a
49:53
Speaker 2: photon depends on its frequency. A photon of ultraviolet light
49:58
Speaker 2: carries more energy than a photon of visible red light.
50:01
Speaker 2: An X-ray photon carries substantially more energy still. This is
50:05
Speaker 2: why different forms of electromagnetic radiation can interact with matter
50:10
Speaker 2: in very different ways. The underlying electromagnetic field is the same,
50:16
Speaker 2: but its excitations can possess different frequencies and energies. To
50:20
Speaker 2: understand how these excitations arise mathematically, physicists often consider the
50:25
Speaker 2: different modes of a field. A mode is a particular
50:29
Speaker 2: pattern in which the field can oscillate. A musical instrument
50:33
Speaker 2: provides a useful comparison. A violin string can vibrate in
50:37
Speaker 2: its fundamental mode or in various higher frequency modes, producing
50:41
Speaker 2: different musical tones. In the simplest description of a free
50:46
Speaker 2: quantum field, each independent field mode behaves mathematically like a
50:50
Speaker 2: quantum harmonic oscillator. A quantum harmonic oscillator has a series
50:55
Speaker 2: of allowed energy levels rather than an unrestricted continuous range.
51:00
Speaker 2: For a bisonic field mode, such as a mode of
51:03
Speaker 2: the electromagnetic field, The number of excitation quanta can be 0, 1, 2,
51:08
Speaker 2: 3 or any other non-negative whole number. Adding one photon
51:13
Speaker 2: increases the energy of that mode by one quantum. Adding
51:16
Speaker 2: another increases it by the same amount. The field itself
51:20
Speaker 2: has not been replaced by a collection of tiny objects.
51:24
Speaker 2: Its quantum state has changed, and this distinction is essential.
51:29
Speaker 2: A single photon does not necessarily occupy one sharply defined
51:33
Speaker 2: little region of space throughout its journey. Quantum states can
51:37
Speaker 2: be spread across extended regions and can combine different field
51:41
Speaker 2: modes into arrangements known as wave packets. A wave packet
51:45
Speaker 2: can provide a description of an excitation that is relatively localised,
51:49
Speaker 2: but is not equivalent to a permanently concentrated lump following
51:53
Speaker 2: an ordinary classical trajectory. The quantum state may spread, interfere,
51:59
Speaker 2: and produce different possible outcomes when a measurement occurs. Nevertheless,
52:04
Speaker 2: when light reaches a suitable detector, the recorded result can
52:08
Speaker 2: be remarkably localized. Consider a photomultiplier tube. When a photon
52:14
Speaker 2: is absorbed at a photosensitive surface, it can release an
52:17
Speaker 2: electron through the photoelectric effect. That electron can then initiate
52:22
Speaker 2: a cascade of additional electrons, producing an electrical signal that
52:26
Speaker 2: the instrument can register. A semiconductor photon detector operates differently,
52:31
Speaker 2: but it can also convert the absorption of light into
52:34
Speaker 2: a measurable electrical response. In sufficiently sensitive instruments, individual photon
52:39
Speaker 2: detection events can be recorded. The detector registers a particular interaction,
52:45
Speaker 2: even though the incoming quantum state may have extended across
52:48
Speaker 2: a much larger region. There is no contradiction between these observations.
52:54
Speaker 2: The extended quantum description determines the probabilities of different outcomes,
52:59
Speaker 2: while the interaction with the detector produces a particular recorded event.
53:04
Speaker 2: The same underlying principles apply to other quantum fields. An
53:08
Speaker 2: electron is an excitation associated with the electron field. But
53:12
Speaker 2: electrons and photons have different intrinsic properties. An electron possesses
53:17
Speaker 2: a non-zero rest mass and carries negative electric charge. A
53:22
Speaker 2: photon has no rest mass and is electrically neutral. These
53:26
Speaker 2: properties are connected to the mathematical structure of their respective fields. Consequently,
53:32
Speaker 2: different fields produce different kinds of particle excitations, and particles
53:37
Speaker 2: cannot simply emerge without regard for the physical circumstances. Energy
53:42
Speaker 2: and momentum must be conserved. An excited atom, for example,
53:47
Speaker 2: can release energy by emitting a photon. and moving into
53:51
Speaker 2: a lower energy state. The emitted photon carries energy and
53:54
Speaker 2: momentum away from the atom. In the reverse process, an
53:59
Speaker 2: atom may absorb a photon and move into a higher
54:01
Speaker 2: energy state, provided the interaction satisfies the relevant physical conditions.
54:07
Speaker 2: In a simple free-field description, excitations do not spontaneously appear
54:13
Speaker 2: or disappear through interactions with other fields, because those interactions
54:17
Speaker 2: have been excluded. Once fields interact, however, energy can be
54:22
Speaker 2: transferred between them. Particle numbers may change, new excitations may
54:28
Speaker 2: be produced, and existing excitations may be absorbed or annihilated.
54:33
Speaker 2: These processes are governed by the laws of quantum field theory,
54:37
Speaker 2: not by random violations of energy conservation. And the resulting
54:42
Speaker 2: picture is quite remarkable. The countless photons arriving from the
54:46
Speaker 2: Sun are not countless independently manufactured pieces of some special
54:52
Speaker 2: luminous substance. They are excitations of the electromagnetic field. Likewise,
54:58
Speaker 2: the electrons found throughout ordinary matter are excitations associated with
55:02
Speaker 2: the electron field. An enormous variety of apparently separate particles
55:08
Speaker 2: can therefore be understood through the different possible states of
55:12
Speaker 2: a relatively small collection of fundamental fields. The universe does
55:17
Speaker 2: not need to manufacture a completely new underlying substance each
55:21
Speaker 2: time another particle is produced. The fields are already part
55:25
Speaker 2: of its physical description. What changes is their quantum state,
55:30
Speaker 2: and from those changes emerge the particles, radiation and interactions
55:35
Speaker 2: that make the familiar world possible. Somewhere inside your body,
55:40
Speaker 2: an electron is moving through the quantum state associated with
55:43
Speaker 2: an ordinary atom. Meanwhile, in a star thousands of light-years away,
55:48
Speaker 2: another electron is participating in the extraordinary processes taking place
55:52
Speaker 2: within that distant object. The two electrons may never encounter
55:57
Speaker 2: one another. They may exist in completely different environments, separated
56:02
Speaker 2: by distances so enormous that light itself would require thousands
56:07
Speaker 2: of years to travel between them, and yet, as far
56:10
Speaker 2: as modern physics can determine, Their intrinsic properties are exactly
56:14
Speaker 2: the same. They possess the same rest mass, the same
56:19
Speaker 2: electric charge, and the same intrinsic spin. There is nothing
56:23
Speaker 2: about an electron that reveals where it came from, how
56:27
Speaker 2: old it is, or which particular atom it happens to occupy.
56:31
Speaker 2: This is one of the most remarkable facts about the
56:34
Speaker 2: microscopic universe. Every electron is identical to every other electron.
56:40
Speaker 2: Not approximately identical. like two manufactured objects produced to an
56:44
Speaker 2: exceptionally high standard, identical in the deeper quantum mechanical sense.
56:51
Speaker 2: An electron's rest mass is approximately 9.11 times 10 to
56:55
Speaker 2: the power of minus 31 kilograms. Its electric charge is negative,
57:00
Speaker 2: with a magnitude of approximately 1.6 times 10 to the
57:04
Speaker 2: power of minus 19 coulombs, and its intrinsic spin is
57:08
Speaker 2: always one half, measured in the conventional quantum units. These
57:13
Speaker 2: properties do not depend on whether the electron belongs to
57:16
Speaker 2: a hydrogen atom, moves through a copper wire, or exists
57:20
Speaker 2: in the atmosphere of a distant star. An electron can
57:24
Speaker 2: possess different energies and momenta, but its fundamental identity remains unchanged.
57:30
Speaker 2: In everyday life, this kind of perfect consistency would be
57:33
Speaker 2: rather extraordinary. Even two supposedly identical grains of sand contain
57:38
Speaker 2: microscopic differences. Two manufactured ball bearings may have slightly different masses,
57:44
Speaker 2: surfaces or internal imperfections, but electrons are not independently manufactured
57:49
Speaker 2: little objects. According to quantum field theory, they are excitations
57:54
Speaker 2: associated with one fundamental electron field, and this provides a
57:59
Speaker 2: surprisingly elegant way of understanding their identical properties. The electron
58:04
Speaker 2: field is described throughout spacetime, an electron detected in one laboratory.
58:10
Speaker 2: and an electron detected on the opposite side of the
58:12
Speaker 2: planet are not excitations of two fundamentally different electron fields.
58:17
Speaker 2: They belong to the same underlying field description. The properties
58:22
Speaker 2: associated with an electron are therefore determined by the structure
58:26
Speaker 2: of that field and its interactions. This does not mean
58:30
Speaker 2: that all electrons are in the same physical condition. Far
58:33
Speaker 2: from it. An electron bound within an atom may occupy
58:37
Speaker 2: a particular energy state. Another electron travelling through a vacuum
58:41
Speaker 2: may possess substantial kinetic energy. A third may be confined
58:46
Speaker 2: within a magnetic field in a laboratory. Their circumstances can
58:50
Speaker 2: be very different. Their quantum states can be different. But
58:55
Speaker 2: their intrinsic properties remain the same, and quantum mechanics takes
58:59
Speaker 2: this consistency even further. It introduces the principle of quantum indistinguishability.
59:06
Speaker 2: Imagine placing two ordinary marbles on a table. Perhaps we'll...
59:10
Speaker 2: one is red and the other is blue. If you
59:12
Speaker 2: exchange their positions, the arrangement visibly changes. Even if both
59:17
Speaker 2: marbles were the same colour, you could theoretically label them
59:21
Speaker 2: and follow their individual movements. Quantum particles do not generally
59:25
Speaker 2: permit this kind of permanent individual labelling. When two identical
59:29
Speaker 2: electrons are exchanged, the exchange does not produce a new
59:33
Speaker 2: physically distinguishable arrangement. Their joint quantum state must obey particular
59:38
Speaker 2: mathematical rules reflecting their indistinguishability, and those rules have enormous consequences.
59:45
Speaker 2: Electrons belong to a category of particles called fermions. A
59:50
Speaker 2: fermion possesses half-integer intrinsic spin, and electrons specifically have spin one-half.
59:57
Speaker 2: Quantum spin is not ordinary mechanical rotation. An electron is
1:00:01
Speaker 2: not known to be a tiny solid sphere spinning around
1:00:04
Speaker 2: its own axis. Spin is an intrinsic quantum property that
1:00:08
Speaker 2: affects how particles behave and interact. For identical fermions, the
1:00:13
Speaker 2: joint quantum state is anti-symmetric under exchange. In simpler language,
1:00:19
Speaker 2: exchanging two identical fermions changes the sign of the mathematical
1:00:23
Speaker 2: state description. That sign change has no direct observable effect
1:00:28
Speaker 2: when the particles are merely relabelled, but it produces a
1:00:31
Speaker 2: remarkable restriction. Two identical fermions cannot occupy the same complete
1:00:37
Speaker 2: single-particle quantum state. This is the Pauli exclusion principle, named
1:00:42
Speaker 2: after the Austrian physicist Wolfgang Pauli, who introduced the principle
1:00:47
Speaker 2: in 1925. And without it, the familiar structure of matter
1:00:52
Speaker 2: would be profoundly different. Consider an atom containing several electrons.
1:00:57
Speaker 2: The electrons cannot all settle into precisely the same complete
1:01:01
Speaker 2: quantum state. Instead, they occupy arrangements determined by the available
1:01:06
Speaker 2: energies and quantum numbers. Within an atomic orbital, two electrons
1:01:10
Speaker 2: can occupy different spin states, but a third electron cannot
1:01:14
Speaker 2: occupy that same orbital with an already occupied spin state.
1:01:19
Speaker 2: This contributes to the organisation of electrons into shells and subshells.
1:01:24
Speaker 2: and those arrangements help explain the periodic table. Hydrogen has
1:01:28
Speaker 2: one electron in its neutral atomic state. Helium has two.
1:01:33
Speaker 2: Larger atoms contain progressively more electrons, occupying increasingly complex arrangements
1:01:38
Speaker 2: of quantum states. The resulting electron configurations strongly influence how
1:01:44
Speaker 2: different chemical elements interact. They help determine why some elements
1:01:48
Speaker 2: are highly reactive, why noble gases are comparatively unreactive, and
1:01:53
Speaker 2: why atoms form particular kinds of chemical bonds. The extraordinary
1:01:58
Speaker 2: variety of chemistry is therefore connected to the quantum statistics
1:02:02
Speaker 2: of identical electrons. The materials making up rocks, oceans and
1:02:07
Speaker 2: living organisms depend on this microscopic principle, and the same
1:02:11
Speaker 2: exclusion principle contributes to the resistance of ordinary matter against compression.
1:02:17
Speaker 2: Quantum field theory also reveals that the electron field describes
1:02:20
Speaker 2: more than electrons alone. It describes their antiparticles, called positrons.
1:02:26
Speaker 2: A positron possesses exactly the same rest mass and spin
1:02:30
Speaker 2: as an electron, but carries positive rather than negative electric charge.
1:02:35
Speaker 2: Electrons and positrons can be created or annihilated through suitable
1:02:39
Speaker 2: physical interactions while respecting conservation laws. They are not excitations
1:02:45
Speaker 2: of unrelated fundamental substances. Both are described within the quantum
1:02:49
Speaker 2: theory of the electron field. This is a particularly striking
1:02:53
Speaker 2: example of how one field can support different particle and
1:02:57
Speaker 2: antiparticle excitations. There is, however, an entertaining historical idea that
1:03:02
Speaker 2: deserves a little clarification. The physicists John Archibald Wheeler and
1:03:07
Speaker 2: Richard Feynman once discussed the possibility that all electrons might
1:03:11
Speaker 2: somehow represent the same individual electron travelling forwards and backwards
1:03:16
Speaker 2: through time. It is a memorable suggestion. sometimes called the
1:03:21
Speaker 2: one electron universe hypothesis. But it is not an established
1:03:25
Speaker 2: explanation of electron identity. Quantum field theory does not require
1:03:30
Speaker 2: every electron to be one particle, repeatedly travelling through time. Instead,
1:03:35
Speaker 2: the identical nature of electrons follows from their association with
1:03:39
Speaker 2: the same quantum field and the rules governing identical quantum particles.
1:03:45
Speaker 2: And this explanation has a remarkable implication. The properties of
1:03:50
Speaker 2: matter are consistent across an extraordinary range of environments. The
1:03:54
Speaker 2: electrons within the atoms of your body obey the same
1:03:57
Speaker 2: fundamental laws as electrons in distant galaxies. Astronomers can examine
1:04:03
Speaker 2: the light emitted or absorbed by remote stars and identify
1:04:06
Speaker 2: familiar atomic transitions. Because the underlying properties of electrons and
1:04:11
Speaker 2: electromagnetic interactions are remarkably consistent. This does not prove that
1:04:17
Speaker 2: every physical constant must remain unchanged under every conceivable circumstance,
1:04:22
Speaker 2: but observations strongly support the consistency of these laws across
1:04:26
Speaker 2: the regions and periods of the universe that we can investigate,
1:04:30
Speaker 2: and perhaps that is the most beautiful aspect of the
1:04:32
Speaker 2: electron field. The countless electrons scattered throughout the observable universe
1:04:39
Speaker 2: do not require countless independent explanations for their intrinsic properties.
1:04:45
Speaker 2: Their remarkable similarity reflects a shared underlying quantum description. An
1:04:50
Speaker 2: electron in your hand and an electron in a distant
1:04:53
Speaker 2: star may occupy entirely different states, participate in different interactions,
1:04:58
Speaker 2: and follow very different physical histories. Yet neither carries any
1:05:02
Speaker 2: intrinsic mark identifying it as a unique little object. They
1:05:06
Speaker 2: are excitations of the same kind of quantum field, and
1:05:10
Speaker 2: from that simple, Rather extraordinary fact emerged the rules that
1:05:14
Speaker 2: help organise atoms, determine chemical behaviour and make the familiar
1:05:18
Speaker 2: structure of ordinary matter possible. Imagine a room filled with people,
1:05:23
Speaker 2: each trying to find a comfortable place to sit. Some
1:05:26
Speaker 2: might happily gather together on the same large sofa, while
1:05:30
Speaker 2: others insist on having an entirely separate seat. It is
1:05:34
Speaker 2: a rather ordinary difference in behaviour, but something vaguely similar
1:05:38
Speaker 2: happens in the quantum world. Except that, for elementary particles,
1:05:44
Speaker 2: these different behaviours are not matters of preference. They are
1:05:47
Speaker 2: consequences of fundamental mathematical rules, and those rules divide the
1:05:52
Speaker 2: known elementary particles into two remarkable categories, fermions and bosons.
1:06:00
Speaker 2: The distinction is one of the most important features of
1:06:02
Speaker 2: quantum field theory, because it helps explain why matter forms
1:06:06
Speaker 2: stable atoms, why light behaves as it does, and even
1:06:10
Speaker 2: why certain dead stars can resist the enormous force of
1:06:14
Speaker 2: their own gravity. At the centre of the distinction is
1:06:18
Speaker 2: a property called spin. In everyday language, spinning means rotating
1:06:22
Speaker 2: around an axis. A planet spins, a bicycle wheel spins,
1:06:27
Speaker 2: a small wooden top spins across a table. But quantum
1:06:30
Speaker 2: spin is something different. It is an intrinsic property of particles.
1:06:35
Speaker 2: Related to their angular momentum and the mathematical way their
1:06:38
Speaker 2: quantum states behave under rotations, an electron has spin one half,
1:06:44
Speaker 2: a photon has spin one, and the Higgs boson has
1:06:47
Speaker 2: spin zero. These values do not mean that electrons are
1:06:52
Speaker 2: physically rotating at half the speed of photons, or that
1:06:55
Speaker 2: a Higgs boson is somehow refusing to turn. Quantum spin
1:06:59
Speaker 2: is not an ordinary mechanical motion. Instead, it determines important
1:07:04
Speaker 2: aspects of how particles behave, including the statistical rules governing
1:07:09
Speaker 2: collections of identical particles. During the 1920s, physicists discovered that
1:07:16
Speaker 2: the familiar statistical methods used to describe ordinary objects could
1:07:20
Speaker 2: not explain every microscopic system. The Indian physicist Satyendra Nath
1:07:26
Speaker 2: Bose developed a new statistical treatment of light particles in 1924,
1:07:32
Speaker 2: Albert Einstein recognised the importance of Bose's work and extended
1:07:36
Speaker 2: the underlying reasoning to particles of matter. The resulting framework
1:07:41
Speaker 2: became known as Bose-Einstein statistics. It describes the collective behaviour
1:07:46
Speaker 2: of identical bosons. One of its most remarkable properties is
1:07:51
Speaker 2: that multiple identical bosons can occupy the same quantum state.
1:07:55
Speaker 2: There is no fundamental exclusion rule preventing a large number
1:07:58
Speaker 2: of photons from sharing the same electromagnetic field mode. In fact,
1:08:04
Speaker 2: under suitable conditions, the presence of bosons in a particular
1:08:07
Speaker 2: state can enhance the probability that additional bosons enter that state.
1:08:13
Speaker 2: This behaviour plays an important role in the physics of lasers.
1:08:16
Speaker 2: A laser produces light through stimulated emission, in which an
1:08:20
Speaker 2: incoming photon can encourage an excited atom or other emitting
1:08:24
Speaker 2: system to release another photon into the same optical mode.
1:08:28
Speaker 2: With suitable amplification and feedback, large numbers of photons can
1:08:33
Speaker 2: occupy a coherent electromagnetic field state. The resulting light possesses
1:08:39
Speaker 2: a high degree of coherence, meaning that its electromagnetic oscillations
1:08:43
Speaker 2: maintain well-defined phase relationships. This helps explain why laser light
1:08:48
Speaker 2: can form such a concentrated, orderly beam. Ordinary lamplight, by comparison,
1:08:54
Speaker 2: contains radiation distributed across many modes with much less organised
1:08:58
Speaker 2: phase relationships. Fermions behave very differently. Their statistical description was
1:09:05
Speaker 2: developed independently by the Italian physicist Enrico Fermi and the
1:09:10
Speaker 2: English physicist Paul Dirac during the 1920s. It became known
1:09:14
Speaker 2: as Fermi-Dirac statistics. Unlike Bosson's, Identical fermions cannot occupy the
1:09:21
Speaker 2: same complete single-particle quantum state. This restriction is a consequence
1:09:26
Speaker 2: of the antisymmetric mathematical behaviour of their joint quantum state.
1:09:30
Speaker 2: When two identical fermions are exchanged, the sign of the
1:09:34
Speaker 2: state description changes. If both fermions attempted to occupy precisely
1:09:39
Speaker 2: the same single-particle state, the required antisymmetry would make the
1:09:43
Speaker 2: combined state vanish. This produces the Pauli exclusion principle. and
1:09:49
Speaker 2: its consequences are extraordinary. Electrons are fermions, they have spin
1:09:54
Speaker 2: one half, and the quantum states available to them within
1:09:57
Speaker 2: atoms are subject to the exclusion principle. An atom containing
1:10:01
Speaker 2: several electrons cannot place all of those electrons into precisely
1:10:05
Speaker 2: the same state. Instead, electrons occupy different allowed combinations of energy,
1:10:11
Speaker 2: orbital properties, and spin. These arrangements create the familiar electron
1:10:16
Speaker 2: shells and subshells that help organise the periodic table. They
1:10:20
Speaker 2: also help explain why chemical elements possess such different properties.
1:10:25
Speaker 2: The behaviour of sodium, oxygen, carbon and gold depends strongly
1:10:29
Speaker 2: on the quantum arrangements of their electrons. Without Fermi-Dirac statistics
1:10:35
Speaker 2: and the exclusion principle, the microscopic foundations of ordinary chemistry
1:10:39
Speaker 2: would be radically different. The relationship between spin and statistical
1:10:44
Speaker 2: behaviour is not simply an experimental coincidence. Within relativistic quantum
1:10:49
Speaker 2: field theory, it is described by the spin statistics theorem.
1:10:54
Speaker 2: This establishes that particles with integer spin behave as bosons,
1:10:58
Speaker 2: while particles with half-integer spin behave as fermions. Under the
1:11:03
Speaker 2: usual assumptions of local relativistic quantum field theory, the distinction
1:11:07
Speaker 2: is connected to the consistency of quantum mechanics with relativity,
1:11:12
Speaker 2: including the requirements governing locality and and physically acceptable quantum states.
1:11:18
Speaker 2: In the standard model of particle physics, electrons and quarks
1:11:22
Speaker 2: are fermions. Photons and gluons are bosons. The particles associated
1:11:28
Speaker 2: with the weak interaction, known as the W and Z bosons,
1:11:32
Speaker 2: are bosons as well. And the Higgs boson, despite having
1:11:36
Speaker 2: spin zero, belongs to the bosonic category. Fermions are often
1:11:41
Speaker 2: described as matter particles, while bosons are commonly associated with
1:11:45
Speaker 2: the interactions between them. This is useful as a broad introduction,
1:11:50
Speaker 2: although it is not an absolute division. Composite particles made
1:11:54
Speaker 2: from several fermions can behave as bosons, depending on their
1:11:57
Speaker 2: total spin, and that leads to another remarkable phenomenon, Bose-Einstein condensation.
1:12:05
Speaker 2: When a suitable gas of bosonic atoms is cooled to
1:12:08
Speaker 2: extraordinarily low temperatures, A large fraction of the atoms can
1:12:12
Speaker 2: accumulate in the same quantum state. Their collective behaviour then
1:12:16
Speaker 2: reveals quantum effects on a scale much larger than that
1:12:20
Speaker 2: of an individual atom. In 1995, Eric Cornell and Carl
1:12:25
Speaker 2: Wieman led a research group that produced a Bose-Einstein condensate
1:12:29
Speaker 2: using a dilute gas of rubidium atoms. Later that year,
1:12:35
Speaker 2: Wolfgang Ketteler's research group produced a condensate using sodium atoms
1:12:39
Speaker 2: These experiments demonstrated that quantum statistical behaviour could create an
1:12:44
Speaker 2: unusual state of matter in which many atoms shared a
1:12:47
Speaker 2: common quantum description. The temperatures involved were extraordinarily close to
1:12:52
Speaker 2: absolute zero, yet the significance of these statistical rules extends
1:12:57
Speaker 2: far beyond extremely cold laboratories. Consider a white dwarf star.
1:13:03
Speaker 2: A white dwarf is the dense stellar remnant left behind
1:13:06
Speaker 2: when a star of sufficiently modest mass has exhausted its
1:13:10
Speaker 2: nuclear fuel and shed its outer layers. Such an object
1:13:14
Speaker 2: can contain a mass comparable to that of the Sun
1:13:17
Speaker 2: compressed into a volume roughly the size of Earth. Its
1:13:21
Speaker 2: gravity is immense, but the star does not immediately collapse
1:13:25
Speaker 2: under its own weight. One important reason is electron degeneracy pressure.
1:13:31
Speaker 2: As matter becomes increasingly compressed, electrons are forced into a
1:13:35
Speaker 2: smaller volume. Because electrons are fermions, the Pauli exclusion principle
1:13:40
Speaker 2: prevents them from all occupying the same available quantum state.
1:13:44
Speaker 2: The electrons must populate a range of momentum states, creating
1:13:49
Speaker 2: pressure that can oppose gravitational compression, even when the star
1:13:53
Speaker 2: is no longer supported by ordinary nuclear fusions. This remarkable
1:13:58
Speaker 2: form of support arises from quantum statistics rather than from
1:14:02
Speaker 2: the familiar thermal pressure of a hot gas. For white
1:14:06
Speaker 2: dwarfs below the appropriate stability limit, electron degeneracy pressure plays
1:14:11
Speaker 2: a central role in resisting gravitational collapse. The behaviour of
1:14:16
Speaker 2: an entire star therefore depends upon the same quantum principles
1:14:20
Speaker 2: that help determine the electronic structure of an ordinary atom.
1:14:24
Speaker 2: And this is perhaps the most extraordinary consequence of the
1:14:27
Speaker 2: distinction between fermions and bosons. A mathematical rule governing the
1:14:32
Speaker 2: exchange of identical particles can influence structures ranging from individual
1:14:37
Speaker 2: atoms to enormous stellar remnants. Bosons can gather into shared
1:14:41
Speaker 2: quantum states, producing phenomena such as coherent laser light and
1:14:46
Speaker 2: Bose-Einstein condensation. Fermions obey an exclusion principle that helps give
1:14:52
Speaker 2: matter its familiar structure, and contributes to the survival of
1:14:55
Speaker 2: some of the densest stars in the universe. Both categories
1:14:59
Speaker 2: arise naturally within quantum field theory. Their differences reflect the
1:15:04
Speaker 2: intrinsic properties of the fields and the mathematical rules governing
1:15:08
Speaker 2: their excitations, and from these seemingly abstract distinctions emerge some
1:15:13
Speaker 2: of the most familiar and remarkable features of the physical world.
1:15:17
Speaker 2: The organisation of the periodic table, the light from a
1:15:20
Speaker 2: laboratory laser, and the stability of a white dwarf all
1:15:24
Speaker 2: reveal the consequences of quantum statistics. Deep beneath the appearance
1:15:29
Speaker 2: of ordinary matter, nature follows rules that are rather unlike
1:15:32
Speaker 2: anything we encounter in everyday life. Yet those rules help
1:15:35
Speaker 2: explain why the everyday world behaves as reliably as it does.
1:15:39
Speaker 2: Imagine two electrons separated by a very small distance. Neither
1:15:44
Speaker 2: electron is touching the other. There is no visible connection
1:15:47
Speaker 2: between them. and nothing resembling a tiny rope or mechanical
1:15:50
Speaker 2: device linking their movements. Nevertheless, the electrons repel one another.
1:15:56
Speaker 2: If they are free to move, each responds to the
1:15:59
Speaker 2: electromagnetic influence of the other, tending to accelerate away. This
1:16:04
Speaker 2: is a familiar consequence of electric charge. Two negatively charged
1:16:08
Speaker 2: objects repel, while objects carrying opposite charges attract. But at
1:16:13
Speaker 2: the deepest level, what actually produces this interaction? According to
1:16:18
Speaker 2: quantum field theory, the answer involves more than the electrons themselves.
1:16:23
Speaker 2: It involves the relationship between the electron field and the
1:16:26
Speaker 2: electromagnetic field. And this introduces one of the most important
1:16:30
Speaker 2: features of the quantum universe. Fields do not simply exist independently,
1:16:35
Speaker 2: each carrying its own collection of particles. They can interact
1:16:39
Speaker 2: with one another. Those interactions help determine how matter behaves,
1:16:44
Speaker 2: how energy moves between systems, and why the familiar forces
1:16:48
Speaker 2: of nature exist. Consider an electron. It possesses negative electric charge,
1:16:55
Speaker 2: a fundamental property that determines how it participates in electromagnetic interactions.
1:17:01
Speaker 2: The electron field is coupled to the electromagnetic field. This
1:17:06
Speaker 2: means that the mathematical laws describing these fields contain terms
1:17:09
Speaker 2: connecting their behaviour. An electron can emit or absorb a
1:17:14
Speaker 2: photon under suitable conditions. and electrically charged particles can influence
1:17:19
Speaker 2: one another through their shared electromagnetic interaction. The theory describing
1:17:25
Speaker 2: these processes is called quantum electrodynamics. Quantum electrodynamics combines quantum mechanics,
1:17:33
Speaker 2: special relativity, and the electromagnetic interaction within a remarkably successful
1:17:38
Speaker 2: mathematical framework. It describes how electrically charged particles interact with
1:17:43
Speaker 2: the electromagnetic field, including processes involving electrons, positrons and photons.
1:17:51
Speaker 2: The modern theory emerged through decades of research, with particularly
1:17:55
Speaker 2: important advances made during the 1940s. Three physicists played leading
1:18:01
Speaker 2: roles in its development. Richard Feynman, Julian Schwinger and Shinichiro
1:18:06
Speaker 2: Tomonaga developed powerful methods for calculating the effects of quantum
1:18:11
Speaker 2: electromagnetic interactions. Their approaches were mathematically different, but they contributed
1:18:17
Speaker 2: to a consistent theory capable of making extraordinarily precise predictions.
1:18:23
Speaker 2: And one of the most interesting features of quantum electrodynamics
1:18:26
Speaker 2: is the way it describes interactions between particles. We often
1:18:30
Speaker 2: hear that two electrons repel each other by exchanging photons.
1:18:35
Speaker 2: This description can be useful, but it requires an important qualification.
1:18:40
Speaker 2: The photons involved in calculations of an electromagnetic force are
1:18:45
Speaker 2: not necessarily ordinary detectable photons travelling between the electrons. Instead,
1:18:51
Speaker 2: physicists frequently describe the interaction using mathematical contributions associated with
1:18:56
Speaker 2: virtual photons. A virtual photon is an internal element of
1:19:01
Speaker 2: a particular calculation. It is not a freely propagating particle
1:19:05
Speaker 2: that could simply be intercepted by a detector. This distinction
1:19:09
Speaker 2: matters because the popular picture of particles continuously throwing invisible
1:19:14
Speaker 2: photons back and forth can create a misleading impression. Quantum
1:19:19
Speaker 2: electrodynamics does not require electrons to behave like tiny people
1:19:23
Speaker 2: passing objects between them. Rather, the theory calculates the probabilities
1:19:28
Speaker 2: and effects of interactions between quantum fields. Virtual photons arise
1:19:33
Speaker 2: within mathematical methods used to organise those calculations. And one
1:19:38
Speaker 2: of the most recognisable of these methods involves Feynman diagrams.
1:19:42
Speaker 2: A Feynman diagram uses lines and connecting points to represent
1:19:45
Speaker 2: particular mathematical contributions to a particle interaction. For example, a
1:19:51
Speaker 2: diagram describing electron scattering may contain two electron lines connected
1:19:56
Speaker 2: through an internal photon line. The arrangement helps physicists calculate
1:20:01
Speaker 2: how likely different scattering outcomes are. but the diagram should
1:20:05
Speaker 2: not generally be interpreted as a literal photograph of what happened.
1:20:09
Speaker 2: Its lines are not necessarily the actual paths travelled by
1:20:13
Speaker 2: tiny particles, and its internal connections are not direct observations
1:20:17
Speaker 2: of invisible objects moving through space. Instead, the diagram represents
1:20:23
Speaker 2: terms in a mathematical expansion of the interaction. This is
1:20:27
Speaker 2: an important distinction between a useful physical picture and the
1:20:30
Speaker 2: full quantum description. The mathematics can be extraordinarily sophisticated, but
1:20:36
Speaker 2: its predictions can be tested through real experiments. There is
1:20:40
Speaker 2: another fascinating principle underlying electromagnetic interactions. It is called gauge symmetry.
1:20:47
Speaker 2: In physics, a symmetry describes a transformation that leaves particular
1:20:51
Speaker 2: physical relationships unchanged. Gauge symmetry concerns changes in the mathematical
1:20:57
Speaker 2: description of fields that do not alter observable physical results.
1:21:03
Speaker 2: For electromagnetism, certain changes to the electromagnetic potentials can leave
1:21:08
Speaker 2: the electric and magnetic fields unchanged. In quantum electrodynamics, the
1:21:13
Speaker 2: relevant local gauge symmetry also involves transformations of the phase
1:21:18
Speaker 2: used to describe electrically charged quantum fields. Maintaining this symmetry
1:21:23
Speaker 2: requires a particular relationship between the charged fields and the
1:21:27
Speaker 2: electromagnetic field. This is one reason gauge symmetry plays such
1:21:32
Speaker 2: a central role in the organisation of modern particle physics.
1:21:35
Speaker 2: It is not merely a decorative mathematical feature. It strongly
1:21:39
Speaker 2: constrains the interactions that can appear within the theory. And
1:21:43
Speaker 2: those interactions connect an astonishing variety of familiar phenomena. The
1:21:48
Speaker 2: electromagnetic field is involved when an atom emits light. It
1:21:52
Speaker 2: governs the behaviour of electrons moving through electrical conductors. It
1:21:57
Speaker 2: contributes to the forces responsible for chemical bonding and the
1:22:01
Speaker 2: structure of ordinary matter. The magnetic field surrounding a household
1:22:05
Speaker 2: magnet and the radiation emitted by a distant star are
1:22:09
Speaker 2: both connected to electromagnetic physics. Quantum electrodynamics provides the microscopic
1:22:16
Speaker 2: quantum description of this interaction. Its accuracy is particularly impressive
1:22:21
Speaker 2: when scientists examine the magnetic properties of electrons. An electron
1:22:26
Speaker 2: possesses an intrinsic magnetic moment associated with its spin. The
1:22:31
Speaker 2: simplest relativistic description predicts a particular relationship between the electron's
1:22:36
Speaker 2: spin and magnetic moment. Quantum electrodynamics predicts small corrections to
1:22:41
Speaker 2: that relationship. These corrections arise from the quantum effects of
1:22:46
Speaker 2: electromagnetic interactions. The resulting quantity is known as the electron's
1:22:51
Speaker 2: anomalous magnetic moment. Physicists have measured this property with extraordinary precision,
1:22:58
Speaker 2: and its comparison with theoretical predictions provides one of the
1:23:02
Speaker 2: most demanding tests of quantum electrodynamics. The agreement is remarkable,
1:23:07
Speaker 2: but obtaining such accurate predictions has not always been straightforward.
1:23:12
Speaker 2: Early calculations sometimes produced expressions containing infinities. Certain mathematical contributions
1:23:19
Speaker 2: appeared to grow without limit. creating difficulties when scientists attempted
1:23:23
Speaker 2: to calculate measurable quantities. The solution involved a framework known
1:23:28
Speaker 2: as renormalisation. Renormalisation provides a systematic way of relating the
1:23:33
Speaker 2: parameters appearing in theoretical calculations to physically measurable quantities. It
1:23:39
Speaker 2: also explains how effective interaction strengths can depend on the
1:23:43
Speaker 2: energy scale at which they are investigated. The process is
1:23:47
Speaker 2: considerably more sophisticated than simply removing inconvenient infinities and hoping
1:23:52
Speaker 2: nobody notices. It allows quantum field theories to produce finite,
1:23:58
Speaker 2: consistent predictions for observable phenomena when the relevant theory is
1:24:02
Speaker 2: renormalisable and the calculation is properly organised. This mathematical achievement
1:24:09
Speaker 2: helped transform quantum electrodynamics into one of the most accurately
1:24:13
Speaker 2: tested theories in science. and it reveals something important about
1:24:18
Speaker 2: the fundamental fields of nature. Their interactions are not arbitrary.
1:24:23
Speaker 2: They follow definite mathematical rules, including conservation of energy, momentum,
1:24:30
Speaker 2: electric charge, and other relevant quantities. These rules determine which
1:24:35
Speaker 2: processes are possible and how likely different outcomes may be.
1:24:40
Speaker 2: The result is a remarkably unified description of electromagnetic phenomena.
1:24:45
Speaker 2: Two electrons repelling one another, an excited atom emitting light,
1:24:50
Speaker 2: and an electrical current passing through a wire may seem
1:24:53
Speaker 2: like very different events, yet all can be understood through
1:24:56
Speaker 2: the electromagnetic interaction. And beneath that interaction lies a relationship
1:25:02
Speaker 2: between quantum fields. The familiar world, therefore, depends not only
1:25:06
Speaker 2: on which fields exist, but on the ways those fields
1:25:09
Speaker 2: are allowed to influence one another. Without their interactions, the
1:25:13
Speaker 2: extraordinary variety of matter would be impossible to explain. Atoms
1:25:17
Speaker 2: would not possess their familiar electromagnetic structure. Light would not
1:25:22
Speaker 2: be emitted and absorbed in the same way, and the
1:25:25
Speaker 2: chemical processes supporting ordinary life would be profoundly different. What
1:25:30
Speaker 2: we experience as a collection of separate objects and forces is,
1:25:34
Speaker 2: at a deeper level, a world of interacting quantum fields.
1:25:39
Speaker 2: And from those interactions emerge many of the physical properties
1:25:42
Speaker 2: that make the universe recognisable. There is something rather comforting
1:25:46
Speaker 2: about the idea that physical objects cannot simply disappear. A
1:25:51
Speaker 2: stone may break into smaller pieces. A piece of wood
1:25:54
Speaker 2: may burn, releasing gases and leaving ashes behind. A drop
1:25:58
Speaker 2: of water may evaporate, becoming invisible to the eye. But
1:26:02
Speaker 2: in each case the underlying matter has not simply vanished.
1:26:06
Speaker 2: Its constituent atoms have been rearranged, separated or combined with
1:26:10
Speaker 2: other substances. For a long time, scientists expected the smallest
1:26:15
Speaker 2: particles of matter to possess a similar kind of permanence.
1:26:19
Speaker 2: An electron might travel from one atom to another, but
1:26:23
Speaker 2: surely the electron itself would continue to exist. Quantum field
1:26:27
Speaker 2: theory reveals a considerably stranger possibility. Under suitable conditions, particles
1:26:33
Speaker 2: can be created, destroyed and transformed into other kinds of particles.
1:26:37
Speaker 2: And remarkably, none of this requires the laws of physics
1:26:40
Speaker 2: to be broken. The explanation begins with Albert Einstein's special
1:26:45
Speaker 2: theory of relativity. One of its most famous consequences is
1:26:49
Speaker 2: the relationship between mass and energy. Ng equals mass multiplied
1:26:55
Speaker 2: by the speed of light-squared. This equation describes the rest
1:26:59
Speaker 2: energy associated with an object's mass. It means that mass
1:27:03
Speaker 2: represents a particular form of energy, and that physical processes
1:27:07
Speaker 2: can convert energy between different forms while respecting the relevant
1:27:11
Speaker 2: conservation laws. For elementary particles, this relationship becomes especially important.
1:27:18
Speaker 2: If sufficient energy is available during an interaction, it may
1:27:21
Speaker 2: be possible to produce particles that were not present beforehand.
1:27:25
Speaker 2: Consider a photon travelling through space. A photon has no
1:27:29
Speaker 2: rest mass, but it carries energy and momentum. The energy
1:27:33
Speaker 2: depends on its frequency, with higher frequency photons carrying more
1:27:37
Speaker 2: energy than lower frequency photons. Under suitable circumstances, a sufficiently
1:27:43
Speaker 2: energetic photon can participate in a process that produces two
1:27:47
Speaker 2: massive particles, an electron and a positron. The positron is
1:27:52
Speaker 2: the electron's antiparticle. It possesses the same rest mass as
1:27:57
Speaker 2: an electron, but carries positive rather than negative electric charge.
1:28:02
Speaker 2: The process is called electron-positron pair production. At first this
1:28:06
Speaker 2: sounds almost impossible. How can something with no rest mass
1:28:10
Speaker 2: contribute to the creation of two particles that possess mass?
1:28:14
Speaker 2: The answer is that the incoming photon carries energy. That
1:28:18
Speaker 2: energy can become part of the rest energy and motion
1:28:21
Speaker 2: of the newly produced electron and positron. Each has a
1:28:24
Speaker 2: rest energy of approximately 511,000 electron volts. Together, their combined
1:28:30
Speaker 2: rest energy is just over one billion electron volts. The
1:28:34
Speaker 2: incoming photon must therefore provide at least this amount of energy,
1:28:38
Speaker 2: with additional energy required in practical circumstances to satisfy the
1:28:43
Speaker 2: full conservation conditions. There is, however, a subtle complication. An
1:28:48
Speaker 2: isolated photon cannot simply transform into an electron and a
1:28:52
Speaker 2: positron in otherwise empty space. Although the energy might appear sufficient,
1:28:57
Speaker 2: the process cannot simultaneously conserve both energy and momentum. For
1:29:03
Speaker 2: ordinary single photon pair production, another participant is needed. This
1:29:08
Speaker 2: is often a nearby atomic nucleus. The nucleus can recoil slightly,
1:29:13
Speaker 2: carrying away some momentum and allowing the overall interaction to
1:29:16
Speaker 2: satisfy the conservation laws. Two sufficiently energetic photons can also
1:29:21
Speaker 2: produce an electron-positron pair when their combined energy and momentum
1:29:26
Speaker 2: permit it. The important point is that particle creation is
1:29:30
Speaker 2: not unrestricted. The physical conditions must be suitable and the
1:29:35
Speaker 2: conservation laws remain firmly in place. Now consider the reverse process.
1:29:41
Speaker 2: Suppose an electron encounters a positron. Because they are particles
1:29:45
Speaker 2: and antiparticles, they can annihilate one another through an electromagnetic interaction.
1:29:51
Speaker 2: The electron and positron disappear from the outgoing particle state,
1:29:55
Speaker 2: while their energy and momentum are transferred into newly produced photons.
1:29:59
Speaker 2: This is called electron-positron annihilation. When an electron and positron
1:30:04
Speaker 2: annihilate after coming approximately to rest, the usual outcome is
1:30:08
Speaker 2: two gamma-ray photons travelling in nearly opposite directions. Each photon
1:30:14
Speaker 2: carries approximately 511,000 electron volts of energy. The combined energy
1:30:20
Speaker 2: of the photons corresponds to the rest energy of the
1:30:22
Speaker 2: original particles together with any additional energy that may have
1:30:26
Speaker 2: been present. Electric charge is conserved as well. Before annihilation,
1:30:32
Speaker 2: the negative charge of the electron and the positive charge
1:30:34
Speaker 2: of the positron add to zero. Afterwards, the photons carry
1:30:39
Speaker 2: no electric charge. Nothing has escaped the accounting required by physics.
1:30:44
Speaker 2: The identities and numbers of particles have changed. but the
1:30:48
Speaker 2: conserved quantities remain consistent. Quantum field theory provides a natural
1:30:53
Speaker 2: description of these events. An electron is an excitation associated
1:30:58
Speaker 2: with the electron field. A positron is the corresponding antiparticle
1:31:02
Speaker 2: excitation described by that same field. Photons are excitations of
1:31:08
Speaker 2: the electromagnetic field. When these fields interact, energy and momentum
1:31:13
Speaker 2: can be transferred between them. changing the excitations present in
1:31:17
Speaker 2: the initial and final states. Mathematically physicists describe such changes
1:31:23
Speaker 2: using operators. A creation operator adds a quantum excitation to
1:31:27
Speaker 2: an appropriate field state. An annihilation operator removes one. These
1:31:32
Speaker 2: operators are not physical machines manufacturing or destroying tiny objects.
1:31:37
Speaker 2: They are mathematical elements of the quantum field description. allowing
1:31:41
Speaker 2: calculations involving states with different particle numbers, and they reveal
1:31:46
Speaker 2: why quantum field theory is particularly well suited to processes
1:31:50
Speaker 2: that ordinary, fixed particle quantum mechanics cannot describe conveniently. There
1:31:56
Speaker 2: is also an important misconception worth avoiding. Quantum particles are
1:32:01
Speaker 2: sometimes described as appearing from nothing by borrowing energy for
1:32:05
Speaker 2: a very short time. The explanation usually appeals to the
1:32:08
Speaker 2: uncertainty principle. suggesting that energy conservation can temporarily be violated,
1:32:14
Speaker 2: but this is not an accurate description of genuine particle production.
1:32:19
Speaker 2: The energy-time uncertainty relationship does not provide permission to create
1:32:24
Speaker 2: detectable particles without the energy required by the process. Real
1:32:29
Speaker 2: particle creation must satisfy the relevant conservation laws. The distinction
1:32:34
Speaker 2: matters because these processes are not merely theoretical curiosities. Electron-positron
1:32:41
Speaker 2: annihilation has an important practical application in medical imaging. It
1:32:45
Speaker 2: is used in positron emission tomography. During this procedure, a
1:32:50
Speaker 2: patient receives a small quantity of a radioactive tracer containing
1:32:54
Speaker 2: a positron-emitting isotope, such as fluorine-18. When the isotope decays,
1:33:00
Speaker 2: it can emit a positron. The positron travels a short
1:33:04
Speaker 2: distance through nearby tissue gradually losing kinetic energy before eventually
1:33:08
Speaker 2: annihilating with an electron. The resulting gamma-ray photons usually travel
1:33:14
Speaker 2: in approximately opposite directions. Detectors arranged around the patient can
1:33:19
Speaker 2: identify pairs of photons arriving within a very short time interval.
1:33:24
Speaker 2: By analysing many such detection events, a computer reconstructs information
1:33:29
Speaker 2: about the distribution of the radioactive tracer, This allows physicians
1:33:33
Speaker 2: to investigate processes such as tissue metabolism and assess certain diseases.
1:33:39
Speaker 2: An interaction between elementary quantum fields has become a powerful
1:33:42
Speaker 2: medical tool. Particle creation also occurs on a much grander scale.
1:33:48
Speaker 2: Inside high-energy particle accelerators, collisions can concentrate enormous amounts of
1:33:53
Speaker 2: energy within extremely small regions. The outgoing particles may include
1:33:58
Speaker 2: species that were absent from the incoming beams. at the
1:34:01
Speaker 2: Large Hadron Collider, operated by the European Organisation for Nuclear
1:34:06
Speaker 2: Research near Geneva, energetic proton collisions can produce a wide
1:34:10
Speaker 2: variety of particles, including heavy and unstable ones. These particles
1:34:16
Speaker 2: subsequently interact or decay, leaving measurable signals inside enormous detectors.
1:34:22
Speaker 2: Similar principles were important in the early universe, when temperatures
1:34:27
Speaker 2: and particle energies were sufficiently high for many kinds of
1:34:30
Speaker 2: particle-producing interactions to occur frequently. Particle creation and annihilation also
1:34:37
Speaker 2: influence processes involving cosmic rays and energetic astrophysical environments. The
1:34:43
Speaker 2: microscopic universe is therefore not a permanent collection of tiny
1:34:47
Speaker 2: objects whose identities can never change. It is a physical
1:34:51
Speaker 2: system in which quantum fields interact, exchange energy, and occupy
1:34:56
Speaker 2: different excitation states. Particles can emerge when the necessary conditions
1:35:01
Speaker 2: are satisfied. They can disappear through annihilation or transformation, and
1:35:07
Speaker 2: the resulting processes remain governed by remarkably precise physical laws.
1:35:12
Speaker 2: What appears at first to be the creation or destruction
1:35:15
Speaker 2: of matter is, at a deeper level, a change in
1:35:19
Speaker 2: the quantum state of interacting fields. The individual particles need
1:35:23
Speaker 2: not endure forever. but the laws describing their appearance, disappearance
1:35:28
Speaker 2: and interactions remain wonderfully consistent. Imagine travelling far beyond Earth,
1:35:33
Speaker 2: beyond the Moon and beyond the outermost planets of the
1:35:36
Speaker 2: solar system. Eventually you reach a region of space where
1:35:40
Speaker 2: there are no nearby stars, no drifting clouds of gas
1:35:44
Speaker 2: and almost no detectable matter. Suppose you could somehow remove
1:35:48
Speaker 2: the remaining atoms, stray photons and every other identifiable particle
1:35:52
Speaker 2: from a small region. What would be left? The obvious
1:35:56
Speaker 2: answer is nothing. And according to our everyday understanding of emptiness,
1:36:01
Speaker 2: that seems perfectly reasonable. But quantum field theory gives us
1:36:04
Speaker 2: a rather different answer. Even when every detectable particle has
1:36:08
Speaker 2: been removed, the fundamental quantum fields remain part of the
1:36:12
Speaker 2: physical description. The region may be empty of particles, but
1:36:16
Speaker 2: it is not necessarily empty of physical properties. This distinction
1:36:20
Speaker 2: leads to one of the strangest ideas in modern physics.
1:36:24
Speaker 2: The quantum vacuum. In ordinary language, a vacuum is simply
1:36:29
Speaker 2: a region containing very little matter. A laboratory vacuum chamber,
1:36:33
Speaker 2: for example, uses pumps to remove most of the air
1:36:36
Speaker 2: from an enclosed space. Even the most sophisticated chambers cannot
1:36:40
Speaker 2: eliminate every remaining molecule, and radiation may still pass through
1:36:45
Speaker 2: the evacuated region. The vacuum of quantum field theory is
1:36:49
Speaker 2: something more precise. It refers to a lowest energy state
1:36:53
Speaker 2: of the relevant quantum fields under particular physical conditions. In
1:36:58
Speaker 2: the simplest description, this is a state containing no ordinary
1:37:02
Speaker 2: particle excitations. But the absence of particles does not mean
1:37:06
Speaker 2: that the underlying fields have disappeared. The electron field remains
1:37:10
Speaker 2: part of the description, even when there are no electrons present.
1:37:14
Speaker 2: The electromagnetic field remains, even when there are no photons.
1:37:19
Speaker 2: and the other fundamental fields continue to possess quantum properties.
1:37:24
Speaker 2: The distinction becomes especially important when we consider how quantum
1:37:28
Speaker 2: mechanics describes the lowest possible energy of a system. Imagine
1:37:33
Speaker 2: a small mass attached to a spring. In classical physics,
1:37:37
Speaker 2: the mass can eventually come completely to rest at its
1:37:40
Speaker 2: equilibrium position. Its motion stops, and the energy associated with
1:37:45
Speaker 2: its vibration can become zero. But a quantum harmonic oscillator
1:37:50
Speaker 2: behaves differently. Its allowed energy levels are quantized, and its
1:37:54
Speaker 2: lowest possible state generally possesses non-zero energy. This is called
1:37:59
Speaker 2: zero-point energy. The uncertainty principle prevents the oscillator from simultaneously
1:38:05
Speaker 2: having an exactly defined equilibrium position and exactly zero momentum.
1:38:12
Speaker 2: In quantum field theory the independent modes of a simple
1:38:15
Speaker 2: free field can be described mathematically using quantum harmonic oscillators.
1:38:20
Speaker 2: Even when none of those modes contains ordinary particle excitations
1:38:25
Speaker 2: their lowest energy states retain quantum properties. This is part
1:38:29
Speaker 2: of the foundation of what physicists call vacuum fluctuations. The
1:38:34
Speaker 2: phrase can be misleading. It might suggest that empty space
1:38:37
Speaker 2: is filled with tiny objects continuously trembling, appearing and disappearing.
1:38:44
Speaker 2: That is not an accurate picture. Vacuum fluctuations refer to
1:38:48
Speaker 2: the non-zero quantum uncertainties and correlations associated with field observables
1:38:54
Speaker 2: in the vacuum state. They do not necessarily describe a
1:38:58
Speaker 2: field physically shaking back and forth in time, nor do
1:39:02
Speaker 2: they mean that particles are continually breaking the law of
1:39:05
Speaker 2: energy conservation. A common popular explanation claims that particles can
1:39:10
Speaker 2: borrow energy from the vacuum, exist briefly, and then return
1:39:14
Speaker 2: the borrowed energy before anyone notices. It is an entertaining image,
1:39:19
Speaker 2: but quantum mechanics does not grant temporary permission to violate
1:39:22
Speaker 2: energy conservation. The uncertainty principle describes limitations and relationships involving
1:39:30
Speaker 2: quantum observables. It is not a license for energy to
1:39:34
Speaker 2: appear from nowhere. The genuine physics is subtler and considerably
1:39:39
Speaker 2: more interesting. One of the most famous examples involves a
1:39:43
Speaker 2: phenomenon called the Casimir effect. In 1948 the Dutch physicist
1:39:48
Speaker 2: Hendrik Casimir predicted that two closely spaced conducting plates could
1:39:53
Speaker 2: experience a force arising from quantum electromagnetic effects. Imagine two
1:39:59
Speaker 2: extremely flat metal plates positioned parallel to one another, separated
1:40:05
Speaker 2: by a tiny distance. When the separation becomes sufficiently small,
1:40:09
Speaker 2: electromagnetic interactions can produce a measurable attraction. For ideal conducting
1:40:16
Speaker 2: plates at zero temperature, a standard calculation describes the force
1:40:20
Speaker 2: through changes in the allowed electromagnetic field modes associated with
1:40:25
Speaker 2: the boundaries. Real experiments are more complicated because actual materials
1:40:30
Speaker 2: possess finite conductivity, surface imperfections and temperature-dependent properties. Nevertheless, Casimir
1:40:39
Speaker 2: forces have been measured and studied extensively. They provide an
1:40:43
Speaker 2: important demonstration of quantum electromagnetic behaviour at very small separations,
1:40:48
Speaker 2: but the Casimir effect should not be interpreted as evidence
1:40:52
Speaker 2: that empty space contains an unlimited supply of energy waiting
1:40:56
Speaker 2: to be collected. Its theoretical description can involve vacuum energy,
1:41:00
Speaker 2: material interactions, and different but equivalent ways of organising the calculation,
1:41:06
Speaker 2: and any attempt to extract useful work must still respect
1:41:09
Speaker 2: the conservation of energy. The quantum vacuum is not a
1:41:13
Speaker 2: particularly generous invisible battery. Another important example comes from the
1:41:19
Speaker 2: hydrogen atom. During the 1940s, physicists Willis Lamb and Robert
1:41:25
Speaker 2: Rutherford investigated tiny differences between atomic energy levels. In 1947,
1:41:32
Speaker 2: they measured a small separation between levels that had the
1:41:35
Speaker 2: same energy in the simplest treatment based on Paul Dirac's
1:41:39
Speaker 2: relativistic electron equation. This difference became known as the Lamb shift.
1:41:44
Speaker 2: Quantum electrodynamics explains the effect through corrections associated with interactions
1:41:50
Speaker 2: between the electron and electromagnetic field, including contributions often described
1:41:55
Speaker 2: through vacuum fluctuations and electron self-energy. The effect is extremely small,
1:42:02
Speaker 2: yet measurable. It demonstrates that quantum field interactions influence even
1:42:07
Speaker 2: the delicate energy structure of an ordinary hydrogen atom. The
1:42:11
Speaker 2: vacuum also possesses correlations. In quantum mechanics, correlations describe relationships
1:42:17
Speaker 2: between measurement outcomes or observables. The vacuum state of a
1:42:22
Speaker 2: field can contain correlations between measurements associated with different regions,
1:42:27
Speaker 2: even when no ordinary particles are present. These properties form
1:42:31
Speaker 2: part of the mathematical structure of the vacuum and can
1:42:34
Speaker 2: influence how quantum systems interact with their surroundings. And there
1:42:38
Speaker 2: is a further complication. The meaning of a particle is
1:42:42
Speaker 2: not always completely independent of the observer. In flat spacetime,
1:42:47
Speaker 2: inertial observers can agree on a natural vacuum state, but
1:42:51
Speaker 2: accelerating observers may describe the same quantum field state differently.
1:42:56
Speaker 2: The Unruh effect, predicted by William Unruh, suggests that a
1:43:00
Speaker 2: uniformly accelerating detector can respond as though it is immersed
1:43:04
Speaker 2: in thermal radiation. even when an inertial observer describes the
1:43:08
Speaker 2: field as being in its vacuum state. This is a
1:43:12
Speaker 2: theoretical consequence of quantum field theory, whose direct experimental observation
1:43:17
Speaker 2: remains challenging. In curved spacetime, particularly around black holes, identifying
1:43:23
Speaker 2: a preferred vacuum state can become even more subtle. The
1:43:27
Speaker 2: definitions of particles and vacuum depend upon the spacetime geometry
1:43:30
Speaker 2: and the physical circumstances. All of this changes our understanding
1:43:35
Speaker 2: of emptiness. A region containing no ordinary particles can still
1:43:39
Speaker 2: possess quantum fields, fluctuations and correlations. It can participate in
1:43:45
Speaker 2: measurable physical phenomena without being filled with an invisible collection
1:43:50
Speaker 2: of ordinary objects. Yet none of this proves that absolute
1:43:54
Speaker 2: nothingness is impossible. The quantum vacuum is still a physical
1:43:58
Speaker 2: state described by a theory containing fields, spacetime, and mathematical laws.
1:44:05
Speaker 2: It is not the complete absence of everything, and perhaps
1:44:09
Speaker 2: that is the most remarkable distinction. What we casually call
1:44:13
Speaker 2: empty space is not simply a blank region where physics
1:44:16
Speaker 2: has nothing left to describe. Even when matter and radiation
1:44:20
Speaker 2: have been removed, quantum field theory still has something meaningful
1:44:24
Speaker 2: to say. The universe, it seems, can be extraordinarily interesting,
1:44:29
Speaker 2: even in places where there appears to be absolutely nothing.
1:44:32
Speaker 2: Almost everything around us possesses mass. A stone has mass.
1:44:37
Speaker 2: The water in a glass has mass. Even the air
1:44:41
Speaker 2: filling a room contains an enormous number of particles, whose
1:44:44
Speaker 2: combined mass can be measured. Mass helps determine how an
1:44:48
Speaker 2: object responds when a force acts upon it, and how
1:44:51
Speaker 2: much gravitational influence it produces. But at the microscopic level,
1:44:57
Speaker 2: this familiar property raises a surprisingly difficult question. Why do
1:45:01
Speaker 2: elementary particles have mass at all? An electron possesses a
1:45:06
Speaker 2: definite rest mass. Quarks have their own masses, while photons,
1:45:10
Speaker 2: the particles associated with light, have no rest mass. These
1:45:15
Speaker 2: differences are essential to the structure of the physical world.
1:45:18
Speaker 2: Without the properties of electrons and quarks, atoms would not
1:45:22
Speaker 2: behave as they do, and ordinary matter would be profoundly different.
1:45:27
Speaker 2: Quantum field theory offers a remarkable explanation for an important
1:45:31
Speaker 2: part of this mystery. It involves a field that exists
1:45:35
Speaker 2: throughout space, even in its lowest energy state, the Higgs field.
1:45:40
Speaker 2: Unlike many familiar fields, whose simplest vacuum states have a
1:45:44
Speaker 2: zero average field value, the Higgs field possesses a non-zero
1:45:49
Speaker 2: vacuum expectation value. This means that, in the usual mathematical description,
1:45:54
Speaker 2: The field has a non-zero background value even when no
1:45:58
Speaker 2: Higgs particles are present. The idea is subtle, but its
1:46:02
Speaker 2: consequences are extraordinary. During the 1960s, physicists were trying to
1:46:08
Speaker 2: understand how the electromagnetic and weak nuclear interactions could fit
1:46:12
Speaker 2: within a unified mathematical description. One difficulty involved the particles
1:46:18
Speaker 2: responsible for weak interactions. These particles needed to possess substantial mass.
1:46:23
Speaker 2: but introducing their masses directly into the theory appeared to
1:46:27
Speaker 2: create problems with its underlying gauge symmetry. A new mechanism
1:46:32
Speaker 2: was required. In 1964, Robert Brut and François Englert published
1:46:38
Speaker 2: important work demonstrating how a field could generate masses for
1:46:42
Speaker 2: certain interaction carriers through spontaneous symmetry breaking. Peter Higgs independently
1:46:48
Speaker 2: developed related ideas. identifying an additional consequence involving a massive
1:46:53
Speaker 2: particle excitation. Gerald Guralnik, Carl Hagen and Tom Kibble also
1:47:00
Speaker 2: made essential contributions to the theoretical development. The resulting explanation
1:47:05
Speaker 2: became known as the Brutenglert-Higgs mechanism. Its central idea is
1:47:10
Speaker 2: that a field can possess a non-zero vacuum value, changing
1:47:14
Speaker 2: the way other fields behave. Within electroweak theory, the Higgs
1:47:18
Speaker 2: field plays precisely this role. At sufficiently high energies, the
1:47:22
Speaker 2: electromagnetic and weak interactions are described through a unified mathematical framework.
1:47:28
Speaker 2: But the non-zero vacuum value of the Higgs field produces
1:47:32
Speaker 2: a phenomenon known as electroweak symmetry breaking. This gives mass
1:47:36
Speaker 2: to the charged weak interaction carriers, known as W bosons,
1:47:41
Speaker 2: and the neutral Z boson. The photon, however, remains massless.
1:47:46
Speaker 2: This distinction is not accidental. It follows from the structure
1:47:50
Speaker 2: of the electroweak theory and the particular symmetry that remains unbroken.
1:47:55
Speaker 2: The result helps explain why electromagnetic radiation can propagate across
1:48:00
Speaker 2: enormous distances while the weak interaction has an extremely short
1:48:05
Speaker 2: effective range. The Higgs mechanism also provides masses for elementary
1:48:09
Speaker 2: fermions including electrons and quarks. These particles interact with the
1:48:14
Speaker 2: Higgs field through mathematical relationships called Yukawa couplings. The strength
1:48:20
Speaker 2: of each coupling determines the corresponding elementary particle's mass within
1:48:24
Speaker 2: the standard model. An electron has a comparatively small coupling
1:48:28
Speaker 2: and a relatively small mass. The top quark has a
1:48:32
Speaker 2: much stronger coupling and is considerably heavier. But this does
1:48:36
Speaker 2: not mean particles are physically dragging themselves through a sticky
1:48:39
Speaker 2: invisible substance. That familiar analogy can be misleading. A massive
1:48:45
Speaker 2: particle does not continually lose energy simply because it moves
1:48:49
Speaker 2: through the Higgs field. There is no ordinary friction slowing
1:48:52
Speaker 2: it down. Instead, the interaction contributes to the particle's rest mass,
1:48:58
Speaker 2: an intrinsic property appearing in the equations describing its behaviour.
1:49:02
Speaker 2: And although the Higgs field exists throughout space, the Higgs
1:49:06
Speaker 2: boson is something different. The boson is a quantum excitation
1:49:11
Speaker 2: of the field itself. Just as a photon is an
1:49:14
Speaker 2: excitation of the electromagnetic field, the Higgs boson represents an
1:49:19
Speaker 2: observable excitation associated with the Higgs field. For decades this
1:49:23
Speaker 2: particle remained experimentally undiscovered. Then, on the 4th of July 2012,
1:49:30
Speaker 2: scientists announced a remarkable breakthrough. Two major research collaborations at
1:49:35
Speaker 2: the European Organization for Nuclear Research, using the ATLAS and
1:49:40
Speaker 2: compact muon solenoid detectors at the Large Hadron Collider near Geneva,
1:49:46
Speaker 2: reported the discovery of a new particle consistent with the
1:49:49
Speaker 2: long-predicted Higgs boson. Subsequent measurements established that its properties closely
1:49:55
Speaker 2: matched those expected for the standard model Higgs particle. Its
1:49:59
Speaker 2: mass corresponds to an energy of approximately 125 billion electron volts.
1:50:06
Speaker 2: The discovery provided powerful experimental confirmation of the mechanism through
1:50:11
Speaker 2: which elementary particles acquire mass. But there is an important qualification.
1:50:17
Speaker 2: The Higgs field does not account directly for most of
1:50:19
Speaker 2: the mass of ordinary matter. Consider a proton. It contains quarks.
1:50:25
Speaker 2: those quarks acquire their elementary masses through interactions with the
1:50:29
Speaker 2: Higgs field. Yet the combined elementary masses of the proton's
1:50:33
Speaker 2: principal constituent quarks account for only a small fraction of
1:50:37
Speaker 2: the proton's total mass. Most of its mass arises from
1:50:41
Speaker 2: the energy associated with the strong nuclear interaction. Inside a proton,
1:50:47
Speaker 2: quarks and gluons participate in an extraordinarily energetic quantum system.
1:50:52
Speaker 2: Their motion, interactions and the dynamics of the strong force
1:50:56
Speaker 2: contribute enormously to the proton's total energy. Through Einstein's relationship
1:51:02
Speaker 2: between mass and energy, that energy contributes to the proton's mass.
1:51:07
Speaker 2: The same general principle applies to neutrons. Since protons and
1:51:11
Speaker 2: neutrons account for most of the mass of ordinary atoms,
1:51:15
Speaker 2: much of the mass of familiar objects ultimately comes from
1:51:18
Speaker 2: strong interaction energy rather than directly from the Higgs mechanism.
1:51:23
Speaker 2: The Higgs field nevertheless remains essential to the structure of
1:51:27
Speaker 2: the universe. Without its familiar interactions, electrons and quarks would
1:51:32
Speaker 2: not possess their observed elementary masses, and the weak interaction
1:51:36
Speaker 2: would behave very differently. But important questions remain unanswered. Why
1:51:42
Speaker 2: does the Higgs field possess its particular vacuum value? Why
1:51:46
Speaker 2: do elementary particles have such different coupling strengths? And might
1:51:50
Speaker 2: the Higgs field be connected to a deeper theory that
1:51:53
Speaker 2: has not yet been discovered? Modern physics can describe many
1:51:57
Speaker 2: consequences of the Higgs mechanism with extraordinary precision, but it
1:52:01
Speaker 2: does not yet explain every feature of the field itself.
1:52:05
Speaker 2: And perhaps that is what makes the Higgs field so fascinating.
1:52:10
Speaker 2: A property that seems as ordinary as mass can be
1:52:12
Speaker 2: traced in part to the quantum behaviour of a field
1:52:15
Speaker 2: extending throughout the universe. Something invisible, present even in apparently
1:52:20
Speaker 2: empty space, helps determine the properties of the particles from
1:52:24
Speaker 2: which familiar matter is constructed. And the remarkable discovery of
1:52:28
Speaker 2: the Higgs boson showed that this extraordinary idea was far
1:52:32
Speaker 2: more than an elegant mathematical possibility. Deep inside every atom
1:52:37
Speaker 2: of ordinary matter, there is a remarkably small region where
1:52:40
Speaker 2: an extraordinary amount of activity is taking place, the atomic nucleus.
1:52:45
Speaker 2: It contains protons and, in most atoms, neutrons, packed together
1:52:50
Speaker 2: within a space that is typically only a few millionths
1:52:53
Speaker 2: of a billionth of a metre across. These particles are
1:52:56
Speaker 2: responsible for nearly all the mass of an ordinary atom,
1:53:00
Speaker 2: and yet protons and neutrons are not fundamental particles. They
1:53:05
Speaker 2: have an internal structure governed by one of the most
1:53:07
Speaker 2: powerful interactions known to physics, the strong interaction. Without it,
1:53:13
Speaker 2: the familiar world would be almost unrecognisable. Atomic nuclei could
1:53:18
Speaker 2: not possess their ordinary structure, and the elements from which planets,
1:53:22
Speaker 2: oceans and living organisms are constructed would not exist in
1:53:26
Speaker 2: their familiar forms. To understand this interaction, we must look
1:53:31
Speaker 2: inside the proton itself. During the 1960s, the American physicist
1:53:36
Speaker 2: Murray Gell-Mann helped develop a theoretical description in which particles
1:53:40
Speaker 2: such as protons and neutrons were composed of smaller constituents.
1:53:46
Speaker 2: These constituents became known as quarks. George Zweig independently developed
1:53:51
Speaker 2: a closely related model. The idea initially seemed rather unusual
1:53:56
Speaker 2: because no experiment had revealed an isolated quark. Nevertheless, the
1:54:01
Speaker 2: quark model provided an elegant explanation for patterns in the
1:54:05
Speaker 2: properties of numerous particles. Today, physicists recognise six varieties of
1:54:12
Speaker 2: quark commonly called flavours. They are up, down, strange, charm,
1:54:18
Speaker 2: bottom and top. These names are simply labels. They do
1:54:23
Speaker 2: not indicate that quarks possess particular tastes, directions or ordinary
1:54:27
Speaker 2: physical characteristics corresponding to their names. The up and down
1:54:31
Speaker 2: quarks are especially important because they form the principal constituents
1:54:35
Speaker 2: of ordinary protons and neutrons. A proton contains two up
1:54:40
Speaker 2: valence quarks and one down valence quark, A neutron contains
1:54:44
Speaker 2: two down-valence quarks and one up-valence quark, but these three
1:54:48
Speaker 2: quarks are not the whole internal picture. Protons and neutrons
1:54:52
Speaker 2: also contain gluons and fluctuating contributions involving additional quark-antiquark pairs.
1:54:59
Speaker 2: Their interiors are dynamic quantum systems rather than tidy collections
1:55:03
Speaker 2: of three stationary objects. The theory describing these interactions is
1:55:08
Speaker 2: called quantum chromodynamics. It is the quantum field theory of
1:55:12
Speaker 2: the strong interaction, and at its foundation lies a property
1:55:16
Speaker 2: known as colour charge. Despite the name, colour charge has
1:55:20
Speaker 2: nothing to do with the colours we see. Quarks are
1:55:23
Speaker 2: not genuinely red, green or blue. These names describe three
1:55:27
Speaker 2: possible labels associated with a particular kind of quantum charge.
1:55:32
Speaker 2: Colour charge plays a role in the strong interaction somewhat
1:55:35
Speaker 2: analogous to electric charge in electromagnetism, although the mathematical behaviour
1:55:40
Speaker 2: of the two interactions is very different. Quarks possess colour charge,
1:55:45
Speaker 2: and they interact through quantum fields associated with particles called gluons.
1:55:51
Speaker 2: Gluons are the carriers of the strong interaction. The standard
1:55:56
Speaker 2: model contains eight gluon states arising from the mathematical structure
1:56:01
Speaker 2: of quantum chromodynamics, and gluons possess a particularly important property.
1:56:08
Speaker 2: Unlike photons, which carry no electric charge, gluons themselves carry
1:56:12
Speaker 2: colour charge. This means that gluons can interact directly with
1:56:16
Speaker 2: other gluons. The strong interaction therefore has a much more
1:56:20
Speaker 2: complicated internal structure than ordinary electromagnetism. Its field dynamics can
1:56:26
Speaker 2: become extraordinarily intense. One consequence is a phenomenon called quark confinement.
1:56:34
Speaker 2: In ordinary circumstances quarks are not observed travelling freely through
1:56:38
Speaker 2: space as isolated particles. Instead they are found within composite
1:56:42
Speaker 2: particles called hadrons. Protons and neutrons are examples of hadrons.
1:56:48
Speaker 2: The reason involves the unusual behaviour of the strong interaction
1:56:52
Speaker 2: as coloured particles are separated. Imagine attempting to pull two
1:56:56
Speaker 2: quarks apart. For ordinary electric charges the electromagnetic interaction becomes
1:57:02
Speaker 2: weaker as the separation increases, but the strong interaction behaves differently.
1:57:09
Speaker 2: As colour charge constituents are separated, the quantum chromodynamic field
1:57:14
Speaker 2: can form a narrow concentration of energy, often described as
1:57:17
Speaker 2: a colour flux tube. Increasing the separation can increase the
1:57:22
Speaker 2: energy stored in this field configuration. Eventually enough energy may
1:57:26
Speaker 2: become available to produce additional quark-antiquark pairs, Instead of obtaining
1:57:31
Speaker 2: an isolated quark, the system can reorganise into new colour-neutral hadrons.
1:57:36
Speaker 2: This process helps explain why particle detectors observe collections of
1:57:40
Speaker 2: hadrons rather than individual free quarks. But at extremely short
1:57:45
Speaker 2: distances something surprising happens. The strong interaction becomes weaker. This
1:57:51
Speaker 2: property is called asymptotic freedom. It was discovered theoretically in
1:57:55
Speaker 2: 1973 through the work of David Gross, Frank Wilczek, and
1:58:00
Speaker 2: David Politzer. At sufficiently high momentum transfers, corresponding to very
1:58:06
Speaker 2: short distance scales, quarks interact more weakly This allows certain
1:58:11
Speaker 2: high-energy particle processes to be calculated using methods that would
1:58:16
Speaker 2: be unsuitable for strongly interacting quarks at larger distances. Asymptotic
1:58:22
Speaker 2: freedom is one of the great achievements of quantum field theory.
1:58:26
Speaker 2: It explains how quarks can behave almost like weakly interacting
1:58:30
Speaker 2: particles in some high-energy experiments while remaining confined inside hadrons
1:58:36
Speaker 2: under ordinary conditions. This brings us to another remarkable consequence.
1:58:41
Speaker 2: The mass of a proton is approximately 938 million electron
1:58:46
Speaker 2: volts when expressed using its equivalent rest energy. Yet the
1:58:51
Speaker 2: combined elementary masses of its two up-valence quarks and one
1:58:54
Speaker 2: down-valence quark account for only a small fraction of that total.
1:59:00
Speaker 2: Most of the proton's mass arises from the energy associated
1:59:03
Speaker 2: with its internal quantum chromodynamic dynamics. The motion and interactions
1:59:10
Speaker 2: of quarks and gluons contribute to the total energy of
1:59:12
Speaker 2: the bound system, and through Einstein's relationship between mass and energy,
1:59:17
Speaker 2: that energy contributes to the proton's mass. This is why
1:59:21
Speaker 2: most of the mass of ordinary matter does not come
1:59:24
Speaker 2: directly from the elementary masses generated through the Higgs mechanism.
1:59:29
Speaker 2: It emerges from the extraordinary internal dynamics of the strong interaction.
1:59:34
Speaker 2: There is also an important distinction between the force-binding quarks
1:59:37
Speaker 2: inside protons and neutrons and the force-binding protons and neutrons
1:59:42
Speaker 2: together inside atomic nuclei. The fundamental colour interaction confines quarks
1:59:48
Speaker 2: within hadrons But protons and neutrons are themselves colour-neutral composite particles.
1:59:54
Speaker 2: A residual effect of the strong interaction can act between them.
1:59:59
Speaker 2: In nuclear physics, part of this interaction can be described
2:00:02
Speaker 2: through the exchange of particles called pions, although the complete
2:00:06
Speaker 2: nuclear force involves additional contributions. This residual strong force helps
2:00:12
Speaker 2: hold atomic nuclei together, despite the electrical repulsion between positively
2:00:16
Speaker 2: charged protons. it is essential to the existence of stable
2:00:21
Speaker 2: nuclei containing more than one proton. The consequences extend across
2:00:26
Speaker 2: almost every scale of ordinary matter. Without quantum chromodynamics, protons
2:00:31
Speaker 2: and neutrons would not possess their familiar structure. Without the
2:00:35
Speaker 2: residual strong nuclear interaction, the landscape of atomic nuclei would
2:00:40
Speaker 2: be profoundly different. And without stable atomic nuclei, the chemistry
2:00:44
Speaker 2: responsible for familiar materials and living systems could not exist
2:00:49
Speaker 2: as we know it. The extraordinary part is that all
2:00:52
Speaker 2: of this depends on quantum fields operating at distances far
2:00:56
Speaker 2: smaller than anything we can directly perceive. The stone resting
2:01:00
Speaker 2: on a beach, the water moving through an ocean, and
2:01:04
Speaker 2: the atoms throughout the human body all contain matter whose
2:01:07
Speaker 2: internal structure depends upon the strong interaction. What appears solid
2:01:12
Speaker 2: and permanent on the outside is sustained by an immensely
2:01:15
Speaker 2: energetic quantum world within, And much of the mass we
2:01:19
Speaker 2: associate with ordinary physical substance is not simply the sum
2:01:23
Speaker 2: of tiny constituent masses. It is the energy of interacting
2:01:27
Speaker 2: quantum fields, organised into the stable particles that make up
2:01:31
Speaker 2: the world around us. Every morning, sunlight arrives at Earth
2:01:35
Speaker 2: after travelling approximately 150 million kilometres through space. It illuminates
2:01:40
Speaker 2: the landscape, warms the oceans, and provides the energy supporting
2:01:45
Speaker 2: almost every living ecosystem on our planet. The Sun has
2:01:48
Speaker 2: been shining for approximately 4.6 billion years, and remarkably, it
2:01:54
Speaker 2: is expected to continue producing energy through hydrogen fusion for
2:01:58
Speaker 2: roughly another 5 billion years. But the explanation for this
2:02:03
Speaker 2: extraordinary endurance involves one of the least familiar interactions in physics,
2:02:08
Speaker 2: the weak interaction. Without it, the Sun would not shine
2:02:12
Speaker 2: in the way it does, and the history of the
2:02:15
Speaker 2: universe would be profoundly different. To understand why, consider the
2:02:19
Speaker 2: forces that govern the microscopic world. The electromagnetic interaction acts
2:02:24
Speaker 2: between electrically charged particles. It explains the behaviour of light, electricity,
2:02:30
Speaker 2: magnetism and much of ordinary chemistry. The strong interaction binds
2:02:36
Speaker 2: quarks together within protons and neutrons, while its residual effects
2:02:41
Speaker 2: help hold atomic nuclei together. but another interaction allows certain
2:02:46
Speaker 2: fundamental particles to transform into different kinds of particles. This
2:02:51
Speaker 2: is the weak interaction. It is responsible for processes including
2:02:55
Speaker 2: beta decay, and it plays an essential role in the
2:02:59
Speaker 2: nuclear reactions occurring inside stars. During the 20th century, physicists
2:03:05
Speaker 2: discovered that electromagnetism and the weak interaction were not completely
2:03:10
Speaker 2: separate phenomena. The American physicist Sheldon Glashow, together with the
2:03:14
Speaker 2: Pakistani physicist Abdus Salam, and the American physicist Steven Weinberg,
2:03:21
Speaker 2: made fundamental contributions to the development of the electroweak theory.
2:03:26
Speaker 2: This framework describes electromagnetic and weak interactions as different aspects
2:03:31
Speaker 2: of a unified quantum field theory. At the energies familiar
2:03:36
Speaker 2: to everyday life, their behaviour appears very different. Electromagnetic interactions
2:03:43
Speaker 2: can extend across enormous distances. Weak interactions, by comparison, operate
2:03:49
Speaker 2: over extremely short distances. The explanation involves the particles associated
2:03:55
Speaker 2: with these interactions. Electromagnetism is mediated by photons which have
2:04:00
Speaker 2: no rest mass. The weak interaction involves three massive particles,
2:04:05
Speaker 2: the positively charged W boson, the negatively charged W boson,
2:04:11
Speaker 2: and the electrically neutral Z boson. The W bosons possess
2:04:16
Speaker 2: masses corresponding to approximately 80 billion electron volts of rest energy,
2:04:22
Speaker 2: while the Z boson is heavier still, at approximately 91
2:04:26
Speaker 2: billion electron volts. These substantial masses contribute to the weak interaction,
2:04:32
Speaker 2: having a characteristic range of roughly one thousandth of a
2:04:35
Speaker 2: millionth of a millionth of a millimetre. That is approximately
2:04:39
Speaker 2: one thousandth of a typical proton's diameter. Although this interaction
2:04:43
Speaker 2: operates over such tiny distances, its consequences can extend across
2:04:47
Speaker 2: entire galaxies. One familiar example is beta decay. A free
2:04:53
Speaker 2: neutron is unstable, with an average lifetime of approximately 15
2:04:58
Speaker 2: minutes before it decays. In a typical beta decay, a
2:05:02
Speaker 2: neutron transforms into a proton, an electron, and an electron antineutrino.
2:05:08
Speaker 2: Inside the neutron, one of its down quarks changes into
2:05:12
Speaker 2: an up quark through the weak interaction. This transformation involves
2:05:17
Speaker 2: a negatively charged W boson, which subsequently produces an electron
2:05:22
Speaker 2: and an electron antineutrino. The overall process conserves electric charge, energy,
2:05:29
Speaker 2: momentum and other relevant quantum quantities. It also demonstrates something
2:05:34
Speaker 2: rather extraordinary. The identity of an elementary quark can change
2:05:38
Speaker 2: during an interaction. Such transformations are among the essential features
2:05:42
Speaker 2: of the weak force. The neutrino produced during beta decay
2:05:46
Speaker 2: is itself a remarkable particle. Neutrinos possess no electric charge
2:05:51
Speaker 2: and interact with ordinary matter only through the weak interaction
2:05:55
Speaker 2: and gravity, apart from any still undiscovered interactions. Their masses
2:06:00
Speaker 2: are extraordinarily small. As a result, they can travel through
2:06:04
Speaker 2: enormous amounts of matter without interacting. Trillions of solar neutrinos
2:06:09
Speaker 2: pass through your body every second, yet almost all continue
2:06:13
Speaker 2: onwards without leaving any detectable trace. Most will also pass
2:06:17
Speaker 2: straight through Earth, and these elusive particles provide a direct
2:06:21
Speaker 2: connection to the nuclear reactions occurring deep inside the Sun.
2:06:26
Speaker 2: At the centre of our star, temperatures reach approximately 15
2:06:30
Speaker 2: million degrees Celsius. The immense gravitational pressure produces an extraordinarily
2:06:36
Speaker 2: dense environment in which hydrogen nuclei move rapidly and repeatedly
2:06:41
Speaker 2: approach one another. Hydrogen nuclei are positively charged protons, because
2:06:46
Speaker 2: like electric charges repel, two protons naturally resist approaching closely
2:06:51
Speaker 2: enough for the strong interaction to bind them. Quantum tunnelling
2:06:55
Speaker 2: makes it possible for some protons to overcome this effective
2:06:58
Speaker 2: barrier even when their energies would be insufficient according to
2:07:02
Speaker 2: classical physics. But approaching closely is only part of the challenge.
2:07:07
Speaker 2: In the Sun, the principal energy-producing sequence is called the
2:07:10
Speaker 2: proton-proton chain. Its opening reaction requires two protons to combine,
2:07:16
Speaker 2: with one proton changing into a neutron. The resulting proton
2:07:20
Speaker 2: and neutron form a deuterium nucleus, accompanied by the production
2:07:25
Speaker 2: of a positron and an electron neutrino. and that transformation
2:07:29
Speaker 2: requires the weak interaction. Within one of the protons, an
2:07:33
Speaker 2: up quark changes into a down quark. Because this process
2:07:37
Speaker 2: involves the weak interaction, it happens relatively rarely. A typical
2:07:42
Speaker 2: proton in the Sun's core may participate in countless close
2:07:45
Speaker 2: encounters before successfully beginning the fusion sequence. This slowness is essential.
2:07:52
Speaker 2: If the initial reaction proceeded much more rapidly, the Sun
2:07:55
Speaker 2: would consume its available hydrogen far more quickly. Instead, the
2:08:00
Speaker 2: weak interaction helps regulate the rate at which stellar hydrogen
2:08:03
Speaker 2: fusion can begin. The deuterium nucleus can subsequently combine with
2:08:08
Speaker 2: another proton to form helium-3. Additional nuclear reactions eventually produce
2:08:13
Speaker 2: helium-4 containing two protons and two neutrons. Across the complete
2:08:19
Speaker 2: proton-proton chain, four hydrogen nuclei are effectively converted into one
2:08:24
Speaker 2: helium nucleus, with positrons, neutrinos, and released energy among the products.
2:08:30
Speaker 2: The final products have slightly less total mass than the
2:08:33
Speaker 2: original hydrogen nuclei. That mass difference appears as released energy.
2:08:38
Speaker 2: The energy released equals the mass difference multiplied by the
2:08:42
Speaker 2: speed of light squared. Across the Sun, these reactions generate
2:08:46
Speaker 2: an extraordinary amount of power. approximately 3.8 times 10 to
2:08:51
Speaker 2: the power of 26 watts. Yet the energy is produced
2:08:55
Speaker 2: gradually enough for the Sun to maintain hydrogen fusion over
2:08:59
Speaker 2: a main sequence lifetime of approximately 10 billion years. The
2:09:04
Speaker 2: photons carrying energy outward undergo an immense number of interactions
2:09:09
Speaker 2: within the solar interior before energy eventually escapes from the surface.
2:09:14
Speaker 2: Neutrinos behave very differently. Because they interact so weakly, Most
2:09:19
Speaker 2: escape directly from the Sun's interior and travel towards Earth.
2:09:24
Speaker 2: They provide evidence of nuclear reactions occurring deep inside a
2:09:28
Speaker 2: region that ordinary telescopes cannot see, and neutrinos have revealed
2:09:33
Speaker 2: another important discovery. Experiments at the Super Kamiokande Observatory in
2:09:39
Speaker 2: Japan and the Sudbury Neutrino Observatory in Canada demonstrated that
2:09:43
Speaker 2: neutrinos can change between different types, known as flavours, as
2:09:48
Speaker 2: they travel. This phenomenon is called neutrino oscillation. Its observation
2:09:54
Speaker 2: provided compelling evidence that neutrinos possess non-zero mass. It also
2:09:59
Speaker 2: demonstrated that the simplest original version of the Standard Model,
2:10:03
Speaker 2: which treated neutrinos as massless, was incomplete. The discovery was
2:10:08
Speaker 2: an extraordinary achievement, connecting quantum physics, enormous underground detectors, and
2:10:14
Speaker 2: the behaviour of particles travelling across astronomical distances. The weak
2:10:20
Speaker 2: interaction also influences radioactive decay, nuclear processes in extreme stellar environments,
2:10:27
Speaker 2: and the transformations of matter occurring during powerful astrophysical events.
2:10:33
Speaker 2: Its effects are not limited to laboratories or particle accelerators.
2:10:38
Speaker 2: They are woven into the history of stars. And perhaps
2:10:42
Speaker 2: the most remarkable example remains the familiar sunlight reaching Earth.
2:10:48
Speaker 2: A process taking place inside an unimaginably small region of
2:10:53
Speaker 2: quantum physics helps determine the lifetime of an enormous star.
2:10:58
Speaker 2: The conversion of one kind of quark into another allows
2:11:01
Speaker 2: hydrogen fusion to begin. The resulting chain of reactions releases
2:11:06
Speaker 2: the energy that eventually warms our planet. and the exceptional
2:11:10
Speaker 2: rarity of that first transformation helps ensure that the sun
2:11:13
Speaker 2: continues shining across billions of years. Quantum field theory therefore
2:11:18
Speaker 2: describes far more than the strange behaviour of microscopic particles.
2:11:23
Speaker 2: It helps explain why stars can produce light, why their
2:11:27
Speaker 2: nuclear reactions proceed at particular rates, and why the universe
2:11:31
Speaker 2: possesses the long-lived sources of energy upon which so much
2:11:35
Speaker 2: of its complexity depends. The gentle warmth of an ordinary
2:11:39
Speaker 2: sunny afternoon is connected to an interaction so subtle that
2:11:44
Speaker 2: most neutrinos can pass through an entire planet almost unnoticed.
2:11:49
Speaker 2: And deep within the sun, that quiet interaction continues helping
2:11:53
Speaker 2: to make the light of our nearest star possible. There
2:11:56
Speaker 2: is a rather interesting difficulty with the idea that the
2:11:59
Speaker 2: universe is fundamentally described by quantum fields. Nobody has ever
2:12:04
Speaker 2: taken an ordinary photograph of one You cannot point a
2:12:07
Speaker 2: camera towards the electron field and capture an image of
2:12:11
Speaker 2: its invisible structure. Nor can you place the Higgs field
2:12:15
Speaker 2: beneath a microscope and examine it in the familiar way.
2:12:19
Speaker 2: So how can physicists be so confident that quantum fields
2:12:23
Speaker 2: provide an accurate description of nature? The answer lies in
2:12:27
Speaker 2: one of the most powerful principles of science. We do
2:12:30
Speaker 2: not always need to see something directly to investigate its existence. Instead,
2:12:35
Speaker 2: we can examine what it does. A magnetic field, for example,
2:12:40
Speaker 2: is not directly visible to the human eye, but its
2:12:43
Speaker 2: effects can be measured through the movement of charged particles,
2:12:46
Speaker 2: the behaviour of compass needles, and the electrical currents induced
2:12:50
Speaker 2: in conducting materials. Quantum fields are investigated through a similar principle,
2:12:56
Speaker 2: although the measurements are considerably more sophisticated. Physicists calculate what
2:13:02
Speaker 2: should happen if particular quantum fields exist and interact according
2:13:07
Speaker 2: to certain mathematical rules. They then perform experiments to determine
2:13:12
Speaker 2: whether nature behaves as predicted, and in the case of
2:13:15
Speaker 2: quantum field theory, the agreement has been extraordinary. One of
2:13:20
Speaker 2: the clearest examples comes from the electron. Electrons possess a
2:13:25
Speaker 2: property called a magnetic moment associated with their intrinsic spin.
2:13:31
Speaker 2: theory of quantum electrodynamics predicts that interactions involving the electromagnetic
2:13:37
Speaker 2: field produce extremely small corrections to this magnetic behaviour. These
2:13:42
Speaker 2: corrections contribute to a quantity known as the electron's anomalous
2:13:46
Speaker 2: magnetic moment. The calculations are remarkably demanding, accounting for increasingly
2:13:52
Speaker 2: subtle contributions from quantum interactions. Yet when physicists compare these
2:13:57
Speaker 2: predictions with precise laboratory measurements, The agreement is astonishing. The
2:14:03
Speaker 2: magnetic properties of an individual electron can be measured with
2:14:06
Speaker 2: precision approaching parts per trillion. And theoretical predictions can be
2:14:12
Speaker 2: tested at a similarly extraordinary level, although their accuracy also
2:14:16
Speaker 2: depends on independently measured physical constants. This is not simply
2:14:21
Speaker 2: a theory that provides a convincing explanation after an experiment
2:14:24
Speaker 2: has already happened. It makes numerical predictions precise enough for
2:14:29
Speaker 2: researchers to detect Extraordinarily small differences. But scientists have also
2:14:35
Speaker 2: developed ways to investigate quantum fields at much higher energies.
2:14:39
Speaker 2: Particle accelerators allow researchers to explore interactions occurring at distances
2:14:44
Speaker 2: far smaller than those accessible to ordinary microscopes. The principle
2:14:49
Speaker 2: involves the relationship between a particle's momentum and its associated
2:14:54
Speaker 2: quantum wavelength. Particles carrying greater momentum generally possess shorter wavelengths.
2:15:00
Speaker 2: and shorter wavelengths allow experiments to investigate smaller physical structures.
2:15:05
Speaker 2: This is why high-energy particle collisions have become such an
2:15:09
Speaker 2: important part of modern physics. At the Large Hadron Collider,
2:15:14
Speaker 2: operated by the European Organisation for Nuclear Research near Geneva,
2:15:18
Speaker 2: protons are accelerated to energies of several trillion electron volts.
2:15:23
Speaker 2: The protons travel around an underground circular accelerator approximately 27
2:15:28
Speaker 2: kilometres in circumference. When they collide, their constituent quarks and
2:15:34
Speaker 2: gluons participate in enormously energetic interactions. The resulting collisions can
2:15:40
Speaker 2: produce many different particles, and those particles provide evidence about
2:15:44
Speaker 2: the quantum fields involved. Two enormous experiments, ATLAS and the
2:15:50
Speaker 2: compact muonsolenoid, investigate these events using sophisticated layers of particle detectors.
2:15:57
Speaker 2: The instruments do not take ordinary photographs of the collisions. Instead,
2:16:02
Speaker 2: they record quantities such as particle energies, momenta, electrical charges,
2:16:08
Speaker 2: and the positions at which detectable interactions occur. Charge particles
2:16:14
Speaker 2: leave tracks in tracking detectors. Other particles deposit energy within
2:16:18
Speaker 2: specialised calorimeters. By combining these measurements, scientists reconstruct the likely
2:16:25
Speaker 2: processes responsible for the observed signals. One of the most
2:16:29
Speaker 2: famous achievements of this approach occurred in 2012. For decades,
2:16:34
Speaker 2: physicists had predicted the existence of a particle associated with
2:16:37
Speaker 2: the Higgs field. If the theoretical description was correct, sufficiently
2:16:42
Speaker 2: energetic collisions should occasionally produce a Higgs boson. But this
2:16:47
Speaker 2: particle would be extremely short-lived. Scientists would not observe it
2:16:52
Speaker 2: quietly travelling through a detector. Instead, they would identify the
2:16:56
Speaker 2: particles produced when it decayed. On 4 July 2012, the
2:17:02
Speaker 2: Atlas and Compact Muon Solenoid collaborations announced the discovery of
2:17:07
Speaker 2: a new particle consistent with the predicted Higgs boson. The
2:17:11
Speaker 2: evidence included distinctive decay signatures, such as pairs of energetic
2:17:16
Speaker 2: photons and combinations of charged leptons. the statistical significance reached
2:17:22
Speaker 2: the demanding threshold normally required for a particle physics discovery.
2:17:27
Speaker 2: Subsequent measurements strengthened the conclusion that the particle was the
2:17:31
Speaker 2: Higgs boson predicted by the standard model. This provided remarkable
2:17:36
Speaker 2: experimental support for the Higgs mechanism, and it was not
2:17:40
Speaker 2: the first time quantum field theory had successfully predicted previously
2:17:44
Speaker 2: unknown particles. In 1983, experiments at the European Organisation for
2:17:50
Speaker 2: Nuclear Research discovered the charged W bosons and the neutral
2:17:54
Speaker 2: Z boson. The discoveries were made through the Underground Area
2:17:58
Speaker 2: 1 and Underground Area 2 experiments, providing major confirmation of
2:18:03
Speaker 2: the electroweak theory developed by Sheldon Glashow, Abdus Salam and
2:18:08
Speaker 2: Steven Weinberg. Earlier still, experiments at the Stanford Linear Accelerator
2:18:13
Speaker 2: Center revealed evidence of smaller constituents within protons. During the
2:18:18
Speaker 2: late 1960s, researchers directed high-energy electrons towards protons and examined
2:18:25
Speaker 2: the resulting scattering patterns. These deep, inelastic scattering experiments revealed
2:18:31
Speaker 2: evidence of point-like constituents, supporting the developing quark model. Jerome Friedman,
2:18:38
Speaker 2: Henry Kendall and Richard Taylor made particularly important contributions to
2:18:42
Speaker 2: this experimental program. The results helped establish the foundation for
2:18:47
Speaker 2: our modern understanding of the strong interaction. None of these
2:18:51
Speaker 2: achievements depended on physically seeing an individual quantum field in
2:18:55
Speaker 2: the ordinary sense. Instead, they depended on predictions, measurements and
2:19:00
Speaker 2: the remarkable consistency of experimental evidence. Of course, One successful
2:19:06
Speaker 2: prediction is never enough to establish an entire physical theory.
2:19:10
Speaker 2: Scientists must consider measurement uncertainties, possible experimental errors and alternative explanations.
2:19:18
Speaker 2: Results must be examined independently and important discoveries require evidence
2:19:23
Speaker 2: that can withstand repeated testing. This is particularly important when
2:19:28
Speaker 2: experiments investigate phenomena near the limits of current technology. A
2:19:33
Speaker 2: convincing theory must explain many different observations without requiring a
2:19:38
Speaker 2: new collection of arbitrary assumptions for every experiment. Quantum field
2:19:43
Speaker 2: theory has proved exceptionally successful in this respect. It connects
2:19:48
Speaker 2: atomic energy levels, electromagnetic interactions, particle scattering, radioactive processes, and
2:19:56
Speaker 2: the production of elementary particles within a remarkably coherent mathematical framework.
2:20:02
Speaker 2: yet there is an important distinction between experimental success and
2:20:06
Speaker 2: philosophical certainty. The evidence strongly supports quantum field theory as
2:20:12
Speaker 2: an accurate description of the phenomena it addresses. It does
2:20:15
Speaker 2: not establish beyond all possible doubt that quantum fields must
2:20:19
Speaker 2: be the ultimate substances of reality. Future discoveries may reveal
2:20:23
Speaker 2: deeper physical structures, but whatever those discoveries turn out to be,
2:20:28
Speaker 2: they will need to explain why quantum field theory works
2:20:31
Speaker 2: so extraordinarily well. And that is perhaps the most convincing
2:20:35
Speaker 2: evidence of all. Across decades of experiments involving enormously different
2:20:41
Speaker 2: energies and physical environments, the theory has repeatedly described nature
2:20:46
Speaker 2: with remarkable precision. The quantum fields themselves remain beyond the
2:20:51
Speaker 2: reach of an ordinary camera, but their predicted effects are measurable, repeatable.
2:20:57
Speaker 2: and woven throughout the physical world. We may not be
2:21:01
Speaker 2: able to see these fields directly, yet through their interactions
2:21:05
Speaker 2: we have learned an extraordinary amount about how they behave.
2:21:08
Speaker 2: For thousands of years, human beings have wondered what the
2:21:11
Speaker 2: universe is ultimately made of. The ancient Greeks imagined indivisible atoms.
2:21:17
Speaker 2: Later scientists discovered that atoms contained smaller particles, and those
2:21:21
Speaker 2: particles revealed structures of their own. Each discovery seemed to
2:21:26
Speaker 2: bring us closer to the fundamental ingredients of reality. But
2:21:30
Speaker 2: quantum field theory has changed the nature of the question.
2:21:34
Speaker 2: Instead of describing everything as a collection of permanently existing
2:21:38
Speaker 2: miniature objects, modern physics describes elementary particles as excitations of
2:21:44
Speaker 2: underlying quantum fields. Electrons, photons, quarks, and the other known
2:21:49
Speaker 2: elementary particles can all be understood within this framework. And
2:21:53
Speaker 2: that leads to a rather extraordinary question. If particles are
2:21:57
Speaker 2: excitations of quantum fields, what exactly are the fields made of?
2:22:02
Speaker 2: The surprising answer is that we do not know whether
2:22:05
Speaker 2: they are made of anything more fundamental. Within the standard
2:22:09
Speaker 2: model of particle physics, quantum fields are treated as fundamental
2:22:13
Speaker 2: components of nature. They describe the known elementary particles and
2:22:18
Speaker 2: three of the four fundamental interactions. The electromagnetic interaction, the
2:22:24
Speaker 2: weak interaction, and the strong interaction all fit within this
2:22:28
Speaker 2: remarkably successful description. Its predictions have been confirmed through decades
2:22:34
Speaker 2: of experiments, sometimes with extraordinary precision. But scientific success does
2:22:40
Speaker 2: not necessarily mean that a theory represents the final, possible
2:22:45
Speaker 2: description of reality, and there are important reasons to suspect
2:22:49
Speaker 2: that our present understanding remains incomplete. Perhaps the greatest difficulty
2:22:54
Speaker 2: involves gravity. Albert Einstein's general theory of relativity describes gravity
2:22:59
Speaker 2: through the curvature of spacetime. Matter and energy influence the
2:23:04
Speaker 2: geometry of spacetime, while that geometry determines how objects and
2:23:08
Speaker 2: light move. Quantum field theory, meanwhile, describes microscopic interactions through
2:23:14
Speaker 2: quantum fields, usually within a specified spacetime background. Both frameworks
2:23:20
Speaker 2: are extraordinarily successful. but they have not yet been combined
2:23:24
Speaker 2: into a complete experimentally confirmed theory capable of describing every
2:23:29
Speaker 2: gravitational and quantum phenomenon. This is the challenge of quantum gravity.
2:23:35
Speaker 2: At ordinary energies, the distinction rarely causes difficulties. But under
2:23:40
Speaker 2: extreme conditions, such as those associated with the earlier stages
2:23:45
Speaker 2: of cosmic history, or the deepest interiors of black holes,
2:23:49
Speaker 2: both quantum effects and strong gravitational effects may become essential,
2:23:54
Speaker 2: and our current theories cannot provide a complete account of
2:23:57
Speaker 2: everything occurring in such environments. One important scale associated with
2:24:02
Speaker 2: this problem is the Planck length. It measures approximately 1.6
2:24:07
Speaker 2: times 10 to the power of minus 35 metres. This
2:24:11
Speaker 2: extraordinarily small length is constructed from fundamental constants associated with gravity,
2:24:17
Speaker 2: quantum mechanics, and the speed of light. It provides a
2:24:21
Speaker 2: characteristic scale at which quantum gravitational effects may become important,
2:24:26
Speaker 2: but it is worth being precise. Physics has not demonstrated
2:24:30
Speaker 2: that the Planck length is the smallest possible distance, or
2:24:34
Speaker 2: that space must consist of individual pieces of that size.
2:24:38
Speaker 2: Whether spacetime possesses a deeper microscopic structure remains unknown. Black
2:24:43
Speaker 2: holes offer another fascinating connection between quantum fields and gravity.
2:24:48
Speaker 2: During the 1970s, Stephen Hawking investigated quantum fields in curved spacetime.
2:24:55
Speaker 2: His calculations predicted that black holes should emit thermal radiation.
2:25:00
Speaker 2: This phenomenon became known as Hawking radiation. The radiation arises
2:25:04
Speaker 2: from quantum effects associated with the geometry of spacetime around
2:25:08
Speaker 2: a black hole. It is often explained through a simplified
2:25:12
Speaker 2: picture involving particle-antiparticle pairs appearing near the event horizon. But
2:25:18
Speaker 2: the actual theoretical description concerns quantum field states and the
2:25:23
Speaker 2: way different observers identify particles in curved spacetime. Hawking radiation
2:25:29
Speaker 2: has not yet been directly detected from an astrophysical black hole. Nevertheless,
2:25:34
Speaker 2: its theoretical prediction reveals a profound connection between quantum physics,
2:25:40
Speaker 2: gravity and thermodynamics. And it raises questions that neither ordinary
2:25:45
Speaker 2: quantum field theory nor classical general relativity can fully resolve alone.
2:25:52
Speaker 2: Another mystery concerns the quantum vacuum. Quantum fields possess lowest
2:25:57
Speaker 2: energy states, and those states can exhibit fluctuations and correlations
2:26:02
Speaker 2: even without ordinary particles. In general relativity, energy contributes to
2:26:08
Speaker 2: the gravitational behaviour of spacetime. This creates a difficult question
2:26:12
Speaker 2: about how the energy associated with quantum fields relates to
2:26:16
Speaker 2: the cosmological constant. Observations of the universe's accelerating expansion indicate
2:26:22
Speaker 2: a remarkably small effective cosmological constant, yet straightforward estimates of
2:26:28
Speaker 2: quantum vacuum contributions can suggest enormously larger values depending on
2:26:33
Speaker 2: the assumptions and mathematical treatment involved. The discrepancy is part
2:26:38
Speaker 2: of what physicists call the cosmological constant problem. It remains
2:26:43
Speaker 2: one of the deepest unresolved questions connecting quantum field theory
2:26:47
Speaker 2: with cosmology. There are other mysteries. Astronomical observations strongly support
2:26:52
Speaker 2: the existence of dark matter, but its fundamental identity remains unknown.
2:26:58
Speaker 2: Dark energy is the name given to whatever is responsible
2:27:01
Speaker 2: for the observed acceleration of cosmic expansion, and its physical
2:27:05
Speaker 2: nature is also unresolved. Researchers have proposed additional quantum fields
2:27:10
Speaker 2: that might help explain these phenomena, but none has yet
2:27:14
Speaker 2: been established as the complete answer. Even within ordinary particle physics,
2:27:19
Speaker 2: important questions remain. Neutrino oscillations demonstrate that neutrinos possess mass,
2:27:26
Speaker 2: requiring physics beyond the simplest original standard model. The theory
2:27:31
Speaker 2: also does not explain why electrons, quarks and other elementary
2:27:35
Speaker 2: particles possess their particular pattern of masses. These gaps suggest
2:27:40
Speaker 2: that the currently known quantum fields may not tell the
2:27:42
Speaker 2: entire story. One possibility is that our existing field theories
2:27:47
Speaker 2: are effective descriptions. An effective field theory provides an accurate
2:27:51
Speaker 2: account of physical phenomena within a particular range of energies
2:27:55
Speaker 2: and distances. It need not describe every conceivable scale. Just
2:28:00
Speaker 2: as the behaviour of water can be described without following
2:28:03
Speaker 2: every individual molecule, familiar quantum fields might conceivably emerge from
2:28:08
Speaker 2: deeper physical structures. That possibility remains speculative, but it is
2:28:13
Speaker 2: scientifically meaningful. And it brings us back to the original question.
2:28:18
Speaker 2: What are quantum fields actually made of? Perhaps future experiments
2:28:23
Speaker 2: will reveal more fundamental constituents. Perhaps quantum fields and space-time
2:28:28
Speaker 2: will emerge together from a deeper mathematical description. Or perhaps
2:28:33
Speaker 2: the expectation that everything must be made from smaller pieces
2:28:37
Speaker 2: is itself a habit inherited from our everyday experience of matter.
2:28:41
Speaker 2: At present, physics cannot establish which possibility is correct. What
2:28:46
Speaker 2: we can say is that quantum field theory provides one
2:28:50
Speaker 2: of the most powerful descriptions of nature ever developed. It
2:28:53
Speaker 2: connects the behaviour of elementary particles with the structure of atoms,
2:28:57
Speaker 2: the properties of light, and the interactions that make ordinary
2:29:01
Speaker 2: matter possible. The fields may not be the final foundation
2:29:05
Speaker 2: of reality, but they have already revealed that the universe
2:29:09
Speaker 2: is considerably stranger than a collection of tiny solid objects
2:29:14
Speaker 2: moving through empty space. And somewhere beneath the familiar appearance
2:29:19
Speaker 2: of everything around us there may still be deeper principles
2:29:23
Speaker 2: waiting to be discovered. For now, the quantum fields remain
2:29:27
Speaker 2: an extraordinary achievement of human understanding. Not necessarily the final
2:29:32
Speaker 2: answer to what everything is made of, but perhaps the
2:29:35
Speaker 2: clearest glimpse we have yet obtained of the remarkable structure
2:29:38
Speaker 2: beneath the world we know. The world around us has
2:29:41
Speaker 2: a reassuring sense of solidity. The floor beneath your feet,
2:29:45
Speaker 2: the air moving gently through a room, the water filling
2:29:49
Speaker 2: an ocean– and the warmth of sunlight reaching your skin
2:29:53
Speaker 2: all seem like familiar, separate parts of physical reality. Yet
2:29:58
Speaker 2: beneath their ordinary appearance lies something quite extraordinary. According to
2:30:03
Speaker 2: quantum field theory, the elementary particles from which matter is
2:30:07
Speaker 2: constructed can be understood as excitations of underlying quantum fields.
2:30:12
Speaker 2: An electron is not simply a miniature solid object travelling
2:30:16
Speaker 2: through empty space. It is an excitation associated with the
2:30:20
Speaker 2: electron field. A photon is an excitation of the electromagnetic field.
2:30:26
Speaker 2: And the quarks found inside protons and neutrons are described
2:30:29
Speaker 2: through their own quantum fields, interacting through the powerful forces
2:30:34
Speaker 2: that help hold ordinary matter together. What appears to be
2:30:38
Speaker 2: an enormous collection of independent objects can therefore be described
2:30:42
Speaker 2: through a remarkably consistent set of quantum principles. Perhaps one
2:30:47
Speaker 2: of the most beautiful consequences is the identity of elementary particles.
2:30:52
Speaker 2: An electron inside your body possesses the same intrinsic properties
2:30:56
Speaker 2: as an electron in a galaxy billions of light-years away.
2:31:00
Speaker 2: Their circumstances may be completely different, but the fundamental laws
2:31:04
Speaker 2: describing them remain remarkably consistent. And those laws have consequences
2:31:10
Speaker 2: extending far beyond the microscopic world. Electromagnetic interactions help determine
2:31:16
Speaker 2: the structure of atoms and the behaviour of light. The
2:31:19
Speaker 2: strong interaction binds quarks within protons and neutrons, contributing most
2:31:25
Speaker 2: of the mass of ordinary matter through its internal energy.
2:31:28
Speaker 2: The weak interaction allows certain particles to transform, making radioactive
2:31:34
Speaker 2: decay possible and helping sustain the nuclear reactions that illuminate stars. Meanwhile,
2:31:41
Speaker 2: the Higgs field contributes to the masses of elementary particles
2:31:45
Speaker 2: through the Brute-Englert-Higgs mechanism. Together, these interactions help explain an
2:31:51
Speaker 2: extraordinary variety of familiar physical phenomena. Even apparently empty space
2:31:57
Speaker 2: is more interesting than it first appears. In quantum field theory,
2:32:01
Speaker 2: a vacuum is not necessarily the complete absence of everything.
2:32:05
Speaker 2: It is a lowest energy state of the relevant fields–
2:32:09
Speaker 2: which can still possess quantum fluctuations and correlations. And the
2:32:13
Speaker 2: remarkable success of this description is supported by experimental evidence.
2:32:18
Speaker 2: Quantum electrodynamics predicts the electron's magnetic properties with astonishing precision.
2:32:25
Speaker 2: The discoveries of the W and Z bosons confirmed essential
2:32:29
Speaker 2: features of electroweak theory, and the discovery of the Higgs
2:32:32
Speaker 2: boson in 2012 provided compelling evidence for another fundamental part
2:32:37
Speaker 2: of the standard model. yet our understanding remains incomplete. Gravity
2:32:42
Speaker 2: has not been incorporated into a fully established quantum theory.
2:32:47
Speaker 2: The identities of dark matter and dark energy remain unresolved,
2:32:51
Speaker 2: and the relationship between quantum vacuum energy and the expansion
2:32:54
Speaker 2: of the universe presents profound difficulties. We also cannot say
2:32:59
Speaker 2: with certainty whether the known quantum fields represent the deepest
2:33:03
Speaker 2: possible description of reality. Perhaps there are more fundamental structures
2:33:08
Speaker 2: still waiting to be discovered. But there is something rather
2:33:12
Speaker 2: wonderful about how far science has already come. By studying
2:33:16
Speaker 2: extraordinarily small interactions, physicists have developed a description connecting the
2:33:22
Speaker 2: behaviour of individual particles with the structure of atoms, the
2:33:26
Speaker 2: light of distant stars, and the matter composing entire galaxies.
2:33:32
Speaker 2: and all around us those same physical principles continue to
2:33:35
Speaker 2: operate quietly and reliably without requiring our attention. The apparently
2:33:40
Speaker 2: ordinary world is supported by a remarkable underlying quantum description.
2:33:45
Speaker 2: We may not yet know what the universe is ultimately
2:33:48
Speaker 2: made of, but quantum field theory has revealed that the
2:33:52
Speaker 2: answer is far more fascinating than a simple collection of
2:33:55
Speaker 2: tiny objects, and somewhere within those invisible fields there are
2:34:00
Speaker 2: excitations and their countless interactions lies much of the explanation
2:34:04
Speaker 2: for the extraordinary universe we call home.