Latest / Night Science Stories / quantum field theory, what is quantum field theory, quantum physics, quantum mechanics, quantum fields explained, particle physics, fundamen

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quantum field theory, what is quantum field theory, quantum physics, quantum mechanics, quantum fields explained, particle physics, fundamen · Night Science Stories

• 0:00
  Speaker 1: Good evening, and welcome to the Sleepy Scientist. It's lovely
• 0:03
  Speaker 1: to have you here tonight. I'm always curious where these
• 0:06
  Speaker 1: evenings end up, so if it's not too much to ask,
• 0:09
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• 0:11
  Speaker 1: the world you're listening from. A dark bedroom with one
• 0:14
  Speaker 1: window slightly open. A chair you've been meaning to get
• 0:18
  Speaker 1: out of for an hour. A kitchen table long after
• 0:21
  Speaker 1: the dishes were done. Wherever you are, I hope you're warm, comfortable,
• 0:26
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• 0:36
  Speaker 1: whenever you like. I'll carry on talking either way. And
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  Speaker 1: if you enjoy these slow wanderings through science, a like
• 0:44
  Speaker 1: and a subscribe help this little channel drift a bit
• 0:47
  Speaker 1: further out into the world. Two very small ripples, but
• 0:50
  Speaker 1: they travel. If you're already with us, thank you. Your
• 0:54
  Speaker 1: company on these evenings means a great deal. Now, I'll
• 0:59
  Speaker 1: do my best not to make tonight's subject so interesting
• 1:02
  Speaker 1: that it keeps you awake, though it does have a
• 1:04
  Speaker 1: habit of making the ordinary world seem rather less ordinary
• 1:07
  Speaker 1: than it did an hour ago. Because underneath everything around you,
• 1:12
  Speaker 1: the air in the room, the walls, the bed, your
• 1:14
  Speaker 1: own body, modern physics doesn't actually describe reality as tiny
• 1:19
  Speaker 1: solid particles drifting about in empty space. It describes something
• 1:25
  Speaker 1: far stranger, a universe filled everywhere and always with invisible fields.
• 1:32
  Speaker 1: Picture the surface of a very still pond stretching endlessly
• 1:36
  Speaker 1: in every direction. An electron in this picture is a
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  Speaker 1: little ripple in one of those fields. A particle of
• 1:43
  Speaker 1: light is a ripple in another. The building blocks inside
• 1:47
  Speaker 1: every atom of your body are ripples in fields of
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  Speaker 1: their own. And even in the emptiest stretch of space imaginable,
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  Speaker 1: the fields are still there. Perfectly calm but present. Which
• 2:00
  Speaker 1: means the particles we spent a century thinking of as
• 2:03
  Speaker 1: the smallest things in existence may not be the deepest
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  Speaker 1: layer of nature at all. They may simply be what
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  Speaker 1: these fields look like when something disturbs them. So, shall
• 2:14
  Speaker 1: we begin? What exactly is quantum field theory? Most of
• 2:18
  Speaker 1: us first meet the microscopic world as a collection of
• 2:20
  Speaker 1: tiny particles. Matter is made from atoms. Atoms contain electrons
• 2:25
  Speaker 1: and nuclei. Nuclei contain protons and neutrons. Protons and neutrons
• 2:31
  Speaker 1: contain quarks. Light arrives in packets called photons. It is
• 2:35
  Speaker 1: a wonderfully useful picture, and for many purposes it works
• 2:39
  Speaker 1: extremely well. But modern physics eventually asks us to turn
• 2:43
  Speaker 1: that picture almost completely inside out. At the deepest level
• 2:48
  Speaker 1: currently described by our most successful theories, particles are not
• 2:53
  Speaker 1: necessarily the fundamental things from which everything else is built. Instead,
• 2:58
  Speaker 1: physicists describe nature in terms of quantum fields that extend
• 3:02
  Speaker 1: throughout space. What we call a particle can then appear
• 3:06
  Speaker 1: as a particular excitation of one of those fields. An electron,
• 3:09
  Speaker 1: for example, is associated with the electron field. A photon
• 3:14
  Speaker 1: is associated with the electromagnetic field. Quarks are associated with
• 3:20
  Speaker 1: quark fields. The fields are present throughout space. while the
• 3:24
  Speaker 1: particles are particular quantum states of those fields. That idea
• 3:28
  Speaker 1: sits at the heart of quantum field theory. In the simplest,
• 3:32
  Speaker 1: useful definition, quantum field theory is the framework that combines
• 3:36
  Speaker 1: quantum mechanics, the concept of a physical field, and the
• 3:40
  Speaker 1: requirements of Albert Einstein's special theory of relativity. It allows
• 3:45
  Speaker 1: physics to describe particles being created, destroyed and transformed while
• 3:50
  Speaker 1: still respecting laws such as the conservation of energy momentum
• 3:54
  Speaker 1: and electric charge. The word field can sound more exotic
• 3:58
  Speaker 1: than it really is. Imagine measuring the temperature at every
• 4:02
  Speaker 1: point in a room. Near a radiator it might be
• 4:04
  Speaker 1: slightly warmer, beside a window it might be cooler. Instead
• 4:08
  Speaker 1: of describing the room with a single temperature, you could
• 4:11
  Speaker 1: assign a temperature to every location. That collection of values
• 4:16
  Speaker 1: forms what mathematicians would call a field. Physics contains more
• 4:21
  Speaker 1: substantial examples. Earth has a gravitational field. At every location
• 4:27
  Speaker 1: around the planet, that field tells you something about how
• 4:30
  Speaker 1: an object would respond to gravity there. An electrically charged
• 4:34
  Speaker 1: object produces an electromagnetic field around it. The field can
• 4:39
  Speaker 1: vary from place to place and change with time. Those
• 4:42
  Speaker 1: examples help build intuition. Although a quantum field is not
• 4:45
  Speaker 1: simply a classical field with a few strange quantum rules
• 4:49
  Speaker 1: sprinkled on top, A quantum field is itself a quantum
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  Speaker 1: mechanical object. It possesses quantum states, probabilities, uncertainties and quantist excitations.
• 5:01
  Speaker 1: In the mathematical description, the field provides physical degrees of
• 5:04
  Speaker 1: freedom throughout spacetime. That phrase, degrees of freedom, simply refers
• 5:10
  Speaker 1: to the possible ways a physical system can vary. It
• 5:13
  Speaker 1: is important not to imagine the electron field as a
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  Speaker 1: kind of invisible fog filling the room, or the electromagnetic
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  Speaker 1: field as a transparent liquid poured through empty space. Those
• 5:24
  Speaker 1: pictures can be useful for a few seconds, but they
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  Speaker 1: quickly become misleading. A quantum field is not known to
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  Speaker 1: be made from some finer ordinary substance. It is part
• 5:35
  Speaker 1: of the fundamental mathematical and physical description itself. Wherever you're
• 5:40
  Speaker 1: listening right now, the electron field is present. So is
• 5:44
  Speaker 1: the electromagnetic field. so are the different quark fields and
• 5:48
  Speaker 1: the other fields used in modern particle physics. Yet that
• 5:51
  Speaker 1: does not mean your bedroom is crowded with electrons simply
• 5:54
  Speaker 1: because the electron field exists there. The existence of a
• 5:58
  Speaker 1: field and the existence of a particle excitation of that
• 6:01
  Speaker 1: field are different ideas. A calm lake provides a rough analogy.
• 6:06
  Speaker 1: The lake exists whether or not a particular wave is
• 6:09
  Speaker 1: travelling across its surface. When a wave appears, It is
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  Speaker 1: a particular state of the lake rather than a new
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  Speaker 1: substance added on top of it. The analogy is imperfect
• 6:19
  Speaker 1: because water itself is made from molecules, whereas quantum fields
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  Speaker 1: are being used as part of our fundamental description. Still,
• 6:27
  Speaker 1: it captures something important. The field is not the same
• 6:31
  Speaker 1: thing as the excitation. The particle picture remains extremely useful
• 6:36
  Speaker 1: because these excitations behave experimentally like particles. An electron arriving
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  Speaker 1: at a detector can produce a localised event. It possesses
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  Speaker 1: a definite electric charge, a definite intrinsic spin and a
• 6:49
  Speaker 1: particular mass. Photons can be emitted and absorbed in individual packets.
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  Speaker 1: Particle accelerators record discrete tracks and energy deposits that are
• 6:59
  Speaker 1: naturally described using the language of particles. Quantum field theory
• 7:04
  Speaker 1: does not tell us to throw that language away. Instead,
• 7:07
  Speaker 1: it explains why particle-like behaviour appears. The need for this
• 7:12
  Speaker 1: deeper description becomes especially clear when quantum mechanics meets special relativity.
• 7:18
  Speaker 1: Albert Einstein's special theory of relativity tells us that the
• 7:22
  Speaker 1: laws of physics must take the same basic form for
• 7:25
  Speaker 1: observers moving at constant velocity relative to one another. It
• 7:30
  Speaker 1: also establishes the speed of a light in vacuum as
• 7:33
  Speaker 1: the limiting speed at which causal influences can propagate. Ordinary
• 7:38
  Speaker 1: non-relativistic quantum mechanics works beautifully for many systems moving much
• 7:42
  Speaker 1: more slowly than light. It describes atoms, chemical bonds and
• 7:47
  Speaker 1: a vast range of laboratory phenomena with astonishing accuracy. But
• 7:52
  Speaker 1: once particles move at relativistic speeds, a deeper difficulty appears.
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  Speaker 1: Relativity allows energy and mass to be converted into one another.
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  Speaker 1: With enough energy concentrated into an interaction, entirely new particles
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  Speaker 1: can be produced. Conversely, particles can disappear as identifiable particles
• 8:13
  Speaker 1: and their energy can reappear in other forms. An electron
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  Speaker 1: and its antimatter counterpart, the positron, can annihilate and produce photons.
• 8:22
  Speaker 1: Energetic photons can participate in processes that create particle and
• 8:27
  Speaker 1: antiparticle pairs. Inside powerful particle accelerators, collisions routinely produce particles
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  Speaker 1: that were not present before the collision. A theory built
• 8:38
  Speaker 1: around the assumption that there will always be exactly one
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  Speaker 1: particle or exactly two begins to look rather uncomfortable. Quantum
• 8:47
  Speaker 1: field theory handles this naturally. The underlying fields remain part
• 8:51
  Speaker 1: of the theory while their quantum states can contain different
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  Speaker 1: numbers of particle excitations. One state might correspond to no
• 8:59
  Speaker 1: particles of a particular type. Another might correspond to one
• 9:04
  Speaker 1: another might correspond to several. Particles can therefore appear and
• 9:08
  Speaker 1: disappear without requiring the fundamental entities of the theory themselves
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  Speaker 1: to pop into and out of existence. This framework forms
• 9:16
  Speaker 1: the foundation of the standard model of particle physics. The
• 9:20
  Speaker 1: standard model describes the known fundamental particles and three of
• 9:24
  Speaker 1: the four known fundamental interactions. These are the electromagnetic interaction,
• 9:30
  Speaker 1: the weak interaction, and the strong interaction. Electromagnetism governs phenomena
• 9:36
  Speaker 1: involving electrically charged particles and light. The weak interaction is
• 9:42
  Speaker 1: involved in processes such as radioactive beta decay and in
• 9:46
  Speaker 1: reactions taking place within stars. The strong interaction binds quarks
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  Speaker 1: together and ultimately helps hold atomic nuclei together. All three
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  Speaker 1: are described using quantum fields. Gravity remains the conspicuous expression. exception.
• 10:01
  Speaker 1: Albert Einstein's general theory of relativity describes gravity extremely successfully
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  Speaker 1: as the curvature of spacetime produced by matter and energy,
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  Speaker 1: but gravity has not yet been incorporated into the same
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  Speaker 1: experimentally established quantum field framework as the other fundamental interactions.
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  Speaker 1: Physicists possess promising ideas about quantum gravity, but no complete
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  Speaker 1: theory has yet received the kind of experimental confirmation enjoyed
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  Speaker 1: by the Standard Model. So the important shift is not
• 10:31
  Speaker 1: simply that particles happen to be surrounded by invisible fields.
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  Speaker 1: That would still leave the particles as the main actors.
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  Speaker 1: Quantum field theory suggests something more fundamental. The fields themselves
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  Speaker 1: occupy centre stage. The electron field is fundamental to the description.
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  Speaker 1: and an electron is a particular excitation of it. The
• 10:51
  Speaker 1: electromagnetic field is fundamental, and a photon is one of
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  Speaker 1: its quantum excitations. The tiny particle we once imagined as
• 10:59
  Speaker 1: the basic building block turns out to be something the
• 11:01
  Speaker 1: field can do. And beneath the apparently solid world of tables, stars, air,
• 11:07
  Speaker 1: skin and stone, modern physics describes a restless collection of
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  Speaker 1: quantum fields whose excitations become the particles from which familiar
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  Speaker 1: matter is built. Long before physicists began talking about quantum fields,
• 11:22
  Speaker 1: the idea of a field was already quietly changing the
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  Speaker 1: way they thought about nature. For much of the history
• 11:28
  Speaker 1: of physics, forces were often pictured as acting directly between objects.
• 11:33
  Speaker 1: Two masses attracted one another through gravity. Two electric charges
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  Speaker 1: could attract or repel. Two magnets could exert forces even
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  Speaker 1: when they were not touching. The mathematics could describe these
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  Speaker 1: effects very well, but there was an uncomfortable question sitting
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  Speaker 1: underneath them. What exactly was happening in the space between
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  Speaker 1: the objects? One of the people who helped transform that
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  Speaker 1: question was Michael Faraday. Faraday was born in England in
• 12:01
  Speaker 1: 1791 and became one of the most influential experimental physicists
• 12:05
  Speaker 1: of the 19th century. He worked extensively with electricity and magnetism,
• 12:11
  Speaker 1: and he developed a remarkably physical way of thinking about
• 12:14
  Speaker 1: the space surrounding electric charges, currents and magnets. Rather than
• 12:19
  Speaker 1: imagining forces as somehow jumping instantly across empty space, Faraday
• 12:24
  Speaker 1: pictured what he called lines of force extending through the
• 12:27
  Speaker 1: surrounding region. Today, we would describe this in the language
• 12:31
  Speaker 1: of fields. A magnetic field, for example, is not limited
• 12:35
  Speaker 1: to the magnet itself. It exists throughout the region around
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  Speaker 1: the magnet. At each location, the field has a strength
• 12:43
  Speaker 1: and a direction, and another magnetic object placed there can
• 12:47
  Speaker 1: respond to it. The same idea applies to electric fields.
• 12:51
  Speaker 1: An electric charge alters the physical conditions around it. Another
• 12:55
  Speaker 1: charge placed nearby responds to the electric field at its
• 12:58
  Speaker 1: own location. This may sound perfectly ordinary now, but it
• 13:03
  Speaker 1: represented a major conceptual shift. Empty space was no longer
• 13:07
  Speaker 1: merely the blank gap between objects. The region around those
• 13:11
  Speaker 1: objects could possess physical properties of its own. Faraday's ideas
• 13:16
  Speaker 1: were largely intuitive and experimental. The mathematical structure that transformed
• 13:21
  Speaker 1: them into one of the great theories of physics came
• 13:23
  Speaker 1: from James Clerk Maxwell. Maxwell was a Scottish physicist born
• 13:28
  Speaker 1: in 1831. During the 19th century, he developed a mathematical
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  Speaker 1: theory that brought electricity and magnetism together into a single framework.
• 13:38
  Speaker 1: Before Maxwell, Electric and magnetic phenomena had often been studied
• 13:42
  Speaker 1: as related but distinct subjects. Maxwell showed that they were
• 13:47
  Speaker 1: parts of one connected system. A changing electric field can
• 13:51
  Speaker 1: be associated with a magnetic field, and a changing magnetic
• 13:55
  Speaker 1: field can be associated with an electric field. The two
• 13:59
  Speaker 1: are woven together as aspects of what we now call
• 14:02
  Speaker 1: the electromagnetic field. The consequences were enormous. Maxwell's equations showed
• 14:08
  Speaker 1: that disturbances in the electromagnetic field could travel through space
• 14:12
  Speaker 1: as waves. When Maxwell calculated the speed of those waves,
• 14:17
  Speaker 1: he found that it matched the known speed of light.
• 14:20
  Speaker 1: That was not a small coincidence. It suggested that light
• 14:23
  Speaker 1: itself was an electromagnetic wave. Visible light, radio waves, microwaves,
• 14:30
  Speaker 1: infrared radiation, ultraviolet radiation, X-rays and gamma rays, are all
• 14:36
  Speaker 1: now understood as different forms of electromagnetic radiation. They differ
• 14:41
  Speaker 1: primarily in wavelength and frequency, but they belong to the
• 14:44
  Speaker 1: same electromagnetic family. So by the late 19th century, physics
• 14:50
  Speaker 1: already possessed a powerful field theory. The difference between a
• 14:54
  Speaker 1: particle and a field is worth making very clear. A
• 14:57
  Speaker 1: classical particle can be imagined as occupying a particular position
• 15:02
  Speaker 1: and following a trajectory through space. If you throw a
• 15:05
  Speaker 1: ball across a room, you can in principle describe where
• 15:08
  Speaker 1: the ball is and how its position changes over time.
• 15:11
  Speaker 1: A field is different. A field assigns physical information throughout
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  Speaker 1: an extended region. Temperature offers a simple example. Imagine measuring
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  Speaker 1: the temperature at every point in a room. The result
• 15:25
  Speaker 1: would not be one object travelling from place to place.
• 15:29
  Speaker 1: It would be a distribution of values throughout the room.
• 15:31
  Speaker 1: An electromagnetic field is richer. because it has direction as
• 15:36
  Speaker 1: well as magnitude, but the underlying idea is similar. The
• 15:41
  Speaker 1: field exists across space and can vary from one location
• 15:44
  Speaker 1: to another. Most importantly, a field can have dynamics of
• 15:48
  Speaker 1: its own. It can change, it can carry energy, it
• 15:52
  Speaker 1: can carry momentum, and disturbances in it can propagate from
• 15:56
  Speaker 1: one place to another. This is one of the most
• 16:00
  Speaker 1: important ideas to keep hold of when thinking about quantum
• 16:03
  Speaker 1: field theory. Fields were not invented simply to explain quantum particles.
• 16:09
  Speaker 1: They were already doing serious physical work before quantum theory arrived.
• 16:14
  Speaker 1: In classical electromagnetism, light can be imagined as a continuous
• 16:18
  Speaker 1: wave travelling through the electromagnetic field. The electric and magnetic
• 16:23
  Speaker 1: components oscillate as the wave moves carrying energy across space.
• 16:28
  Speaker 1: Within the classical theory, that energy appears capable of varying smoothly.
• 16:33
  Speaker 1: A weak beam of light can carry less energy. A
• 16:36
  Speaker 1: stronger beam can carry more. There is no obvious requirement
• 16:40
  Speaker 1: that the energy must arrive in indivisible packets. For a while,
• 16:44
  Speaker 1: this continuous picture seemed entirely reasonable. Classical field theory was
• 16:48
  Speaker 1: spectacularly successful. It explained electromagnetic induction, electric circuits, magnetic effects,
• 16:57
  Speaker 1: radio transmission and the propagation of light. It connected laboratory
• 17:02
  Speaker 1: experiments with a mathematical theory of remarkable elegance. But as
• 17:06
  Speaker 1: physicists began examining matter and radiation more carefully near the
• 17:10
  Speaker 1: end of the 19th century and the beginning of the 20th,
• 17:13
  Speaker 1: cracks started to appear. Certain experiments refused to behave as
• 17:18
  Speaker 1: though energy could always be exchanged in completely smooth amounts.
• 17:23
  Speaker 1: Radiation interacting with matter began revealing a strange graininess. The
• 17:28
  Speaker 1: problem was not that Maxwell's theory had suddenly become useless.
• 17:32
  Speaker 1: Far from it. Classical electromagnetism remained extraordinarily accurate across a
• 17:38
  Speaker 1: huge range of situations. The problem was that the classical
• 17:42
  Speaker 1: theory was not the whole story. Nature seemed to contain
• 17:46
  Speaker 1: both continuity and discreteness. Fields could spread through space-like waves,
• 17:52
  Speaker 1: yet interaction sometimes seemed to involve distinct packets of energy.
• 17:56
  Speaker 1: That tension eventually became one of the central pressures driving
• 18:00
  Speaker 1: the development of quantum physics. And it created a very
• 18:03
  Speaker 1: specific question. If a field is something that can exist
• 18:07
  Speaker 1: throughout space, carry energy, change with time and support travelling waves,
• 18:13
  Speaker 1: what happens when the field itself is required to obey
• 18:16
  Speaker 1: the rules of quantum mechanics? That is the essential step
• 18:20
  Speaker 1: behind the phrase quantum field. The field does not disappear.
• 18:26
  Speaker 1: Its ability to propagate does not disappear. Its ability to
• 18:30
  Speaker 1: carry energy and momentum does not disappear. Instead, the field
• 18:36
  Speaker 1: is given a quantum description. The result is not merely
• 18:39
  Speaker 1: a particle theory with a decorative field added around it.
• 18:43
  Speaker 1: It is a theory in which the field itself becomes
• 18:46
  Speaker 1: quantum mechanical. And that idea only makes sense because physics
• 18:50
  Speaker 1: had already spent much of the 19th century learning that
• 18:53
  Speaker 1: fields were real, dynamic, and capable of carrying physical influence
• 18:59
  Speaker 1: across space. At the beginning of the 20th century, physics
• 19:03
  Speaker 1: was in an awkward position. The great theories of the
• 19:07
  Speaker 1: 19th century had worked remarkably well. Newtonian mechanics described motion.
• 19:13
  Speaker 1: Thermodynamics described heat. James Clerk Maxwell's electromagnetic theory explained electricity,
• 19:20
  Speaker 1: magnetism and light. Yet when physicists examined matter and radiation
• 19:25
  Speaker 1: more closely, some experiments simply refuse to fit the classical picture.
• 19:31
  Speaker 1: One of the first serious cracks appeared in the study
• 19:33
  Speaker 1: of blackbody radiation. A blackbody is an idealised object that
• 19:38
  Speaker 1: absorbs incoming electromagnetic radiation and, depending on its temperature, emits
• 19:45
  Speaker 1: radiation with a characteristic spectrum. Heated metal provides an appropriate
• 19:50
  Speaker 1: everyday example. As its temperature rises, it begins glowing red,
• 19:56
  Speaker 1: then orange, and eventually much brighter colours. By the late
• 20:00
  Speaker 1: 19th century, physicists could measure this emitted radiation very accurately.
• 20:05
  Speaker 1: The problem was explaining it. Classical physics predicted that a
• 20:09
  Speaker 1: hot object should emit increasingly enormous amounts of energy at
• 20:14
  Speaker 1: very short wavelengths. Taken literally, the predicted energy would grow
• 20:19
  Speaker 1: without limit. Nature did nothing of the sort. In 1900,
• 20:24
  Speaker 1: the German physicist Max Planck found a mathematical solution. He
• 20:28
  Speaker 1: proposed that matter could exchange energy with electromagnetic radiation only
• 20:33
  Speaker 1: in discrete amounts. These amounts depended on the frequency of
• 20:37
  Speaker 1: the radiation. The essential relationship can be spoken simply as
• 20:42
  Speaker 1: the energy of one quantum equals Planck's constant multiplied by
• 20:45
  Speaker 1: the frequency. This was a radical departure from the classical picture.
• 20:51
  Speaker 1: Energy had generally been treated as something continuously divisible. In principle,
• 20:56
  Speaker 1: you could imagine transferring any amount you liked, however small.
• 21:01
  Speaker 1: Planck's proposal instead introduced a certain graininess into the exchange
• 21:05
  Speaker 1: of energy. At first, even Planck did not necessarily regard
• 21:10
  Speaker 1: this as a declaration that nature itself was fundamentally quantist
• 21:14
  Speaker 1: in the modern sense. It was an ingenious way of
• 21:17
  Speaker 1: reproducing the observed spectrum. But the idea refused to remain
• 21:21
  Speaker 1: merely mathematical. Five years later, Albert Einstein pushed it much further.
• 21:27
  Speaker 1: In 1905… Einstein examined the photoelectric effect in which light
• 21:33
  Speaker 1: striking a material can eject electrons from its surface. Classical
• 21:38
  Speaker 1: wave theory led to a reasonable expectation. If light is
• 21:42
  Speaker 1: a continuous electromagnetic wave then increasing its intensity should deliver
• 21:47
  Speaker 1: more energy to the material eventually allowing electrons to escape
• 21:51
  Speaker 1: regardless of the light's frequency. Experiments told a stranger story.
• 21:57
  Speaker 1: Below a certain frequency Increasing the intensity did not release
• 22:02
  Speaker 1: electrons in the expected way. Above that threshold, electrons could
• 22:08
  Speaker 1: be emitted, and their maximum kinetic energy depended primarily on
• 22:12
  Speaker 1: the light's frequency rather than simply on its brightness. Einstein
• 22:17
  Speaker 1: proposed that light interacted with matter in discrete packets of energy.
• 22:21
  Speaker 1: Higher frequency light carried more energy in each packet. Increasing
• 22:26
  Speaker 1: the brightness mainly increased the number of packets arriving. while
• 22:30
  Speaker 1: changing the frequency changed the energy carried by each one.
• 22:34
  Speaker 1: These light quanta would later become known as photons. This
• 22:38
  Speaker 1: created a wonderfully uncomfortable situation. For decades, evidence had shown
• 22:43
  Speaker 1: that light behaved like a wave. Light could interfere with itself.
• 22:48
  Speaker 1: It could diffract around obstacles. Maxwell's theory described it beautifully
• 22:52
  Speaker 1: as an electromagnetic wave. And yet, when light exchanged energy
• 22:57
  Speaker 1: with matter … it sometimes behaved as though that energy
• 23:01
  Speaker 1: arrived in individual units. Light had not stopped being wave-like,
• 23:06
  Speaker 1: nor could it simply be declared an ordinary classical particle.
• 23:10
  Speaker 1: It appeared to possess properties that belonged to both categories,
• 23:14
  Speaker 1: depending on how it was prepared and measured. Then the
• 23:18
  Speaker 1: difficulty became even more symmetrical. In 1924, the French physicist
• 23:24
  Speaker 1: Louis de Broglie proposed that matter itself might possess wave-like characteristics.
• 23:29
  Speaker 1: If light, traditionally considered a wave, could exhibit particle-like behaviour,
• 23:35
  Speaker 1: perhaps particles such as electrons could display wave-like behaviour as well.
• 23:41
  Speaker 1: The proposal sounded bold, but experiments soon supported it. In
• 23:44
  Speaker 1: the late 1920s, electron diffraction experiments associated with Clinton-Davison and Lester-Germa,
• 23:51
  Speaker 1: and independently with George Paget-Thompson, showed that beams of electrons
• 23:55
  Speaker 1: could produce diffraction patterns. Electrons could behave in ways strikingly
• 24:00
  Speaker 1: similar to waves passing through a crystal structure. The comfortable
• 24:05
  Speaker 1: 19th century division between particles and waves was collapsing from
• 24:08
  Speaker 1: both directions. Light could behave like particles. Matter could behave
• 24:13
  Speaker 1: like waves. The solution was not to decide which label
• 24:17
  Speaker 1: nature secretly preferred. The deeper lesson was that the classical
• 24:21
  Speaker 1: categories themselves were incomplete. During the 1920s, a new mathematical
• 24:26
  Speaker 1: framework began taking shape. Werner Heisenberg developed matrix mechanics, one
• 24:32
  Speaker 1: of the first complete formulations of quantum mechanics. It focused
• 24:36
  Speaker 1: directly on quantities connected with observable transitions, rather than trying
• 24:41
  Speaker 1: to preserve classical pictures of microscopic trajectories. Erwin Schrödinger developed
• 24:47
  Speaker 1: wave mechanics, described through what became known as the Schrödinger equations.
• 24:52
  Speaker 1: His formulation introduced a mathematical object called the wave function,
• 24:56
  Speaker 1: whose evolution could be calculated with extraordinary precision. At first glance,
• 25:01
  Speaker 1: the wave function looked temptingly like an ordinary physical wave.
• 25:06
  Speaker 1: But Max Born proposed a different interpretation. The wave function
• 25:10
  Speaker 1: was connected not simply to some material substance spreading through space,
• 25:14
  Speaker 1: but to probabilities. More precisely … Quantum mechanics allows the
• 25:19
  Speaker 1: quantum state to determine probabilities for the possible results of measurements.
• 25:23
  Speaker 1: This is a crucial distinction. Suppose you do not know
• 25:27
  Speaker 1: where an ordinary hidden coin has landed. There is a
• 25:29
  Speaker 1: difference between saying that the coin definitely shows heads or tails,
• 25:34
  Speaker 1: and you simply have not looked, and saying that the
• 25:37
  Speaker 1: physical theory itself assigns a quantum state from which probabilities
• 25:41
  Speaker 1: for measurement outcomes are calculated. Quantum uncertainty is not merely
• 25:46
  Speaker 1: ordinary ignorance disguised with more complicated mathematics. The structure of
• 25:51
  Speaker 1: quantum theory is fundamentally probabilistic in the way it connects
• 25:55
  Speaker 1: states to possible measurement results. The word quantization gradually acquired
• 25:59
  Speaker 1: a much broader meaning as these ideas developed. It did
• 26:03
  Speaker 1: not simply refer to Planck's original proposal about radiation. Quantization
• 26:08
  Speaker 1: came to describe situations in which certain physical quantities, states
• 26:12
  Speaker 1: or excitations, appear in discrete units, rather than varying with
• 26:17
  Speaker 1: complete classical continuity. An atom, for example, does not allow
• 26:21
  Speaker 1: an electron bound within it to possess just any energy whatsoever.
• 26:26
  Speaker 1: Particular quantum states have particular allowed energies. Electromagnetic radiation can
• 26:32
  Speaker 1: exchange energy in photon-sized units. Yet this discreteness does not
• 26:36
  Speaker 1: mean that everything in the theory becomes a collection of
• 26:39
  Speaker 1: tiny blocks. A quantum wave function can extend smoothly through space.
• 26:44
  Speaker 1: A quantum field can be defined throughout space. Quantization concerns
• 26:48
  Speaker 1: the structure of the allowed quantum states and excitations, not
• 26:52
  Speaker 1: necessarily a claim that space itself has been chopped into
• 26:55
  Speaker 1: microscopic cubes. The new quantum mechanics quickly became spectacularly successful.
• 27:00
  Speaker 1: It explained the structure of atoms, atomic spectra and chemical
• 27:04
  Speaker 1: behaviour with a precision classical physics could never provide. But
• 27:09
  Speaker 1: an important problem remained. The original formulations worked most naturally
• 27:13
  Speaker 1: in situations where particles were moving much more slowly than light,
• 27:17
  Speaker 1: and where the number of particles could effectively be treated
• 27:20
  Speaker 1: as fixed. That is perfectly adequate for a great deal
• 27:24
  Speaker 1: of atomic physics. It is not adequate for the whole universe.
• 27:28
  Speaker 1: Special relativity allows energy to be converted into matter, and
• 27:32
  Speaker 1: matter into other forms of energy. At sufficiently high energies,
• 27:36
  Speaker 1: particles can be created. Existing particles can annihilate. Entirely different
• 27:42
  Speaker 1: sets of particles can emerge from a collision, a theory
• 27:45
  Speaker 1: that assumes from the beginning that one particular particle simply
• 27:49
  Speaker 1: exists forever, cannot naturally describe every such possibility. Physics therefore
• 27:54
  Speaker 1: needed something broader. It needed a framework capable of preserving
• 27:59
  Speaker 1: the wave-like behaviour revealed by quantum mechanics, the discrete particle-like
• 28:04
  Speaker 1: behaviour revealed in experiments, and the relativistic possibility that particles
• 28:08
  Speaker 1: can appear and disappear. Quantum field theory would eventually provide
• 28:13
  Speaker 1: that framework. Instead of treating waves and particles as two
• 28:17
  Speaker 1: separate stories that nature somehow alternates between, the field itself
• 28:21
  Speaker 1: could be quantum mechanical. The wave-like behaviour belongs naturally to
• 28:26
  Speaker 1: the field. The particle-like behaviour appears through quantist excitations of
• 28:30
  Speaker 1: that field. What had looked like a contradiction begins to
• 28:34
  Speaker 1: resemble two different ways of encountering the same underlying quantum structure.
• 28:39
  Speaker 1: The new quantum mechanics of the 1920s was extraordinarily successful.
• 28:44
  Speaker 1: It could explain atomic spectra, chemical bonds and the behaviour
• 28:48
  Speaker 1: of electrons inside atoms with a precision that classical physics
• 28:52
  Speaker 1: had never approached. But there was an important restriction hiding
• 28:56
  Speaker 1: beneath that success. Most of this early quantum mechanics was
• 29:00
  Speaker 1: not fully compatible with Albert Einstein's special theory of relativity.
• 29:04
  Speaker 1: Special relativity had been introduced in 1905 and had fundamentally
• 29:09
  Speaker 1: changed the relationship between space, time, energy and motion. One
• 29:14
  Speaker 1: of its central requirements is that the laws of physics
• 29:17
  Speaker 1: should take the same basic form for observers moving uniformly
• 29:20
  Speaker 1: relative to one another. There is no special state of
• 29:23
  Speaker 1: absolute rest against which everything else in the universe can
• 29:27
  Speaker 1: be measured. Relativity also establishes a causal speed limit. Light
• 29:32
  Speaker 1: in a vacuum travels at approximately 300,000 km per second,
• 29:37
  Speaker 1: and physical influences cannot simply communicate information arbitrarily quickly across space.
• 29:43
  Speaker 1: For slow-moving particles, ordinary non-relativistic quantum mechanics works beautifully. An
• 29:49
  Speaker 1: electron bound inside an atom can often be treated without
• 29:52
  Speaker 1: worrying about the full machinery of relativity. But nature does
• 29:56
  Speaker 1: not promise that particles will always remain slow, In high
• 30:00
  Speaker 1: energy collisions, particles can approach the speed of light. Under
• 30:04
  Speaker 1: those conditions, quantum mechanics and special relativity both have to
• 30:08
  Speaker 1: be respected simultaneously. And relativity introduces an even deeper complication.
• 30:14
  Speaker 1: Einstein's famous relationship between mass and energy tells us that
• 30:18
  Speaker 1: mass is a form of energy. Sufficient energy concentrated into
• 30:23
  Speaker 1: an interaction can therefore contribute to producing new massive particles
• 30:27
  Speaker 1: Provided the necessary conservation laws and quantum rules are satisfied,
• 30:32
  Speaker 1: the reverse process can happen as well. A particle and
• 30:36
  Speaker 1: its corresponding antiparticle can annihilate with their energy appearing in
• 30:40
  Speaker 1: other particles or radiation. An electron and a positron, for example,
• 30:45
  Speaker 1: can annihilate into photons. So particle number is not a
• 30:49
  Speaker 1: permanent feature of nature. Imagine trying to describe a collision
• 30:53
  Speaker 1: while insisting from the beginning that exactly two particles must
• 30:56
  Speaker 1: exist for the entire calculation. That may work perfectly well
• 31:01
  Speaker 1: if the physical process genuinely contains two particles throughout, but
• 31:05
  Speaker 1: it becomes inadequate if those two particles collide and produce
• 31:08
  Speaker 1: four particles, or annihilate and produce photons, or participate in
• 31:13
  Speaker 1: some other reaction where the identities and numbers of particles change.
• 31:18
  Speaker 1: A deeper theory had to be capable of describing those
• 31:21
  Speaker 1: possibilities naturally. One of the decisive steps came from the
• 31:25
  Speaker 1: English theoretical physicist Paul Dirac. In 1928, Dirac introduced an
• 31:31
  Speaker 1: equation describing electrons in a way that incorporated both quantum
• 31:34
  Speaker 1: mechanics and special relativity. The Dirac equation was an extraordinary achievement.
• 31:40
  Speaker 1: It correctly described electrons as particles with an intrinsic quantum
• 31:44
  Speaker 1: property called spin. In particular, electrons possess spin one-half, a
• 31:50
  Speaker 1: type of angular momentum that has no exact classical equivalent.
• 31:55
  Speaker 1: The equation also helped explain subtle details in the energy
• 31:58
  Speaker 1: levels of hydrogen, but it contained something unexpected. The mathematics
• 32:03
  Speaker 1: admitted solutions that initially appeared associated with negative energies. These
• 32:09
  Speaker 1: were not easy to interpret using the familiar picture of
• 32:11
  Speaker 1: a single electron moving through space. Dirac eventually connected the
• 32:16
  Speaker 1: structure of his theory with the possibility of an entirely
• 32:19
  Speaker 1: new kind of matter, antimatter. For the electron, the corresponding
• 32:24
  Speaker 1: antiparticle is the positron. A positron has the same mass
• 32:28
  Speaker 1: as an electron, but carries positive rather than negative electric charge.
• 32:33
  Speaker 1: That prediction soon acquired experimental support. In 1932, the American
• 32:39
  Speaker 1: physicist Carl Anderson was studying cosmic rays using a cloud
• 32:43
  Speaker 1: chamber when he observed the track of a particle behaving
• 32:46
  Speaker 1: like an electron with the opposite electric charge. The positron
• 32:50
  Speaker 1: had been discovered. Antimatter was not merely a mathematical curiosity.
• 32:55
  Speaker 1: Relativistic quantum physics seemed to be telling us that particles
• 32:59
  Speaker 1: and antiparticles belonged naturally within the same underlying framework. At
• 33:04
  Speaker 1: roughly the same period, another important distinction was becoming clear.
• 33:09
  Speaker 1: In 1925, the Austrian physicist Wolfgang Pauli formulated what became
• 33:14
  Speaker 1: known as the Pearly Exclusion Principle. It states that identical
• 33:18
  Speaker 1: fermions cannot occupy the same quantum states simultaneously. Electrons are fermions,
• 33:25
  Speaker 1: so are quarks and neutrinos. This rule is one reason
• 33:29
  Speaker 1: atoms possess their familiar structure. Electrons cannot all simply collapse
• 33:34
  Speaker 1: into the same lowest available state. Instead, they occupy different
• 33:38
  Speaker 1: allowed quantum states simultaneously. helping to produce the shells and
• 33:41
  Speaker 1: chemical behaviour that ultimately give ordinary matter its structure. Particles
• 33:46
  Speaker 1: known as bosons behave differently. Photons are bosons. Unlike fermions,
• 33:52
  Speaker 1: identical bosons are permitted to occupy the same quantum state
• 33:55
  Speaker 1: in large numbers. This distinction between fermions and bosons is
• 34:00
  Speaker 1: not merely a convenient classification. It is deeply embedded in
• 34:04
  Speaker 1: relativistic quantum field theory. and connected with the particle's spin
• 34:09
  Speaker 1: through what is known as the spin-statistics relationship. The more
• 34:13
  Speaker 1: physicists tried to reconcile quantum mechanics with relativity, the more
• 34:17
  Speaker 1: difficult it became to preserve the image of permanent little
• 34:21
  Speaker 1: particles simply following quantum versions of ordinary trajectories. Relativity also
• 34:27
  Speaker 1: demands careful respect for causality. If something happens at one location,
• 34:32
  Speaker 1: it cannot instantaneously produce an observable causal effect arbitrarily far away.
• 34:39
  Speaker 1: The speed of light limits how quickly information or physical
• 34:42
  Speaker 1: influence can propagate. This requirement is closely related to locality.
• 34:48
  Speaker 1: A local physical theory describes interactions through quantities associated with
• 34:53
  Speaker 1: particular places and times, rather than allowing controllable influences to
• 34:58
  Speaker 1: leap instantaneously across arbitrary distances. Quantum mechanics famously contains correlations
• 35:05
  Speaker 1: that can stretch across enormous separations, particularly in quantum entanglement,
• 35:11
  Speaker 1: but those correlations cannot be used to send usable information
• 35:14
  Speaker 1: faster than light. Relativistic quantum field theory is constructed so
• 35:19
  Speaker 1: that measurable causal influences remain consistent with special relativity. The
• 35:24
  Speaker 1: field picture turns out to be remarkably well suited to
• 35:27
  Speaker 1: all of this. Instead of beginning with a permanent electron
• 35:31
  Speaker 1: and asking where that particular little object travels forever, we
• 35:35
  Speaker 1: begin with an electron field. The electron field persists as
• 35:40
  Speaker 1: part of the physical description. Its quantum state can correspond
• 35:44
  Speaker 1: to no electron excitations in a particular situation. Another state
• 35:48
  Speaker 1: can correspond to one electron. Another can contain several electrons.
• 35:53
  Speaker 1: States containing positrons are included within the same relativistic field framework.
• 35:59
  Speaker 1: The number of particle excitations is allowed to change. That
• 36:03
  Speaker 1: is exactly what high-energy physics requires. A collision can begin
• 36:07
  Speaker 1: with one collection of particles and finish with a different collection,
• 36:12
  Speaker 1: because the underlying fields can exchange energy and momentum and
• 36:15
  Speaker 1: move between different quantum states. Particles may be created. Particles
• 36:20
  Speaker 1: may be annihilated. Their identities may change through interactions permitted
• 36:25
  Speaker 1: by the theory. The underlying quantum fields remain the entities
• 36:29
  Speaker 1: from which all of those possible particle states arise. This
• 36:33
  Speaker 1: is why quantum field theory was not invented simply because
• 36:37
  Speaker 1: physicists fancied a more abstract way of describing reality. Ordinary
• 36:41
  Speaker 1: quantum mechanics had worked wonderfully within its appropriate domain, and
• 36:46
  Speaker 1: it continues to do so. But relativity forced the theory
• 36:50
  Speaker 1: into territory, where a fixed number of permanent particles was
• 36:54
  Speaker 1: no longer enough. Antimatter existed. Particles could be created and destroyed.
• 37:01
  Speaker 1: Causal influences still had to respect the speed of light.
• 37:04
  Speaker 1: Quantum statistics had to distinguish fermions from bosons. A successful
• 37:10
  Speaker 1: theory had to accommodate all of these facts at once.
• 37:13
  Speaker 1: Quantum field theory did exactly that by changing the starting point.
• 37:18
  Speaker 1: Instead of treating particles as the permanent foundations of the theory,
• 37:22
  Speaker 1: It treated relativistic quantum fields as fundamental and allowed particles
• 37:27
  Speaker 1: to emerge as particular excitations of those fields. The particle
• 37:31
  Speaker 1: could appear. It could disappear. It could meet its antiparticle
• 37:35
  Speaker 1: and annihilate. But the field remained. The phrase quantising a
• 37:40
  Speaker 1: field can sound as though physicists have taken something already
• 37:43
  Speaker 1: rather abstract and decided to make it considerably less friendly.
• 37:48
  Speaker 1: But the central idea is surprisingly approachable. Start with an
• 37:52
  Speaker 1: ordinary classical field. Such a field can support different patterns
• 37:56
  Speaker 1: of motion or oscillation. The electromagnetic field, for example, can
• 38:01
  Speaker 1: carry waves with different wavelengths, frequencies and directions of travel.
• 38:07
  Speaker 1: In classical physics, each possible wave pattern can possess essentially
• 38:11
  Speaker 1: any amount of energy. You can imagine separating a complicated
• 38:15
  Speaker 1: wave into simpler patterns. A vibrating guitar string provides a
• 38:19
  Speaker 1: useful comparison. The string can vibrate in different modes. One
• 38:24
  Speaker 1: mode might involve the whole string moving in a broad arc.
• 38:28
  Speaker 1: Higher modes contain additional nodes and shorter wavelengths. A complicated
• 38:33
  Speaker 1: vibration can be understood as a combination of these simpler patterns.
• 38:37
  Speaker 1: A field works in a mathematically richer way, but a
• 38:40
  Speaker 1: similar idea applies. The field can be decomposed into modes.
• 38:45
  Speaker 1: Each mode represents a particular way in which the field
• 38:48
  Speaker 1: can vary through space and time. For a free field,
• 38:51
  Speaker 1: those modes can be associated with particular wavelengths, frequencies and momenta. Classically,
• 38:57
  Speaker 1: the strength of each mode can vary continuously. Quantum theory
• 39:02
  Speaker 1: changes that. The essential mathematical model is something called the
• 39:06
  Speaker 1: quantum harmonic oscillator. A classical harmonic oscillator is one of
• 39:11
  Speaker 1: the simplest systems in physics. A mass attached to a
• 39:14
  Speaker 1: spring is the usual example. Pull the mass away from
• 39:18
  Speaker 1: equilibrium and release it. and it moves backwards and forwards. Classically,
• 39:22
  Speaker 1: the oscillator can possess any amount of energy you care
• 39:25
  Speaker 1: to give it. A quantum harmonic oscillator behaves different Its
• 39:31
  Speaker 1: allowed energy levels come in discrete steps. There is a
• 39:34
  Speaker 1: lowest possible energy state, called the ground state. Above that,
• 39:39
  Speaker 1: a progressively higher level separated by fixed amounts determined by
• 39:43
  Speaker 1: the oscillator's frequency. The relationship can be expressed conceptually as
• 39:48
  Speaker 1: the energy levels increase in steps proportional to Planck's constant
• 39:52
  Speaker 1: multiplied by the oscillator frequency. The important part is not
• 39:56
  Speaker 1: the algebra, it is the staircase. Instead of energy being
• 40:01
  Speaker 1: able to slide smoothly through every imaginable value, the quantum
• 40:05
  Speaker 1: oscillator moves between distinct allowed levels. When physicists quantise a
• 40:10
  Speaker 1: free field, something remarkable happens. Each independent mode of the
• 40:15
  Speaker 1: field behaves mathematically rather like one of these quantum harmonic oscillators.
• 40:20
  Speaker 1: A field therefore becomes comparable to an enormous collection of
• 40:24
  Speaker 1: quantum oscillators, with different modes corresponding to different possible patterns
• 40:29
  Speaker 1: of the field. Suppose one particular mode is in its
• 40:33
  Speaker 1: lowest energy state. There are no particle excitations associated with
• 40:38
  Speaker 1: that mode. Raise that mode by one quantum of excitation.
• 40:43
  Speaker 1: and the resulting state can correspond to one particle carrying
• 40:46
  Speaker 1: the energy and momentum associated with that mode. Raise it again,
• 40:51
  Speaker 1: where the type of field permits this, and there can
• 40:54
  Speaker 1: be two particles in that same mode. Another step can
• 40:57
  Speaker 1: correspond to three. This is where the familiar particle picture
• 41:01
  Speaker 1: emerges from the mathematics of a field. A particle is
• 41:05
  Speaker 1: not inserted into the theory as a tiny object separate
• 41:08
  Speaker 1: from the field. It appears as a quantum of excitation
• 41:12
  Speaker 1: of the field itself. For the electromagnetic field, those quanta
• 41:16
  Speaker 1: are photons. A particular electromagnetic mode can contain no photons,
• 41:22
  Speaker 1: one photon, two photons, or many photons. Changing the excitation
• 41:27
  Speaker 1: level changes the number of photons associated with that mode.
• 41:31
  Speaker 1: Physicists have an elegant mathematical language for describing this. Creation
• 41:36
  Speaker 1: operators are mathematical operations that increase the number of quanta
• 41:40
  Speaker 1: in an appropriate field state. Annihilation operators decrease it. Despite
• 41:47
  Speaker 1: the names, it is worth resisting the temptation to picture
• 41:50
  Speaker 1: some microscopic machine stamping particles into existence or plucking them
• 41:55
  Speaker 1: out of space. Creation and annihilation operators are mathematical tools.
• 42:00
  Speaker 1: They act on quantum states. If a state describes a
• 42:03
  Speaker 1: certain number of photons applying the appropriate creation, operator produces
• 42:08
  Speaker 1: a new state corresponding to one additional photon in a
• 42:12
  Speaker 1: specified mode. An annihilation operator does the reverse, provided there
• 42:17
  Speaker 1: is an excitation available to remove. This allows changing particle
• 42:21
  Speaker 1: number to be built directly into the formalism. The states
• 42:25
  Speaker 1: themselves can be organised using what are known as Fox states.
• 42:29
  Speaker 1: A Fock state specifies how many particles occupy particular quantum modes.
• 42:35
  Speaker 1: For a simple example, one state might describe zero photons
• 42:39
  Speaker 1: in a chosen mode, another describes one, another two, another
• 42:43
  Speaker 1: ten thousand. The underlying electromagnetic field is the same field
• 42:48
  Speaker 1: in every case. What changes is its quantum state. This
• 42:52
  Speaker 1: is one of the reasons quantum field theory handles particle
• 42:55
  Speaker 1: creation and destruction so naturally. Different particle numbers are simply
• 43:00
  Speaker 1: different possible states of the field. There is, however, an
• 43:04
  Speaker 1: important distinction depending on the type of particle involved. Photons
• 43:08
  Speaker 1: are bosons. Bosons can occupy the same quantum state in
• 43:12
  Speaker 1: unrestricted numbers. A single electromagnetic mode can therefore contain many photons.
• 43:18
  Speaker 1: This ability is central to phenomena such as laser light,
• 43:22
  Speaker 1: where large numbers of photons can populate closely related quantum states.
• 43:27
  Speaker 1: Electrons behave differently. Electrons are fermions. They obey the Pauli
• 43:32
  Speaker 1: exclusion principle, named after Wolfgang Pauli, which prevents identical fermions
• 43:38
  Speaker 1: from occupying the same complete quantum state. So an electron
• 43:42
  Speaker 1: field cannot be excited in precisely the same way as
• 43:45
  Speaker 1: a bosonic field. For a particular fermionic mode, the occupancy
• 43:49
  Speaker 1: is restricted. The basic distinction between bosons and fermions is
• 43:53
  Speaker 1: therefore built directly into the structure of their quantum fields.
• 43:57
  Speaker 1: Photons arise from a bosonic field, electrons arise from a
• 44:01
  Speaker 1: fermionic field. Quarks are also fermions. The statistical behaviour of
• 44:07
  Speaker 1: particles is not something added later as an optional rule.
• 44:11
  Speaker 1: It follows from the mathematical character of the fields and,
• 44:15
  Speaker 1: in relativistic quantum field theory, is deeply connected with particle spin.
• 44:20
  Speaker 1: At this point, one misconception is especially worth avoiding. Quantising
• 44:25
  Speaker 1: a field does not mean chopping ordinary space into tiny cubes.
• 44:29
  Speaker 1: The word quantum sometimes encourages the idea that absolutely everything
• 44:33
  Speaker 1: must come in little blocks. But quantum field theory does
• 44:36
  Speaker 1: not by itself say that space is made from discrete pixels.
• 44:41
  Speaker 1: A quantum field can still be defined continuously throughout spacetime.
• 44:45
  Speaker 1: What becomes quantised are the allowed states and excitations of
• 44:49
  Speaker 1: that field. The field can extend smoothly across space. while
• 44:54
  Speaker 1: its excitations display discrete particle-like behavior. There are several mathematical
• 45:00
  Speaker 1: ways of constructing this quantum description. One traditional approach is
• 45:04
  Speaker 1: called canonical quantization. Very roughly, canonical quantization begins with a
• 45:10
  Speaker 1: classical field theory and replaces certain classical quantities with quantum
• 45:15
  Speaker 1: operators obeying specific mathematical relations. It resembles the procedure used
• 45:20
  Speaker 1: to move from classical mechanics to ordinary quantum mechanics, except
• 45:25
  Speaker 1: now the object being quantized is a field rather than
• 45:29
  Speaker 1: the position and momentum of one particle. There is also
• 45:32
  Speaker 1: another powerful formulation associated especially with Richard Feynman. This is
• 45:37
  Speaker 1: the path integral approach. In ordinary quantum mechanics, the path
• 45:42
  Speaker 1: integral describes a process by considering contributions associated with all
• 45:47
  Speaker 1: possible paths connecting an initial situation with a final one.
• 45:52
  Speaker 1: In quantum field theory, the idea becomes broader. Rather than
• 45:56
  Speaker 1: summing over ordinary particle trajectories alone, the mathematical formulation considers
• 46:02
  Speaker 1: possible field configurations. Different configurations contribute quantum amplitudes, and those
• 46:09
  Speaker 1: amplitudes combine according to precise rules. Canonical quantization and the
• 46:15
  Speaker 1: path integral formulation may look very different on paper, but
• 46:18
  Speaker 1: in situations where both are applicable, They provide equivalent descriptions
• 46:23
  Speaker 1: of the same underlying physics. Neither method is particularly gentle
• 46:28
  Speaker 1: once the mathematics become serious. Interacting quantum fields can require
• 46:33
  Speaker 1: enormous technical sophistication. Real calculations involve symmetries, operators, propagators, perturbation theory,
• 46:42
  Speaker 1: and a considerable amount of mathematics that tends not to
• 46:45
  Speaker 1: improve anyone's chances of falling asleep. but the physical idea
• 46:50
  Speaker 1: beneath that machinery remains wonderfully simple. A classical field possesses
• 46:55
  Speaker 1: possible modes of oscillation. Quantum mechanics turns those modes into
• 46:59
  Speaker 1: quantum systems with discrete excitation structures. The lowest state is
• 47:04
  Speaker 1: the ground state. Adding an excitation can correspond to adding
• 47:08
  Speaker 1: a particle. Removing an excitation can correspond to removing one.
• 47:13
  Speaker 1: The field remains. Its quantum state changes. And from those
• 47:17
  Speaker 1: changes emerge the particles that detectors see as individual electrons,
• 47:22
  Speaker 1: photons and other quanta. So quantising a field does not
• 47:26
  Speaker 1: mean replacing a smooth universe with a pile of microscopic
• 47:29
  Speaker 1: building blocks. It means taking something already capable of vibrating,
• 47:34
  Speaker 1: propagating and carrying energy through space, and allowing it to
• 47:38
  Speaker 1: obey quantum rules. Once that is done, particles no longer
• 47:42
  Speaker 1: need to be placed into the theory separately. They appear
• 47:46
  Speaker 1: naturally as the countable excitations of the fields themselves. A
• 47:50
  Speaker 1: particle sounds like a wonderfully simple thing. We tend to
• 47:54
  Speaker 1: imagine something small, compact and definite. A grain of sand
• 47:59
  Speaker 1: is a particle. A speck of dust is a particle.
• 48:03
  Speaker 1: Shrink that picture far enough and it seems natural to
• 48:07
  Speaker 1: imagine an electron or photon as an impossibly tiny bead
• 48:11
  Speaker 1: moving from one location to another. Quantum field theory replaces
• 48:16
  Speaker 1: that picture with something subtler. In this framework, a particle
• 48:20
  Speaker 1: is best understood as a quantumist excitation of an underlying
• 48:24
  Speaker 1: quantum field. An electron is an excitation of the electron field.
• 48:29
  Speaker 1: A photon is an excitation of the electromagnetic field. Other
• 48:34
  Speaker 1: fundamental particles are associated with their own quantum fields. This
• 48:39
  Speaker 1: does not mean that the familiar particle concept becomes useless.
• 48:43
  Speaker 1: Far from it. Detectors register particles as individual events. Particle
• 48:49
  Speaker 1: accelerators count collisions. Photons arrive one by one at sensitive detectors.
• 48:55
  Speaker 1: Electrons leave distinct tracks or deposits of energy. But the
• 48:59
  Speaker 1: underlying description is not quite that of a tiny permanent
• 49:03
  Speaker 1: ball carrying its identity through space. Consider the photon. The
• 49:08
  Speaker 1: electromagnetic field extends throughout space. It can possess different quantum states.
• 49:14
  Speaker 1: One particular kind of excitation of that field is what
• 49:17
  Speaker 1: we call a photon. A photon can carry a certain
• 49:21
  Speaker 1: amount of energy and momentum. It can be emitted by
• 49:24
  Speaker 1: an atom, absorbed by matter or scattered during an interaction.
• 49:28
  Speaker 1: But the photon is not imagined as a little lump
• 49:31
  Speaker 1: of electromagnetic substance separated from the field itself. It is
• 49:35
  Speaker 1: a quantum state of the field. The same basic idea
• 49:39
  Speaker 1: applies to electrons. The electron field exists throughout spacetime, and
• 49:44
  Speaker 1: an electron is an excitation of that field with a
• 49:47
  Speaker 1: very specific set of properties. Every electron ever measured has
• 49:51
  Speaker 1: the same electric charge. Every electron has the same rest mass.
• 49:56
  Speaker 1: Every electron has the same intrinsic spin. This remarkable uniformity
• 50:01
  Speaker 1: is one of the most striking features of particle physics.
• 50:05
  Speaker 1: If electrons were tiny independent objects manufactured separately throughout the universe,
• 50:10
  Speaker 1: their perfect similarity might seem rather surprising. Quantum field theory
• 50:15
  Speaker 1: gives us a natural way to understand it. Electrons are
• 50:19
  Speaker 1: not individually assembled objects with slightly different internal ingredients. They
• 50:24
  Speaker 1: are excitations of the same electron field. That field has
• 50:28
  Speaker 1: particular properties built into the theory, and every electron excitation
• 50:32
  Speaker 1: inherits them. An electron in your body and an electron
• 50:36
  Speaker 1: in a distant galaxy are not copies in the ordinary
• 50:39
  Speaker 1: manufactured sense. They are excitations of the same kind of field,
• 50:44
  Speaker 1: obeying the same rules. This is one reason physicists regard
• 50:48
  Speaker 1: quantum fields as more fundamental than the particles associated with them.
• 50:53
  Speaker 1: The field provides the identity. The particle is one way
• 50:57
  Speaker 1: that a field can be excited. Even so, particles often
• 51:01
  Speaker 1: appear localised. an electron can strike a detector at a
• 51:05
  Speaker 1: particular place. A photon can be absorbed by one atom
• 51:09
  Speaker 1: rather than another. How can that happen if the underlying
• 51:13
  Speaker 1: field extends throughout space? The answer involves quantum states known
• 51:18
  Speaker 1: as wave packets. A wave packet can be formed by
• 51:21
  Speaker 1: combining different momentum states in such a way that the
• 51:25
  Speaker 1: resulting excitation is concentrated within a particular region. It is
• 51:29
  Speaker 1: not infinitely localised. Quantum mechanics prevents us from assigning both
• 51:34
  Speaker 1: perfectly definite position and perfectly definite momentum to a particle,
• 51:40
  Speaker 1: but it can be localised well enough that, for many
• 51:42
  Speaker 1: practical purposes, it behaves like a particle travelling through a
• 51:46
  Speaker 1: particular region of space. This is where the particle picture
• 51:50
  Speaker 1: earns its keep. A carefully prepared electron state can move
• 51:54
  Speaker 1: through an experimental apparatus and produce a localised detection event.
• 51:59
  Speaker 1: The electron can possess momentum It can carry energy. It
• 52:04
  Speaker 1: can have a particular spin state. All of these properties
• 52:08
  Speaker 1: are encoded in the quantum state of the electron field.
• 52:11
  Speaker 1: So the field is extended, while a particular excitation can
• 52:15
  Speaker 1: behave in a strongly particle-like way. That combination is one
• 52:20
  Speaker 1: of the features that makes quantum field theory so powerful.
• 52:23
  Speaker 1: A useful analogy is a musical instrument. Imagine a piano
• 52:27
  Speaker 1: sitting silently in a room. The instrument itself exists whether
• 52:32
  Speaker 1: or not anyone is playing it. Strike one key and
• 52:35
  Speaker 1: a particular note appears. Strike another and a different note appears.
• 52:40
  Speaker 1: You would not normally say that the note was a
• 52:42
  Speaker 1: tiny object stored somewhere inside the piano waiting to escape.
• 52:47
  Speaker 1: The note is a particular excitation of the instrument. The
• 52:51
  Speaker 1: comparison is not perfect, but it points in the right direction.
• 52:55
  Speaker 1: A quantum field is somewhat like the underlying system. The
• 52:59
  Speaker 1: particle is somewhat like one of the allowed excitations. The
• 53:03
  Speaker 1: analogy breaks down if we push it too far. A
• 53:06
  Speaker 1: piano is made from wood, metal, felt and other materials.
• 53:11
  Speaker 1: Its vibrations ultimately depend on atoms and electromagnetic forces. A
• 53:17
  Speaker 1: fundamental quantum field is not known to be made from
• 53:20
  Speaker 1: some deeper ordinary substance. Within our best current theories, it
• 53:25
  Speaker 1: is already part of the fundamental descriptions. The field does
• 53:29
  Speaker 1: not need a hidden material beneath it in order to vibrate.
• 53:33
  Speaker 1: Its quantum states are simply what the theory describes. Another
• 53:37
  Speaker 1: important difference appears when particles interact. In ordinary life, objects
• 53:42
  Speaker 1: tend to preserve their identity. A tennis ball does not
• 53:46
  Speaker 1: normally vanish halfway across a court and turn into three
• 53:49
  Speaker 1: smaller tennis balls and a flash of light. At high energies,
• 53:53
  Speaker 1: particles are under no obligation to be so polite. Quantum
• 53:57
  Speaker 1: fields can exchange energy and momentum. Their excitations can appear
• 54:02
  Speaker 1: and disappear. A photon can be absorbed. A particle and
• 54:07
  Speaker 1: antiparticle can annihilate. New particles can be produced if an
• 54:11
  Speaker 1: interaction contains enough energy and satisfies the relevant conservation laws.
• 54:17
  Speaker 1: This means particle number is not always an absolute feature
• 54:21
  Speaker 1: of a relativistic quantum system. A state might begin with
• 54:25
  Speaker 1: two particle excitations and end with several different excitations. The
• 54:30
  Speaker 1: underlying fields provide a natural framework for that change. The
• 54:34
  Speaker 1: fields remain available, their states change, particle number changes with them.
• 54:40
  Speaker 1: This is much harder to describe if particles are treated
• 54:43
  Speaker 1: as permanently existing little objects that must somehow survive every interaction.
• 54:48
  Speaker 1: There is an even deeper wrinkle. The meaning of a
• 54:51
  Speaker 1: particle is not always completely universal. In the simple conditions
• 54:56
  Speaker 1: of laboratory particle physics, the particle concept works extremely well.
• 55:01
  Speaker 1: Physicists can prepare states containing electrons or photons and calculate
• 55:06
  Speaker 1: how those states evolve and interact. But in more unusual situations,
• 55:12
  Speaker 1: particularly involving accelerating observers or curved spacetimes, What counts as
• 55:17
  Speaker 1: a particle can become dependent on the observer and the
• 55:20
  Speaker 1: physical setting. An accelerating observer can describe a quantum field
• 55:25
  Speaker 1: differently from an inertial observer. Likewise, quantum fields around black
• 55:30
  Speaker 1: holes or in an expanding universe introduce complications that do
• 55:34
  Speaker 1: not arise in an ordinary laboratory. This does not mean
• 55:38
  Speaker 1: that particles are imaginary or arbitrary. It means that the
• 55:41
  Speaker 1: particle concept is derived from the deeper quantum field description
• 55:45
  Speaker 1: and derived concepts can depend on circumstances. The field is
• 55:50
  Speaker 1: the more fundamental object. The particle is a particularly useful
• 55:54
  Speaker 1: way of describing certain states of that field. And useful
• 55:58
  Speaker 1: does not mean unreal. A photon detector clicks. An electron
• 56:03
  Speaker 1: deposits energy. A particle accelerator records discrete events with measurable momenta,
• 56:09
  Speaker 1: charges and masses. These observations are real, reproducible and extraordinarily precise.
• 56:17
  Speaker 1: Quantum field theory does not ask us to pretend those
• 56:20
  Speaker 1: events never happened. It explains why they happen in the
• 56:24
  Speaker 1: form they do. A particle, then, is not best thought
• 56:28
  Speaker 1: of as a microscopic pebble moving through otherwise empty space.
• 56:33
  Speaker 1: It is a quantist excitation of a field. It can
• 56:36
  Speaker 1: be localised into a wave packet. It can carry energy
• 56:40
  Speaker 1: and momentum. It can be created or absorbed. and when
• 56:44
  Speaker 1: it interacts with a detector, it can produce a distinct,
• 56:47
  Speaker 1: localised event. The particle picture survives, but underneath it sits
• 56:53
  Speaker 1: something broader, a field that extends through space-time, carrying within
• 56:59
  Speaker 1: its possible quantum states everything required for that particle to
• 57:02
  Speaker 1: exist at all. Empty space sounds simple. Take away the air,
• 57:07
  Speaker 1: remove the dust, switch off the light, and keep clearing
• 57:11
  Speaker 1: things away until nothing remains. In everyday language, that would
• 57:15
  Speaker 1: seem to be a vacuum. Quantum field theory gives the
• 57:19
  Speaker 1: word a more precise meaning. A quantum vacuum is not
• 57:23
  Speaker 1: a place where the fields themselves have disappeared. It is
• 57:26
  Speaker 1: the lowest energy quantum state available to those fields. The
• 57:31
  Speaker 1: electron field is still present. The electromagnetic field is still present.
• 57:35
  Speaker 1: The other fundamental fields are still present. What has changed
• 57:40
  Speaker 1: is their state. If there are no ordinary particle excitations present,
• 57:45
  Speaker 1: the field can be in its vacuum state. This is
• 57:49
  Speaker 1: an important distinction, because quantum fields are not treated as
• 57:53
  Speaker 1: objects that only appear when particles are around. The field
• 57:57
  Speaker 1: is fundamental to the description. Removing all photons does not
• 58:02
  Speaker 1: remove the electromagnetic field. Removing all electrons does not remove
• 58:07
  Speaker 1: the electron field. The vacuum is therefore not the absence
• 58:11
  Speaker 1: of the quantum fields. It is their lowest energy configuration.
• 58:16
  Speaker 1: To understand why that lowest state is still interesting, it
• 58:20
  Speaker 1: helps to return to the quantum harmonic oscillator. A classical
• 58:24
  Speaker 1: oscillator can be imagined as something like a mass attached
• 58:28
  Speaker 1: to a spring. If the mass is perfectly still at
• 58:31
  Speaker 1: the equilibrium position, then in an idealised classical description the
• 58:36
  Speaker 1: oscillator can have its minimum possible energy. Nothing is moving.
• 58:41
  Speaker 1: Nothing is oscillating. The system simply sits there. A quantum
• 58:48
  Speaker 1: harmonic oscillator cannot behave in quite the same way. Its
• 58:52
  Speaker 1: energy comes in discrete allowed levels, and the lowest of
• 58:56
  Speaker 1: those is called the ground state. Crucially, the ground state
• 59:01
  Speaker 1: is not equivalent to a classical oscillator, possessing exactly zero
• 59:06
  Speaker 1: motion and perfectly definite physical quantities. Quantum uncertainty prevents that.
• 59:13
  Speaker 1: The lowest quantum state still possesses what is known as
• 59:16
  Speaker 1: zero point energy. For a harmonic oscillator this lowest energy
• 59:21
  Speaker 1: is proportional to its natural frequency. When a quantum field
• 59:25
  Speaker 1: is decomposed into its possible modes Each mode behaves mathematically
• 59:30
  Speaker 1: rather like a quantum oscillator. Each mode therefore has its
• 59:34
  Speaker 1: own ground state. Put all of those modes into their
• 59:37
  Speaker 1: lowest possible states and the result is the vacuum state
• 59:40
  Speaker 1: of the field. This is where the phrase vacuum fluctuations
• 59:45
  Speaker 1: often appears. Unfortunately, it is also where popular explanations sometimes
• 59:51
  Speaker 1: become rather too enthusiastic. You may have heard that empty
• 59:55
  Speaker 1: space is constantly filled with particles popping into existence from
• 59:59
  Speaker 1: nothing and then disappearing again before anyone notices. That picture
• 1:00:04
  Speaker 1: can be memorable. It is also dangerously easy to take
• 1:00:07
  Speaker 1: too literally. Vacuum fluctuations are better understood as quantum uncertainties
• 1:00:13
  Speaker 1: and correlations in the vacuum state of the field. Even
• 1:00:17
  Speaker 1: in its lowest energy state, A quantum field does not
• 1:00:21
  Speaker 1: behave like a classical field possessing perfectly fixed values everywhere.
• 1:00:26
  Speaker 1: The quantum state has a non-trivial structure. Certain quantities fluctuate
• 1:00:31
  Speaker 1: in the quantum sense, meaning repeated measurements are described by
• 1:00:35
  Speaker 1: probability distributions rather than by one perfectly fixed classical value.
• 1:00:41
  Speaker 1: This does not require us to imagine ordinary detectable particles
• 1:00:45
  Speaker 1: continually springing out of literal nothingness. The vacuum contains no
• 1:00:52
  Speaker 1: ordinary real particles according to the appropriate particle definition, yet
• 1:00:57
  Speaker 1: its quantum fields still possess ground-state structure. One of the
• 1:01:02
  Speaker 1: most famous phenomena associated with this idea is the Casimir effect.
• 1:01:08
  Speaker 1: In 1948, the Dutch physicist Hendrik Casimir predicted that two
• 1:01:13
  Speaker 1: closely spaced conducting plates could experience an attractive force arising
• 1:01:18
  Speaker 1: from quantum electromagnetic effects. Experiments have since measured forces consistent
• 1:01:25
  Speaker 1: with this prediction. A common explanation says that fewer vacuum
• 1:01:29
  Speaker 1: modes are allowed between the plates than outside them, producing
• 1:01:33
  Speaker 1: a difference in vacuum energy that pushes the plates together.
• 1:01:37
  Speaker 1: That picture can be useful, but it should not be
• 1:01:40
  Speaker 1: treated as the only possible interpretation. The Casimir effect can
• 1:01:45
  Speaker 1: also be described in terms of quantum electromagnetic interactions and
• 1:01:49
  Speaker 1: correlations involving the conducting materials. The important point is that
• 1:01:54
  Speaker 1: the geometry and boundary conditions alter the quantum electromagnetic system
• 1:01:58
  Speaker 1: in a measurable way. The effect becomes significant only at
• 1:02:02
  Speaker 1: very small separations. When conducting surfaces are separated by distances
• 1:02:08
  Speaker 1: on the scale of micrometers or less, the resulting forces
• 1:02:11
  Speaker 1: can become experimentally important. So the vacuum is not simply
• 1:02:15
  Speaker 1: a featureless stage on which physics happens. Its quantum state
• 1:02:20
  Speaker 1: can respond to physical boundaries. Another important example comes from
• 1:02:24
  Speaker 1: atomic physics. In the 1940s, the American physicist Willis Lamb
• 1:02:29
  Speaker 1: and his collaborators measured a tiny difference between energy levels
• 1:02:34
  Speaker 1: in hydrogen and that earlier versions of relativistic quantum theory
• 1:02:38
  Speaker 1: had predicted should have exactly the same energy. This became
• 1:02:43
  Speaker 1: known as the Lamb shift. Quantum electrodynamics explains the shift
• 1:02:48
  Speaker 1: through interactions between the electron and the quantus electromagnetic field.
• 1:02:54
  Speaker 1: The correction is small but measurable, and its successful calculation
• 1:02:58
  Speaker 1: became one of the major achievements of quantum electrodynamics. This
• 1:03:03
  Speaker 1: is another reminder that the quantum vacuum is not simply
• 1:03:07
  Speaker 1: mathematical decoration. Its structure enters into experimentally testable predictions. At
• 1:03:13
  Speaker 1: this point, another phrase usually arrives, virtual particles. They often
• 1:03:18
  Speaker 1: appear in descriptions of the vacuum, interactions and forces. But
• 1:03:23
  Speaker 1: they require some care. A real particle can be detected.
• 1:03:27
  Speaker 1: A photon can strike a detector. An electron can leave
• 1:03:31
  Speaker 1: a measurable track. A virtual particle is different. Virtual particles
• 1:03:37
  Speaker 1: appear as internal components of certain perturbative quantum field calculations,
• 1:03:43
  Speaker 1: particularly in the mathematical language represented by Feynman diagrams. They
• 1:03:48
  Speaker 1: are not directly observable particles travelling around in secret. They
• 1:03:53
  Speaker 1: do not have to obey the same relationship between energy
• 1:03:56
  Speaker 1: and momentum as freely propagating real particles. and calling them
• 1:04:01
  Speaker 1: particles can encourage a picture that is far more literal
• 1:04:05
  Speaker 1: than the mathematics requires. It can still be convenient to
• 1:04:09
  Speaker 1: speak about virtual photons or virtual electrons when discussing particular calculations.
• 1:04:16
  Speaker 1: Physicists do this all the time, but the phrase is
• 1:04:19
  Speaker 1: part of a computational framework, not evidence that empty space
• 1:04:24
  Speaker 1: is physically packed with tiny ordinary particles constantly borrowing energy
• 1:04:29
  Speaker 1: and vanishing. The vacuum remains strange enough without giving it
• 1:04:34
  Speaker 1: extra hobbies. There is, however, an even deeper problem associated
• 1:04:38
  Speaker 1: with vacuum energy. If every mode of a quantum field
• 1:04:42
  Speaker 1: contributes some ground state energy, then a naive attempt to
• 1:04:46
  Speaker 1: add the contributions from arbitrarily high frequency modes produces an
• 1:04:50
  Speaker 1: enormous result. In quantum field calculations without gravity, only differences
• 1:04:56
  Speaker 1: in energy are often physically relevant. allowing constant contributions to
• 1:05:01
  Speaker 1: be handled in ways that do not affect ordinary particle experiments.
• 1:05:06
  Speaker 1: Gravity changes the situation. According to Albert Einstein's general theory
• 1:05:11
  Speaker 1: of relativity, energy contributes to the curvature of spacetime. So
• 1:05:16
  Speaker 1: if the quantum vacuum possesses an energy density, gravity should
• 1:05:20
  Speaker 1: respond to it. Cosmological observations indicate that the expansion of
• 1:05:25
  Speaker 1: the universe is accelerating. Within the standard cosmological model, this
• 1:05:31
  Speaker 1: behaviour can be described using a cosmological constant, often associated
• 1:05:36
  Speaker 1: with an approximately constant energy density of empty space, and
• 1:05:40
  Speaker 1: commonly discussed under the broader label dark energy. The difficulty
• 1:05:45
  Speaker 1: is that straightforward estimates of quantum vacuum contributions do not
• 1:05:49
  Speaker 1: naturally reproduce the very small value inferred from cosmology. Depending
• 1:05:56
  Speaker 1: on how the comparison is formulated and what energy scale
• 1:05:59
  Speaker 1: is used, the discrepancy can be astonishingly large. This is
• 1:06:04
  Speaker 1: known as the cosmological constant problem. It remains unresolved. Quantum
• 1:06:10
  Speaker 1: field theory does not currently provide a complete explanation for
• 1:06:15
  Speaker 1: why the observed effective vacuum energy associated with cosmic acceleration
• 1:06:19
  Speaker 1: has the value it does, and that uncertainty is important.
• 1:06:24
  Speaker 1: It would be tempting to say that quantum field theory
• 1:06:26
  Speaker 1: has revealed exactly what empty space contains. It has not.
• 1:06:31
  Speaker 1: What it has revealed is that the physical concept of
• 1:06:34
  Speaker 1: vacuum is far richer than classical emptiness. The vacuum is
• 1:06:38
  Speaker 1: the lowest energy state of quantum fields. It contains no
• 1:06:43
  Speaker 1: ordinary particle excitations in the relevant description, yet it still
• 1:06:47
  Speaker 1: possesses quantum uncertainty, correlations and ground state structure. that structure
• 1:06:53
  Speaker 1: can contribute to measurable phenomena such as atomic energy shifts
• 1:06:59
  Speaker 1: and forces between closely separated conducting surfaces. So when quantum
• 1:07:04
  Speaker 1: field theory speaks of empty space, it does not mean
• 1:07:07
  Speaker 1: that absolutely nothing is there. It means something much more specific.
• 1:07:13
  Speaker 1: The particles may be absent, the fields are not. A
• 1:07:16
  Speaker 1: universe containing quantum fields but no interactions would be a
• 1:07:20
  Speaker 1: rather uneventful place. The fields could still possess excitations. Electrons
• 1:07:26
  Speaker 1: could exist. Photons could exist. Quarks could exist. But if
• 1:07:32
  Speaker 1: the fields were completely independent of one another, their excitations
• 1:07:36
  Speaker 1: would simply propagate without exchanging energy, changing direction, binding together
• 1:07:42
  Speaker 1: or producing anything new. An electron would pass another electron
• 1:07:46
  Speaker 1: without noticing. Light would travel past charged matter without being absorbed.
• 1:07:52
  Speaker 1: Atoms could not hold together. Chemistry would not exist. The
• 1:07:56
  Speaker 1: universe we actually observe is interesting precisely because its quantum
• 1:08:00
  Speaker 1: fields interact. An interaction means that the behaviour of one
• 1:08:05
  Speaker 1: field is connected to the behaviour of another, or sometimes
• 1:08:09
  Speaker 1: to itself. Energy and momentum can be exchanged. Particle excitations
• 1:08:15
  Speaker 1: can scatter. Some can be absorbed. Others can be created.
• 1:08:20
  Speaker 1: The probabilities for these processes are determined by the structure
• 1:08:24
  Speaker 1: and strengths of the interactions built into the theory. Physicists
• 1:08:29
  Speaker 1: organise much of this information using an object called the Lagrangian.
• 1:08:34
  Speaker 1: Despite the rather formidable name, the underlying idea is straightforward.
• 1:08:39
  Speaker 1: A Lagrangian is a mathematical expression that contains a compact
• 1:08:42
  Speaker 1: description of a physical system. In quantum field theory it
• 1:08:47
  Speaker 1: identifies which fields exist. how those fields behave when they
• 1:08:51
  Speaker 1: are free, and how they interact with one another. You
• 1:08:54
  Speaker 1: can think of it as an extraordinarily compressed description of
• 1:08:58
  Speaker 1: the rules. Part of the Lagrangian describes how a field
• 1:09:01
  Speaker 1: propagates when nothing is disturbing it. Other parts describe interactions
• 1:09:06
  Speaker 1: between fields. Change those interaction terms and you change the physics.
• 1:09:13
  Speaker 1: The Lagrangian is closely connected to another important quantity called
• 1:09:16
  Speaker 1: the action. The action is obtained by combining the Lagrangian
• 1:09:20
  Speaker 1: across space and time. In classical physics, the actual evolution
• 1:09:24
  Speaker 1: of a system can be derived from a principle usually
• 1:09:27
  Speaker 1: called the principle of stationary action. It is often introduced
• 1:09:31
  Speaker 1: as the principle of least action, although stationary action is
• 1:09:35
  Speaker 1: the more precise phrase because the physical path does not
• 1:09:38
  Speaker 1: always literally minimise the action. The striking idea is that
• 1:09:42
  Speaker 1: instead of writing down every equation of motion independently, physicists
• 1:09:47
  Speaker 1: can often begin with the action and derive the equations
• 1:09:51
  Speaker 1: governing the entire system. This approach works beautifully for fields.
• 1:09:57
  Speaker 1: Once the field content, symmetries and interactions have been encoded
• 1:10:00
  Speaker 1: into an appropriate Lagrangian, the corresponding equations of motion and
• 1:10:06
  Speaker 1: much of the quantum theory can be developed systematically. A
• 1:10:10
  Speaker 1: particularly useful example is quantum electrodynamics, Quantum electrodynamics describes interactions
• 1:10:17
  Speaker 1: involving electrically charged particles and the electromagnetic field. For electrons,
• 1:10:23
  Speaker 1: the theory contains the electron field and the electromagnetic field.
• 1:10:28
  Speaker 1: If these fields did not interact, an electron could exist
• 1:10:32
  Speaker 1: and a photon could exist, but they would ignore one
• 1:10:35
  Speaker 1: another completely. The crucial ingredient is a coupling between the fields.
• 1:10:41
  Speaker 1: That coupling allows an electron to interact with the electromagnetic field.
• 1:10:46
  Speaker 1: From it come familiar physical processes. An electron can emit
• 1:10:50
  Speaker 1: a photon. An electron can absorb a photon. A photon
• 1:10:54
  Speaker 1: can scatter from a charged particle. An electron and a
• 1:10:58
  Speaker 1: positron can annihilate and produce photons. Under suitable conditions, photons
• 1:11:04
  Speaker 1: can also participate in processes that produce charged particle and
• 1:11:08
  Speaker 1: antiparticle pairs. All of these phenomena emerge from the underlying
• 1:11:13
  Speaker 1: interaction encoded in the theory. The word coupling is important.
• 1:11:18
  Speaker 1: A coupling strength tells us, roughly speaking, how strongly particular
• 1:11:23
  Speaker 1: fields interact. For electromagnetism, the interaction strength is related to
• 1:11:28
  Speaker 1: the electric charge and is commonly characterised by a dimensionless
• 1:11:32
  Speaker 1: quantity called the fine structure constant. Its value at ordinary
• 1:11:38
  Speaker 1: low energies is roughly one part in 137. That relatively
• 1:11:43
  Speaker 1: small number is one reason quantum electrodynamics can be treated
• 1:11:47
  Speaker 1: so successfully using perturbation theory, where calculations are organised as
• 1:11:53
  Speaker 1: a series of increasingly small corrections. But coupling strengths are
• 1:11:57
  Speaker 1: not necessarily fixed in the simple way we might expect
• 1:12:01
  Speaker 1: from everyday intuition. In quantum field theory, The effective strength
• 1:12:06
  Speaker 1: of an interaction can depend on the energy scale at
• 1:12:09
  Speaker 1: which it is measured. Probe a system at one energy,
• 1:12:13
  Speaker 1: and the interaction can appear to have one strength. Probe
• 1:12:16
  Speaker 1: it at a much higher energy, and quantum effects can
• 1:12:20
  Speaker 1: slightly alter the effective value. This scale dependence becomes extremely
• 1:12:25
  Speaker 1: important in modern particle physics. The clearest way to study
• 1:12:29
  Speaker 1: interactions experimentally is often through scattering. The basic idea is
• 1:12:34
  Speaker 1: almost beautifully simple. Prepare something known, let it interact, then
• 1:12:40
  Speaker 1: measure what comes out. Particle accelerators do this on an
• 1:12:44
  Speaker 1: enormous and sophisticated scale. Beams of particles are accelerated to
• 1:12:49
  Speaker 1: high energies and directed toward targets, or collided with one another.
• 1:12:55
  Speaker 1: Detectors surrounding the interaction region then record the energy, momentum,
• 1:13:00
  Speaker 1: charge and other properties of the particles emerging from those collisions.
• 1:13:05
  Speaker 1: Physicists compare the resulting patterns with the probabilities predicted by
• 1:13:09
  Speaker 1: quantum field theory. The word probability matters. Quantum field theory
• 1:13:14
  Speaker 1: generally does not say that one precisely specified collision must
• 1:13:19
  Speaker 1: always produce one unique final result. Instead, it predicts amplitudes
• 1:13:25
  Speaker 1: associated with possible outcomes. From those amplitudes, physicists calculate probabilities,
• 1:13:32
  Speaker 1: repeat the same kind of collision many millions or billions
• 1:13:35
  Speaker 1: of times, and those probabilities reveal themselves statistically in the
• 1:13:40
  Speaker 1: distribution of events recorded by the detectors. Some outcomes are common,
• 1:13:46
  Speaker 1: others are rare, some are forbidden entirely. The forbidden processes
• 1:13:51
  Speaker 1: often reveal something especially important about the structure of the theory.
• 1:13:55
  Speaker 1: Interactions must respect conservation laws. Energy is conserved. Momentum is conserved.
• 1:14:02
  Speaker 1: Electric charge is conserved. Other quantum numbers may also be
• 1:14:06
  Speaker 1: conserved depending on the interaction and the circumstances. If two
• 1:14:11
  Speaker 1: particles collide, the total energy before the collision must match
• 1:14:15
  Speaker 1: the total energy afterward. The same applies to momentum. New
• 1:14:20
  Speaker 1: massive particles can appear, but their mass does not emerge
• 1:14:24
  Speaker 1: from nowhere. The required energy must already be present in
• 1:14:28
  Speaker 1: the initial state. Conservation laws might initially look like separate
• 1:14:33
  Speaker 1: rules added to physics because experiments tell us that nature
• 1:14:37
  Speaker 1: behaves that way. But there is a deeper connection. That
• 1:14:42
  Speaker 1: connection was revealed by the German mathematician Emmy Noether. In 1918,
• 1:14:47
  Speaker 1: Noether published a theorem showing that continuous symmetries of physical
• 1:14:51
  Speaker 1: laws are intimately related to conservation laws. If the laws
• 1:14:56
  Speaker 1: of physics do not change from one moment in time
• 1:14:59
  Speaker 1: to another, the corresponding symmetry is connected with conservation of energy.
• 1:15:04
  Speaker 1: If the laws remain the same when an experiment is
• 1:15:07
  Speaker 1: moved from one location in space to another, the spatial
• 1:15:11
  Speaker 1: symmetry is connected with conservation of momentum. Rotational symmetry is
• 1:15:16
  Speaker 1: connected with conservation of angular momentum. Noether's theorem is one
• 1:15:21
  Speaker 1: of the deepest organising principles in theoretical physics. It tells
• 1:15:26
  Speaker 1: us that conservation laws are not merely a collection of
• 1:15:28
  Speaker 1: fortunate habits, displayed by the universe. They are linked to
• 1:15:33
  Speaker 1: the symmetries built into the mathematical structure of physical law.
• 1:15:37
  Speaker 1: Quantum field theory makes extensive use of this relationship. The
• 1:15:42
  Speaker 1: allowed interactions between fields are strongly constrained by symmetry. A
• 1:15:47
  Speaker 1: theory cannot simply contain any interaction one happens to fancy.
• 1:15:51
  Speaker 1: The interaction terms must fit the symmetries and consistency requirements
• 1:15:56
  Speaker 1: of the theory. This helps explain why certain particle processes
• 1:16:00
  Speaker 1: occur while others do not. It also brings us to
• 1:16:03
  Speaker 1: an important caution about the word interaction. When two ordinary
• 1:16:09
  Speaker 1: objects interact, we often imagine them touching. Two billiard balls collide.
• 1:16:15
  Speaker 1: A hand pushes a door. A spoon taps the side
• 1:16:18
  Speaker 1: of a cup. that picture becomes unreliable in quantum field theory.
• 1:16:23
  Speaker 1: Two particles do not necessarily need to behave like tiny
• 1:16:27
  Speaker 1: solid objects making physical surface contact. An electron scattering from
• 1:16:33
  Speaker 1: another charged particle is better described through the dynamics of
• 1:16:36
  Speaker 1: interacting quantum fields. The theory begins with an initial quantum
• 1:16:41
  Speaker 1: state and determines amplitudes for possible final states. Between those
• 1:16:47
  Speaker 1: two descriptions lies the interaction governed by the fields and
• 1:16:50
  Speaker 1: the terms in the Lagrangian. There may not be a
• 1:16:54
  Speaker 1: meaningful little classical story in which two microscopic balls approach,
• 1:16:59
  Speaker 1: touch at their edges and bounce away. The interaction is
• 1:17:03
  Speaker 1: fundamentally field-theoretic. This becomes especially clear with electromagnetism. Two electrons
• 1:17:11
  Speaker 1: can repel one another, even though they never touch in
• 1:17:15
  Speaker 1: the ordinary sense. Both are excitations of the electron field,
• 1:17:19
  Speaker 1: and both interact with the electromagnetic field. The combined quantum
• 1:17:24
  Speaker 1: dynamics determine how the initial state develops into possible final states.
• 1:17:30
  Speaker 1: The same general principle extends across particle physics. Quark fields
• 1:17:35
  Speaker 1: interact through the strong interaction. Electrons and neutrinos participate in
• 1:17:40
  Speaker 1: weak interactions. Charged fields interact electromagnetically. The fields are not
• 1:17:47
  Speaker 1: independent layers silently occupying the same space-time. They are coupled,
• 1:17:52
  Speaker 1: and those couplings are what allow the universe to do
• 1:17:55
  Speaker 1: almost everything interesting. They let particles scatter and decay. They
• 1:18:00
  Speaker 1: allow atoms to form. They permit stars to shine. They
• 1:18:04
  Speaker 1: make chemistry possible. At the deepest level described by modern
• 1:18:08
  Speaker 1: particle physics, an interaction is therefore not simply one tiny
• 1:18:13
  Speaker 1: object bumping into another. It is a transition between quantum
• 1:18:18
  Speaker 1: states governed by the dynamics of coupled fields. The Lagrangian
• 1:18:23
  Speaker 1: contains the rules. Symmetry constrains those rules. Conservation laws restrict
• 1:18:29
  Speaker 1: what can happen. And experiments reveal the probabilities with remarkable precision.
• 1:18:37
  Speaker 1: What looks, from a distance, like particles colliding, is part
• 1:18:41
  Speaker 1: of a much deeper story. in which quantum fields continually
• 1:18:46
  Speaker 1: influence one another and reshape the possible state of matter
• 1:18:50
  Speaker 1: and radiation. Few images in modern physics are as recognisable
• 1:18:56
  Speaker 1: as a Feynman diagram. A handful of straight or wavy
• 1:19:00
  Speaker 1: lines meet at simple points. An electron approaches from one side.
• 1:19:05
  Speaker 1: A photon appears between two lines. Other particles leave in
• 1:19:09
  Speaker 1: different directions. At first glance, the picture looks almost like
• 1:19:14
  Speaker 1: a tiny map of an event happening somewhere deep inside nature.
• 1:19:19
  Speaker 1: That interpretation is tempting. It is also not quite what
• 1:19:23
  Speaker 1: the diagram means. Feynman diagrams were developed by the American
• 1:19:27
  Speaker 1: physicist Richard Feynman during the 1940s as part of a
• 1:19:32
  Speaker 1: new way of organising calculations in quantum field theory. Their
• 1:19:36
  Speaker 1: great strength is that they turn complicated mathematical expressions into
• 1:19:41
  Speaker 1: relatively simple visual structures. But they are not photographs of
• 1:19:46
  Speaker 1: what particles literally do. They are pieces of mathematics drawn
• 1:19:50
  Speaker 1: as pictures. To understand why they are useful, imagine a
• 1:19:55
  Speaker 1: simple scattering experiment involving electrons. Two electrons approach one another.
• 1:20:01
  Speaker 1: They interact electromagnetically. After the interaction, two electrons emerge with
• 1:20:06
  Speaker 1: different momenta. Quantum field theory allows physicists to calculate the
• 1:20:11
  Speaker 1: probability of obtaining particular outgoing states from particular incoming states.
• 1:20:18
  Speaker 1: The calculation involves quantum amplitudes. A Feynman diagram is one
• 1:20:23
  Speaker 1: convenient way of representing a particular mathematical contribution to that amplitude.
• 1:20:29
  Speaker 1: The lines entering or leaving the diagram are called external lines.
• 1:20:34
  Speaker 1: These correspond to the particles in the initial and final
• 1:20:38
  Speaker 1: states of the calculation. An incoming electron might be represented
• 1:20:43
  Speaker 1: by one line. An outgoing electron is represented by another.
• 1:20:47
  Speaker 1: A photon entering or leaving an experiment would have its
• 1:20:50
  Speaker 1: own external line. Where lines meet, the diagram contains a vertex.
• 1:20:56
  Speaker 1: A vertex represents an interaction allowed by the theory. In
• 1:21:00
  Speaker 1: quantum electrodynamics, a vertex can represent the coupling between the
• 1:21:05
  Speaker 1: electron field and the electromagnetic field. That coupling allows processes
• 1:21:10
  Speaker 1: involving electrons, positrons and photons. Then there are the lines
• 1:21:16
  Speaker 1: running between vertices. These are called internal lines. Mathematically, they
• 1:21:21
  Speaker 1: correspond to objects known as propagators. A propagator describes, in
• 1:21:27
  Speaker 1: a precise quantum field sense, how the influence associated with
• 1:21:31
  Speaker 1: a field connects different interaction points within a calculation. This
• 1:21:36
  Speaker 1: is where the diagrams are often interpreted too literally. An
• 1:21:40
  Speaker 1: internal photon line is sometimes described as a virtual photon
• 1:21:44
  Speaker 1: travelling from one electron to another. That language can be
• 1:21:48
  Speaker 1: useful shorthand. But the line should not be pictured as
• 1:21:52
  Speaker 1: a directly observable little photon secretly following a definite path
• 1:21:56
  Speaker 1: through space. The internal line is part of the mathematical calculation.
• 1:22:02
  Speaker 1: The object associated with it is not an ordinary freely
• 1:22:05
  Speaker 1: propagating photon that could simply be intercepted with a detector
• 1:22:10
  Speaker 1: halfway along its supposed journey. This distinction becomes especially important
• 1:22:15
  Speaker 1: when people say that forces work because particles are constantly
• 1:22:20
  Speaker 1: exchanging virtual particles. There is a useful piece of intuition
• 1:22:25
  Speaker 1: hiding in that phrase. but it can easily become misleading
• 1:22:28
  Speaker 1: if treated as a literal microscopic movie. Quantum electrodynamics fundamentally
• 1:22:34
  Speaker 1: describes interacting electron and electromagnetic fields. Feynman diagrams provide one
• 1:22:40
  Speaker 1: way of organising calculations involving those interactions. Virtual particles belong
• 1:22:46
  Speaker 1: to that calculational language. They are not hidden ordinary particles
• 1:22:51
  Speaker 1: that have somehow escaped direct observation. Consider another simple process.
• 1:22:56
  Speaker 1: An electron and a positron can meet and annihilate. Their
• 1:23:00
  Speaker 1: initial state might eventually produce another particle, an antiparticle pair.
• 1:23:06
  Speaker 1: Depending on the process being calculated, physicists can draw different
• 1:23:10
  Speaker 1: diagrams representing different mathematical contributions connecting the same initial and
• 1:23:16
  Speaker 1: final states. And this introduces one of the most important
• 1:23:20
  Speaker 1: features of quantum theory. If several contributions lead to the
• 1:23:24
  Speaker 1: same measurable outcome, Their probabilities are not simply calculated separately
• 1:23:29
  Speaker 1: and then added. The quantum amplitudes are combined first. Only
• 1:23:34
  Speaker 1: after the amplitudes have been combined is the resulting probability obtained.
• 1:23:39
  Speaker 1: Because amplitudes can reinforce or partially cancel one another, different
• 1:23:44
  Speaker 1: contributions can interfere. This is closely related to the interference
• 1:23:49
  Speaker 1: familiar from the double-slit experiment. There, too, Quantum alternatives contribute
• 1:23:55
  Speaker 1: amplitudes that combine before an observable probability pattern emerges. In
• 1:24:01
  Speaker 1: quantum field theory, the alternatives can be far more complicated.
• 1:24:06
  Speaker 1: Different Feynman diagrams can represent different contributions to the same
• 1:24:10
  Speaker 1: overall transition amplitude. The measurable result depends on their combined effect.
• 1:24:17
  Speaker 1: A single diagram therefore does not usually represent the complete process.
• 1:24:21
  Speaker 1: It is one term in a larger calculation. This becomes
• 1:24:25
  Speaker 1: important because quantum field theory calculations are often performed using
• 1:24:30
  Speaker 1: perturbation theory. Perturbation theory works by beginning with a relatively
• 1:24:35
  Speaker 1: simple situation that can be solved and then adding corrections
• 1:24:40
  Speaker 1: associated with interactions. When the relevant coupling strength is sufficiently small,
• 1:24:45
  Speaker 1: those corrections can often be arranged in a useful hierarchy.
• 1:24:50
  Speaker 1: The simplest Feynman diagrams usually represent the leading contributions. Then
• 1:24:55
  Speaker 1: more elaborate diagrams provide higher order corrections. A simple diagram
• 1:25:00
  Speaker 1: might contain only a small number of vertiges. A more
• 1:25:03
  Speaker 1: advanced correction might contain extra internal lines, loops and additional vertices.
• 1:25:09
  Speaker 1: Each increase in complexity usually comes with extra powers of
• 1:25:12
  Speaker 1: the interaction strength. In quantum electrodynamics, where the effective electromagnetic
• 1:25:19
  Speaker 1: coupling is relatively weak at ordinary energies, the higher order
• 1:25:23
  Speaker 1: terms often become progressively smaller. This is why perturbation theory
• 1:25:27
  Speaker 1: works so extraordinarily well there. Physicists can calculate the leading contribution,
• 1:25:33
  Speaker 1: then the next correction, then another, steadily improving the precision.
• 1:25:38
  Speaker 1: Some quantities predicted this way agree with laboratory measurements to
• 1:25:42
  Speaker 1: an astonishing number of decimal places. but there is no
• 1:25:46
  Speaker 1: rule saying perturbation theory must always work. In situations where
• 1:25:52
  Speaker 1: interactions become strong, the higher order contributions may not shrink
• 1:25:56
  Speaker 1: rapidly enough to provide a useful expansion. The strong interaction
• 1:26:01
  Speaker 1: at relatively low energies provides an important example. There, physicists
• 1:26:06
  Speaker 1: often need non-perturbative methods rather than simply drawing increasingly complicated
• 1:26:11
  Speaker 1: Feynman diagrams and hoping for the best. So Feynman diagrams
• 1:26:16
  Speaker 1: are powerful, but they are not the whole of quantum
• 1:26:19
  Speaker 1: field theory. Their influence comes partly from how efficiently they
• 1:26:23
  Speaker 1: organise the mathematics. Before this diagrammatic language became widespread, high-order
• 1:26:30
  Speaker 1: calculations in quantum electrodynamics could become extraordinarily difficult to track.
• 1:26:36
  Speaker 1: Feynman's method associated mathematical ingredients with visual elements. External particles
• 1:26:43
  Speaker 1: became lines. Interactions became vertices. Propagators became internal lines. Closed
• 1:26:50
  Speaker 1: loops represented particular types of quantum correction. Each visual feature
• 1:26:56
  Speaker 1: corresponded to a precise mathematical factor. A complicated expression could
• 1:27:01
  Speaker 1: therefore be built systematically from the structure of its diagram.
• 1:27:06
  Speaker 1: The pictures also provide intuition. A physicist can look at
• 1:27:09
  Speaker 1: the interactions permitted by a theory and quickly identify possible
• 1:27:14
  Speaker 1: contributions to a particular process. They can see which fields participate,
• 1:27:19
  Speaker 1: how many interaction vertices are involved, and what order of
• 1:27:23
  Speaker 1: perturbation theory the contribution belongs to. But intuition has to
• 1:27:28
  Speaker 1: remain attached to the mathematics. The diagram is not telling
• 1:27:32
  Speaker 1: us that nature literally selects one picture and performs it
• 1:27:36
  Speaker 1: behind the scenes. An electron does not necessarily travel along
• 1:27:40
  Speaker 1: the exact line drawn on the page. A virtual photon
• 1:27:45
  Speaker 1: does not have to be a tiny luminous object racing
• 1:27:48
  Speaker 1: along the wavy line between two vertitors. The location of
• 1:27:53
  Speaker 1: a vertex on the paper is not automatically a physical
• 1:27:56
  Speaker 1: coordinate in a laboratory. The diagram encodes a contribution to
• 1:28:01
  Speaker 1: a quantum amplitude. That distinction protects us from turning a
• 1:28:05
  Speaker 1: sophisticated quantum calculation back into a classical story that the
• 1:28:10
  Speaker 1: theory itself does not actually tell. Feynman diagrams are therefore
• 1:28:15
  Speaker 1: rather unusual scientific pictures. They show us an enormous amount
• 1:28:19
  Speaker 1: without literally showing us what happened. They are diagrams of mathematics,
• 1:28:25
  Speaker 1: not microscopic photographs, and that is precisely why they are
• 1:28:29
  Speaker 1: so useful. A few modest lines on a page can
• 1:28:33
  Speaker 1: organise interactions among quantum fields, Keep track of difficult calculations
• 1:28:39
  Speaker 1: and reveal which contributions can affect an experiment. Nature is
• 1:28:44
  Speaker 1: not secretly drawing them. Fortunately, physicists are. Quantum electrodynamics is
• 1:28:51
  Speaker 1: one of the great success stories of modern physics. It
• 1:28:54
  Speaker 1: is the relativistic quantum field theory. describing how electrically charged
• 1:28:59
  Speaker 1: particles interact with the electromagnetic field. Electrons belong to the
• 1:29:05
  Speaker 1: electron field. Photons are excitations of the electromagnetic field. And
• 1:29:12
  Speaker 1: quantum electrodynamics provides the mathematical framework for describing how those
• 1:29:17
  Speaker 1: fields influence one another. That includes processes such as an
• 1:29:22
  Speaker 1: electron emitting a photon, absorbing a photon, scattering from another
• 1:29:28
  Speaker 1: charged particle, or annihilating with a positron. In principle, the
• 1:29:33
  Speaker 1: theory sounds beautifully tidy. In practice, early calculations ran into
• 1:29:39
  Speaker 1: a rather alarming problem. Infinity. Physicists found that when they
• 1:29:44
  Speaker 1: tried to calculate certain quantities in full detail, some expressions
• 1:29:49
  Speaker 1: did not settle down to sensible finite numbers. Instead, they diverged.
• 1:29:55
  Speaker 1: Roughly speaking, contributions associated with extremely short distances or extremely
• 1:30:01
  Speaker 1: high energies could keep growing without bound. This was not
• 1:30:05
  Speaker 1: the sort of answer one could quietly ignore. If a
• 1:30:08
  Speaker 1: theory predicts an infinite correction to the mass or charge
• 1:30:12
  Speaker 1: of an electron, something has clearly gone wrong in the
• 1:30:15
  Speaker 1: way the calculation is being interpreted. The difficulty became especially serious.
• 1:30:21
  Speaker 1: when physicists began trying to include increasingly subtle quantum corrections.
• 1:30:27
  Speaker 1: A simple approximation might produce a finite result, but once
• 1:30:31
  Speaker 1: the calculation included loops and higher order processes, divergent integrals
• 1:30:36
  Speaker 1: often appeared. At first, this made quantum field theory look
• 1:30:40
  Speaker 1: dangerously incomplete. The theory seemed capable of describing the right
• 1:30:45
  Speaker 1: physical ideas while producing mathematical expressions that refused to behave.
• 1:30:50
  Speaker 1: The resolution came through the development of renormalisation. Several physicists
• 1:30:55
  Speaker 1: played central roles. Sinitiro Tominaga developed a relativistically consistent approach
• 1:31:01
  Speaker 1: to quantum electrodynamics during and after the Second World War.
• 1:31:06
  Speaker 1: Julian Schwinger developed an extremely powerful operator-based formalism. Richard Feynman
• 1:31:12
  Speaker 1: introduced his path-integral approach and the diagrammatic methods that made
• 1:31:17
  Speaker 1: many calculations far easier to organise. Freeman Dyson later demonstrated
• 1:31:22
  Speaker 1: how the apparently different approaches developed by Tominaga, Schwinger and
• 1:31:26
  Speaker 1: Feynman could be understood as equivalent formulations of the same
• 1:31:30
  Speaker 1: underlying theory. Together, these developments transformed quantum electrodynamics from a
• 1:31:36
  Speaker 1: promising but troublesome framework into a precise predictive theory. Renormalisation
• 1:31:42
  Speaker 1: is often described as a trick for getting rid of infinities.
• 1:31:46
  Speaker 1: That description is understandable, but it misses the deeper point.
• 1:31:50
  Speaker 1: The basic idea is that some of the parameters appearing
• 1:31:52
  Speaker 1: in the mathematical theory are not themselves directly measurable quantities.
• 1:31:58
  Speaker 1: What experiments actually give us are physical masses, charges and
• 1:32:02
  Speaker 1: interaction strengths. Renormalisation provides a systematic procedure for relating the
• 1:32:08
  Speaker 1: formal parameters inside the theory to those measurable quantities. Suppose
• 1:32:13
  Speaker 1: a calculation of an electron's properties contains divergent contributions. Rather
• 1:32:19
  Speaker 1: than treating the bare mass and bare charge appearing in
• 1:32:21
  Speaker 1: the starting equations as directly observable, physicists define the physically
• 1:32:27
  Speaker 1: measured mass and charge after the quantum corrections are taken
• 1:32:30
  Speaker 1: into account. The divergent pieces can then be reorganised and
• 1:32:35
  Speaker 1: absorbed into the definitions of the underlying parameters. What remains
• 1:32:39
  Speaker 1: is a finite prediction for measurable quantities. This sounds suspicious
• 1:32:43
  Speaker 1: the first time one hears it, and historically many physicists
• 1:32:47
  Speaker 1: were suspicious. There was something uncomfortable about beginning with quantities
• 1:32:51
  Speaker 1: that could themselves become formally infinite and then combining them
• 1:32:55
  Speaker 1: with other infinities to obtain a finite observable answer. But
• 1:33:00
  Speaker 1: the procedure was not arbitrary. Renormalisation works only when the
• 1:33:04
  Speaker 1: structure of the theory allows the divergences to be controlled
• 1:33:08
  Speaker 1: using a finite number of measurable parameters. Quantum electrodynamics has
• 1:33:13
  Speaker 1: exactly that property. More importantly, the resulting predictions work. extremely well.
• 1:33:21
  Speaker 1: A classic example is the magnetic behaviour of the electron.
• 1:33:24
  Speaker 1: An electron possesses intrinsic spin and behaves as though it
• 1:33:28
  Speaker 1: carries a tiny magnetic moment. The Dirac equation predicts a
• 1:33:33
  Speaker 1: particular relationship between the electron's spin and magnetic moment. But
• 1:33:38
  Speaker 1: quantum electrodynamics predicts a small correction to that simple result.
• 1:33:43
  Speaker 1: This correction is known as the electron's anomalous magnetic moment.
• 1:33:48
  Speaker 1: The anomaly is tiny, yet it can be measured with
• 1:33:51
  Speaker 1: extraordinary precision. Quantum electrodynamics calculations include contributions from increasingly complicated
• 1:33:58
  Speaker 1: processes involving virtual photons, electron and positron loops, and other
• 1:34:03
  Speaker 1: quantum corrections. When these contributions are added carefully, theory and
• 1:34:08
  Speaker 1: experiment agree to an astonishing degree. The exact comparison changes
• 1:34:13
  Speaker 1: as both experimental measurements and theoretical calculations improve. but the
• 1:34:18
  Speaker 1: agreement extends across many significant digits. It is one of
• 1:34:22
  Speaker 1: the most precise tests of any physical theory ever performed.
• 1:34:27
  Speaker 1: That precision depends partly on another important quantity, the fine
• 1:34:31
  Speaker 1: structure constant. The fine structure constant is a dimensionless number
• 1:34:38
  Speaker 1: that characterises the strength of the electromagnetic interaction. At relatively
• 1:34:43
  Speaker 1: low energies, Its value is close to 1 divided by 137.
• 1:34:49
  Speaker 1: Because that number is small, perturbation theory works especially well
• 1:34:53
  Speaker 1: in quantum electrodynamics. Each additional order in the perturbative expansion
• 1:34:58
  Speaker 1: is typically suppressed by extra powers of this small coupling.
• 1:35:03
  Speaker 1: So the simplest contribution gives the largest part of the answer.
• 1:35:07
  Speaker 1: The next correction is smaller, the next is smaller again,
• 1:35:11
  Speaker 1: and so on. This makes it possible to calculate electromagnetic
• 1:35:16
  Speaker 1: processes with exceptional precision. But there is another layer to
• 1:35:20
  Speaker 1: the story. The fine structure constant is not perfectly fixed
• 1:35:24
  Speaker 1: in the naive sense. Its effective value depends on the
• 1:35:28
  Speaker 1: energy scale at which the interaction is probed. At higher energies,
• 1:35:32
  Speaker 1: quantum fluctuations modify the apparent strength of the electromagnetic interaction.
• 1:35:38
  Speaker 1: One way to picture this is to imagine the electron
• 1:35:41
  Speaker 1: surrounded by the quantum response of the vacuum. At larger distances,
• 1:35:45
  Speaker 1: the measured charge appears slightly screened by those quantum effects.
• 1:35:50
  Speaker 1: Probe closer to the electron, corresponding to higher energies, and
• 1:35:54
  Speaker 1: the effective electric charge changes slightly, so the coupling runs
• 1:35:57
  Speaker 1: with energy. This idea is known as a running coupling.
• 1:36:02
  Speaker 1: It reveals that renormalisation is not merely about removing troublesome infinities.
• 1:36:07
  Speaker 1: It tells us something physically profound. The parameters used to
• 1:36:11
  Speaker 1: describe interactions depend on scale. This became much clearer through
• 1:36:16
  Speaker 1: the work of the American physicist Kenneth Wilson in the
• 1:36:19
  Speaker 1: 20th century. Wilson developed the modern renormalisation group viewpoint. The
• 1:36:25
  Speaker 1: central idea is that a physical theory can look different
• 1:36:28
  Speaker 1: depending on the length scale or energy scale at which
• 1:36:31
  Speaker 1: it is examined. At low energies, some details of the
• 1:36:35
  Speaker 1: microscopic theory may become irrelevant. At higher energies, those details
• 1:36:39
  Speaker 1: may matter greatly. The effective parameters used to describe the
• 1:36:43
  Speaker 1: system therefore change as the scale changes. This viewpoint transformed
• 1:36:48
  Speaker 1: renormalization from an awkward repair procedure into one of the
• 1:36:53
  Speaker 1: deepest organizing ideas in modern theoretical physics. It also connected
• 1:36:58
  Speaker 1: particle physics with areas such as condensed matter physics and
• 1:37:01
  Speaker 1: the study of phase transitions. A magnet near a critical
• 1:37:05
  Speaker 1: temperature and a quantum field describing fundamental particles may seem
• 1:37:10
  Speaker 1: to have very little in common. Yet the same renormalisation
• 1:37:15
  Speaker 1: group ideas can help explain how both systems behave across
• 1:37:18
  Speaker 1: different scales. There is another useful way to think about
• 1:37:22
  Speaker 1: the original infinities. Quantum field calculations often attempt to account
• 1:37:27
  Speaker 1: for contributions from arbitrarily short distances. But a theory may
• 1:37:32
  Speaker 1: only be intended to describe physics accurately up to some
• 1:37:35
  Speaker 1: range of energies. From the modern effective field theory viewpoint,
• 1:37:39
  Speaker 1: this is not necessarily a defect. A theory can be
• 1:37:42
  Speaker 1: extraordinarily accurate at accessible energies without claiming to remain the
• 1:37:46
  Speaker 1: final description at infinitely high energies. Quantum electrodynamics is understood
• 1:37:52
  Speaker 1: as part of the larger standard model, and even the
• 1:37:55
  Speaker 1: standard model itself may eventually be replaced or embedded within
• 1:37:59
  Speaker 1: a deeper theory. Renormalisation tells us how the predictions at
• 1:38:04
  Speaker 1: one scale can remain reliable. without requiring complete knowledge of
• 1:38:08
  Speaker 1: every possible higher energy detail. This is a remarkably powerful idea.
• 1:38:14
  Speaker 1: It means physics can often separate scales. You do not
• 1:38:18
  Speaker 1: need to understand the deepest possible structure of nature in
• 1:38:21
  Speaker 1: order to calculate how an electron scatters from a photon
• 1:38:24
  Speaker 1: at laboratory energies. The relevant low energy theory can still
• 1:38:28
  Speaker 1: make extraordinarily accurate predictions, and quantum electrodynamics does exactly that.
• 1:38:35
  Speaker 1: Its success is one of the strongest pieces of evidence
• 1:38:37
  Speaker 1: that quantum field theory is not merely an elegant conceptual framework.
• 1:38:43
  Speaker 1: It works quantitatively. It predicts subtle shifts in atomic energy levels.
• 1:38:48
  Speaker 1: It describes scattering probabilities. It accounts for quantum corrections to
• 1:38:53
  Speaker 1: magnetic moments. And it does so with a level of
• 1:38:56
  Speaker 1: precision that would be difficult to overstate. The infinities that
• 1:39:00
  Speaker 1: once threatened the theory did not disappear because physicists simply
• 1:39:04
  Speaker 1: looked away from them. They forced a deeper understanding of
• 1:39:07
  Speaker 1: what the parameters in a physical theory mean. Mass, charge
• 1:39:11
  Speaker 1: and coupling strength are not isolated numbers floating outside the physics.
• 1:39:16
  Speaker 1: They are connected to the scale of measurement and to
• 1:39:18
  Speaker 1: the quantum field surrounding the particles being studied. Renormalisation made
• 1:39:23
  Speaker 1: that clear, and what began as an attempt to rescue
• 1:39:27
  Speaker 1: quantum electrodynamics from infinity eventually became one of the most
• 1:39:31
  Speaker 1: important insights in all of modern theoretical physics. Symmetry is
• 1:39:36
  Speaker 1: one of those words that sounds simple until physics gets
• 1:39:39
  Speaker 1: hold of it. In ordinary life, we usually think of
• 1:39:42
  Speaker 1: symmetry as visual balance. A butterfly has roughly matching wings.
• 1:39:47
  Speaker 1: A circle looks the same after it has been rotated.
• 1:39:51
  Speaker 1: A snowflake can possess a repeating pattern around its centre.
• 1:39:54
  Speaker 1: In physics, the idea is broader. A symmetry is a
• 1:39:58
  Speaker 1: transformation that leaves some essential feature of a physical system
• 1:40:02
  Speaker 1: or the laws describing it unchanged. Imagine performing an experiment
• 1:40:06
  Speaker 1: today and repeating exactly the same experiment tomorrow under identical conditions.
• 1:40:12
  Speaker 1: We expect the fundamental laws to work in the same way.
• 1:40:16
  Speaker 1: Move the experiment from one side of the laboratory to
• 1:40:19
  Speaker 1: the other, and again we expect the same underlying laws.
• 1:40:23
  Speaker 1: Turn the entire apparatus around, and provided nothing else relevant
• 1:40:27
  Speaker 1: has changed, the basic laws should still apply. These simple
• 1:40:32
  Speaker 1: ideas lead to surprisingly deep consequences. One of the people
• 1:40:36
  Speaker 1: who revealed just how deep was the German mathematician Emmy Noether.
• 1:40:41
  Speaker 1: In 1918, Nethe published a theorem showing that continuous symmetries
• 1:40:46
  Speaker 1: of physical laws are connected to conservation laws. The connection
• 1:40:50
  Speaker 1: is remarkably elegant. If the laws of physics remain unchanged
• 1:40:54
  Speaker 1: when we shift an experiment forward or backward in time,
• 1:40:58
  Speaker 1: that time translation symmetry is associated with conservation of energy.
• 1:41:03
  Speaker 1: If the laws remain unchanged when the experiment is moved
• 1:41:06
  Speaker 1: from one location to another, Spatial translation symmetry is associated
• 1:41:11
  Speaker 1: with conservation of momentum. If the laws remain unchanged when
• 1:41:15
  Speaker 1: the experiment is rotated, rotational symmetry is associated with conservation
• 1:41:20
  Speaker 1: of angular momentum. So conservation laws are not simply isolated
• 1:41:24
  Speaker 1: rules that nature happens to obey. They emerge from deeper symmetries.
• 1:41:30
  Speaker 1: Energy conservation is connected to the fact that the fundamental
• 1:41:33
  Speaker 1: laws do not seem to care whether an experiment is
• 1:41:36
  Speaker 1: performed today or tomorrow. Momentum conservation is connected to the
• 1:41:40
  Speaker 1: fact that the laws do not prefer one ordinary location
• 1:41:44
  Speaker 1: in space over another. Angular momentum conservation is connected to
• 1:41:49
  Speaker 1: the fact that the laws do not fundamentally favour one
• 1:41:52
  Speaker 1: direction of orientation over another. Noether's theorem became one of
• 1:41:57
  Speaker 1: the central organising ideas of modern theoretical physics. But quantum
• 1:42:02
  Speaker 1: field theory takes symmetry even further. One especially important type
• 1:42:07
  Speaker 1: is gauge symmetry. Gauge symmetry is not a visual symmetry,
• 1:42:11
  Speaker 1: like a mirror reflection. It is a symmetry in the
• 1:42:14
  Speaker 1: mathematical description of fields. To see the basic idea, consider
• 1:42:18
  Speaker 1: the electron field. In quantum mechanics, the state of an
• 1:42:22
  Speaker 1: electron involves a quantity called a phase. The absolute value
• 1:42:27
  Speaker 1: of that phase cannot normally be observed directly. If the
• 1:42:30
  Speaker 1: phase of the electron field is changed everywhere by the
• 1:42:33
  Speaker 1: same amount, measurable predictions remain unchanged. That is already a
• 1:42:38
  Speaker 1: kind of symmetry. But physicists can demand something stronger. Suppose
• 1:42:43
  Speaker 1: the phase is allowed to change differently from one point
• 1:42:46
  Speaker 1: in spacetime to another. Now the mathematical description no longer
• 1:42:50
  Speaker 1: remains consistent by itself. To restore the symmetry, another field
• 1:42:55
  Speaker 1: has to be introduced. That field is associated with electromagnetism.
• 1:43:01
  Speaker 1: In this way, the requirement of a local gauge symmetry
• 1:43:03
  Speaker 1: naturally introduces the electromagnetic gauge field, and determines how it
• 1:43:08
  Speaker 1: couples to electrically charged matter. This is one of the
• 1:43:12
  Speaker 1: remarkable features of modern field theory. A symmetry requirement can
• 1:43:17
  Speaker 1: help dictate the form of an interaction. The electromagnetic case
• 1:43:21
  Speaker 1: is mathematically associated with a relatively simple type of gauge symmetry.
• 1:43:26
  Speaker 1: It belongs to what mathematicians call an abelian symmetry group.
• 1:43:29
  Speaker 1: The word abelian means, roughly speaking, that the order in
• 1:43:33
  Speaker 1: which two symmetry transformations are applied does not matter. do
• 1:43:38
  Speaker 1: one transformation and then another, or reverse the order and
• 1:43:42
  Speaker 1: you end up with the same result. That simplicity is
• 1:43:46
  Speaker 1: reflected in the behaviour of the electromagnetic field. The photon
• 1:43:50
  Speaker 1: is the gauge boson associated with electromagnetism. Photons carry energy
• 1:43:56
  Speaker 1: and momentum, but they do not themselves carry electric charge.
• 1:44:00
  Speaker 1: As a result, photons do not directly interact with one
• 1:44:03
  Speaker 1: another through the simplest electromagnetic coupling in the same way
• 1:44:07
  Speaker 1: that electrically charged particles interact with the electromagnetic field. The
• 1:44:13
  Speaker 1: story becomes richer with the work of Chen Ning Yang
• 1:44:16
  Speaker 1: and Robert Mills. In 1954, Yang and Mills developed a
• 1:44:21
  Speaker 1: more general kind of gauge theory, based on what are
• 1:44:24
  Speaker 1: called non-abelian symmetries. Here, the order of transformations can matter.
• 1:44:30
  Speaker 1: Apply one transformation, and then another, and you may obtain
• 1:44:34
  Speaker 1: a different result from performing them in the opposite order.
• 1:44:37
  Speaker 1: that apparently abstract mathematical change has major physical consequences. In
• 1:44:42
  Speaker 1: non-abelian gauge theories, the gauge bosons can themselves carry the
• 1:44:47
  Speaker 1: relevant type of gauge charge. This means the force-carrying fields
• 1:44:51
  Speaker 1: can interact with one another. The strong interaction provides the
• 1:44:55
  Speaker 1: clearest example. Its gauge bosons are gluons. Quarks carry colour charge,
• 1:45:00
  Speaker 1: the type of charge associated with the strong interaction. But
• 1:45:04
  Speaker 1: unlike photons, Gluons themselves also carry colour charge, so gluons
• 1:45:09
  Speaker 1: can interact directly with other gluons. This self-interaction is one
• 1:45:13
  Speaker 1: of the central reasons the strong force behaves so differently
• 1:45:16
  Speaker 1: from electromagnetism. It contributes to phenomena such as quark confinement,
• 1:45:21
  Speaker 1: which prevents isolated quarks from being observed under ordinary conditions.
• 1:45:26
  Speaker 1: Non-abelian gauge theory also became essential to the modern description
• 1:45:30
  Speaker 1: of the weak interaction. The weak force is mediated by
• 1:45:33
  Speaker 1: massive gauge bosons known as the charged weak bosons and
• 1:45:37
  Speaker 1: the neutral weak boson. These particles participate in processes such
• 1:45:42
  Speaker 1: as radioactive beta decay and in reactions involving neutrinos. The
• 1:45:48
  Speaker 1: weak interaction and electromagnetism are now understood as parts of
• 1:45:52
  Speaker 1: a larger electroweak gauge theory. At sufficiently high energies, the
• 1:45:56
  Speaker 1: distinction between them becomes less fundamental than it appears in
• 1:46:00
  Speaker 1: ordinary low-energy conditions. Gauge symmetry therefore provides much more than
• 1:46:06
  Speaker 1: an elegant mathematical decoration. It strongly constrains what kinds of
• 1:46:10
  Speaker 1: interactions can appear in a theory. Once the relevant fields
• 1:46:14
  Speaker 1: and symmetries are chosen, the possible interaction terms are limited.
• 1:46:19
  Speaker 1: The theory cannot simply contain any imaginable coupling. The structure
• 1:46:24
  Speaker 1: has to respect the symmetry. This is one reason the
• 1:46:27
  Speaker 1: standard model is so tightly organised. The photon belongs to
• 1:46:31
  Speaker 1: the electromagnetic gauge field. The gluons belong to the gauge
• 1:46:35
  Speaker 1: fields of the strong interaction. The weak gauge bosons belong
• 1:46:39
  Speaker 1: to the electroweak structure. Their properties are tied to the
• 1:46:43
  Speaker 1: symmetries from which the theory is built. But gauge symmetry
• 1:46:47
  Speaker 1: does not answer every question by itself. One important complication
• 1:46:52
  Speaker 1: concerns mass. The photon is massless. The weak gauge bosons
• 1:46:56
  Speaker 1: are not. If mass terms are inserted into the theory
• 1:47:00
  Speaker 1: in the simplest way, they can spoil the gauge structure
• 1:47:03
  Speaker 1: that made the theory work in the first place. This
• 1:47:06
  Speaker 1: is one reason the Higgs field became so important. Through
• 1:47:10
  Speaker 1: spontaneous symmetry breaking and interactions with the Higgs field, the
• 1:47:14
  Speaker 1: electroweak theory can retain its underlying gauge structure while the
• 1:47:18
  Speaker 1: weak gauge bosons acquire mass. So even here, symmetry is
• 1:47:22
  Speaker 1: not simply broken and forgotten. It remains embedded in the
• 1:47:25
  Speaker 1: deeper mathematical structure. This is a recurring theme in particle physics.
• 1:47:31
  Speaker 1: Symmetry tells physicists what must remain unchanged. Breaking or hiding
• 1:47:35
  Speaker 1: a symmetry can explain why the world we observe does
• 1:47:38
  Speaker 1: not look perfectly symmetric. And the relationship between the two
• 1:47:42
  Speaker 1: can determine which particles exist, which interactions are allowed, and
• 1:47:47
  Speaker 1: which quantities are conserved. That makes symmetry one of the
• 1:47:50
  Speaker 1: most powerful ideas in quantum field theory. Instead of treating
• 1:47:54
  Speaker 1: every particle and every interaction as a separate rule to
• 1:47:58
  Speaker 1: be memorised, physicists can search for a smaller collection of
• 1:48:02
  Speaker 1: principles from which many of those rules emerge. The electron field,
• 1:48:07
  Speaker 1: the photon field, the quark fields and the gluon fields
• 1:48:10
  Speaker 1: may look like a complicated catalogue. Symmetry reveals that they
• 1:48:15
  Speaker 1: are organised. Their interactions are not random. Their conservation law
• 1:48:20
  Speaker 1: are not independent coincidences. And the force-carrying fields themselves arise
• 1:48:25
  Speaker 1: within mathematical structures that restrict how nature is allowed to behave.
• 1:48:30
  Speaker 1: At first glance, symmetry may seem like little more than
• 1:48:32
  Speaker 1: an aesthetic idea. In quantum field theory, it becomes something
• 1:48:37
  Speaker 1: far more powerful. It helps build the laws themselves. Most
• 1:48:42
  Speaker 1: of the visible matter around us is built from particles
• 1:48:45
  Speaker 1: that cannot normally be pulled apart and examined individually. Protons
• 1:48:49
  Speaker 1: and neutrons sit inside atomic nuclei. Inside those protons and
• 1:48:54
  Speaker 1: neutrons are quarks. And binding the quarks together is one
• 1:48:59
  Speaker 1: of the strangest interactions described by modern physics. The theory
• 1:49:03
  Speaker 1: is called quantum chromodynamics. Quantum chromodynamics is the quantum field
• 1:49:08
  Speaker 1: theory of the strong interaction. It describes quark fields, gluon fields,
• 1:49:14
  Speaker 1: and the rules governing how they interact. The name chromodynamics
• 1:49:18
  Speaker 1: comes from a property known as colour charge. This has
• 1:49:21
  Speaker 1: nothing to do with ordinary colour. Quarks are not literally red, green,
• 1:49:26
  Speaker 1: or blue in the visual sense. The words are simply
• 1:49:30
  Speaker 1: labels physicists use to distinguish different types of strong interaction charge.
• 1:49:35
  Speaker 1: In the mathematical language of the theory, quarks can carry
• 1:49:38
  Speaker 1: three varieties of colour charge, traditionally called red, green and blue.
• 1:49:44
  Speaker 1: Antiquarks carry corresponding anti-colours. The strong interaction acts on these
• 1:49:49
  Speaker 1: colour charges in much the same broad sense that electromagnetism
• 1:49:54
  Speaker 1: acts on electric charge. But the similarity does not go
• 1:49:59
  Speaker 1: very far. Electromagnetism is mediated by photons. The strong interaction
• 1:50:05
  Speaker 1: is mediated by gluons. And gluons behave in a much
• 1:50:08
  Speaker 1: more complicated way. There are eight independent gluon states in
• 1:50:13
  Speaker 1: quantum chromodynamics. These gluons are the gauge bosons of the
• 1:50:17
  Speaker 1: strong interaction. A photon carries no electric charge. That means
• 1:50:21
  Speaker 1: photons do not directly interact with one another through the
• 1:50:24
  Speaker 1: simplest electromagnetic coupling. Gluons are different. They themselves carry colour charge.
• 1:50:32
  Speaker 1: so gluons can interact directly with other gluons. This single
• 1:50:35
  Speaker 1: difference makes the strong interaction far more complicated than the electromagnetism.
• 1:50:41
  Speaker 1: The field that carries the interaction also participates in the interaction.
• 1:50:45
  Speaker 1: Gluons influence quarks, quarks influence gluons, and gluons influence one another.
• 1:50:52
  Speaker 1: The result is a highly dynamic quantum field system. One
• 1:50:56
  Speaker 1: of the most remarkable consequences is confinement. Under ordinary conditions,
• 1:51:02
  Speaker 1: isolated quarks are not observed freely. Neither are isolated gluons. Instead,
• 1:51:08
  Speaker 1: they appear bound together inside composite particles. Three quarks can
• 1:51:13
  Speaker 1: combine to form particles such as protons and neutrons. A
• 1:51:17
  Speaker 1: quark and an antiquark can combine to form particles known
• 1:51:20
  Speaker 1: as mesons. The combinations observed in nature are colour neutral overall.
• 1:51:26
  Speaker 1: This is sometimes loosely compared with combining ordinary colours to
• 1:51:29
  Speaker 1: make white. Although again, the word colour here is purely
• 1:51:33
  Speaker 1: a mathematical label. Confinement means that trying to pull quarks
• 1:51:37
  Speaker 1: apart does not simply make the strong force gradually fade away.
• 1:51:42
  Speaker 1: At sufficiently large separations, the energy stored in the gluon
• 1:51:46
  Speaker 1: field between the quarks can increase. If enough energy is supplied,
• 1:51:51
  Speaker 1: it becomes energetically favourable to create new quark and antiquark pairs.
• 1:51:56
  Speaker 1: Instead of eventually isolating one quark, you tend to produce
• 1:52:00
  Speaker 1: new colour neutral particles. The harder you pull, the more
• 1:52:03
  Speaker 1: the system rearranges itself. Nature is apparently quite determined not
• 1:52:08
  Speaker 1: to hand over a single quark. This means the familiar
• 1:52:11
  Speaker 1: picture of a proton, as three tiny quarks sitting inside
• 1:52:15
  Speaker 1: a small sphere, is far too simple. A proton does
• 1:52:18
  Speaker 1: contain what are often called three valence quarks. Two are
• 1:52:22
  Speaker 1: up quarks, and one is a down quark. But the
• 1:52:26
  Speaker 1: proton is not a miniature bag containing three static objects.
• 1:52:30
  Speaker 1: It is a highly dynamic quantum field system. Inside it,
• 1:52:34
  Speaker 1: quark fields and gluon fields are constantly interacting. Gluons carry
• 1:52:40
  Speaker 1: a large amount of the proton's momentum. Quantum processes also
• 1:52:44
  Speaker 1: allow temporary quark and antiquark contributions to appear within the
• 1:52:48
  Speaker 1: full quantum state. The proton is therefore better understood as
• 1:52:52
  Speaker 1: a complicated bound state of quantum chromodynamics than as three
• 1:52:56
  Speaker 1: little beads held together by invisible string. This matters enormously
• 1:53:02
  Speaker 1: when we ask where the proton's mass comes from. The
• 1:53:05
  Speaker 1: masses of the up and down quarks contribute only a
• 1:53:08
  Speaker 1: small fraction of the proton's total mass. Most of the
• 1:53:12
  Speaker 1: proton's mass arises from the energy associated with the strong interaction.
• 1:53:17
  Speaker 1: including the motion of the quarks and the energy stored
• 1:53:20
  Speaker 1: in the gluon fields. In other words, much of the
• 1:53:23
  Speaker 1: mass of ordinary matter does not simply come from adding
• 1:53:26
  Speaker 1: up the rest masses of tiny constituents. It emerges from
• 1:53:30
  Speaker 1: the dynamics of interacting quantum fields. That is one of
• 1:53:34
  Speaker 1: the clearest examples of just how physical these fields are.
• 1:53:38
  Speaker 1: There is another remarkable feature of quantum chromodynamics. At extremely
• 1:53:42
  Speaker 1: short distances, or equivalently at very high energies, the strong
• 1:53:46
  Speaker 1: interaction becomes weaker. This property is called asymptotic freedom. It
• 1:53:52
  Speaker 1: was discovered theoretically in the early 1970s by David Gross,
• 1:53:57
  Speaker 1: Frank Wilczek and Hugh David Politzer. At first glance, this
• 1:54:02
  Speaker 1: seems almost backwards. The strong interaction is called strong because,
• 1:54:07
  Speaker 1: at familiar particle physics scales, it is indeed powerful. Yet
• 1:54:12
  Speaker 1: when quarks are probed at extremely short distances, They can
• 1:54:16
  Speaker 1: behave almost as though they are relatively free. The effective
• 1:54:19
  Speaker 1: strength of the interaction decreases as the energy scale increases.
• 1:54:24
  Speaker 1: This helped explain results from high-energy scattering experiments. When energetic
• 1:54:30
  Speaker 1: electrons were fired into protons, the electrons appeared to scatter
• 1:54:34
  Speaker 1: from compact constituents inside. Those constituents were eventually understood as quarks.
• 1:54:40
  Speaker 1: At sufficiently high energies, the quarks behaved enough like weakly
• 1:54:44
  Speaker 1: interacting particles that perturbative calculations became useful. This was a
• 1:54:49
  Speaker 1: major clue that quantum chromodynamics was describing the right physics.
• 1:54:53
  Speaker 1: The situation changes at lower energies. As the energy scale
• 1:54:57
  Speaker 1: falls and the typical distances become larger, the strong interaction
• 1:55:01
  Speaker 1: becomes stronger. Then perturbation theory becomes much less reliable. the
• 1:55:07
  Speaker 1: neat hierarchy of smaller and smaller corrections begins to break down.
• 1:55:12
  Speaker 1: Physicists need other methods. One of the most important is
• 1:55:16
  Speaker 1: lattice quantum chromodynamics. In this approach, spacetime is represented mathematically
• 1:55:22
  Speaker 1: as a finite grid. The quantum fields are defined on
• 1:55:26
  Speaker 1: this lattice, allowing powerful computers to calculate properties of strongly
• 1:55:31
  Speaker 1: interacting systems numerically. The idea is not that spacetime has
• 1:55:36
  Speaker 1: been experimentally proven to be a physical grid. The lattice
• 1:55:40
  Speaker 1: is a computational technique. By making the grid finer and
• 1:55:44
  Speaker 1: the simulated region larger, physicists can work toward predictions that
• 1:55:48
  Speaker 1: approximate the continuous theory. Lattice calculations have become essential for
• 1:55:54
  Speaker 1: studying phenomena that cannot easily be handled using perturbation theory.
• 1:55:59
  Speaker 1: They allow researchers to calculate quantities such as the masses
• 1:56:02
  Speaker 1: of hadrons, properties of the strong interaction, and aspects of
• 1:56:06
  Speaker 1: how quarks and gluons behave when they are tightly bound.
• 1:56:11
  Speaker 1: The agreement between many lattice calculations and experiment provides strong
• 1:56:15
  Speaker 1: support for quantum chromodynamics, and the theory has an unusual range.
• 1:56:21
  Speaker 1: It begins with abstract ideas about gauge symmetry and colour charge.
• 1:56:26
  Speaker 1: From those ideas come gluon self-interactions. From those interactions come
• 1:56:31
  Speaker 1: confinement and the complicated internal structure of protons and neutrons.
• 1:56:37
  Speaker 1: And from that structure comes most of the mass in
• 1:56:40
  Speaker 1: the ordinary matter surrounding us. The chair beneath you, the
• 1:56:44
  Speaker 1: walls around you, and the atoms in your body all
• 1:56:46
  Speaker 1: contain nuclei whose mass is dominated by strong interaction energy.
• 1:56:53
  Speaker 1: So quantum chromodynamics is not simply an exotic theory for
• 1:56:58
  Speaker 1: distant particle accelerators. It is part of the explanation for
• 1:57:02
  Speaker 1: why ordinary matter weighs what it does. The world around
• 1:57:05
  Speaker 1: us feels solid and familiar, but inside each proton and
• 1:57:09
  Speaker 1: neutron sits an extraordinarily active system of quark and gluon fields.
• 1:57:15
  Speaker 1: Those fields interact, exchange energy, and bind themselves into stable structures,
• 1:57:21
  Speaker 1: and from that restless quantum activity emerges much of the
• 1:57:25
  Speaker 1: mass of the visible world. For most of everyday life,
• 1:57:29
  Speaker 1: Electromagnetism and the weak interaction seem to have almost nothing
• 1:57:33
  Speaker 1: in common. Electromagnetism reaches across enormous distances. It holds electrons
• 1:57:39
  Speaker 1: within atoms, allows atoms to form molecules, carries light across
• 1:57:44
  Speaker 1: the universe, and produces everything from magnets to radio waves.
• 1:57:50
  Speaker 1: The weak interaction behaves very differently. It acts over distances
• 1:57:54
  Speaker 1: far smaller than an atomic nucleus. and participates in processes
• 1:57:58
  Speaker 1: such as radioactive beta decay. It is also essential to
• 1:58:03
  Speaker 1: reactions involving neutrinos and to the chain of nuclear processes
• 1:58:06
  Speaker 1: that ultimately allows stars such as the Sun to shine.
• 1:58:10
  Speaker 1: Yet modern particle physics describes these apparently different forces as
• 1:58:14
  Speaker 1: parts of a single underlying structure. This is the electroweak theory.
• 1:58:19
  Speaker 1: Its development involved several physicists, particularly Sheldon Glashow, Abdus Salam
• 1:58:25
  Speaker 1: and Steven Weinberg. During the 1960s, Their work helped establish
• 1:58:31
  Speaker 1: a gauge theory in which electromagnetism and the weak interaction
• 1:58:34
  Speaker 1: emerge from a common electroweak framework. At very high energies,
• 1:58:39
  Speaker 1: the distinction between the electromagnetic and weak interactions becomes less pronounced.
• 1:58:45
  Speaker 1: Under the conditions we usually encounter, however, the symmetry between
• 1:58:49
  Speaker 1: them is hidden, leaving one interaction long-ranged and the other
• 1:58:53
  Speaker 1: extraordinarily short-ranged. That difference presented physicists with a serious problem,
• 1:59:00
  Speaker 1: The photon, which carries the electromagnetic interaction, has no rest mass.
• 1:59:06
  Speaker 1: The particles associated with the weak interaction are very different.
• 1:59:10
  Speaker 1: The charged weak bosons have masses of roughly 80 billion
• 1:59:14
  Speaker 1: electron volts when expressed as energy, while the neutral weak
• 1:59:18
  Speaker 1: boson has a mass of roughly 91 billion electron volts.
• 1:59:23
  Speaker 1: Those are enormous masses by elementary particle standards. And this mattered.
• 1:59:29
  Speaker 1: because gauge theories work only when their mathematical structure is
• 1:59:33
  Speaker 1: carefully preserved. Physicists could not simply write down a mass
• 1:59:37
  Speaker 1: for the weak gauge bosons and carry on as though
• 1:59:40
  Speaker 1: nothing had happened. Adding such masses directly to the equations
• 1:59:44
  Speaker 1: in the obvious way would spoil the gauge symmetry on
• 1:59:47
  Speaker 1: which the theory depended and would undermine the mathematical consistency
• 1:59:52
  Speaker 1: required for a renormalisable quantum field theory. Something subtler was needed.
• 1:59:58
  Speaker 1: The solution involved a remarkable idea called spontaneous symmetry breaking.
• 2:00:03
  Speaker 1: The phrase can sound as though a symmetry is simply destroyed,
• 2:00:06
  Speaker 1: but that is not quite what happens. The fundamental equations
• 2:00:10
  Speaker 1: can retain their symmetry even when the lowest energy state
• 2:00:14
  Speaker 1: of the system does not display that symmetry in an
• 2:00:17
  Speaker 1: obvious way. A familiar analogy is a perfectly balanced object
• 2:00:21
  Speaker 1: sitting at the top of a rounded hill. The hill
• 2:00:24
  Speaker 1: itself may look identical in every horizontal direction. Nothing in
• 2:00:29
  Speaker 1: its shape chooses north, south, east or west. But once
• 2:00:33
  Speaker 1: the object rolls down and settles somewhere around the bottom,
• 2:00:36
  Speaker 1: it has selected one particular direction. The underlying system was symmetric.
• 2:00:42
  Speaker 1: The state it settled into was not. This is roughly
• 2:00:47
  Speaker 1: the idea behind spontaneous symmetry breaking, although the actual electroweak
• 2:00:51
  Speaker 1: theory is considerably more sophisticated than a ball on a hill.
• 2:00:55
  Speaker 1: During the 1960s, several researchers developed mechanisms showing how this
• 2:01:00
  Speaker 1: could work in quantum field theory. François Englert and Robert
• 2:01:04
  Speaker 1: Brut published one version. Peter Higgs independently developed a closely
• 2:01:09
  Speaker 1: related formulation. Other important contributions came from Gerald Guralnik, Carl
• 2:01:14
  Speaker 1: Richard Hagen and Thomas Kibble. The central ingredient became known
• 2:01:19
  Speaker 1: as the Higgs field. Unlike many familiar fields, the Higgs
• 2:01:22
  Speaker 1: field has an unusual vacuum state. Recall that the vacuum
• 2:01:26
  Speaker 1: in quantum field theory is the lowest energy state of
• 2:01:29
  Speaker 1: the fields. For the Higgs field, the lowest energy configuration
• 2:01:33
  Speaker 1: does not correspond to the field simply averaging to zero everywhere. Instead,
• 2:01:38
  Speaker 1: the Higgs field has a non-zero vacuum expectation value. In
• 2:01:42
  Speaker 1: simple terms, even empty space contains a persistent Higgs field background.
• 2:01:48
  Speaker 1: The universe does not need to contain a Higgs boson
• 2:01:50
  Speaker 1: at every location for the Higgs field to be present.
• 2:01:54
  Speaker 1: The field itself occupies its non-zero vacuum state throughout space.
• 2:01:59
  Speaker 1: This changes the behaviour of other fields that interact with it.
• 2:02:04
  Speaker 1: Within the electroweak theory, the interaction with the Higgs field
• 2:02:07
  Speaker 1: gives mass to the weak gauge bosons. The underlying electroweak
• 2:02:12
  Speaker 1: gauge structure remains present, but the vacuum state distinguishes the
• 2:02:16
  Speaker 1: electromagnetic direction from the weak ones. One combination of the
• 2:02:21
  Speaker 1: original electroweak gauge fields remains massless. That combination becomes the photon.
• 2:02:27
  Speaker 1: Other combinations acquire mass and become the charged weak bosons
• 2:02:30
  Speaker 1: and the neutral weak boson. This is why electromagnetism remains
• 2:02:35
  Speaker 1: long-ranged while the weak interaction has such a short range.
• 2:02:39
  Speaker 1: A massive force-carrying particle cannot mediate an interaction over unlimited
• 2:02:43
  Speaker 1: distances in the same way as a massless photon. The
• 2:02:48
  Speaker 1: Higgs field also contributes to the masses of fundamental matter particles. Electrons,
• 2:02:53
  Speaker 1: quarks and other fermions can interact with the Higgs field
• 2:02:57
  Speaker 1: through couplings known as Yukawa interactions, named after the Japanese
• 2:03:01
  Speaker 1: physicist Hideki Yukawa. When the Higgs field settles into its
• 2:03:06
  Speaker 1: non-zero vacuum state, these interactions generate effective mass terms for
• 2:03:11
  Speaker 1: the fermions. Different fermions interact with the Higgs field with
• 2:03:15
  Speaker 1: different strengths. The electron couples relatively weakly. The top quark
• 2:03:20
  Speaker 1: couples far more strongly. and is therefore much more massive.
• 2:03:24
  Speaker 1: Why those coupling strengths have the particular values observed in
• 2:03:28
  Speaker 1: nature remains an open question. The standard model contains those values,
• 2:03:33
  Speaker 1: but it does not currently explain why they form the
• 2:03:36
  Speaker 1: pattern they do. This is also where a very common
• 2:03:39
  Speaker 1: misunderstanding should be cleared away. The Higgs field does not
• 2:03:43
  Speaker 1: provide all or even most of the mass of familiar
• 2:03:46
  Speaker 1: ordinary matter. It does contribute to the masses of fundamental
• 2:03:50
  Speaker 1: quarks and electrons. But most of the mass of a
• 2:03:53
  Speaker 1: proton or neutron does not come directly from the Higgs-generated
• 2:03:57
  Speaker 1: masses of the quarks inside it. Most of that mass
• 2:04:00
  Speaker 1: arises from the energy of the strong interaction. Quarks and
• 2:04:04
  Speaker 1: gluons move and interact inside protons and neutrons, and the
• 2:04:08
  Speaker 1: energy stored in those quantum chromodynamic fields contributes the overwhelming
• 2:04:13
  Speaker 1: majority of the total mass. So it would be misleading
• 2:04:16
  Speaker 1: to say that the Higgs field simply gives everything its mass.
• 2:04:20
  Speaker 1: It plays an essential role, but the full story is
• 2:04:23
  Speaker 1: more interesting. The Higgs field also provides one of the
• 2:04:27
  Speaker 1: clearest examples of what a particle means in quantum field theory.
• 2:04:32
  Speaker 1: The field exists throughout space. A Higgs boson is a
• 2:04:36
  Speaker 1: quantist excitation of that field. The particle is not the
• 2:04:40
  Speaker 1: field itself. It is one particular excited state of the field.
• 2:04:45
  Speaker 1: For decades, the Higgs boson remained the missing experimental piece
• 2:04:48
  Speaker 1: of the electroweak theory. If the Higgs field existed in
• 2:04:52
  Speaker 1: the way the standard model predicted, sufficiently energetic collisions should
• 2:04:56
  Speaker 1: occasionally excite it strongly enough to produce detectable Higgs bosons.
• 2:05:01
  Speaker 1: That search eventually focused on the Large Hedron Collider at
• 2:05:05
  Speaker 1: the European Organisation for Nuclear Research near Geneva. Protons travelling
• 2:05:10
  Speaker 1: in opposite directions around the collider were accelerated to enormous
• 2:05:13
  Speaker 1: energies and brought into collision. Most collisions produced familiar particles.
• 2:05:19
  Speaker 1: Very rarely, however, The conditions could produce a Higgs boson.
• 2:05:24
  Speaker 1: The Higgs boson itself is extremely short-lived. It decays almost
• 2:05:29
  Speaker 1: immediately into other particles, so experiments cannot simply watch one
• 2:05:33
  Speaker 1: sitting inside a detector. Instead, researchers reconstruct its existence from
• 2:05:39
  Speaker 1: the energies, momenta and identities of its decay products. In 2012,
• 2:05:45
  Speaker 1: two large experiments at the Large Hadron Collider the Ateroidal
• 2:05:49
  Speaker 1: Large Hadron Collider Apparatus Experiment and the Compact Muon Solenoid
• 2:05:54
  Speaker 1: Experiment independently announced evidence for a new particle with a
• 2:05:59
  Speaker 1: mass of roughly 125 billion electron volts. Further measurements of
• 2:06:04
  Speaker 1: its spin, decay patterns and interactions increasingly matched the behaviour
• 2:06:09
  Speaker 1: predicted for the standard model Higgs boson. The discovery completed
• 2:06:13
  Speaker 1: a remarkable chain of reasonings. Gauge symmetry had helped organise
• 2:06:17
  Speaker 1: the electroweak interaction. Massive weak bosons had created a mathematical difficulty,
• 2:06:24
  Speaker 1: spontaneous symmetry breaking, and the Higgs field had provided a solution.
• 2:06:29
  Speaker 1: The vacuum itself turned out to possess a non-zero Higgs
• 2:06:33
  Speaker 1: field value. Interactions with that background produced masses for the
• 2:06:37
  Speaker 1: weak gauge bosons and contributed to fermion masses, and an
• 2:06:41
  Speaker 1: excitation of the same field could be created experimentally and
• 2:06:45
  Speaker 1: observed through its decay products. Few examples bring the ideas
• 2:06:50
  Speaker 1: of quantum field theory together so neatly. Fields fill spacetime,
• 2:06:55
  Speaker 1: their vacuum states matter, symmetry restricts the laws, interactions between
• 2:07:01
  Speaker 1: fields determine measurable properties, and something as apparently particle-like as
• 2:07:06
  Speaker 1: the Higgs boson emerges as an excitation of a field
• 2:07:10
  Speaker 1: that is present everywhere. The Higgs story is therefore about
• 2:07:13
  Speaker 1: much more than one particle discovered in 2012. It is
• 2:07:17
  Speaker 1: an example of how modern physics builds the visible properties
• 2:07:21
  Speaker 1: of nature, from the behaviour, interactions and symmetries of quantum fields.
• 2:07:26
  Speaker 1: The standard model is often introduced as though it were
• 2:07:29
  Speaker 1: a list. There are electrons, neutrinos and quarks. There are
• 2:07:34
  Speaker 1: photons and gluons. There are the particles responsible for the
• 2:07:38
  Speaker 1: weak interaction, along with the Higgs boson. placed together on
• 2:07:42
  Speaker 1: a chart, they can look rather like a collection of
• 2:07:44
  Speaker 1: microscopic ingredients that physicists have gradually discovered and catalogued. But
• 2:07:51
  Speaker 1: that is not really what the Standard Model is. At
• 2:07:54
  Speaker 1: a deeper level, the Standard Model is a quantum field theory.
• 2:07:58
  Speaker 1: Its fundamental ingredients are quantum fields, and the familiar particles
• 2:08:02
  Speaker 1: appear as excitations of those fields. The theory brings together
• 2:08:07
  Speaker 1: three of the four known fundamental interactions, electromagnetism, the weak interaction,
• 2:08:13
  Speaker 1: and the strong interaction. Gravity is not included within the
• 2:08:18
  Speaker 1: standard model. The matter fields can first be divided into
• 2:08:22
  Speaker 1: two broad families. There are quarks and there are leptons.
• 2:08:26
  Speaker 1: Quarks include the up quark, down quark, charm quark, strange quark,
• 2:08:32
  Speaker 1: top quark and bottom quark. They participate in the strong
• 2:08:36
  Speaker 1: interaction as well as the weak interaction. And electrically charged
• 2:08:41
  Speaker 1: quarks also participate in electromagnetism. The most familiar leptons include
• 2:08:46
  Speaker 1: the electron and the neutrinos. There are three charged leptons,
• 2:08:50
  Speaker 1: the electron, muon and tau. Each has an associated neutrino.
• 2:08:55
  Speaker 1: Unlike quarks, leptons do not participate in the strong interaction.
• 2:08:59
  Speaker 1: All of these matter particles are fermions. They possess spin
• 2:09:03
  Speaker 1: one half and obey the Pauli exclusion principle. But in
• 2:09:07
  Speaker 1: quantum field theory, it is more precise to say that
• 2:09:10
  Speaker 1: each type of fundamental fermion is associated with a corresponding
• 2:09:14
  Speaker 1: quantum field. An electron is an excitation of the electron field.
• 2:09:19
  Speaker 1: An up quark is an excitation of the up quark field.
• 2:09:23
  Speaker 1: A neutrino corresponds to an excitation of its relevant neutrino field.
• 2:09:28
  Speaker 1: The interactions between these matter fields are described using gauge fields.
• 2:09:32
  Speaker 1: The electromagnetic gauge field is associated with photons. The strong
• 2:09:37
  Speaker 1: interaction gauge fields are associated with gluons. The electroweak gauge
• 2:09:42
  Speaker 1: structure produces the photon along with the massive weak gauge
• 2:09:46
  Speaker 1: bosons after electroweak symmetry breaking. These gauge bosons are bosons
• 2:09:51
  Speaker 1: rather than fermions. They possess integer spin and follow different
• 2:09:56
  Speaker 1: quantum statistical rules from the matter particles. So the standard
• 2:10:00
  Speaker 1: model is not simply saying that several different species of
• 2:10:03
  Speaker 1: tiny object happen to exist. It specifies fields, their symmetries,
• 2:10:09
  Speaker 1: and the allowed ways those fields can interact. Then there
• 2:10:12
  Speaker 1: is the Higgs field. The Higgs field occupies a special
• 2:10:16
  Speaker 1: place in the theory, because its vacuum state has a
• 2:10:19
  Speaker 1: non-zero value. Its interactions with the electroweak gauge fields allow
• 2:10:24
  Speaker 1: the weak gauge bosons to acquire mass while leaving the
• 2:10:27
  Speaker 1: photon massless. Its Yukawa interactions with fundamental fermions also contribute
• 2:10:33
  Speaker 1: to the masses of particles, such as electrons and quarks.
• 2:10:38
  Speaker 1: The Higgs boson is simply the detectable quantum excitation associated
• 2:10:42
  Speaker 1: with this same Higgs field. Together, the matter fields, gauge
• 2:10:46
  Speaker 1: fields and Higgs field form the field content of the
• 2:10:50
  Speaker 1: standard model. Antimatter also fits naturally into this framework. For
• 2:10:55
  Speaker 1: every fundamental matter particle, there is a corresponding antiparticle. The
• 2:10:59
  Speaker 1: electron has the positron. Quarks have antiquarks. Neutrinos have corresponding
• 2:11:05
  Speaker 1: antineutrino states. Although the deeper question of whether neutrinos may
• 2:11:09
  Speaker 1: also possess a Majorana component remains experimentally unresolved. A particle
• 2:11:14
  Speaker 1: and its antiparticle have the same mass. They carry opposite
• 2:11:18
  Speaker 1: values of appropriate additive quantum numbers, such as electric charge
• 2:11:23
  Speaker 1: in the case of an electron and positron. In relativistic
• 2:11:27
  Speaker 1: quantum field theory, antiparticles are not awkward extras that have
• 2:11:31
  Speaker 1: to be attached to the theory afterward. They arise naturally
• 2:11:35
  Speaker 1: from the structure of the relativistic fields themselves. The standard
• 2:11:40
  Speaker 1: model can then be tested by examining what those fields
• 2:11:43
  Speaker 1: predict will happen. High-energy particle colliders provide some of the
• 2:11:47
  Speaker 1: clearest examples. Physicists accelerate particles to enormous energies, collide them
• 2:11:54
  Speaker 1: and surround the interaction points with detectors. Each collision can
• 2:11:58
  Speaker 1: produce a different final state. Quantum field theory does not
• 2:12:03
  Speaker 1: generally predict one inevitable outcome for every collision. Instead, it
• 2:12:08
  Speaker 1: calculates probabilities for possible processes. A particular reaction may happen frequently.
• 2:12:15
  Speaker 1: Another may occur only once in millions or billions of collisions.
• 2:12:19
  Speaker 1: Some possibilities are forbidden by conservation laws or by the
• 2:12:22
  Speaker 1: structure of the interactions. Testing the standard model therefore requires
• 2:12:27
  Speaker 1: enormous collections of events. Physicists compare the observed frequencies, energies,
• 2:12:33
  Speaker 1: momenta and angular distributions of particles with the statistical predictions
• 2:12:39
  Speaker 1: derived from quantum field theory. One of the largest facilities
• 2:12:43
  Speaker 1: performing these tests is the Large Hadron Collider at the
• 2:12:46
  Speaker 1: European Organisation for Nuclear Research near Geneva. Protons are accelerated
• 2:12:51
  Speaker 1: around a ring roughly 27 kilometres in circumference and brought
• 2:12:55
  Speaker 1: into collision at extremely high energies. Large detectors surround the
• 2:13:00
  Speaker 1: collision points. Two of the best known are the atoroidal
• 2:13:03
  Speaker 1: Large Hadron Collider Apparatus Experiment and the Compact Muon Solenoid Experiment.
• 2:13:10
  Speaker 1: These machines do not simply take photographs of fundamental particles.
• 2:13:14
  Speaker 1: Layers of detectors measure tracks, deposited energy, momentum and other signatures.
• 2:13:20
  Speaker 1: Researchers then reconstruct what processes most likely occurred and compare
• 2:13:24
  Speaker 1: enormous sets of measurements with standard model predictions. The discovery
• 2:13:29
  Speaker 1: of the Higgs boson in 2012 was one dramatic example,
• 2:13:33
  Speaker 1: but most tests of the standard model are less famous.
• 2:13:36
  Speaker 1: They involve painstaking comparisons between theoretical calculations and vast numbers
• 2:13:41
  Speaker 1: of ordinary collision events. Again and again, the predictions have survived.
• 2:13:46
  Speaker 1: Electromagnetic processes have been measured to extraordinary precision. The predicted
• 2:13:51
  Speaker 1: properties of weak interactions have been confirmed across many experiments.
• 2:13:56
  Speaker 1: Quantum chromodynamics successfully describes an enormous range of phenomena involving
• 2:14:01
  Speaker 1: quarks and gluons. The Higgs boson's measured properties have so
• 2:14:05
  Speaker 1: far remained broadly consistent with the standard model Higgs field.
• 2:14:09
  Speaker 1: That success is remarkable, but it does not mean fundamental
• 2:14:13
  Speaker 1: physics is finished. The standard model does not provide a
• 2:14:17
  Speaker 1: quantum description of gravity. It does not identify the substance
• 2:14:21
  Speaker 1: responsible for dark matter. Its simplest original form did not
• 2:14:26
  Speaker 1: include the non-zero neutrino masses revealed by neutrino oscillation experiments.
• 2:14:31
  Speaker 1: It does not explain why the universe contains so much
• 2:14:34
  Speaker 1: more matter than antimatter. And it contains several measured parameters,
• 2:14:39
  Speaker 1: including particle masses and interaction strengths, whose particular numerical values
• 2:14:45
  Speaker 1: are entered from experiment rather than derived from some deeper
• 2:14:48
  Speaker 1: principle within the model. So the standard model is both
• 2:14:51
  Speaker 1: enormously successful and clearly incomplete, yet its quantum fields already
• 2:14:56
  Speaker 1: account for most of the microscopic physics underlying the visible world.
• 2:15:01
  Speaker 1: Electrons occupying quantum states around nuclei give atoms their structure.
• 2:15:06
  Speaker 1: The electron field and electromagnetic field make chemistry possible. Quark
• 2:15:11
  Speaker 1: and gluon fields build the protons and neutrons inside atomic nuclei.
• 2:15:16
  Speaker 1: Strong interaction energy provides most of the mass of those
• 2:15:19
  Speaker 1: protons and neutrons. Photons carry electromagnetic radiation, including the visible
• 2:15:25
  Speaker 1: light entering your eyes. Even the solidity of an ordinary
• 2:15:29
  Speaker 1: table ultimately depends on quantum behaviour involving electrons, electromagnetic interactions
• 2:15:35
  Speaker 1: and the Paley exclusion principle. Quantum field theory is therefore
• 2:15:39
  Speaker 1: not something that belongs only inside a particle accelerator. The
• 2:15:43
  Speaker 1: accelerator merely lets us probe the fields under unusually energetic conditions.
• 2:15:49
  Speaker 1: Those same fields are already present in every atom around us.
• 2:15:53
  Speaker 1: The standard model takes the familiar world and, underneath it,
• 2:15:57
  Speaker 1: replaces the image of permanent microscopic objects with something more fundamental.
• 2:16:02
  Speaker 1: Matter fields, gauge fields, the Higgs field, and a remarkably
• 2:16:07
  Speaker 1: precise set of rules describing how their quantum excitations interact
• 2:16:11
  Speaker 1: to produce almost everything we can see and touch. Quantum
• 2:16:15
  Speaker 1: field theory is extraordinarily successful. It describes electromagnetic interactions with
• 2:16:21
  Speaker 1: remarkable precision. It explains the strong interaction between quarks and gluons.
• 2:16:27
  Speaker 1: It provides the framework for the weak interaction and the
• 2:16:30
  Speaker 1: Higgs field. Combined within the standard model, it accounts for
• 2:16:35
  Speaker 1: an enormous range of experiments carried out over many decades,
• 2:16:39
  Speaker 1: but success does not mean completion. There is an important
• 2:16:43
  Speaker 1: distinction between the limitations of the standard model and the
• 2:16:47
  Speaker 1: limitations of quantum field theory itself. The standard model is
• 2:16:51
  Speaker 1: one particular collection of quantum field theories describing known particles,
• 2:16:56
  Speaker 1: and three of the four fundamental interactions. Quantum field theory
• 2:17:00
  Speaker 1: is the broader framework, so when physicists discover something the
• 2:17:05
  Speaker 1: standard model cannot explain, that does not automatically mean quantum
• 2:17:09
  Speaker 1: field theory has failed. Sometimes the standard model can simply
• 2:17:13
  Speaker 1: be extended by adding new fields, new particles, or new interactions.
• 2:17:18
  Speaker 1: Other problems are deeper. The clearest example is gravity. Albert
• 2:17:23
  Speaker 1: Einstein's general theory of relativity describes gravity in a profoundly
• 2:17:27
  Speaker 1: different way from the other interactions. In the standard model,
• 2:17:32
  Speaker 1: quantum fields exist and interact within spacetime. In general relativity,
• 2:17:37
  Speaker 1: spacetime itself is dynamical. Matter and energy influence the geometry
• 2:17:42
  Speaker 1: of spacetime, and that geometry influences how matter and radiation move.
• 2:17:47
  Speaker 1: Gravity is not simply another force field situated inside a
• 2:17:50
  Speaker 1: fixed background. It is tied directly to the structure of
• 2:17:54
  Speaker 1: spacetime itself. This creates a difficult conceptual and mathematical problem
• 2:18:00
  Speaker 1: when physicists try to combine gravity with quantum theory. One
• 2:18:04
  Speaker 1: natural idea is to treat small disturbances in the gravitational
• 2:18:08
  Speaker 1: field in much the same way as other quantum fields.
• 2:18:12
  Speaker 1: In weak gravity situations this can be useful. Physicists can
• 2:18:16
  Speaker 1: even describe hypothetical quantum excitations of the gravitational field called gravitons.
• 2:18:23
  Speaker 1: But if ordinary perturbative techniques are applied to general relativity
• 2:18:27
  Speaker 1: at arbitrarily high energies, The resulting theory develops divergences that
• 2:18:31
  Speaker 1: cannot be absorbed into a finite number of measurable parameters
• 2:18:35
  Speaker 1: in the same straightforward way used for quantum electrodynamics. In
• 2:18:40
  Speaker 1: technical language, general relativity is not perturbatively renormalisable in the
• 2:18:45
  Speaker 1: conventional sense. That does not mean quantum field theory becomes
• 2:18:49
  Speaker 1: useless whenever gravity appears. At sufficiently low energies, general relativity
• 2:18:54
  Speaker 1: can be treated as an effective quantum field theory and
• 2:18:57
  Speaker 1: can produce meaningful quantum corrections. The real difficulty appears when
• 2:19:02
  Speaker 1: quantum gravitational effects become strong. Physicists expect this to matter
• 2:19:07
  Speaker 1: at extraordinarily small distance scales and extremely high energy densities.
• 2:19:14
  Speaker 1: Black holes provide one important setting. Far from a black hole,
• 2:19:18
  Speaker 1: general relativity works extremely well. Quantum field theory can also
• 2:19:23
  Speaker 1: be applied to fields propagating through curved spacetime. This combination
• 2:19:27
  Speaker 1: leads to remarkable results. Stephen Hawking showed that quantum fields
• 2:19:32
  Speaker 1: around black holes imply that black holes can emit thermal radiation.
• 2:19:36
  Speaker 1: Yet near the deepest regions of a black hole, particularly
• 2:19:40
  Speaker 1: where classical general relativity predicts a singularity, the combination of
• 2:19:44
  Speaker 1: strong curvature and quantum physics suggests that neither ordinary quantum
• 2:19:48
  Speaker 1: field theory on a fixed space-time nor classical general relativity
• 2:19:52
  Speaker 1: is likely to be sufficient by itself. A similar problem
• 2:19:56
  Speaker 1: appears when we trace the history of the universe backwards.
• 2:20:00
  Speaker 1: The early universe was hotter and denser than it is today.
• 2:20:04
  Speaker 1: At sufficiently early times, quantum effects associated with matter and
• 2:20:09
  Speaker 1: fields become unavoidable, while space-time curvature may also become extreme. Eventually,
• 2:20:15
  Speaker 1: the assumptions behind our established theories are pushed beyond the
• 2:20:18
  Speaker 1: regimes where they have been experimentally tested. A complete theory
• 2:20:23
  Speaker 1: of quantum gravity would need to describe both quantum physics
• 2:20:26
  Speaker 1: and dynamical spacetime within one consistent framework. No experimentally confirmed
• 2:20:33
  Speaker 1: complete theory currently does that, but gravity is not the
• 2:20:37
  Speaker 1: only sign that the standard model is incomplete. Neutrinos provide
• 2:20:41
  Speaker 1: another clear example. For many years, the simplest formulation of
• 2:20:45
  Speaker 1: the Standard Model treated neutrinos as massless. Experiments studying neutrinos
• 2:20:51
  Speaker 1: produced by the Sun, Earth's atmosphere, nuclear reactors and particle
• 2:20:56
  Speaker 1: accelerators revealed something different. Neutrinos can change from one flavour
• 2:21:01
  Speaker 1: into another as they travel. This phenomenon is called neutrino oscillation.
• 2:21:08
  Speaker 1: Oscillations require differences between neutrino mass states which means at
• 2:21:12
  Speaker 1: least two neutrinos must have non-zero mass. That observation tells
• 2:21:17
  Speaker 1: us that the simplest original standard model cannot be the
• 2:21:21
  Speaker 1: entire story, yet it does not require abandoning quantum field theory.
• 2:21:27
  Speaker 1: Quantum field theories can be extended to include neutrino masses.
• 2:21:31
  Speaker 1: The unanswered questions concern exactly how those masses arise, and
• 2:21:35
  Speaker 1: whether neutrinos possess additional properties not included in the simplest model.
• 2:21:40
  Speaker 1: Dark matter presents a different challenge. Astronomical observations show that
• 2:21:45
  Speaker 1: visible stars, gas and dust cannot account for all the
• 2:21:49
  Speaker 1: gravitational effects seen in galaxies and galaxy clusters. Galaxy rotation,
• 2:21:54
  Speaker 1: gravitational lensing, the motions of galaxies within clusters and the
• 2:21:58
  Speaker 1: large-scale structure of the universe all point toward additional gravitating matter.
• 2:22:04
  Speaker 1: Cosmological measurements also indicate that this dark matter makes up
• 2:22:08
  Speaker 1: substantially more of the universe's matter content than ordinary atomic matter.
• 2:22:13
  Speaker 1: No confirmed standard model particle has the required combination of
• 2:22:17
  Speaker 1: properties to explain all of that evidence. Physicists have proposed
• 2:22:22
  Speaker 1: many possibilities. Dark matter might consist of one or more
• 2:22:26
  Speaker 1: new quantum fields and their associated particles. If so, the
• 2:22:31
  Speaker 1: solution could remain comfortably. within an expanded quantum field framework.
• 2:22:36
  Speaker 1: But its actual microscopic nature is still unknown. The imbalance
• 2:22:41
  Speaker 1: between matter and antimatter raises another unresolved question. Relativistic quantum
• 2:22:46
  Speaker 1: field theory naturally contains both particles and antiparticles. Yet the
• 2:22:52
  Speaker 1: observable universe today appears overwhelmingly dominated by matter. Stars, planets,
• 2:22:58
  Speaker 1: gas clouds and people are built mainly from matter rather
• 2:23:02
  Speaker 1: than equal amounts of matter and antimatter. To generate such
• 2:23:07
  Speaker 1: an asymmetry from an initially more symmetric state, certain conditions
• 2:23:12
  Speaker 1: must be satisfied. These are associated with ideas introduced by
• 2:23:16
  Speaker 1: the physicist Andrei Sakharov. One ingredient is a difference in
• 2:23:21
  Speaker 1: the behaviour of matter and antimatter, known as charge parity violation.
• 2:23:26
  Speaker 1: The standard model does contain charge parity violations. But under
• 2:23:31
  Speaker 1: standard cosmological scenarios, the known amount appears insufficient to explain
• 2:23:36
  Speaker 1: the observed dominance of matter. Something else may therefore be missing.
• 2:23:41
  Speaker 1: The vacuum creates perhaps the most dramatic mismatch of all.
• 2:23:44
  Speaker 1: Quantum fields possess vacuum states. In quantum field calculations, zero-point
• 2:23:51
  Speaker 1: contributions naturally appear. General relativity, meanwhile, tells us that energy
• 2:23:56
  Speaker 1: affects spacetime. Cosmological observations show that the expansion of the
• 2:24:01
  Speaker 1: universe is accelerating. The simplest explanation within general relativity is
• 2:24:07
  Speaker 1: a cosmological constant, equivalent to a very small, approximately constant
• 2:24:12
  Speaker 1: vacuum energy density spread through space. The puzzle is that
• 2:24:17
  Speaker 1: naive estimates of quantum vacuum contributions can be enormously larger
• 2:24:22
  Speaker 1: than the observed effective value. The disagreement can span an
• 2:24:26
  Speaker 1: almost absurd number of orders of magnitude depending on how
• 2:24:29
  Speaker 1: the calculation is framed. This is the cosmological constant problem.
• 2:24:35
  Speaker 1: It is one of the sharpest signs that our present
• 2:24:37
  Speaker 1: understanding of quantum fields, gravity and the vacuum is incomplete.
• 2:24:42
  Speaker 1: None of these problems erase the achievements of quantum field theory.
• 2:24:46
  Speaker 1: A theory does not become useless simply because it has
• 2:24:49
  Speaker 1: a boundary. Newtonian mechanics remains extremely useful. even though relativity
• 2:24:56
  Speaker 1: provides a deeper description. Classical electromagnetism remains indispensable, even though
• 2:25:02
  Speaker 1: quantum electrodynamics extends it. In much the same way, quantum
• 2:25:07
  Speaker 1: field theory may be part of a broader future framework,
• 2:25:10
  Speaker 1: without becoming wrong inside the enormous range where it has
• 2:25:13
  Speaker 1: already been tested. The standard model may need additional fields.
• 2:25:18
  Speaker 1: Gravity may require a deeper quantum description. The vacuum may
• 2:25:23
  Speaker 1: be telling us that our understanding of space-time and quantum
• 2:25:26
  Speaker 1: energy is incomplete. Black holes in the early universe may
• 2:25:29
  Speaker 1: reveal principles that cannot be seen under ordinary laboratory conditions.
• 2:25:34
  Speaker 1: What quantum field theory has given physics is not a
• 2:25:37
  Speaker 1: finished answer to every question. It has given us an
• 2:25:40
  Speaker 1: extraordinarily successful language for describing much of the microscopic world.
• 2:25:45
  Speaker 1: And where that language begins to struggle, the difficulty is useful.
• 2:25:49
  Speaker 1: It marks the places where nature may still be hiding
• 2:25:52
  Speaker 1: something deeper. Quantum field theory is, at its heart, a
• 2:25:57
  Speaker 1: way of describing nature in which fields are the fundamental
• 2:26:00
  Speaker 1: objects and particles emerge from the quantum states of those fields.
• 2:26:05
  Speaker 1: That sentence sounds compact, but it contains almost everything we
• 2:26:09
  Speaker 1: have been trying to understand. A quantum field is not
• 2:26:13
  Speaker 1: merely an invisible substance added to a universe already filled
• 2:26:17
  Speaker 1: with tiny particles. The field is more fundamental than the particle.
• 2:26:22
  Speaker 1: An electron is an excitation of the electron field. A
• 2:26:25
  Speaker 1: photon is an excitation of the electromagnetic field. A gluon
• 2:26:30
  Speaker 1: is an excitation associated with the strong interaction gauge field.
• 2:26:35
  Speaker 1: The Higgs boson is an excitation of the Higgs field.
• 2:26:38
  Speaker 1: What we call a particle is one of the ways
• 2:26:40
  Speaker 1: a quantum field can behave. That is the central shift.
• 2:26:45
  Speaker 1: Classical physics encouraged us to think in terms of objects first.
• 2:26:49
  Speaker 1: Quantum field theory asks us to think in terms of
• 2:26:52
  Speaker 1: fields first. The framework developed because three great ideas had
• 2:26:56
  Speaker 1: to be made compatible. The first was quantum mechanics. Quantum
• 2:27:01
  Speaker 1: mechanics show that nature does not behave like a collection
• 2:27:04
  Speaker 1: of perfectly predictable classical objects. Physical systems are described by
• 2:27:09
  Speaker 1: quantum states, probabilities, uncertainty and quantist outcomes. The second ingredient
• 2:27:17
  Speaker 1: was the classical field concept. Michael Faraday and James Clerk
• 2:27:22
  Speaker 1: Maxwell had already shown that fields could possess physical dynamics
• 2:27:25
  Speaker 1: of their own. The electromagnetic field could vary throughout space,
• 2:27:30
  Speaker 1: carry energy and momentum, and support travelling waves. The third
• 2:27:35
  Speaker 1: ingredient was Albert Einstein's special theory of relativity. Special relativity
• 2:27:41
  Speaker 1: required the laws of physics to respect the structure of
• 2:27:43
  Speaker 1: spacetime and the finite speed of light. It also established
• 2:27:48
  Speaker 1: that mass and energy are related. Once quantum mechanics and
• 2:27:51
  Speaker 1: relativity were combined, the idea of a permanently fixed number
• 2:27:55
  Speaker 1: of particles became increasingly difficult to maintain. Sufficient energy can
• 2:28:00
  Speaker 1: produce new particles. Particles and antiparticles can annihilate. One collection
• 2:28:06
  Speaker 1: of particles can interact and emerge as a completely different collection.
• 2:28:11
  Speaker 1: Quantum field theory accommodates this naturally. The fields remain. Their
• 2:28:15
  Speaker 1: quantum states change. A field can occupy a state corresponding
• 2:28:19
  Speaker 1: to no particle excitations. It can occupy another state corresponding
• 2:28:24
  Speaker 1: to one particle. Another state may contain two, ten, or
• 2:28:28
  Speaker 1: an enormous number. Interactions can move the fields between those states.
• 2:28:33
  Speaker 1: This is why creation and annihilation are not awkward exceptions
• 2:28:37
  Speaker 1: within quantum field theory. They are built into its basic structure.
• 2:28:42
  Speaker 1: The same framework also explains why particles of the same
• 2:28:45
  Speaker 1: type are perfectly identical. Every electron possesses the same electric charge,
• 2:28:51
  Speaker 1: the same rest mass and the same intrinsic spin. An
• 2:28:56
  Speaker 1: electron in an atom on Earth is not merely very
• 2:28:59
  Speaker 1: similar to an electron produced in some distant galaxy. As
• 2:29:03
  Speaker 1: far as physics can determine, they are identical. Quantum field
• 2:29:07
  Speaker 1: theory makes that less surprising. They are excitations of the
• 2:29:11
  Speaker 1: same type of field. The electron field provides the common properties.
• 2:29:15
  Speaker 1: There is no need to imagine each electron being individually
• 2:29:18
  Speaker 1: assembled and somehow emerging with exactly the same specifications. The
• 2:29:24
  Speaker 1: same reasoning applies to photons. Every photon is associated with
• 2:29:28
  Speaker 1: the electromagnetic field. Photons may have different energies, frequencies, momenta
• 2:29:34
  Speaker 1: and polarisations, but they are excitations of the same underlying field.
• 2:29:39
  Speaker 1: Gluons emerge from the quantum fields responsible for the strong interaction.
• 2:29:44
  Speaker 1: They carry colour charge and can interact with one another,
• 2:29:47
  Speaker 1: helping produce the complicated behaviour that binds quarks into protons,
• 2:29:51
  Speaker 1: neutrons and other composite particles. The Higgs boson provides another
• 2:29:56
  Speaker 1: particularly clear example. The Higgs field possesses a non-zero value
• 2:30:01
  Speaker 1: even in the vacuum. An excitation of that field is
• 2:30:04
  Speaker 1: what we call a Higgs boson. The particle is not
• 2:30:08
  Speaker 1: an independent object sitting on top of the field. It
• 2:30:11
  Speaker 1: is a quantum state of the field itself. Quantum field
• 2:30:15
  Speaker 1: theory also provides the natural language for particle scattering. Physicists
• 2:30:20
  Speaker 1: can prepare an initial quantum state containing particular particle excitations.
• 2:30:26
  Speaker 1: The fields interact according to the terms in the theory.
• 2:30:29
  Speaker 1: The mathematics then provides amplitudes for different possible final states.
• 2:30:34
  Speaker 1: Those amplitudes can interfere. From them, probabilities can be calculated.
• 2:30:40
  Speaker 1: Experiments test those probabilities by repeating collisions enormous numbers of times.
• 2:30:46
  Speaker 1: This is one reason quantum field theory is so powerful.
• 2:30:50
  Speaker 1: It does not merely give us a philosophical picture of
• 2:30:53
  Speaker 1: what particles might be. It produces numbers, and those numbers
• 2:30:57
  Speaker 1: can be compared directly with experiment. There is also a
• 2:31:01
  Speaker 1: more modern way of thinking about quantum field theory that
• 2:31:04
  Speaker 1: makes the framework even broader. This is the idea of
• 2:31:07
  Speaker 1: effective field theory. An effective field theory does not necessarily
• 2:31:13
  Speaker 1: claim to be the final description of nature at every
• 2:31:16
  Speaker 1: possible energy. Instead, it describes physics accurately within a particular
• 2:31:21
  Speaker 1: range of scales. This may sound like a compromise. In fact,
• 2:31:26
  Speaker 1: it is one of the most useful ideas in modern physics.
• 2:31:29
  Speaker 1: Imagine studying waves on the surface of an ocean. You
• 2:31:33
  Speaker 1: can describe the waves without calculating the motion of every
• 2:31:36
  Speaker 1: molecule of water individually. At the scale of the waves,
• 2:31:41
  Speaker 1: many microscopic details do not matter. the larger-scale behaviour has
• 2:31:46
  Speaker 1: its own effective description. Quantum field theory often works in
• 2:31:50
  Speaker 1: a similar way. At relatively low energies, physics may become
• 2:31:54
  Speaker 1: insensitive to many details of whatever happens at much shorter distances.
• 2:32:01
  Speaker 1: The American physicist Kenneth Wilson helped make this scale-dependent viewpoint
• 2:32:05
  Speaker 1: precise through the renormalisation group. Wilson's work showed how the
• 2:32:10
  Speaker 1: parameters And even the most useful description of a system
• 2:32:14
  Speaker 1: can change as the scale of observation changes. Certain microscopic
• 2:32:19
  Speaker 1: details can become unimportant at long distances. Other features become dominant.
• 2:32:25
  Speaker 1: This helps explain why the same field-theoretic ideas can be
• 2:32:28
  Speaker 1: useful far beyond particle physics. In condensed matter physics, for example,
• 2:32:34
  Speaker 1: researchers often describe collective behaviour using effective quantum fields. A
• 2:32:39
  Speaker 1: solid contains enormous numbers of atoms and electrons. Yet under
• 2:32:44
  Speaker 1: suitable conditions, the combined behaviour of those constituents can produce quasiparticles.
• 2:32:51
  Speaker 1: A quasiparticle behaves in many respects like a particle, even
• 2:32:55
  Speaker 1: though it emerges from the collective motion of a much
• 2:32:58
  Speaker 1: more complicated underlying system. Phonons provide a familiar example. A
• 2:33:03
  Speaker 1: phonon is a quantist collective vibration in a solid, It
• 2:33:07
  Speaker 1: is not a fundamental particle like an electron, yet treating
• 2:33:11
  Speaker 1: it as a particle like excitation can be extraordinarily useful.
• 2:33:16
  Speaker 1: Other condensed matter systems produce even stranger quasiparticles and effective fields.
• 2:33:22
  Speaker 1: This reinforces an important lesson. Quantum field theory is not
• 2:33:28
  Speaker 1: only a theory of tiny fundamental particles. It is also
• 2:33:32
  Speaker 1: a general framework for understanding how particle-like behaviour can emerge
• 2:33:37
  Speaker 1: from quantum systems, with many interacting degrees of freedom. And
• 2:33:41
  Speaker 1: even the word particle becomes more subtle when spacetime itself
• 2:33:45
  Speaker 1: enters the story. Stephen Hawking famously showed that quantum fields
• 2:33:51
  Speaker 1: around a black hole lead to what is now called
• 2:33:53
  Speaker 1: Hawking radiation. A black hole that appears completely dark in
• 2:33:57
  Speaker 1: classical general relativity can, when quantum field effects are included,
• 2:34:02
  Speaker 1: emit approximately thermal radiation. William Unruh discovered another striking result.
• 2:34:08
  Speaker 1: An observer accelerating through what an inertial observer would describe
• 2:34:12
  Speaker 1: as the vacuum can perceive a thermal bath of particles.
• 2:34:16
  Speaker 1: This is known as the Unruh effect. The deeper lesson
• 2:34:20
  Speaker 1: is not simply that black holes or acceleration somehow create
• 2:34:23
  Speaker 1: ordinary particles in an obvious mechanical way. It is that
• 2:34:27
  Speaker 1: the definition of a particle can depend on how a
• 2:34:30
  Speaker 1: quantum field is being observed and on the structure of spacetime.
• 2:34:34
  Speaker 1: In ordinary laboratory physics, this subtlety rarely causes trouble. Electrons
• 2:34:39
  Speaker 1: and photons remain extremely useful and well-defined concepts, but at
• 2:34:44
  Speaker 1: a deeper level, the field is more fundamental than the
• 2:34:47
  Speaker 1: particle description. That brings us back to the original question.
• 2:34:52
  Speaker 1: What is quantum field theory? It is the framework in
• 2:34:57
  Speaker 1: which quantum fields are the fundamental dynamical ingredients. Those fields
• 2:35:02
  Speaker 1: obey quantum principles. They exist within relativistic spacetime. Their excitations
• 2:35:08
  Speaker 1: appear as particles. Their interactions allow particles to scatter, appear, disappear,
• 2:35:15
  Speaker 1: and transform. Their symmetries help determine the laws those interactions obey.
• 2:35:20
  Speaker 1: Their vacuum states possess physical structure. and their collective behaviour
• 2:35:24
  Speaker 1: can give rise to everything, from photons and electrons to
• 2:35:28
  Speaker 1: quasiparticles in solid matter. Modern physics does not currently reveal
• 2:35:34
  Speaker 1: a hidden world of tiny classical objects underneath everything else.
• 2:35:38
  Speaker 1: At the best-tested fundamental level available to us, matter and
• 2:35:43
  Speaker 1: three of the known fundamental interactions are described through interacting
• 2:35:47
  Speaker 1: quantum fields. The world looks like particles when we measure
• 2:35:50
  Speaker 1: it in certain ways. Underneath that particle picture lies something broader,
• 2:35:56
  Speaker 1: not little objects moving through an otherwise empty stage, fields
• 2:36:00
  Speaker 1: extending through spacetime, capable of occupying different quantum states, interacting
• 2:36:06
  Speaker 1: with one another and producing the particle-like world we observe.
• 2:36:12
  Speaker 1: Quantum field theory begins with a deceptively simple question. What
• 2:36:17
  Speaker 1: is the most fundamental way we currently know to describe
• 2:36:20
  Speaker 1: matter and the forces acting between its components? The answer
• 2:36:25
  Speaker 1: is that quantum field theory is the framework modern physics
• 2:36:28
  Speaker 1: uses to describe quantum matter, and three of the four
• 2:36:32
  Speaker 1: known fundamental interactions in a way that is compatible with
• 2:36:35
  Speaker 1: special relativity. That framework did not appear all at once.
• 2:36:40
  Speaker 1: It grew from a long sequence of ideas. Michael Faraday
• 2:36:44
  Speaker 1: helped establish the physical importance of fields. James Clerk Maxwell
• 2:36:49
  Speaker 1: showed that electric and magnetic fields form a unified electromagnetic
• 2:36:53
  Speaker 1: system and that light itself is an electromagnetic wave. Max
• 2:36:58
  Speaker 1: Planck introduced the idea that energy exchange could occur in
• 2:37:01
  Speaker 1: discrete amounts. Albert Einstein pushed that idea further and showed
• 2:37:06
  Speaker 1: that light could behave as though its energy arrived in
• 2:37:09
  Speaker 1: individual quanta. Louis de Broglie proposed that matter could display
• 2:37:13
  Speaker 1: wave-like behaviour. Erwin Schrödinger Werner Heisenberg and Max Born helped
• 2:37:19
  Speaker 1: build the mathematical foundations of quantum mechanics. Then Paul Dirac
• 2:37:23
  Speaker 1: showed how quantum theory and special relativity could be brought
• 2:37:27
  Speaker 1: together in a way that naturally pointed toward antimatter. From there,
• 2:37:31
  Speaker 1: the need for a full theory of quantum fields became
• 2:37:34
  Speaker 1: increasingly difficult to avoid. The deepest conceptual change is a
• 2:37:39
  Speaker 1: reversal of the ordinary picture. We tend to imagine tiny
• 2:37:43
  Speaker 1: particles first, then think of forces and fields as things
• 2:37:47
  Speaker 1: surrounding them. Quantum field theory begins with the fields. An
• 2:37:51
  Speaker 1: electron is an excitation of the electron field. A photon
• 2:37:55
  Speaker 1: is an excitation of the electromagnetic field. Quarks belong to
• 2:37:59
  Speaker 1: quark fields. Gluons arise from the strong interaction gauge fields.
• 2:38:04
  Speaker 1: The Higgs boson is an excitation of the Higgs field.
• 2:38:07
  Speaker 1: The particles are not discarded. They emerge naturally from the
• 2:38:11
  Speaker 1: quantum states of the fields. This is not just a
• 2:38:14
  Speaker 1: prettier way of describing the same old picture. The field
• 2:38:18
  Speaker 1: framework is necessary because relativistic quantum physics allows particle number
• 2:38:22
  Speaker 1: to change. Particles can be created. Particles can be annihilated.
• 2:38:28
  Speaker 1: Matter and antimatter can transform into other forms of energy.
• 2:38:32
  Speaker 1: High energy collisions can begin with one set of particles
• 2:38:36
  Speaker 1: and end with another. A theory built around a permanently
• 2:38:39
  Speaker 1: fixed number of particles would struggle to describe that world.
• 2:38:43
  Speaker 1: Quantum field theory handles it naturally because the underlying fields
• 2:38:47
  Speaker 1: remain while their quantum states change. Even empty space takes
• 2:38:51
  Speaker 1: on a different meaning. The quantum vacuum is not a
• 2:38:54
  Speaker 1: place where the fundamental fields have vanished. It is the
• 2:38:58
  Speaker 1: lowest energy state available to those fields. There may be
• 2:39:01
  Speaker 1: no ordinary particles present, but the fields remain, along with
• 2:39:05
  Speaker 1: quantum uncertainties and correlations associated with their vacuum state. From
• 2:39:11
  Speaker 1: that same framework come some of the most successful theories
• 2:39:14
  Speaker 1: in all of physics. Quantum electrodynamics describes interactions between charged
• 2:39:19
  Speaker 1: particles and the electromagnetic field with extraordinary precision. Quantum chromodynamics
• 2:39:26
  Speaker 1: describes quarks, gluons, and the strong interaction that binds quarks
• 2:39:31
  Speaker 1: into protons, neutrons and other particles. The electroweak theory brings electromagnetism,
• 2:39:38
  Speaker 1: and the weak interaction into a common gauge framework. The
• 2:39:43
  Speaker 1: Higgs field explains how the weak gauge bosons acquire mass
• 2:39:47
  Speaker 1: and contributes to the masses of fundamental fermions. Together, these
• 2:39:51
  Speaker 1: ideas form the standard model of particle physics. And the
• 2:39:55
  Speaker 1: standard model has survived an extraordinary range of experimental tests.
• 2:40:00
  Speaker 1: Its predictions have been checked in atomic measurements, scattering experiments,
• 2:40:04
  Speaker 1: particle decays, and enormous collider experiments. The agreement between theory
• 2:40:10
  Speaker 1: and observation is often astonishing, but the standard model is
• 2:40:14
  Speaker 1: not the final theory of nature. Gravity remains outside its
• 2:40:18
  Speaker 1: quantum framework. A complete theory of quantum gravity has not
• 2:40:23
  Speaker 1: yet been experimentally established. Dark matter appears to exert gravitational
• 2:40:28
  Speaker 1: influence throughout galaxies and the large-scale universe, yet no confirmed
• 2:40:33
  Speaker 1: standard model particle accounts for it. Neutrino oscillations show that
• 2:40:38
  Speaker 1: neutrinos possess non-zero masses, requiring extensions beyond the simplest original
• 2:40:44
  Speaker 1: version of the standard model. The matter-antimatter imbalance of the
• 2:40:48
  Speaker 1: observable universe remains only partly explained, and the cosmological constant
• 2:40:53
  Speaker 1: problem reveals a deep mismatch between quantum field expectations for
• 2:40:57
  Speaker 1: vacuum contributions and the very small effective vacuum energy associated
• 2:41:02
  Speaker 1: with cosmic accelerations. These open questions do not erase the
• 2:41:07
  Speaker 1: achievements of quantum field theory. They show us where the
• 2:41:10
  Speaker 1: present framework reaches its boundaries. And perhaps the most profound
• 2:41:15
  Speaker 1: thing quantum field theory changes is our intuitive idea of
• 2:41:18
  Speaker 1: matter itself. The world around us appears solid. Objects seem localised.
• 2:41:25
  Speaker 1: Particles appear to arrive at detectors as distinct individual events.
• 2:41:30
  Speaker 1: At human scales, that picture works beautifully. But at a
• 2:41:34
  Speaker 1: deeper level, modern physics describes matter in terms of quantum
• 2:41:38
  Speaker 1: fields extending throughout space-time. Those fields occupy different states. They interact.
• 2:41:44
  Speaker 1: They exchange energy and momentum. Their excitations behave as the
• 2:41:48
  Speaker 1: particles we observe. So beneath the familiar world of tables, stars,
• 2:41:54
  Speaker 1: atoms and bodies, the current best-tested picture is not one
• 2:41:57
  Speaker 1: of tiny classical objects moving through an empty background. It
• 2:42:01
  Speaker 1: is a world of quantum fields. And what we call
• 2:42:04
  Speaker 1: matter is one of the remarkable things those fields can do.