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What Is a Photon_ Really · Night Science Stories

• 0:00
  Speaker 1: Good evening, and welcome to the Sleepy Scientist. It's lovely
• 0:04
  Speaker 1: to have you here tonight. I'm always curious where these
• 0:07
  Speaker 1: evenings end up, so do leave a comment and tell
• 0:10
  Speaker 1: me where in the world you're listening from, and whether
• 0:12
  Speaker 1: your sun has only just gone down, or is already
• 0:15
  Speaker 1: thinking about coming back up. Maybe you're in a dark bedroom,
• 0:19
  Speaker 1: maybe a hotel room far from home, maybe at a
• 0:22
  Speaker 1: kitchen table long after the dish is done. Wherever you are,
• 0:27
  Speaker 1: I hope you're warm, comfortable and ready to let the
• 0:29
  Speaker 1: day drift away. Now, if there's a lamp still on
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  Speaker 1: somewhere near you, or a phone screen glowing softly in
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  Speaker 1: the dark, then right now you're being gently showered with photons.
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  Speaker 1: Millions upon millions of them, every single second, landing on
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  Speaker 1: your skin and slipping into your eyes. And if you'd
• 0:50
  Speaker 1: rather not be showered with anything tonight, you can let
• 0:53
  Speaker 1: the room go completely dark and listen on Spotify instead.
• 0:57
  Speaker 1: There's a link in the description. The photons won't mind.
• 1:01
  Speaker 1: But if you can keep the lights on just a
• 1:03
  Speaker 1: little longer, long enough to tap like and subscribe, it
• 1:07
  Speaker 1: really does help this little channel shine a bit further
• 1:10
  Speaker 1: out into the world. And if you're already subscribed, thank you.
• 1:14
  Speaker 1: You're the ones keeping the lights on around here. Then,
• 1:17
  Speaker 1: whenever you're ready, switch everything off, eyelids included. I'll carry
• 1:21
  Speaker 1: on talking either way. We talk about light all the time.
• 1:24
  Speaker 1: We switch it on, we switch it off, we sit
• 1:27
  Speaker 1: in it all day long, and yet hardly anyone stops
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  Speaker 1: to ask what a photon actually is. I'll do my
• 1:33
  Speaker 1: best not to make tonight's subject too illuminating. Sorry. But
• 1:37
  Speaker 1: if it does keep you up, at least you'll drift
• 1:40
  Speaker 1: off knowing how something that weighs absolutely nothing can still
• 1:43
  Speaker 1: carry energy and give things a push. Why light can't
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  Speaker 1: quite make up its mind, whether it's a wave or
• 1:49
  Speaker 1: a particle, and why a photon is not the tiny
• 1:52
  Speaker 1: glowing speck we all tend to imagine. Not the worst
• 1:56
  Speaker 1: reason to lose a little sleep. So, whether the light
• 1:59
  Speaker 1: around you is still quietly arriving, or you've already sent
• 2:02
  Speaker 1: it packing, what is a photon, really? When sunlight slips
• 2:06
  Speaker 1: through a gap in the curtains, when a laser beam
• 2:09
  Speaker 1: crosses a laboratory, or when the faint glow of a
• 2:12
  Speaker 1: distant star finally reaches a telescope, after travelling for centuries,
• 2:17
  Speaker 1: something has moved from one place to another. Energy has
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  Speaker 1: crossed space. Information about the object that produced the light
• 2:26
  Speaker 1: has arrived with it. But what exactly has made the journey?
• 2:29
  Speaker 1: It sounds like the sort of question that ought to
• 2:33
  Speaker 1: have a simple answer. Light is so familiar that we
• 2:36
  Speaker 1: rarely give it much thought. We switch it on, block
• 2:40
  Speaker 1: it with curtains, bend it through lenses and watch it
• 2:43
  Speaker 1: scatter from dust in the air. Yet the question of
• 2:46
  Speaker 1: what light actually has occupied some of the greatest minds
• 2:49
  Speaker 1: in physics for hundreds of years and the answer turned
• 2:53
  Speaker 1: out to be stranger than either side of the early
• 2:55
  Speaker 1: debate expected. One of the most famous views came from
• 3:00
  Speaker 1: Isaac Newton. During the 17th century, Newton developed what became
• 3:05
  Speaker 1: known as the corpuscular theory of light. He imagined light
• 3:09
  Speaker 1: as being made from exceedingly small particles, or corpuscles, emitted
• 3:14
  Speaker 1: by luminous objects and travelling through space. The idea seemed
• 3:18
  Speaker 1: capable of explaining several familiar behaviours, Light usually travels along
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  Speaker 1: straight lines. It can bounce from a mirror. It can
• 3:27
  Speaker 1: cast sharp shadows. Those qualities make it quite tempting to
• 3:31
  Speaker 1: picture light as a stream of tiny projectiles. Newton's enormous
• 3:36
  Speaker 1: scientific reputation gave the copuscular picture considerable influence, although he
• 3:42
  Speaker 1: was not the only person thinking about light. Christian Huygens
• 3:47
  Speaker 1: developed a very different approach. Huygens argued that light behaved
• 3:52
  Speaker 1: as a wave. In his picture, light propagated through space
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  Speaker 1: somewhat as waves spread outward across water, although the proposed
• 4:00
  Speaker 1: physical medium carrying those waves was very different from water itself.
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  Speaker 1: Neither Newton's corpuscles nor Huygens' waves were the photon of
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  Speaker 1: modern physics. Newton was not describing quantum particles, and Huygens
• 4:14
  Speaker 1: was not describing a quantum electromagnetic field. They were working
• 4:20
  Speaker 1: centuries before quantum mechanics existed. Their disagreement was about which
• 4:24
  Speaker 1: familiar classical picture best captured the behaviour of light. For
• 4:29
  Speaker 1: a time, Newton's particle picture held considerable sway. But experiments
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  Speaker 1: gradually began revealing features that were much easier to understand
• 4:39
  Speaker 1: if light behaved like a wave. One of the most
• 4:42
  Speaker 1: important came from Thomas Young in the early 19th century.
• 4:46
  Speaker 1: Young allowed light to pass through two closely spaced openings
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  Speaker 1: and observed what happened beyond them. If light consisted simply
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  Speaker 1: of independent classical particles travelling through one opening or the other,
• 5:00
  Speaker 1: one might expect two bright regions to appear behind the openings. Instead,
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  Speaker 1: the light formed a pattern of alternating bright and dark bands.
• 5:10
  Speaker 1: This was interference. Interference is a deeply wave-like phenomenon. When
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  Speaker 1: two waves overlap, their effects can reinforce one another in
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  Speaker 1: some places and cancel one another in others. Two water
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  Speaker 1: waves meeting crest to crest can produce a larger wave.
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  Speaker 1: A crest meeting a trough can reduce or even cancel
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  Speaker 1: the disturbance. Light passing through two openings behaved in much
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  Speaker 1: the same mathematical way. Bright regions appeared where the waves
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  Speaker 1: reinforced one another, while dark regions appeared where they cancelled.
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  Speaker 1: That did not sit comfortably with the idea of ordinary
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  Speaker 1: little particles simply flying through space like grains of sand.
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  Speaker 1: The wave case became still stronger through the work of
• 5:56
  Speaker 1: Augustin-Jean Fresnel. Fresnel developed a detailed mathematical theory of diffraction
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  Speaker 1: and interference. Diffraction occurs when waves spread around obstacles or
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  Speaker 1: through openings, and light displays precisely this sort of behaviour.
• 6:13
  Speaker 1: The details of the resulting patterns could be predicted with
• 6:16
  Speaker 1: impressive accuracy using wave mathematics. By the 19th century, the
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  Speaker 1: wave description of light had become extremely powerful. But another
• 6:25
  Speaker 1: question remained. If light was a wave, what kind of
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  Speaker 1: wave was it? Michael Faraday helped provide an important clue.
• 6:35
  Speaker 1: Faraday had spent years studying electricity and magnetism, and had
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  Speaker 1: become convinced that the two were deeply connected through physical
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  Speaker 1: fields extending through space. In 1845, he discovered that a
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  Speaker 1: magnetic field could affect the polarisation of light travelling through
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  Speaker 1: certain materials. This Faraday effect provided striking evidence that light
• 6:57
  Speaker 1: and electromagnetism were related. James Clerk Maxwell then transformed that
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  Speaker 1: connection into one of the great theoretical achievements in physics.
• 7:08
  Speaker 1: During the 19th century, Maxwell developed a unified mathematical description
• 7:12
  Speaker 1: of electricity and magnetism. His equations showed that changing electric
• 7:17
  Speaker 1: fields can produce magnetic fields, and changing magnetic fields can
• 7:21
  Speaker 1: produce electric fields. Under the right conditions, these changing fields
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  Speaker 1: can sustain one another and travel through space as a wave.
• 7:31
  Speaker 1: Then something remarkable appeared in the mathematics. The speed predicted
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  Speaker 1: for these electromagnetic waves was determined by measurable electrical and
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  Speaker 1: magnetic properties of empty space. When Maxwell calculated that speed,
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  Speaker 1: it was essentially the same as the already measured speed
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  Speaker 1: of light. That was a rather large clue. Light, Maxwell concluded,
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  Speaker 1: was an electromagnetic wave. The modern measured speed of light
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  Speaker 1: in vacuum is just under 300,000 kilometres per second. At
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  Speaker 1: that speed, Light could circle Earth more than seven times
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  Speaker 1: in a single second. It takes only a little over
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  Speaker 1: one second for light reflected from the Moon to reach Earth,
• 8:12
  Speaker 1: while sunlight takes a little over eight minutes to cross
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  Speaker 1: the distance from the Sun. Maxwell's theory did something even
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  Speaker 1: more profound than explain visible light. The equations allowed electromagnetic
• 8:24
  Speaker 1: waves with many different frequencies and wavelengths. Visible light turned
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  Speaker 1: out to occupy only a small part of an enormous
• 8:32
  Speaker 1: electromagnetic spectrum. Radio waves are electromagnetic radiation, so are microwaves.
• 8:39
  Speaker 1: Infrared radiation, which is strongly associated with the thermal radiation
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  Speaker 1: emitted by warm objects, belongs to the same family. Visible
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  Speaker 1: light occupies the narrow range our eyes happen to detect.
• 8:53
  Speaker 1: Beyond it lie ultraviolet radiation, X-rays and gamma rays. These
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  Speaker 1: are not fundamentally separate substances. They are all electromagnetic radiation.
• 9:05
  Speaker 1: What primarily distinguishes the different regions is frequency and wavelength,
• 9:09
  Speaker 1: with higher frequency corresponding to shorter wavelength. A radio signal
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  Speaker 1: crossing a city and an X-ray passing through soft tissue
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  Speaker 1: can therefore seem like completely different things in everyday life.
• 9:22
  Speaker 1: yet both belong to the same electromagnetic framework. Their dramatically
• 9:28
  Speaker 1: different interactions with matter arise largely because their frequencies, wavelengths
• 9:34
  Speaker 1: and energies are very different. By the end of the
• 9:38
  Speaker 1: 19th century, this picture seemed astonishingly complete. Light was an
• 9:42
  Speaker 1: electromagnetic wave. Maxwell's theory could explain reflection, refraction, interference, diffraction
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  Speaker 1: and and polarization. It could describe how electromagnetic radiation propagated
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  Speaker 1: through space, and how electric and magnetic fields evolved. And importantly,
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  Speaker 1: none of that has become useless. Classical electromagnetism remains one
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  Speaker 1: of the most successful theories ever developed. Engineers use it
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  Speaker 1: to design antennas, fibre optic systems, microwave equipment, electric motors
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  Speaker 1: and countless other technologies. Astronomers use electromagnetic theory to understand
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  Speaker 1: radiation arriving from stars and galaxies. When enormous numbers of
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  Speaker 1: photons behave collectively, Maxwell's equations often describe the result with
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  Speaker 1: extraordinary precision. Modern physics did not discover that Maxwell was
• 10:42
  Speaker 1: simply wrong. It discovered that Maxwell's description was not the
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  Speaker 1: whole story. At very small scales, and particularly when light
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  Speaker 1: exchanges energy with matter in individual events, the smooth classical
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  Speaker 1: wave picture begins to reveal its limits. Nature does something
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  Speaker 1: that a purely continuous electromagnetic wave cannot fully explain. So
• 11:04
  Speaker 1: the old argument between particles and waves did not end
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  Speaker 1: with one side defeating the other forever. Newton's classical particles
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  Speaker 1: were not quite right. Huygens' classical waves were not the
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  Speaker 1: final answer either. Maxwell brought light into the electromagnetic world,
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  Speaker 1: but even that magnificent theory left one deeper layer still hidden.
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  Speaker 1: The light crossing a room really is electromagnetic radiation. It
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  Speaker 1: really does interfere, diffract and propagate as Maxwell's theory predicts.
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  Speaker 1: But when we ask what that radiation is at the
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  Speaker 1: quantum level, the answer becomes something far less familiar, and
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  Speaker 1: that is where the photon begins. For all the success
• 11:46
  Speaker 1: of 19th century physics, Light still had one awkward habit.
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  Speaker 1: It refused to behave exactly as classical theory said it
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  Speaker 1: should whenever physicists looked closely at the radiation produced by
• 11:57
  Speaker 1: hot objects. The problem appeared in something extraordinarily ordinary. Heat
• 12:04
  Speaker 1: a piece of metal and eventually it begins to glow.
• 12:06
  Speaker 1: At first it may appear a dull red. Raise the
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  Speaker 1: temperature further and the colour becomes brighter, shifting through orange
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  Speaker 1: and yellow toward white. Even before the metal becomes visibly luminous, however,
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  Speaker 1: it is already emitting electromagnetic radiation, much of it at
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  Speaker 1: infrared wavelengths that human eyes cannot detect. Every object with
• 12:29
  Speaker 1: a temperature above absolute zero emits thermal radiation. A person
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  Speaker 1: does it. A cup of tea does it. Earth does it.
• 12:38
  Speaker 1: The sun does it rather enthusiastically. To study this phenomenon cleanly,
• 12:43
  Speaker 1: physicists developed an idealised concept called a blackbody. A perfect
• 12:48
  Speaker 1: blackbody absorbs all electromagnetic radiation that falls upon it, regardless
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  Speaker 1: of wavelength or direction. It also emits radiation in a
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  Speaker 1: very particular spectrum determined only by its temperature. No perfectly
• 13:02
  Speaker 1: ideal blackbody exists in nature, but cavities with a small
• 13:06
  Speaker 1: opening can approximate one extremely well. and many real objects
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  Speaker 1: behave sufficiently like black bodies for the concept to be
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  Speaker 1: enormously useful. Imagine a hollow container whose walls are kept
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  Speaker 1: at a particular temperature. Radiation inside the cavity is repeatedly
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  Speaker 1: absorbed and re-emitted by the walls until it reaches thermal equilibrium.
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  Speaker 1: A tiny opening then allows a small sample of that
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  Speaker 1: radiation to escape. Measure the escaping light carefully, and something
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  Speaker 1: beautifully regular appears. The radiation is not distributed equally across
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  Speaker 1: every wavelength. Instead, it forms a characteristic spectrum. At a
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  Speaker 1: given temperature, there is a particular region where the emission
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  Speaker 1: is strongest. Increase the temperature and the object radiates more
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  Speaker 1: energy overall, while the peak shifts towards shorter wavelengths. That
• 13:58
  Speaker 1: behaviour explains something familiar from everyday life. Cooler glowing objects
• 14:02
  Speaker 1: tend to look redder. Hotter objects become progressively whiter and
• 14:07
  Speaker 1: eventually bluer. Stars follow broadly the same physical principle, which
• 14:12
  Speaker 1: is why their surface temperatures are related to their colours.
• 14:16
  Speaker 1: By the late 19th century, experimental measurements of blackbody spectra
• 14:20
  Speaker 1: had become increasingly precise. The difficulty was explaining them. Classical
• 14:25
  Speaker 1: electromagnetic theory could describe radiation as waves, while classical statistical
• 14:31
  Speaker 1: mechanics provided rules for how energy should be shared among
• 14:35
  Speaker 1: the many possible modes of radiation inside a cavity. Unfortunately,
• 14:40
  Speaker 1: applying those ideas in the most straightforward way produced a
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  Speaker 1: result that became increasingly wrong at shorter wavelengths. The later
• 14:48
  Speaker 1: formula associated with Lord Rayleigh and James Jeans worked reasonably
• 14:52
  Speaker 1: well at long wavelengths, but as wavelength became shorter and
• 14:56
  Speaker 1: frequency increased, the classical prediction rose without limit. Taken literally,
• 15:01
  Speaker 1: it suggested that a hot object should pour an absurd
• 15:04
  Speaker 1: amount of energy into very high frequency radiations. Push the
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  Speaker 1: calculation far enough toward ultraviolet frequencies and beyond, and the
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  Speaker 1: total predicted energy becomes infinite. Nature rather sensibly does not
• 15:18
  Speaker 1: do this. Real hot objects produce a spectrum that rises,
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  Speaker 1: reaches a peak and then falls away strongly at high frequencies.
• 15:27
  Speaker 1: The disagreement between classical theory and experiment later became famous
• 15:32
  Speaker 1: as the ultraviolet catastrophe. The name sounds as though physicists
• 15:36
  Speaker 1: arrived at the laboratory one morning, discovered infinity pouring out
• 15:40
  Speaker 1: of an oven, and immediately declared classical physics dead. The
• 15:44
  Speaker 1: history was less theatrical. The difficulty emerged through several theoretical
• 15:49
  Speaker 1: approaches and increasingly accurate measurements, and the phrase ultraviolet catastrophe
• 15:55
  Speaker 1: itself became common only later. But the underlying problem was
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  Speaker 1: very real. One of the physicists trying to solve it
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  Speaker 1: was Max Planck. Planck was deeply interested in thermodynamics and
• 16:07
  Speaker 1: electromagnetic radiation, and he wanted a formula that accurately reproduced
• 16:12
  Speaker 1: the measured blackbody spectrum. In 1900, while working on the problem,
• 16:18
  Speaker 1: he introduced an assumption that would eventually become one of
• 16:21
  Speaker 1: the foundations of quantum physics. The energy exchanged by the
• 16:25
  Speaker 1: material oscillators in the cavity could not vary in completely
• 16:29
  Speaker 1: arbitrary amounts. Instead, it came in discrete portions. The size
• 16:34
  Speaker 1: of each portion depended on frequency. In its simplest spoken form,
• 16:39
  Speaker 1: the relationship is the energy of one quantum equals Planck's
• 16:44
  Speaker 1: constant multiplied by the frequency. Planck's constant is an extraordinarily
• 16:49
  Speaker 1: small fundamental constant of nature. In ordinary units, its value
• 16:53
  Speaker 1: is about 6.626 times 10 to the power of minus
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  Speaker 1: 34 joule seconds. That tiny number helps explain why quantization
• 17:02
  Speaker 1: is so difficult to notice in everyday life. If you
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  Speaker 1: move a chair across a room, pour water into a glass,
• 17:09
  Speaker 1: or roll a ball along the floor, energy appears to
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  Speaker 1: vary continuously for all practical purposes. The individual quantum scale
• 17:17
  Speaker 1: is so small compared with familiar human energies that discreteness
• 17:21
  Speaker 1: is effectively hidden. At atomic scales, however, that small constant
• 17:26
  Speaker 1: becomes crucial. Planck's relationship says that frequency matters directly. A
• 17:32
  Speaker 1: higher frequency oscillation corresponds to a larger quantum of energy.
• 17:36
  Speaker 1: A lower frequency oscillation corresponds to a smaller one. So
• 17:40
  Speaker 1: a quantum associated with ultraviolet radiation carries more energy than
• 17:45
  Speaker 1: one associated with visible red light. An X-ray quantum carries
• 17:49
  Speaker 1: more still. A radio frequency quantum, by comparison, carries an
• 17:54
  Speaker 1: extremely small amount of energy. This connection between energy and
• 17:58
  Speaker 1: frequency would later become central to the modern photon. But
• 18:02
  Speaker 1: there is an important historical detail here. Planck did not
• 18:06
  Speaker 1: initially stand up in 1900 and announce that space was
• 18:10
  Speaker 1: filled with tiny particles of light called photons. The word
• 18:14
  Speaker 1: photon did not yet exist. His original argument concerned the
• 18:18
  Speaker 1: way material oscillators exchanged energy with electromagnetic radiation. The quantization
• 18:25
  Speaker 1: was introduced into the energy of those oscillators and their interactions.
• 18:29
  Speaker 1: Planck was trying to reproduce the observed blackbody spectrum, not
• 18:33
  Speaker 1: immediately proposing the complete modern idea that freely travelling light
• 18:37
  Speaker 1: itself consists of discrete particles. In fact, Planck remained cautious
• 18:43
  Speaker 1: about the deeper meaning of the assumption he had introduced.
• 18:46
  Speaker 1: Yet the mathematical step worked. By restricting energy exchange to
• 18:51
  Speaker 1: discrete amounts linked to frequency, Planck obtained a formula that
• 18:55
  Speaker 1: matched the observed blackbody spectrum across both long and short wavelengths.
• 19:01
  Speaker 1: The impossible runaway prediction at high frequencies disappeared. High frequency
• 19:06
  Speaker 1: modes were no longer able to soak up energy in
• 19:09
  Speaker 1: the unrestricted way the classical calculation had suggested. The blackbody
• 19:13
  Speaker 1: problem had therefore revealed something profound. Energy, at least in
• 19:18
  Speaker 1: this context, could not always be treated as though it
• 19:21
  Speaker 1: changed in perfectly smooth amounts. There was a graininess hidden
• 19:25
  Speaker 1: beneath the classical picture. That graininess was not visible, because
• 19:30
  Speaker 1: physicists had gone looking for tiny particles of light. It
• 19:34
  Speaker 1: emerged because hot objects stubbornly refused to radiate according to
• 19:38
  Speaker 1: classical expectations. A furnace had exposed a crack in the
• 19:42
  Speaker 1: old description. Classical electromagnetism still worked beautifully for the propagation
• 19:47
  Speaker 1: of light. thermodynamics still worked. Statistical mechanics still worked across
• 19:53
  Speaker 1: enormous ranges of phenomena. But at the point where matter
• 19:57
  Speaker 1: and radiation exchanged energy on microscopic scales, a new rule
• 20:01
  Speaker 1: had appeared. Planck had introduced the quantum. He had not
• 20:05
  Speaker 1: yet given physics the full modern photon. But the door
• 20:09
  Speaker 1: was now open. Max Planck had introduced the idea that
• 20:12
  Speaker 1: energy could be exchanged in discrete amounts, but he had
• 20:15
  Speaker 1: done so cautiously. His original goal was to explain the
• 20:19
  Speaker 1: spectrum of radiation emitted by hot objects, not to replace
• 20:23
  Speaker 1: the classical wave picture of light with a swarm of
• 20:26
  Speaker 1: tiny particles. Albert Einstein was willing to push the idea
• 20:31
  Speaker 1: much further. In 1905, during the same extraordinary year in
• 20:36
  Speaker 1: which he published papers on special relativity and Brownian motion,
• 20:40
  Speaker 1: Einstein proposed a radically different way of thinking about light itself.
• 20:45
  Speaker 1: He suggested that, under certain circumstances, electromagnetic radiation behaves as
• 20:50
  Speaker 1: though its energy is concentrated into separate packets. These packets
• 20:54
  Speaker 1: were not yet called photons. Einstein referred to them as
• 20:58
  Speaker 1: light quanta. The idea was unsettling because Maxwell's theory had
• 21:02
  Speaker 1: described light so successfully as a continuous electromagnetic wave. Interference, diffraction,
• 21:10
  Speaker 1: and polarisation all appear to support the wave picture beautifully.
• 21:15
  Speaker 1: Einstein was not disputing those observations. He was arguing that
• 21:19
  Speaker 1: there were situations in which the continuous wave description was
• 21:22
  Speaker 1: not enough. One of the clearest examples was the photoelectric effect.
• 21:27
  Speaker 1: The basic experiment is wonderfully simple. Shine light onto the
• 21:31
  Speaker 1: surface of certain materials, especially metals, and electrons can be
• 21:36
  Speaker 1: knocked out of the material. Those emitted electrons are called photoelectrons.
• 21:41
  Speaker 1: At first glance, the phenomenon might sound exactly like what
• 21:45
  Speaker 1: a classical wave should do. Light carries energy. If enough
• 21:49
  Speaker 1: light falls on a metal surface, perhaps the electrons absorb
• 21:53
  Speaker 1: some of that energy and eventually escape. But the details
• 21:57
  Speaker 1: did not behave that way. One important feature was that
• 22:01
  Speaker 1: the frequency of the incoming light mattered enormously. For a
• 22:05
  Speaker 1: particular material, light below a certain threshold frequency would not
• 22:09
  Speaker 1: eject electrons at all, no matter how bright the light became.
• 22:14
  Speaker 1: You could increase the intensity and deliver more total electromagnetic
• 22:18
  Speaker 1: energy to the surface, but if the frequency remained too low,
• 22:22
  Speaker 1: the electrons stubbornly stayed where they were. Classical intuition might
• 22:27
  Speaker 1: suggest that sufficiently intense low-frequency light should eventually supply enough energy.
• 22:33
  Speaker 1: If light was simply a continuous wave depositing energy in
• 22:36
  Speaker 1: gradually across the material, one might expect an electron to
• 22:40
  Speaker 1: collect energy, until it had enough to escape. That was
• 22:44
  Speaker 1: not what experiments showed. Raise the frequency of the light
• 22:47
  Speaker 1: above the material's threshold, however, and electrons could be emitted.
• 22:52
  Speaker 1: Even relatively weak high-frequency light could do the job. And
• 22:56
  Speaker 1: there was another important clue. Increasing the intensity of light
• 23:00
  Speaker 1: above the threshold generally increased the number of emitted electrons,
• 23:04
  Speaker 1: but it did not determine the maximum energy carried by
• 23:07
  Speaker 1: each electron. Increasing the frequency did. This distinction was exactly
• 23:13
  Speaker 1: the sort of thing Einstein's light quantum idea could explain.
• 23:17
  Speaker 1: He took Max Planck's relationship seriously and applied it directly
• 23:21
  Speaker 1: to radiation. Each light quantum carried an amount of energy
• 23:25
  Speaker 1: determined by its frequency. The energy of one light quantum
• 23:30
  Speaker 1: equals Planck's constant multiplied by the frequency. Higher frequency light
• 23:34
  Speaker 1: therefore arrived in larger individual energy portions. Lower frequency light
• 23:40
  Speaker 1: arrived in smaller ones. Now imagine an electron near the
• 23:43
  Speaker 1: surface of a metal interacting with one of these light quanta.
• 23:47
  Speaker 1: The electron must first receive enough energy to escape from
• 23:50
  Speaker 1: the material. The minimum energy required to remove an electron
• 23:53
  Speaker 1: from a particular surface is called the work function. Different
• 23:57
  Speaker 1: materials have different work functions because their electrons are bound differently.
• 24:02
  Speaker 1: If the energy carried by the incoming light quantum is
• 24:05
  Speaker 1: smaller than the work function, the electron cannot escape. Making
• 24:09
  Speaker 1: the light brighter does not make each individual quantum more energetic.
• 24:13
  Speaker 1: It mainly means that more quanta are arriving. So if
• 24:16
  Speaker 1: each one carries too little energy to release an electron,
• 24:20
  Speaker 1: sending more of them does not solve the problem in
• 24:22
  Speaker 1: the simple photoelectric interaction. It is rather like having a
• 24:26
  Speaker 1: door that requires a single ticket worth a certain amount.
• 24:30
  Speaker 1: Arriving with a larger pile of tickets that are each
• 24:33
  Speaker 1: worth too little does not make any one ticket suddenly
• 24:36
  Speaker 1: valuable enough. Once the frequency is high enough, however, each
• 24:41
  Speaker 1: light quantum can carry sufficient energy to overcome the work function.
• 24:46
  Speaker 1: Any energy left over appears as kinetic energy of the
• 24:49
  Speaker 1: emitted electron. The relationship can be stated very simply. The
• 24:53
  Speaker 1: energy carried by the incoming light quantum equals the work
• 24:57
  Speaker 1: function of the material plus the maximum kinetic energy of
• 25:00
  Speaker 1: the emitted electron. This explains several puzzling observations at once.
• 25:06
  Speaker 1: Below the threshold frequency no electrons were released because each
• 25:10
  Speaker 1: quantum carried insufficient energy. Above the threshold electrons could be
• 25:14
  Speaker 1: emitted immediately. Increasing the frequency increased the energy available to
• 25:19
  Speaker 1: each emitted electron. Increasing the intensity increased the number of
• 25:23
  Speaker 1: available light quanta, and could therefore increase the number of
• 25:27
  Speaker 1: electrons released. The picture was remarkably successful. It was also radical.
• 25:33
  Speaker 1: Einstein was proposing that the quantization Max Planck had introduced
• 25:38
  Speaker 1: was not merely a mathematical feature of matter exchanging energy
• 25:42
  Speaker 1: with radiation. He argued that light itself behaved during emission
• 25:46
  Speaker 1: and absorption as though its energy was spatially concentrated into
• 25:50
  Speaker 1: individual quanta, Not everyone welcomed the idea. One of the
• 25:55
  Speaker 1: scientists who investigated the photoelectric effect with extraordinary precision was
• 26:00
  Speaker 1: Robert Millikan. Millikan was sceptical of Einstein's interpretation. He spent
• 26:06
  Speaker 1: years conducting careful measurements of the relationship between light frequency
• 26:11
  Speaker 1: and the energy of emitted electrons. The irony is rather pleasing.
• 26:16
  Speaker 1: Millikan did not particularly like Einstein's light quantum idea. but
• 26:20
  Speaker 1: his experiments confirmed Einstein's equation with impressive accuracy. The maximum
• 26:27
  Speaker 1: energy of the emitted electrons increased linearly with the frequency
• 26:31
  Speaker 1: of the instant light, just as Einstein had predicted. The
• 26:36
  Speaker 1: slope of that relationship provided an experimental value for Planck's
• 26:40
  Speaker 1: constant that agreed closely with values obtained through other methods.
• 26:45
  Speaker 1: The evidence had become difficult to ignore, Einstein's work on
• 26:49
  Speaker 1: the photoelectric effect was eventually recognised by the Nobel Committee.
• 26:54
  Speaker 1: He received the 1921 Nobel Prize in Physics, awarded for
• 26:58
  Speaker 1: his services to theoretical physics and especially for his discovery
• 27:02
  Speaker 1: of the law of the photoelectric effect. It is worth
• 27:06
  Speaker 1: lingering on that detail because Einstein is now so strongly
• 27:09
  Speaker 1: associated with relativity that people often assume his Nobel Prize
• 27:13
  Speaker 1: must have been awarded for special or general relativity. It
• 27:17
  Speaker 1: was not. The photoelectric effect played the decisive role. The
• 27:23
  Speaker 1: deeper lesson is that the photon did not reveal itself
• 27:26
  Speaker 1: because physicists built a more powerful microscope and finally saw
• 27:30
  Speaker 1: a tiny glowing object. There is no photograph in which
• 27:34
  Speaker 1: somebody zooms in far enough and finds a little bead
• 27:37
  Speaker 1: of light. Instead, the quantum nature of light appeared through
• 27:42
  Speaker 1: the way energy was exchanged. Experiments show that light interacted
• 27:47
  Speaker 1: with matter in discrete amounts tied directly to frequency. That
• 27:51
  Speaker 1: does not mean the photon should immediately be imagined as
• 27:54
  Speaker 1: a microscopic solid pellet with a crisp outer edge. The
• 27:58
  Speaker 1: photoelectric effect tells us something very important about quantization, but
• 28:03
  Speaker 1: it does not by itself tell us that light must
• 28:05
  Speaker 1: behave exactly like an ordinary classical particle in every situation.
• 28:11
  Speaker 1: Light still interferes. It still diffracts. it still displays the
• 28:16
  Speaker 1: wave behaviour that had made Maxwell's theory so successful. Einstein's proposal, therefore,
• 28:22
  Speaker 1: did not replace one simple picture with another. It made
• 28:26
  Speaker 1: the puzzle deeper. Light could behave in ways that were
• 28:30
  Speaker 1: unmistakably wave-like, yet when it exchanged energy with matter, the
• 28:35
  Speaker 1: exchange could occur in discrete portions. The old categories were
• 28:39
  Speaker 1: beginning to strain. A wave was no longer enough. A
• 28:43
  Speaker 1: classical particle was not enough either, and somewhere between those
• 28:47
  Speaker 1: two familiar ideas, the modern photon was beginning to take shape.
• 28:52
  Speaker 1: Einstein's light quantum explained why light could eject electrons from
• 28:57
  Speaker 1: a metal in discrete exchanges of energy. But a difficult
• 29:01
  Speaker 1: question still remained. If light really came in individual quanta,
• 29:06
  Speaker 1: did those quanta possess other particle-like properties as well? In particular,
• 29:12
  Speaker 1: could light-carry momentum. Classical electromagnetic theory already allowed radiation to
• 29:17
  Speaker 1: exert pressure. James Clerk Maxwell's equations implied that electromagnetic waves
• 29:23
  Speaker 1: could transfer momentum to matter, and experiments had confirmed radiation pressure.
• 29:29
  Speaker 1: But in the early 1920s, Arthur Compton found a striking
• 29:33
  Speaker 1: new way to examine momentum at the level of individual interactions.
• 29:37
  Speaker 1: Compton worked with X-rays, a form of electromagnetic radiation with
• 29:42
  Speaker 1: wavelengths much shorter than visible light. He directed X-rays at
• 29:47
  Speaker 1: materials containing electrons and carefully measured the radiation that emerged
• 29:51
  Speaker 1: after scattering. Something interesting happened. The scattered X-rays did not
• 29:57
  Speaker 1: always have the same wavelength as the incoming X-rays. Their
• 30:01
  Speaker 1: wavelength became longer by an amount that depended on the
• 30:03
  Speaker 1: angle through which they had scattered. A longer wavelength means
• 30:07
  Speaker 1: a lower frequency. And because the energy associated with a
• 30:11
  Speaker 1: light quantum depends on frequency, the scattered radiation had lost
• 30:15
  Speaker 1: some energy. That energy had not vanished. It had been
• 30:19
  Speaker 1: transferred to an electron. Compton realised that the observations could
• 30:24
  Speaker 1: be understood remarkably well if the incoming X-ray were treated
• 30:28
  Speaker 1: as an individual quantum carrying both energy and momentum. The
• 30:32
  Speaker 1: quantum would interact with an electron, transfer some of its
• 30:36
  Speaker 1: energy and momentum to that electron, and then continue in
• 30:40
  Speaker 1: a different direction with less energy and a longer wavelength.
• 30:44
  Speaker 1: There is a familiar flavour to this. It resembles a collision.
• 30:48
  Speaker 1: One moving object strikes another. momentum and energy are redistributed.
• 30:54
  Speaker 1: The first object changes direction and loses some energy, while
• 30:58
  Speaker 1: the second object recoils. That comparison is useful, but it
• 31:02
  Speaker 1: needs to be handled with some care. The light quantum
• 31:06
  Speaker 1: is not a microscopic billiard ball. It does not need
• 31:10
  Speaker 1: to possess a hard surface or a tiny spherical shape.
• 31:14
  Speaker 1: Quantum physics would eventually show that familiar mechanical pictures have
• 31:18
  Speaker 1: strict limits. Still, the collision-like conservation of energy and momentum worked.
• 31:24
  Speaker 1: The relationship between photon momentum and wavelength can be stated simply.
• 31:29
  Speaker 1: The momentum of a photon equals Planck's constant divided by
• 31:33
  Speaker 1: its wavelength. This means that shorter wavelength radiation carries more
• 31:37
  Speaker 1: momentum per photon than longer wavelength radiation. An X-ray photon
• 31:42
  Speaker 1: therefore carries considerably more momentum than a visible light photon.
• 31:46
  Speaker 1: A gamma ray photon with an even shorter wavelength, carries
• 31:50
  Speaker 1: more again. Compton's measurements matched the predictions of this quantum
• 31:55
  Speaker 1: treatment extremely well. The change in wavelength depended precisely on
• 32:00
  Speaker 1: the scattering geometry in the way expected when energy and
• 32:03
  Speaker 1: momentum were conserved between an incoming light quantum and an electron.
• 32:08
  Speaker 1: The effect became known as Compton scattering. It provided evidence
• 32:13
  Speaker 1: of a rather awkward combination of properties. Light clearly behaved
• 32:17
  Speaker 1: like a wave, Interference and diffraction had demonstrated that beyond
• 32:21
  Speaker 1: reasonable doubt. Yet here it was also transferring energy and
• 32:25
  Speaker 1: momentum in individual collision-like events. The classical wave picture had
• 32:30
  Speaker 1: not suddenly become useless. It remained excellent for describing an
• 32:34
  Speaker 1: enormous range of optical phenomena. But once again, experiments were
• 32:40
  Speaker 1: showing that it was incomplete. The light quantum was becoming
• 32:43
  Speaker 1: increasingly difficult to dismiss as a mathematical convenience. and soon
• 32:49
  Speaker 1: it acquired a name. In 1926, the American physical chemist
• 32:54
  Speaker 1: Gilbert Newton Lewis published a short paper in which he
• 32:57
  Speaker 1: introduced the word photon. The word was constructed from the
• 33:02
  Speaker 1: Greek root associated with light, and it proved far more
• 33:06
  Speaker 1: durable than the particular theory Lewis was proposing. There is
• 33:10
  Speaker 1: an interesting historical wrinkle here. Lewis's own photon was not
• 33:14
  Speaker 1: quite the photon used in modern physics. His paper proposed
• 33:17
  Speaker 1: ideas about light quanta that did not become part of
• 33:20
  Speaker 1: the standard quantum theory of radiation, but the name survived.
• 33:26
  Speaker 1: Physicists gradually adopted photon as the convenient term for the
• 33:29
  Speaker 1: quantum of electromagnetic radiation. Sometimes scientific vocabulary has a life
• 33:35
  Speaker 1: rather independent of the theory that first produced it. The
• 33:39
  Speaker 1: timing was appropriate because physics itself was changing at extraordinary speed.
• 33:44
  Speaker 1: During the 1920s, The older quantum ideas of Max Planck
• 33:48
  Speaker 1: and Albert Einstein were being transformed into quantum mechanics. Niels
• 33:53
  Speaker 1: Bohr had already developed his early quantum model of the atom,
• 33:57
  Speaker 1: in which electrons occupied particular allowed energy states. Although that
• 34:02
  Speaker 1: model would eventually be replaced by a deeper description, it
• 34:05
  Speaker 1: had helped establish the importance of quantized atomic structure. Werner
• 34:10
  Speaker 1: Heisenberg developed matrix mechanics, one of the first complete mathematical
• 34:15
  Speaker 1: formulations of quantum mechanics. Erwin Schrödinger developed wave mechanics and
• 34:20
  Speaker 1: the equation that now bears his name. Max Born introduced
• 34:24
  Speaker 1: the probabilistic interpretation of the quantum wave function, connecting its
• 34:28
  Speaker 1: mathematical amplitude with the probabilities of measurement outcomes. Paul Dirac
• 34:33
  Speaker 1: developed a powerful abstract formulation of quantum mechanics and later
• 34:38
  Speaker 1: played a central role in the quantum theory of radiation.
• 34:42
  Speaker 1: The photon was entering physics during a period in which
• 34:45
  Speaker 1: the meaning of the words particle and wave was itself
• 34:48
  Speaker 1: being rewritten. This is important because it is easy to
• 34:52
  Speaker 1: tell the history as though physicists had simply changed their minds.
• 34:57
  Speaker 1: First they thought light was made of particles. Then they
• 35:00
  Speaker 1: decided it was a wave. Then Einstein and Compton proved
• 35:04
  Speaker 1: it was actually made of particles after all. But that
• 35:07
  Speaker 1: is not what happened. Newton's corpuscles were classical particles. They
• 35:12
  Speaker 1: were imagined as little objects following definite trajectories. The photon
• 35:17
  Speaker 1: of quantum mechanics is not simply Newton's corpuscle returned under
• 35:21
  Speaker 1: a new name. Likewise, the success of the photon did
• 35:24
  Speaker 1: not erase the wave properties established by Thomas Young, Augustin-Jean
• 35:29
  Speaker 1: Fresnel and James Clerk Maxwell. Interference remained real. Diffraction remained real.
• 35:36
  Speaker 1: Electromagnetic waves remained real and extraordinarily useful. What disappeared was
• 35:41
  Speaker 1: the hope that light could be completely understood using either
• 35:45
  Speaker 1: one of the familiar classical pictures on its own. Compton's
• 35:49
  Speaker 1: scattering made this especially difficult to avoid. An X-ray could
• 35:53
  Speaker 1: be described using a wavelength, something deeply associated with waves.
• 35:58
  Speaker 1: Yet that same wavelength determined the momentum carried by an
• 36:02
  Speaker 1: individual quantum. Wavelanguage and particle language had somehow become tied together.
• 36:08
  Speaker 1: Shorter wavelength meant higher frequency. Higher frequency meant greater quantum energy,
• 36:13
  Speaker 1: and shorter wavelength also meant greater photon momentum. The boundaries
• 36:18
  Speaker 1: between the old categories were becoming rather untidy. By the
• 36:23
  Speaker 1: middle of the 1920s, the question was therefore no longer
• 36:26
  Speaker 1: simply whether light was a wave or a particle. Experiments
• 36:30
  Speaker 1: had already made that choice too crude. Light propagated and
• 36:34
  Speaker 1: interfered in ways that demanded wave-like mathematics. while exchanging energy
• 36:39
  Speaker 1: and momentum in discrete events that looked unmistakably particle-like. The
• 36:45
  Speaker 1: photon was not a compromise in which light spent half
• 36:48
  Speaker 1: its time behaving as one thing and half as another.
• 36:52
  Speaker 1: It was the beginning of a different kind of object entirely.
• 36:56
  Speaker 1: Something quantum. Something that could carry energy and momentum in
• 37:00
  Speaker 1: a discrete interaction without becoming a tiny classical ball. And,
• 37:04
  Speaker 1: as physics would gradually discover, Even the phrase particle of
• 37:08
  Speaker 1: light was only the beginning of the answer. By now,
• 37:12
  Speaker 1: the photon has acquired a rather impressive list of behaviours.
• 37:15
  Speaker 1: It can transfer energy in discrete amounts. It can carry momentum.
• 37:20
  Speaker 1: It can participate in interference and diffraction. It can arrive
• 37:23
  Speaker 1: at a detector in individual events. But if we want
• 37:26
  Speaker 1: to ask what a photon actually is, it helps to
• 37:29
  Speaker 1: stop for a moment and gather the properties that modern
• 37:32
  Speaker 1: physics assigns to it. The most precise short description is
• 37:37
  Speaker 1: that a photon is a quantum of the electromagnetic field.
• 37:41
  Speaker 1: That phrase sounds more abstract than saying that a photon
• 37:44
  Speaker 1: is a particle of light, but it's much closer to
• 37:47
  Speaker 1: the modern physical picture. The electromagnetic field exists throughout space,
• 37:53
  Speaker 1: and in quantum theory that field can exchange energy in
• 37:56
  Speaker 1: discrete excitations. Those excitations are photons. A photon, therefore, is
• 38:02
  Speaker 1: not best imagined as a tiny luminous bead, travelling through
• 38:06
  Speaker 1: an otherwise empty background. It belongs to the electromagnetic field itself.
• 38:11
  Speaker 1: One of its simplest properties is electric charge. A photon
• 38:15
  Speaker 1: has none. It carries zero electric charge, which means a
• 38:19
  Speaker 1: photon does not experience electric forces in the same way
• 38:22
  Speaker 1: that an electron or proton does. An electron has negative
• 38:26
  Speaker 1: electric charge. A proton has positive electric charge. A photon
• 38:31
  Speaker 1: is electrically neutral. Another defining property is its rest mass.
• 38:36
  Speaker 1: According to current experimental evidence, the photon's rest mass is zero.
• 38:41
  Speaker 1: That statement can sound slightly confusing because photons certainly carry energy.
• 38:46
  Speaker 1: In everyday experience, we are accustomed to associating mass and
• 38:50
  Speaker 1: energy very closely. But relativity allows massless particles to possess
• 38:55
  Speaker 1: energy and momentum. A photon does not need rest mass
• 38:59
  Speaker 1: in order to carry either. In fact, a free photon
• 39:02
  Speaker 1: cannot simply be brought to rest so that we can
• 39:05
  Speaker 1: inspect it sitting still. In vacuum, photons propagate at the
• 39:10
  Speaker 1: invariant speed associated with light and special relativity. That speed
• 39:15
  Speaker 1: is approximately 300,000 km per second. More precisely, it is
• 39:20
  Speaker 1: just under that figure, and it is one of the
• 39:23
  Speaker 1: fundamental constants built into the structure of relativity. This speed
• 39:28
  Speaker 1: is not merely the speed that visible light happens to travel.
• 39:32
  Speaker 1: Radio waves, microwaves, infrared radiation, ultraviolet radiation, X-rays and gamma
• 39:40
  Speaker 1: rays all propagate at the same invariant speed when travelling
• 39:44
  Speaker 1: freely through vacuum. The differences between them lie mainly in
• 39:48
  Speaker 1: frequency and wavelength. things become a little subtler inside matter.
• 39:53
  Speaker 1: Light travels more slowly through glass or water than it
• 39:56
  Speaker 1: does through vacuum, at least when we talk about the
• 39:59
  Speaker 1: overall propagation of a light signal through the material, but
• 40:02
  Speaker 1: this should not be imagined as though individual free photons
• 40:06
  Speaker 1: simply decide to reduce their cruising speed because glass is
• 40:09
  Speaker 1: a little crowded. The electromagnetic field interacts with the charged
• 40:14
  Speaker 1: particles inside the material. Those interactions alter the way the
• 40:18
  Speaker 1: disturbance propagates through the medium. The resulting effective speed of
• 40:23
  Speaker 1: light through the material can therefore be lower than the
• 40:26
  Speaker 1: vacuum speed. This is the origin of the refractive index
• 40:30
  Speaker 1: used in optics. The photon also carries energy, and the
• 40:34
  Speaker 1: amount is determined by frequency. The relationship remains the one
• 40:38
  Speaker 1: introduced through Max Planck and Albert Einstein. The energy of
• 40:42
  Speaker 1: a photon equals Planck's constant multiplied by the frequency. A
• 40:47
  Speaker 1: higher frequency photon carries more energy. A lower frequency photon
• 40:51
  Speaker 1: carries less. Visible photons typically have energies of only a
• 40:55
  Speaker 1: few electron volts. An electron volt is a convenient unit
• 40:59
  Speaker 1: used in atomic and particle physics. It corresponds to the
• 41:03
  Speaker 1: energy gained by one electron when it moves through an
• 41:06
  Speaker 1: electric potential difference of one volt. Visible red photons are
• 41:11
  Speaker 1: generally lower in energy than visible blue or violet photons,
• 41:15
  Speaker 1: because red light has a lower frequency. Move beyond visible
• 41:19
  Speaker 1: light into ultraviolet radiation and the energy per photon increases further.
• 41:25
  Speaker 1: X-ray photons can carry thousands of electron volts. Gamma ray
• 41:30
  Speaker 1: photons can carry millions of electron volts and in extreme
• 41:34
  Speaker 1: astrophysical environments they can reach billions or even trillions of
• 41:38
  Speaker 1: electron volts. So when we speak about a photon, There
• 41:42
  Speaker 1: is no single fixed amount of energy attached to the word.
• 41:46
  Speaker 1: The energy depends on the photon's frequency. Momentum works in
• 41:51
  Speaker 1: a similarly elegant way. A photon has zero rest mass,
• 41:55
  Speaker 1: but it still carries momentum. The relationship can be expressed
• 41:59
  Speaker 1: as the momentum of a photon equals Planck's constant divided
• 42:03
  Speaker 1: by its wavelength. Shorter wavelengths correspond to greater momentum. This
• 42:09
  Speaker 1: is why light can exert pressure. A beam of light
• 42:12
  Speaker 1: reflecting from a mirror transfers momentum to that mirror. The
• 42:16
  Speaker 1: effect is tiny in ordinary circumstances, but it is real
• 42:20
  Speaker 1: and measurable. Radiation pressure can influence microscopic particles, dust grains
• 42:25
  Speaker 1: and even astronomical systems. It is also the principle behind
• 42:29
  Speaker 1: solar sails. A solar sail uses a large reflective surface
• 42:33
  Speaker 1: to receive a continuous push from sunlight. No wind is required.
• 42:38
  Speaker 1: The momentum carried by the photons provides the thrust. The
• 42:42
  Speaker 1: acceleration is gentle, but it can continue for long periods
• 42:46
  Speaker 1: without fuel being burned. Photons also possess intrinsic angular momentum.
• 42:52
  Speaker 1: In particle physics, the photon is described as a spin-1 boson.
• 42:56
  Speaker 1: The word spin is slightly dangerous because it invites a
• 42:59
  Speaker 1: very familiar picture. One might imagine a photon as a
• 43:03
  Speaker 1: tiny ball rotating rapidly around its own axis. That is
• 43:07
  Speaker 1: not what quantum spin means. Spin is an intrinsic quantum property.
• 43:12
  Speaker 1: It behaves mathematically like a form of angular momentum, but
• 43:16
  Speaker 1: it does not require a particle to possess a physical
• 43:19
  Speaker 1: surface that is literally turning around. Photons are also members
• 43:23
  Speaker 1: of a broad family of particles called bosons. Bosons obey
• 43:27
  Speaker 1: a statistical framework known as Bose-Einstein statistics, named after Satyendra
• 43:33
  Speaker 1: Nath Bose and Albert Einstein. One important consequence is that
• 43:37
  Speaker 1: many photons can occupy the same quantum state. This behaviour
• 43:41
  Speaker 1: is very different from particles such as electrons, which belong
• 43:45
  Speaker 1: to another family called fermions and are restricted by the
• 43:47
  Speaker 1: Pauli exclusion principle. Photons do not obey that exclusion rule.
• 43:53
  Speaker 1: Large numbers of photons can share the same frequency, direction,
• 43:58
  Speaker 1: phase relationship and polarisation state. that is deeply connected with
• 44:03
  Speaker 1: the extraordinary coherence of laser light. In a laser, stimulated
• 44:08
  Speaker 1: emission can produce photons whose quantum properties are closely matched,
• 44:13
  Speaker 1: allowing the electromagnetic field to develop a highly ordered and
• 44:16
  Speaker 1: coherent structure. The laser beam then behaves very differently from
• 44:20
  Speaker 1: ordinary thermal light, not because laser photons are a different
• 44:24
  Speaker 1: kind of particle, but because their quantum states are organised
• 44:28
  Speaker 1: in a very particular way. A photon's state can be
• 44:31
  Speaker 1: characterised by properties such as its energy, momentum, direction of
• 44:36
  Speaker 1: propagation and polarisation. These are meaningful physical quantities. But there
• 44:42
  Speaker 1: are other questions that come from trying to force the
• 44:44
  Speaker 1: photon into the shape of an everyday object. How wide
• 44:48
  Speaker 1: is a photon? What is its diameter? What does its
• 44:51
  Speaker 1: surface look like? Where is its edge? Modern quantum theory
• 44:55
  Speaker 1: does not assign a photon a rigid little boundary. in
• 44:58
  Speaker 1: the way a marble has a surface. A photon can
• 45:02
  Speaker 1: be prepared in a state that is spread across a
• 45:04
  Speaker 1: region of space. Its wavelength is not simply the diameter
• 45:09
  Speaker 1: of a tiny object. Its spatial description depends on the
• 45:12
  Speaker 1: quantum state in which it has been produced. Even the
• 45:16
  Speaker 1: idea of its exact location can become subtle. That does
• 45:20
  Speaker 1: not make the photon vague in the sense of being
• 45:22
  Speaker 1: poorly understood. Its measurable properties are described with extraordinary precision
• 45:28
  Speaker 1: Quantum electrodynamics, the modern quantum theory of electromagnetic interactions, is
• 45:34
  Speaker 1: among the most accurately tested theories ever developed. The strange
• 45:39
  Speaker 1: part is not that physics knows very little about photons.
• 45:43
  Speaker 1: The strange part is that what physics knows does not
• 45:46
  Speaker 1: fit comfortably into the mental picture of a tiny object
• 45:49
  Speaker 1: moving through space. A photon has no electric charge. It
• 45:53
  Speaker 1: has zero measured rest mass. It propagates through vacuum at
• 45:57
  Speaker 1: the invariant speed of light. It carries energy and momentum.
• 46:01
  Speaker 1: It possesses intrinsic angular momentum. It is a spin-1 boson.
• 46:06
  Speaker 1: It can share a quantum state with enormous numbers of
• 46:09
  Speaker 1: other photons. And yet none of those facts requires it
• 46:13
  Speaker 1: to be a microscopic glowing sphere. The word particle remains useful,
• 46:18
  Speaker 1: but it comes with quantum rules attached. And once those
• 46:21
  Speaker 1: rules are taken seriously, the photon begins to look less
• 46:25
  Speaker 1: like a tiny piece of light and more like something
• 46:28
  Speaker 1: for which everyday objects were never going to provide a
• 46:31
  Speaker 1: perfect analogy. If you open almost any diagram about photons,
• 46:36
  Speaker 1: there is a good chance you will see the same
• 46:38
  Speaker 1: sort of picture. A tiny glowing dot. Perhaps it has
• 46:42
  Speaker 1: a little wavy line attached. Perhaps an arrow points in
• 46:46
  Speaker 1: the direction it is moving. Sometimes it looks like a
• 46:50
  Speaker 1: bright bead fired through space. that image is useful. It
• 46:54
  Speaker 1: is also not what a photon literally is. Modern quantum
• 46:58
  Speaker 1: physics does not describe a photon as a miniature luminous
• 47:02
  Speaker 1: marble with a hard outer surface, a perfectly defined diameter
• 47:07
  Speaker 1: and a precise little path that exists independently of everything
• 47:11
  Speaker 1: around it. The problem is not that the picture is
• 47:14
  Speaker 1: completely useless. The problem is that our everyday ideas of
• 47:18
  Speaker 1: particles were developed by looking at ordinary objects, A grain
• 47:22
  Speaker 1: of sand is a particle. A dust speck is a particle.
• 47:26
  Speaker 1: A tiny ball bearing is a particle. Each one occupies
• 47:29
  Speaker 1: a particular place, has a physical surface and can, at
• 47:33
  Speaker 1: least in principle, be followed along a definite path. Photons
• 47:37
  Speaker 1: do not behave that way. The word particle is still
• 47:41
  Speaker 1: used because photons can exchange energy and momentum in discrete events.
• 47:45
  Speaker 1: A detector may register a single photon at one location.
• 47:49
  Speaker 1: An electron can absorb energy from one photon. A photon
• 47:53
  Speaker 1: can scatter from an electron and transfer a measurable amount
• 47:57
  Speaker 1: of momentum. Those are strongly particle-like features. But if we
• 48:01
  Speaker 1: take the classical particle picture too seriously, it begins to fail.
• 48:06
  Speaker 1: The word wave creates a similar difficulty. Photons can produce
• 48:10
  Speaker 1: interference and diffraction. Light can pass through two openings and
• 48:14
  Speaker 1: form alternating bright and dark regions. Different possible contributions to
• 48:19
  Speaker 1: a photon's behaviour can reinforce or cancel one another in
• 48:23
  Speaker 1: the same mathematical way that waves do. Those are strongly
• 48:27
  Speaker 1: wave-like features. But a photon is not simply a tiny
• 48:31
  Speaker 1: water wave either. It does not consist of some ordinary
• 48:34
  Speaker 1: substance sloshing up and down through space. For historical reasons,
• 48:39
  Speaker 1: this uncomfortable combination became known as wave-particle duality. The phrase
• 48:44
  Speaker 1: is useful because it reminds us that quantum objects can
• 48:47
  Speaker 1: display behaviours associated with both classical particles and classical waves.
• 48:52
  Speaker 1: But it can also suggest the wrong picture. It may
• 48:55
  Speaker 1: sound as though a photon spends some of its time
• 48:58
  Speaker 1: as a little ball, and then, when the experiment changes,
• 49:01
  Speaker 1: quickly changes costume and becomes a wave. Modern quantum theory
• 49:06
  Speaker 1: does not require that kind of transformation. There is one
• 49:09
  Speaker 1: quantum object described by one quantum state. Different experiments reveal
• 49:14
  Speaker 1: different aspects of that state. A quantum state is the
• 49:17
  Speaker 1: mathematical description used to encode what can be predicted about
• 49:21
  Speaker 1: a physical system. For a photon, that state can contain
• 49:24
  Speaker 1: information about possible momentum, direction of travel, frequency, polarisation and
• 49:30
  Speaker 1: spatial distribution. It also determines the probabilities of different outcomes
• 49:35
  Speaker 1: when measurements are made. This is one of the places
• 49:38
  Speaker 1: where quantum physics begins to feel very different from classical physics.
• 49:42
  Speaker 1: In classical mechanics, if you know enough about an object,
• 49:46
  Speaker 1: you can in principle assign it a definite position and momentum,
• 49:49
  Speaker 1: and then calculate how those quantities change over time. Quantum
• 49:54
  Speaker 1: mechanics generally give something different. It gives probability amplitudes. A
• 49:59
  Speaker 1: probability amplitude is not the same thing as an ordinary probability.
• 50:03
  Speaker 1: Amplitudes can combine, they can reinforce one another, they can
• 50:07
  Speaker 1: cancel one another. Only after those amplitudes are combined do
• 50:11
  Speaker 1: we calculate the probabilities of particular measurement outcomes. Max Born
• 50:16
  Speaker 1: was one of the physicists who helped establish this probabilistic
• 50:19
  Speaker 1: interpretation of quantum mechanics during the 1920s. The idea was
• 50:25
  Speaker 1: revolutionary because it changed what the mathematical description seemed to
• 50:29
  Speaker 1: be telling physicists. A wave-like mathematical object could evolve smoothly
• 50:34
  Speaker 1: and interfere, yet the final experimental result might still appear
• 50:38
  Speaker 1: as one localised detection event. That is exactly the sort
• 50:43
  Speaker 1: of behaviour photons display. Imagine sending photons one at a
• 50:47
  Speaker 1: time toward two narrow openings. Each photon eventually produces a
• 50:51
  Speaker 1: single detection somewhere on a screen. One photon does not
• 50:55
  Speaker 1: arrive as a faint wash across the whole detector. It
• 50:58
  Speaker 1: produces one event. Then another photon arrives, another event appears,
• 51:04
  Speaker 1: and so on. At first the pattern may look almost random,
• 51:08
  Speaker 1: but after many detections the familiar interference pattern gradually emerges.
• 51:13
  Speaker 1: The individual events look particle-like. The statistical pattern is governed
• 51:18
  Speaker 1: by interfering probability amplitudes. There is no need to imagine
• 51:22
  Speaker 1: that the photon was secretly a classical particle all along,
• 51:26
  Speaker 1: or secretly a classical wave all along. Both of those
• 51:30
  Speaker 1: descriptions are approximations to something more general. This also helps
• 51:35
  Speaker 1: with one of the most common misunderstandings about wavelength. A
• 51:39
  Speaker 1: photon can be associated with a wavelength. Visible red light
• 51:43
  Speaker 1: has a longer wavelength than visible blue light. Radio photons
• 51:47
  Speaker 1: can have wavelengths measured in metres or much more. X-ray
• 51:51
  Speaker 1: photons can have wavelengths comparable with atomic dimensions. But wavelength
• 51:56
  Speaker 1: is not simply the physical width of a photon. A
• 51:59
  Speaker 1: radio photon with a wavelength of several metres is not
• 52:02
  Speaker 1: a rigid several metre long object with a front edge
• 52:05
  Speaker 1: and a back edge. Wavelength characterises the oscillatory structure of
• 52:09
  Speaker 1: the electromagnetic field and the photon's quantum state. It is
• 52:14
  Speaker 1: connected with momentum and frequency. It should not be treated
• 52:18
  Speaker 1: as a measurement of the diameter of a tiny object.
• 52:21
  Speaker 1: This becomes even clearer when we consider a perfectly monochromatic photon.
• 52:26
  Speaker 1: Monochromatic means having one precisely defined frequency. Because frequency and
• 52:31
  Speaker 1: momentum are connected, a state with an extremely well-defined frequency
• 52:36
  Speaker 1: also has an extremely well-defined momentum magnitude. But such a
• 52:41
  Speaker 1: state is not neatly localised in one tiny region of space.
• 52:45
  Speaker 1: To localise a photon more strongly, physicists often describe a
• 52:49
  Speaker 1: wave packet. A wave packet is formed by combining a
• 52:53
  Speaker 1: range of frequencies and momenta. The different wave components overlap
• 52:58
  Speaker 1: in such a way that the overall quantum state becomes
• 53:01
  Speaker 1: concentrated more strongly in a particular region. There is a trade-off.
• 53:05
  Speaker 1: The more precisely a state is localised, the broader the
• 53:08
  Speaker 1: range of momenta generally needs to be. This is part
• 53:12
  Speaker 1: of the deeper structure associated with the uncertainty principle, developed
• 53:16
  Speaker 1: by Werner Heisenberg. The uncertainty principle is not simply saying
• 53:21
  Speaker 1: that measuring equipment is clumsy. It describes a fundamental relationship
• 53:26
  Speaker 1: between certain pairs of quantum properties. For position and momentum,
• 53:31
  Speaker 1: a state cannot generally possess arbitrarily precise values of both
• 53:35
  Speaker 1: at the same time. This makes another seemingly simple question
• 53:40
  Speaker 1: surprisingly difficult. Exactly which path did the photon take? For
• 53:45
  Speaker 1: a classical particle, that question sounds perfectly reasonable. A tennis
• 53:49
  Speaker 1: ball travels along one path. A raindrop follows one trajectory.
• 53:54
  Speaker 1: A spacecraft has a track through space. But in quantum
• 53:58
  Speaker 1: interference experiments, assigning one definite classical path to the photon
• 54:02
  Speaker 1: can destroy the very description needed to explain the observed interference.
• 54:08
  Speaker 1: If an experiment is arranged so that there is no
• 54:10
  Speaker 1: measurement revealing which path was taken, quantum mechanics allows amplitudes
• 54:15
  Speaker 1: associated with different possibilities to interfere. If the experimental arrangement
• 54:21
  Speaker 1: records reliable which-path information, The interference can disappear. The theory
• 54:27
  Speaker 1: does not need to hide one ordinary trajectory underneath and
• 54:30
  Speaker 1: simply refuse to tell us about it. The quantum state
• 54:34
  Speaker 1: itself contains the alternative possibilities. This is why illustrations must
• 54:39
  Speaker 1: be handled with a little generosity. A wavy line can
• 54:42
  Speaker 1: be useful. A glowing dot can be useful. An arrow
• 54:46
  Speaker 1: can be useful. A little packet moving from left to
• 54:49
  Speaker 1: right can be useful too. All of them help visualise
• 54:53
  Speaker 1: some aspect of the physics, but none of them is
• 54:56
  Speaker 1: a literal photograph of a photon. The wavy line does
• 54:59
  Speaker 1: not show a tiny object physically wriggling through space. The
• 55:03
  Speaker 1: dot does not prove that the photon is a microscopic bead.
• 55:08
  Speaker 1: The arrow does not guarantee one perfectly defined classical trajectory.
• 55:12
  Speaker 1: Quantum physics often forces us to use familiar pictures for
• 55:15
  Speaker 1: objects that do not belong to the familiar world. That
• 55:19
  Speaker 1: is not necessarily a weakness. It simply means the pictures
• 55:22
  Speaker 1: have limits. A photon is particle-like when it transfers energy
• 55:27
  Speaker 1: and momentum in discrete interactions. It is wave-like when probability
• 55:32
  Speaker 1: amplitudes interfere and diffract. Its quantum state can be spread
• 55:36
  Speaker 1: across space. Its detection can occur at one particular place.
• 55:41
  Speaker 1: Its wavelength is not its diameter. Its path is not
• 55:44
  Speaker 1: always a hidden classical line waiting to be uncovered. The
• 55:48
  Speaker 1: easiest mistake is to ask which familiar object the photon
• 55:51
  Speaker 1: really resembles. A ball? A ripple? A bullet? A wave
• 55:56
  Speaker 1: on a pond? The deeper answer is that none of
• 55:59
  Speaker 1: them quite works. The photon is not an awkward mixture
• 56:03
  Speaker 1: of two classical objects. It is a quantum object, and
• 56:07
  Speaker 1: the classical pictures are shadows of its behaviour seen under
• 56:10
  Speaker 1: different experimental conditions. The glowing dot in the diagram is useful.
• 56:16
  Speaker 1: We just should not mistake the drawing for the thing itself.
• 56:20
  Speaker 1: The deepest modern answer to the question of what a
• 56:22
  Speaker 1: photon actually is begins with something more fundamental than the
• 56:26
  Speaker 1: photon itself. It begins with a field. In classical electromagnetism,
• 56:32
  Speaker 1: electric and magnetic fields extend through space. A charged particle
• 56:37
  Speaker 1: can create an electric field around itself. A changing magnetic
• 56:41
  Speaker 1: field can generate an electric field. and a changing electric
• 56:44
  Speaker 1: field can generate a magnetic field. James Clerk Maxwell showed
• 56:49
  Speaker 1: that these fields can support travelling electromagnetic waves, and visible
• 56:53
  Speaker 1: light is one example of such a wave. Quantum physics
• 56:56
  Speaker 1: keeps this basic idea of the electromagnetic field, but changes
• 57:00
  Speaker 1: the way the field is described. Instead of treating the
• 57:03
  Speaker 1: field as something that can vary with completely arbitrary amounts
• 57:07
  Speaker 1: of energy, quantum field theory treats it as a quantum system.
• 57:11
  Speaker 1: That distinction is the key. Quantum field theory is the
• 57:15
  Speaker 1: broader framework in which modern particle physics is formulated. In
• 57:19
  Speaker 1: this framework, fields are not secondary objects that appear only
• 57:23
  Speaker 1: when particles are present. They are fundamental parts of the
• 57:26
  Speaker 1: description of nature. The electromagnetic field exists throughout space. A
• 57:32
  Speaker 1: photon is not something that carries an electromagnetic field around
• 57:35
  Speaker 1: with it like luggage. Rather, the photon is an excitation
• 57:40
  Speaker 1: of the electromagnetic field itself. That is the strongest simple
• 57:44
  Speaker 1: answer modern physics can give. A photon is one quantum
• 57:49
  Speaker 1: of excitation of the electromagnetic field. To make that idea
• 57:53
  Speaker 1: less abstract, it helps to think about the word mode.
• 57:57
  Speaker 1: A field can support different possible patterns of oscillation. These
• 58:01
  Speaker 1: patterns can differ in frequency, direction, wavelength and polarisation. Physicists
• 58:07
  Speaker 1: call them modes. A musical instrument provides a rough analogy.
• 58:12
  Speaker 1: A stretched string can vibrate in different patterns. A pipe
• 58:16
  Speaker 1: can support different resonant sound modes. The electromagnetic field can
• 58:21
  Speaker 1: also be mathematically decomposed into different modes. The analogy is useful,
• 58:26
  Speaker 1: but it should not be pushed too far. There is
• 58:29
  Speaker 1: no physical string hidden inside empty space. There is no
• 58:34
  Speaker 1: invisible material quietly wobbling up and down. The electromagnetic field
• 58:39
  Speaker 1: is not made from a more familiar substance. It is
• 58:42
  Speaker 1: one of the fundamental fields described by physics. When that
• 58:46
  Speaker 1: field is quantized, each mode cannot exchange energy in arbitrary amounts. Instead,
• 58:52
  Speaker 1: its excitations come in discrete units. Those discrete excitations are photons.
• 58:59
  Speaker 1: If a particular mode contains one quantum of excitation, we
• 59:03
  Speaker 1: describe it as containing one photon. If it contains two quanta,
• 59:07
  Speaker 1: it contains two photons. Three quanta correspond to three photons
• 59:11
  Speaker 1: and so on. The energy associated with each quantum depends
• 59:15
  Speaker 1: on the frequency of that mode, just as Max Planck
• 59:18
  Speaker 1: first discovered in a much earlier context. This brings together
• 59:22
  Speaker 1: two ideas that can otherwise seem unrelated. The wave-like electromagnetic
• 59:27
  Speaker 1: field has frequencies wavelengths and modes. The particle-like photon carries
• 59:32
  Speaker 1: discrete energy and momentum. In quantum field theory, those are
• 59:37
  Speaker 1: not two competing descriptions. The discrete particle is an excitation
• 59:41
  Speaker 1: of the field. The road to this description took several decades.
• 59:46
  Speaker 1: Paul Dirac made one of the crucial early advances in
• 59:49
  Speaker 1: the late 1920s by developing a quantum theory of the
• 59:53
  Speaker 1: electromagnetic radiation field. His work showed how the electromagnetic field
• 59:58
  Speaker 1: itself could be quantized and how photons could naturally appear
• 1:00:02
  Speaker 1: as discrete field excitations. Later, quantum electrodynamics developed into the
• 1:00:08
  Speaker 1: extraordinarily precise theory used today to describe interactions between charged
• 1:00:13
  Speaker 1: particles and the electromagnetic field. Richard Feynman, Julian Schwinger and
• 1:00:19
  Speaker 1: Sinatiro Tomonaga developed powerful formulations of the theory during the
• 1:00:23
  Speaker 1: 20th century. while Freeman Dyson demonstrated important connections between different
• 1:00:29
  Speaker 1: approaches and helped establish the modern framework. Quantum electrodynamics became
• 1:00:34
  Speaker 1: one of the most accurately tested theories in all of science.
• 1:00:39
  Speaker 1: Its predictions for certain quantities agree with experiment to extraordinary precision.
• 1:00:44
  Speaker 1: That success matters because the phrase quantum of a field
• 1:00:48
  Speaker 1: can otherwise sound as though physicists have merely taken one
• 1:00:51
  Speaker 1: strange mystery and replaced it with another strange phrase. They
• 1:00:56
  Speaker 1: have not. The field description provides a mathematical framework that
• 1:01:01
  Speaker 1: predicts what experiments actually observe. Within that framework the number
• 1:01:05
  Speaker 1: of photons in a mode can change. Quantum theory contains
• 1:01:09
  Speaker 1: mathematical operations that correspond to this. They are called creation
• 1:01:13
  Speaker 1: operators and annihilation operators. The names can sound rather dramatic,
• 1:01:18
  Speaker 1: but the underlying idea is simple. A creation operator corresponds
• 1:01:22
  Speaker 1: to increasing the number of quanta in a particular mode
• 1:01:25
  Speaker 1: by one. An annihilation operator corresponds to decreasing that number
• 1:01:30
  Speaker 1: by one. If an atom emits a photon into a
• 1:01:33
  Speaker 1: particular electromagnetic mode, the field gains one quantum of excitation.
• 1:01:39
  Speaker 1: If an atom absorbs a photon, the field loses one quantum.
• 1:01:44
  Speaker 1: Nothing needs to imagine a little object being manufactured from
• 1:01:47
  Speaker 1: raw material in a tiny factory. The quantum state of
• 1:01:51
  Speaker 1: the field has changed. States with definite numbers of photons
• 1:01:55
  Speaker 1: are often called photon number states, or Fock states, after
• 1:01:59
  Speaker 1: the physicist Vladimir Fock. A mode can, in principle, be
• 1:02:03
  Speaker 1: in a state containing exactly zero photons, exactly one photon,
• 1:02:08
  Speaker 1: exactly two photons, or another definite number. That sounds almost
• 1:02:13
  Speaker 1: like counting objects in a box, but once again the
• 1:02:16
  Speaker 1: quantum description is deeper than the everyday picture. The photons
• 1:02:20
  Speaker 1: are not tiny independent beads sitting inside a container. The
• 1:02:23
  Speaker 1: number describes how many quanta of excitation are present in
• 1:02:27
  Speaker 1: a particular field mode. The case of zero photons is
• 1:02:30
  Speaker 1: especially interesting. In quantum electrodynamics, the vacuum state is the
• 1:02:35
  Speaker 1: lowest energy state of the quantum electromagnetic field. For a
• 1:02:40
  Speaker 1: chosen set of field modes, it contains no real photons.
• 1:02:44
  Speaker 1: But this does not mean the vacuum is absolute philosophical nothingness.
• 1:02:48
  Speaker 1: the quantum field still exists, its state still has physical structure,
• 1:02:53
  Speaker 1: and because quantum fields obey uncertainty relationships, the vacuum state
• 1:02:58
  Speaker 1: is not equivalent to a perfectly motionless classical field, with
• 1:03:02
  Speaker 1: every possible quantity fixed exactly. This is where popular descriptions
• 1:03:07
  Speaker 1: can become a little too enthusiastic. Vacuum fluctuations are sometimes
• 1:03:12
  Speaker 1: described as particles constantly popping into and out of existence.
• 1:03:16
  Speaker 1: That phrase can be useful as a loose metaphor in
• 1:03:19
  Speaker 1: limited contexts, but it should not be taken literally. There
• 1:03:23
  Speaker 1: is not a hidden swarm of ordinary short-lived particles visibly
• 1:03:27
  Speaker 1: appearing from nowhere and then disappearing before nature notices. Vacuum
• 1:03:32
  Speaker 1: effects are properties of quantum fields and their correlations. They
• 1:03:36
  Speaker 1: can influence measurable phenomena, but their correct description comes from
• 1:03:40
  Speaker 1: quantum field theory rather than from a literal picture of
• 1:03:44
  Speaker 1: tiny objects borrowing energy from nothing. The same caution applies
• 1:03:49
  Speaker 1: to virtual photons. They appear naturally in certain methods of
• 1:03:53
  Speaker 1: calculating electromagnetic interactions, but they are not simply ordinary detectable
• 1:03:58
  Speaker 1: photons briefly hiding between charged particles. The real photon itself
• 1:04:04
  Speaker 1: is already interesting enough without giving empty space and unnecessary
• 1:04:08
  Speaker 1: fireworks display. The field description also explains why classical electromagnetism
• 1:04:14
  Speaker 1: works so well. Many familiar electromagnetic situations involve enormous numbers
• 1:04:20
  Speaker 1: of photons arranged in quantum states, whose collective behaviour closely
• 1:04:24
  Speaker 1: resembles a classical field. A radio transmitter can produce radiation
• 1:04:29
  Speaker 1: involving vast numbers of quanta. A bright beam from a
• 1:04:33
  Speaker 1: lamp contains huge numbers of photons. The fields around antennas,
• 1:04:38
  Speaker 1: inside optical systems and across ordinary communication equipment can often
• 1:04:43
  Speaker 1: be described superbly by Maxwell's equations without tracking individual photons
• 1:04:48
  Speaker 1: at all. This is not because quantum theory has stopped applying.
• 1:04:54
  Speaker 1: It is because the quantum description produces classical behaviour in
• 1:04:58
  Speaker 1: the appropriate limit. Certain quantum states of the electromagnetic field,
• 1:05:03
  Speaker 1: especially coherent states, can behave in ways that closely approximate
• 1:05:07
  Speaker 1: the smooth electromagnetic waves familiar from classical physics. The two
• 1:05:14
  Speaker 1: descriptions therefore fit together. Maxwell's theory describes the large-scale electromagnetic
• 1:05:19
  Speaker 1: field with remarkable accuracy. Quantum electrodynamics describes the deeper quantum
• 1:05:26
  Speaker 1: structure from which photon behaviour and classical electromagnetism can emerge.
• 1:05:32
  Speaker 1: This is why the photon becomes much easier to understand
• 1:05:35
  Speaker 1: once the idea of a tiny flying object is relaxed.
• 1:05:39
  Speaker 1: A photon does not need a rigid surface. It does
• 1:05:42
  Speaker 1: not need to carry its own private wave. It does
• 1:05:45
  Speaker 1: not need to decide whether it feels like being a
• 1:05:47
  Speaker 1: particle or a wave on a particular afternoon. It is
• 1:05:51
  Speaker 1: an excitation of a quantum field. The field can display
• 1:05:55
  Speaker 1: interference because amplitudes associated with different possibilities can combine. Energy
• 1:06:01
  Speaker 1: can be detected in discrete amounts because the field's excitations
• 1:06:05
  Speaker 1: are quantized. Photons can be created and destroyed because the
• 1:06:09
  Speaker 1: number of excitations in the field can change. Many photons
• 1:06:14
  Speaker 1: can collectively produce something that looks extremely close to the
• 1:06:17
  Speaker 1: smooth electromagnetic waves described by classical physics. Seen this way,
• 1:06:23
  Speaker 1: the old contradiction begins to soften. The wave and the
• 1:06:26
  Speaker 1: particle were never two separate substances fighting for ownership of light.
• 1:06:31
  Speaker 1: They were different pieces of evidence pointing toward a more
• 1:06:34
  Speaker 1: general description. The electromagnetic field is quantum, and what we
• 1:06:39
  Speaker 1: call a photon is one discrete excitation of that field.
• 1:06:44
  Speaker 1: If a photon is an excitation of the electromagnetic field,
• 1:06:48
  Speaker 1: then another very natural question appears. Where do photons come from?
• 1:06:53
  Speaker 1: It is tempting to imagine that nature keeps an enormous
• 1:06:56
  Speaker 1: supply of photons hidden inside atoms, stars, lamps and lasers,
• 1:07:02
  Speaker 1: waiting for the right moment to be released. That is
• 1:07:05
  Speaker 1: not the modern picture. Photons are created when physical systems
• 1:07:10
  Speaker 1: transfer energy into the electromagnetic field in a way that
• 1:07:14
  Speaker 1: produces a quantum excitation. They can also disappear as photons
• 1:07:19
  Speaker 1: when that energy is absorbed elsewhere. There is no requirement
• 1:07:23
  Speaker 1: for the total number of photons in the universe to
• 1:07:25
  Speaker 1: remain fixed. Atoms provide one of the clearest examples. An
• 1:07:30
  Speaker 1: atom contains electrons bound to a positively charged nucleus. In
• 1:07:34
  Speaker 1: quantum mechanics, those electrons cannot possess just any arbitrary energy
• 1:07:39
  Speaker 1: while bound to the atom. They occupy allowed quantum states
• 1:07:43
  Speaker 1: with particular energies. This idea has deep historical roots. In
• 1:07:49
  Speaker 1: the early 20th century, Niels Bohr proposed that electrons in
• 1:07:52
  Speaker 1: atoms could occupy certain discrete energy levels. His model pictured
• 1:07:57
  Speaker 1: electrons moving in specific allowed orbits around the nucleus. That
• 1:08:02
  Speaker 1: model was an important step, but the modern picture is
• 1:08:04
  Speaker 1: quite different. Electrons in atoms are not miniature planets following
• 1:08:09
  Speaker 1: sharply defined circular tracks. Modern quantum mechanics describes them using
• 1:08:14
  Speaker 1: quantum states and orbitals, with probabilities for where an electron
• 1:08:19
  Speaker 1: may be found, and with discrete allowed energies arising from
• 1:08:22
  Speaker 1: the structure of the atom. Suppose an atom has an
• 1:08:26
  Speaker 1: electron in a higher energy state. Under the right conditions,
• 1:08:30
  Speaker 1: the atom can move into a lower energy state. The
• 1:08:33
  Speaker 1: energy difference has to go somewhere. One possibility is that
• 1:08:37
  Speaker 1: the electromagnetic field gains one quantum of excitation. A photon
• 1:08:42
  Speaker 1: is emitted. The energy carried by that photon matches the
• 1:08:46
  Speaker 1: difference between the initial and final energy states of the atom.
• 1:08:49
  Speaker 1: The relationship can be expressed in words as The energy
• 1:08:53
  Speaker 1: of the emitted photon equals the difference between the higher
• 1:08:56
  Speaker 1: atomic energy state and the lower atomic energy state. Because
• 1:09:01
  Speaker 1: photon energy is connected directly with frequency, a particular energy
• 1:09:06
  Speaker 1: difference produces radiation of a corresponding frequency. This is why
• 1:09:10
  Speaker 1: atoms can produce distinctive spectral lines. Hydrogen, helium, sodium and
• 1:09:16
  Speaker 1: other elements do not simply emit every possible frequency with
• 1:09:19
  Speaker 1: equal strength. Their allowed quantum states create particular energy differences,
• 1:09:25
  Speaker 1: and those differences correspond to particular photon energies. When the
• 1:09:29
  Speaker 1: light from a glowing gas is separated into its component wavelengths,
• 1:09:34
  Speaker 1: these lines form a kind of atomic fingerprint. Astronomers use
• 1:09:38
  Speaker 1: them constantly. The spectrum of a distant star can reveal
• 1:09:42
  Speaker 1: which chemical elements are present in its atmosphere. because the
• 1:09:46
  Speaker 1: atoms there absorb and emit photons at characteristic frequencies. The
• 1:09:51
  Speaker 1: photon therefore carries information about the process that created it,
• 1:09:56
  Speaker 1: but there is more than one way for an atom
• 1:09:58
  Speaker 1: to emit one. One important mechanism is spontaneous emission. An
• 1:10:03
  Speaker 1: atom in an excited state can make a transition to
• 1:10:06
  Speaker 1: a lower energy state without being triggered by an incoming photon.
• 1:10:11
  Speaker 1: In quantum electrodynamics, this process arises from the interaction between
• 1:10:15
  Speaker 1: the atom and the quantist electromagnetic field. The resulting photon
• 1:10:21
  Speaker 1: is emitted with properties determined by the transition and the
• 1:10:24
  Speaker 1: quantum rules governing the interaction. There is also stimulated emission.
• 1:10:29
  Speaker 1: Albert Einstein analysed spontaneous emission, absorption and stimulated emission in
• 1:10:36
  Speaker 1: the early 20th century. In stimulated emission, An incoming photon
• 1:10:41
  Speaker 1: of suitable energy interacts with an excited atom and encourages
• 1:10:45
  Speaker 1: it to make a transition to a lower energy state.
• 1:10:49
  Speaker 1: A second photon is then emitted. The emitted radiation is
• 1:10:53
  Speaker 1: closely related to the stimulating field in frequency, direction and phase.
• 1:10:58
  Speaker 1: This principle became central to the laser. The word laser
• 1:11:02
  Speaker 1: comes from a phrase describing light amplification by stimulated emission
• 1:11:06
  Speaker 1: of radiation. A laser begins with a material whose atoms,
• 1:11:10
  Speaker 1: molecules or other quantum systems can occupy excited states. Energy
• 1:11:16
  Speaker 1: is supplied to the material through a process often called pumping.
• 1:11:20
  Speaker 1: This creates a population inversion, meaning that more particles occupy
• 1:11:24
  Speaker 1: an appropriate excited state than a lower state involved in
• 1:11:28
  Speaker 1: the laser transition. That situation is unusual because systems left
• 1:11:32
  Speaker 1: to themselves normally favour lower energy states. Once stimulated emission begins,
• 1:11:39
  Speaker 1: photons encourage further excited particles to emit more photons into
• 1:11:43
  Speaker 1: the same optical mode. The radiation becomes amplified. Mirrors placed
• 1:11:49
  Speaker 1: around the active material can form a resonant cavity, allowing
• 1:11:53
  Speaker 1: light to pass repeatedly through the material and encouraging selected
• 1:11:57
  Speaker 1: modes to build up strongly. Some of the radiation is
• 1:12:01
  Speaker 1: allowed to escape, producing the laser beam. There is nothing
• 1:12:05
  Speaker 1: fundamentally different about the photons in that beam. A laser
• 1:12:09
  Speaker 1: photon is not a special species of photon. What is
• 1:12:13
  Speaker 1: special is the quantum state of the light and the
• 1:12:16
  Speaker 1: high degree of coherence produced by the emission process and
• 1:12:20
  Speaker 1: optical cavity. Photons can be created in many other ways
• 1:12:24
  Speaker 1: as well. Molecules can emit photons when they change between
• 1:12:28
  Speaker 1: quantized electronic, vibrational or rotational states. This produces much of
• 1:12:34
  Speaker 1: the rich spectroscopy used in chemistry and astronomy. Accelerating electric
• 1:12:40
  Speaker 1: charges can generate electromagnetic radiation. An electron changing velocity produces
• 1:12:45
  Speaker 1: disturbances in the electromagnetic field and in a quantum description
• 1:12:50
  Speaker 1: that radiation can consist of emitted photons. Thermal radiation is
• 1:12:55
  Speaker 1: another enormous source. The countless charged particles inside a hot
• 1:13:00
  Speaker 1: object are continually moving and interacting. Their collective behaviour produces
• 1:13:05
  Speaker 1: electromagnetic radiation across a range of frequencies. The Sun emits
• 1:13:10
  Speaker 1: an immense number of photons this way, with energy ultimately
• 1:13:13
  Speaker 1: supplied by nuclear fusion in its core, and then repeatedly
• 1:13:17
  Speaker 1: transferred through the solar interior before radiation finally escapes from
• 1:13:21
  Speaker 1: its surface. Atomic nuclei can emit photons too. When a
• 1:13:26
  Speaker 1: nucleus moves from a higher energy state to a lower
• 1:13:29
  Speaker 1: energy state, it can release high energy gamma ray photons.
• 1:13:34
  Speaker 1: Particle physics provides still more examples. When an electron meets
• 1:13:38
  Speaker 1: its antiparticle, the positron, they can annihilate. Their rest energy
• 1:13:43
  Speaker 1: and kinetic energy do not vanish. Under common conditions, that
• 1:13:47
  Speaker 1: energy is converted into photons, often two gamma ray photons
• 1:13:52
  Speaker 1: travelling in directions that allow total momentum to be conserved.
• 1:13:56
  Speaker 1: Charged particles moving through magnetic fields can also emit synchrotron radiation.
• 1:14:02
  Speaker 1: This occurs when fast-moving particles such as electrons are forced
• 1:14:06
  Speaker 1: onto curved trajectories by magnetic fields. Synchrotron radiation is important
• 1:14:11
  Speaker 1: in laboratory particle accelerators and throughout astronomy. It can be
• 1:14:16
  Speaker 1: produced near neutron stars, around black holes, in supernova remnants
• 1:14:21
  Speaker 1: and in powerful jets launched from active galaxies. So photons
• 1:14:26
  Speaker 1: do not all have one birthplace. A red photon from
• 1:14:30
  Speaker 1: a warm object, an X-ray photon from hot gas around
• 1:14:33
  Speaker 1: a black hole, a radio photon from an antenna, and
• 1:14:37
  Speaker 1: a gamma-ray photon from a nuclear transition are all excitations
• 1:14:41
  Speaker 1: of the same electromagnetic field. Their origins differ, their energies differ,
• 1:14:45
  Speaker 1: their frequencies differ, but they are all photons. And just
• 1:14:49
  Speaker 1: as photons can be created, they can also cease to
• 1:14:52
  Speaker 1: exist as photons. When a photon is absorbed by an atom,
• 1:14:56
  Speaker 1: its energy can raise the atom into a higher energy state.
• 1:15:00
  Speaker 1: When light is absorbed by a solid, its energy may
• 1:15:03
  Speaker 1: eventually become vibrations of the material– electrical excitations, chemical energy
• 1:15:09
  Speaker 1: or heat. A photon striking a suitable semiconductor can promote
• 1:15:14
  Speaker 1: an electron into a state where it contributes to an
• 1:15:17
  Speaker 1: electrical current. In every case, the photon does not need
• 1:15:21
  Speaker 1: to survive as a permanent little object hidden somewhere inside
• 1:15:24
  Speaker 1: the material. The electromagnetic field has lost that quantum of excitation,
• 1:15:30
  Speaker 1: while the energy and momentum have been transferred into other
• 1:15:33
  Speaker 1: degrees of freedom. The photon is gone as a photon.
• 1:15:37
  Speaker 1: The conservation laws remain perfectly intact. This is an important
• 1:15:42
  Speaker 1: point because particle number and energy conservation are not the
• 1:15:45
  Speaker 1: same thing. The number of photons does not generally have
• 1:15:49
  Speaker 1: to remain constant. One process can create photons, another can
• 1:15:53
  Speaker 1: absorb them. Two photons can sometimes participate in an interaction
• 1:15:58
  Speaker 1: that produces other particles. If enough energy is available, and
• 1:16:02
  Speaker 1: all relevant, conservation laws are satisfied. Other processes can take
• 1:16:07
  Speaker 1: energy stored in matter and place it into the electromagnetic
• 1:16:11
  Speaker 1: field as newly created photons. Quantum field theory treats this
• 1:16:16
  Speaker 1: as completely ordinary. What is conserved are quantities such as
• 1:16:20
  Speaker 1: total energy, momentum and electric charge where the relevant conservation
• 1:16:24
  Speaker 1: laws apply. There is no universal rule demanding that every
• 1:16:29
  Speaker 1: photon existing now must have existed forever. A photon can
• 1:16:34
  Speaker 1: begin when the electromagnetic field gains the appropriate quantum excitation.
• 1:16:39
  Speaker 1: It can travel across a laboratory through the atmosphere or
• 1:16:42
  Speaker 1: across billions of light-years of intergalactic space. And when it
• 1:16:47
  Speaker 1: is finally absorbed, that particular excitation can end. The energy continues,
• 1:16:53
  Speaker 1: the momentum is accounted for, the physical process carries on,
• 1:16:58
  Speaker 1: but the photon itself does not need to. One of
• 1:17:01
  Speaker 1: the strangest things about a photon is that the less
• 1:17:04
  Speaker 1: we insist on picturing it as a tiny object following
• 1:17:07
  Speaker 1: one definite route, the better its behaviour begins to make sense.
• 1:17:11
  Speaker 1: The double-slit experiment provides perhaps the most famous example. Imagine
• 1:17:16
  Speaker 1: a source of light aimed toward a barrier containing two
• 1:17:20
  Speaker 1: narrow openings. Beyond the barrier is a detector capable of
• 1:17:24
  Speaker 1: recording where the light arrives. With an ordinary bright beam,
• 1:17:28
  Speaker 1: the result is an interference pattern. Some regions receive more light,
• 1:17:33
  Speaker 1: while others receive less. The familiar explanation involves waves emerging
• 1:17:38
  Speaker 1: from both openings and overlapping. Where the wave contributions reinforce
• 1:17:42
  Speaker 1: one another, the detected intensity is high. Where they oppose
• 1:17:47
  Speaker 1: one another, the intensity is reduced. But quantum physics allows
• 1:17:51
  Speaker 1: the experiment to be made considerably stranger. The light source
• 1:17:56
  Speaker 1: can be weakened until photons are being detected one at
• 1:17:59
  Speaker 1: a time. There need not be a crowd of photons
• 1:18:01
  Speaker 1: passing through the apparatus together. One detection occurs, then another,
• 1:18:06
  Speaker 1: then another. Each arrival appears as a localised event. A
• 1:18:10
  Speaker 1: photon does not leave a faint fraction of itself spread
• 1:18:13
  Speaker 1: across the entire detecting screen. One individual detection appears at
• 1:18:18
  Speaker 1: one location. At first, after only a few photons, the
• 1:18:22
  Speaker 1: positions seem scattered almost randomly, but keep sending photons through
• 1:18:26
  Speaker 1: the apparatus one at a time and a pattern gradually appears.
• 1:18:31
  Speaker 1: Thousands of individual detection events begin to form alternating regions
• 1:18:35
  Speaker 1: where photons are more likely and less likely to arrive. Eventually,
• 1:18:39
  Speaker 1: the same interference structure associated with waves becomes visible. This
• 1:18:44
  Speaker 1: is one of the most beautiful demonstrations of the quantum
• 1:18:47
  Speaker 1: nature of light. Each detection is localized. The overall distribution
• 1:18:52
  Speaker 1: is wave-like, and crucially the interference pattern can emerge even
• 1:18:56
  Speaker 1: when the photons pass through the apparatus individually. There does
• 1:19:00
  Speaker 1: not need to be a second photon travelling alongside each
• 1:19:03
  Speaker 1: one for it to interfere with. The quantum state itself
• 1:19:07
  Speaker 1: contains the alternatives required for interference. This is where the
• 1:19:11
  Speaker 1: principle of superposition becomes essential. Quantum superposition means that a
• 1:19:16
  Speaker 1: physical system can be described by a state containing several
• 1:19:20
  Speaker 1: possible alternatives at once. For the double-slit experiment, the photon
• 1:19:25
  Speaker 1: state can contain an amplitude associated with one opening and
• 1:19:29
  Speaker 1: an amplitude associated with the other. It is tempting to
• 1:19:32
  Speaker 1: say that the photon literally splits into two smaller pieces,
• 1:19:36
  Speaker 1: sends one half through each slit and then reassembles itself later.
• 1:19:41
  Speaker 1: That picture is too classical. There is still one quantum
• 1:19:45
  Speaker 1: of electromagnetic excitation involved in the eventual detection. What is
• 1:19:50
  Speaker 1: distributed across the alternatives is the quantum state and its
• 1:19:54
  Speaker 1: probability amplitudes. Those amplitudes can interfere. They can reinforce each
• 1:19:59
  Speaker 1: other in some places and cancel in others. Only when
• 1:20:03
  Speaker 1: a measurement produces detection do we obtain one particular localised outcome.
• 1:20:08
  Speaker 1: A particularly clean way to explore the same principle uses
• 1:20:12
  Speaker 1: a device called the Mach-Zehnder interferometer. A simple Mach-Zehnder interferometer
• 1:20:17
  Speaker 1: contains beam splitters and mirrors arranged so that light can
• 1:20:21
  Speaker 1: follow two distinct routes before those routes are brought together again.
• 1:20:24
  Speaker 1: A beam splitter is an optical device that allows part
• 1:20:28
  Speaker 1: of an incoming light amplitude to continue along one path
• 1:20:32
  Speaker 1: while another part is redirected along a different path. For
• 1:20:35
  Speaker 1: an individual photon, quantum mechanics describes the state after the
• 1:20:40
  Speaker 1: first beam splitter as a superposition involving both possible paths. Again,
• 1:20:45
  Speaker 1: this does not require us to imagine two half photons
• 1:20:49
  Speaker 1: physically travelling around the laboratory. The quantum state contains amplitudes
• 1:20:54
  Speaker 1: associated with both roots. Mirrors redirect the paths toward a
• 1:20:58
  Speaker 1: second beam splitter. When the alternatives meet there, the probability
• 1:21:03
  Speaker 1: amplitudes can interfere. With the apparatus arranged appropriately, the interference
• 1:21:08
  Speaker 1: can make one detector extremely likely to register the photon,
• 1:21:12
  Speaker 1: while another detector may receive almost none. That result is
• 1:21:17
  Speaker 1: remarkable if we insist that the photon must secretly have
• 1:21:20
  Speaker 1: chosen one ordinary classical path immediately after encountering the first
• 1:21:25
  Speaker 1: beam splitter. The experiment behaves as though both alternatives matter
• 1:21:29
  Speaker 1: to the final probabilities. Now change the arrangement. Place equipment
• 1:21:34
  Speaker 1: along the routes capable of determining which path the photon took.
• 1:21:39
  Speaker 1: The situation changes. If reliable which-path information becomes physically available,
• 1:21:44
  Speaker 1: the interference between the alternatives can be reduced or destroyed.
• 1:21:49
  Speaker 1: This is sometimes described rather dramatically as though the photon
• 1:21:52
  Speaker 1: somehow knows it is being watched. There is no need
• 1:21:55
  Speaker 1: for that language. No conscious observer is required. The important
• 1:22:00
  Speaker 1: issue is whether the photon's path becomes physically correlated with
• 1:22:04
  Speaker 1: something else in a way that distinguishes the alternatives. A detector,
• 1:22:09
  Speaker 1: an atom, another photon, or some part of the surrounding
• 1:22:12
  Speaker 1: environment can carry information about which route occurred. Once the
• 1:22:17
  Speaker 1: alternatives have become distinguishable in this way, their amplitudes no
• 1:22:21
  Speaker 1: longer contribute to the same interference pattern in the original manner. Historically,
• 1:22:27
  Speaker 1: Niels Bohr discussed these kinds of situations through the principle
• 1:22:31
  Speaker 1: of complementarity. Bohr argued that wave-like and particle-like experimental arrangements
• 1:22:37
  Speaker 1: reveal complementary aspects of quantum phenomena and cannot always be
• 1:22:42
  Speaker 1: combined into one classical picture. That language played an important
• 1:22:46
  Speaker 1: role in the development of quantum mechanics. Modern treatments can
• 1:22:50
  Speaker 1: describe the same physics more precisely using quantum states, entanglement
• 1:22:54
  Speaker 1: and decoherence. If the path of a photon becomes entangled
• 1:22:59
  Speaker 1: with a measuring device or the environment, information about the
• 1:23:02
  Speaker 1: alternatives can spread into those other systems. Interference then becomes
• 1:23:07
  Speaker 1: inaccessible in the measurement being considered. This is a physical
• 1:23:11
  Speaker 1: quantum process, not simply a matter of a human being
• 1:23:15
  Speaker 1: ignorant of which path was taken. That distinction matters. It
• 1:23:19
  Speaker 1: might be tempting to imagine that the photon always follows
• 1:23:22
  Speaker 1: one perfectly definite classical route, and quantum mechanics merely fails
• 1:23:27
  Speaker 1: to tell us which one. But quantum interference makes that
• 1:23:30
  Speaker 1: simple picture difficult to maintain. The alternatives contribute to measurable
• 1:23:35
  Speaker 1: probabilities in ways that ordinary ignorance about a hidden classical
• 1:23:39
  Speaker 1: route does not naturally reproduce. Quantum experiments have placed increasingly
• 1:23:45
  Speaker 1: strong restrictions on attempts to explain these effects using conventional
• 1:23:50
  Speaker 1: local hidden variable pictures. The quantum state is not merely
• 1:23:54
  Speaker 1: a list of facts that we have failed to discover.
• 1:23:57
  Speaker 1: Its superpositions have observable consequences. Richard Feynman developed another powerful
• 1:24:03
  Speaker 1: way of expressing this idea through the path-integral formulation of
• 1:24:07
  Speaker 1: quantum mechanics. In this approach, calculations do not begin by
• 1:24:13
  Speaker 1: assigning one unique classical trajectory to the quantum object. Instead,
• 1:24:18
  Speaker 1: probability amplitudes associated with possible histories are combined. Very roughly,
• 1:24:24
  Speaker 1: many possible ways of getting from the starting situation to
• 1:24:27
  Speaker 1: the final situation contribute to the calculation. Their amplitudes can
• 1:24:32
  Speaker 1: reinforce or cancel one another. In the appropriate large-scale limit,
• 1:24:37
  Speaker 1: the contributions cluster around paths resembling those predicted by classical physics.
• 1:24:42
  Speaker 1: But at quantum scales, alternatives that classical intuition might want
• 1:24:47
  Speaker 1: to discard can matter. This does not mean that a
• 1:24:50
  Speaker 1: photon is literally a little bead racing along every imaginable
• 1:24:54
  Speaker 1: path in an ordinary mechanical sense. The path integral is
• 1:24:58
  Speaker 1: a mathematical formulation of quantum theory, not a hidden camera
• 1:25:02
  Speaker 1: recording countless miniature journeys. but it provides a powerful reminder
• 1:25:07
  Speaker 1: that demanding one pre-existing classical trajectory can be the wrong question.
• 1:25:12
  Speaker 1: The distinction between the quantum state and the final detection
• 1:25:15
  Speaker 1: is therefore crucial. Before detection, the state can be spatially
• 1:25:19
  Speaker 1: extended and can contain amplitudes for multiple alternatives. Those amplitudes
• 1:25:25
  Speaker 1: evolve and interfere according to quantum rules. At the detector, however,
• 1:25:30
  Speaker 1: energy is transferred in a localised interaction. One detector responds,
• 1:25:35
  Speaker 1: One event is recorded. Repeat the experiment many times and
• 1:25:40
  Speaker 1: the distribution of those individual events reveals the structure of
• 1:25:43
  Speaker 1: the underlying quantum probabilities. This is why a photon can
• 1:25:48
  Speaker 1: appear so contradictory when squeezed into classical language. It spreads
• 1:25:53
  Speaker 1: like something wave-like. It arrives like something particle-like. It can
• 1:25:57
  Speaker 1: be described by several possible paths. Yet the detector ultimately
• 1:26:02
  Speaker 1: records one event. The contradiction comes largely from insisting that
• 1:26:07
  Speaker 1: nature must choose between our familiar classical categories. Quantum mechanics
• 1:26:11
  Speaker 1: does not ask the photon to be a tiny ball
• 1:26:14
  Speaker 1: secretly choosing a road. It gives the photon a quantum
• 1:26:18
  Speaker 1: state containing physical possibilities whose amplitudes can interfere, and only
• 1:26:23
  Speaker 1: when that state finally interacts with a detector does one
• 1:26:26
  Speaker 1: sharply localised event appear. Almost everything we know about an
• 1:26:31
  Speaker 1: individual photon comes from the moment it interacts with something else.
• 1:26:36
  Speaker 1: A photon travelling freely through space does not leave behind
• 1:26:39
  Speaker 1: a glowing trail that can be watched directly. We infer
• 1:26:43
  Speaker 1: its properties from what happens when electromagnetic radiation interacts with matter.
• 1:26:49
  Speaker 1: An atom changes energy state. An electron is released. A
• 1:26:53
  Speaker 1: current begins to flow. A detector produces a pulse. Somewhere
• 1:26:58
  Speaker 1: in that chain, a microscopic quantum event becomes large enough
• 1:27:02
  Speaker 1: for us to notice. Imagine a photon entering a detector.
• 1:27:06
  Speaker 1: Depending on the detector and the photon's energy, it may
• 1:27:10
  Speaker 1: be absorbed by an atom, a molecule, a semiconductor, or
• 1:27:14
  Speaker 1: another carefully chosen material. Its energy is transferred into the detector,
• 1:27:20
  Speaker 1: where it can alter the state of one or more
• 1:27:22
  Speaker 1: charged particles. That first interaction can be astonishingly small, A
• 1:27:27
  Speaker 1: visible photon typically carries only a few electron volts of energy,
• 1:27:31
  Speaker 1: an almost negligible amount on everyday scales. Yet a well-designed
• 1:27:36
  Speaker 1: detector can take that tiny deposit and amplify its consequences
• 1:27:40
  Speaker 1: until an electronic circuit produces a clear signal. One traditional
• 1:27:45
  Speaker 1: device for doing this is the photomultiplier tube. Light enters
• 1:27:49
  Speaker 1: through a transparent window and strikes a photosensitive surface called
• 1:27:53
  Speaker 1: a photocathode. Through the photoelectric effect, an incoming photon can
• 1:27:58
  Speaker 1: cause an electron to be released. That electron is then
• 1:28:02
  Speaker 1: accelerated toward another surface called a dinode. When it strikes
• 1:28:06
  Speaker 1: the dinode, several more electrons can be released. Those electrons
• 1:28:11
  Speaker 1: are accelerated toward another dinode, producing still more electrons. After
• 1:28:17
  Speaker 1: several stages, the original microscopic event has become a sizeable
• 1:28:21
  Speaker 1: electrical pulse. One photon has not somehow grown into thousands
• 1:28:25
  Speaker 1: of photons inside the detector. Instead, the energy and electrical
• 1:28:30
  Speaker 1: arrangement of the detector provide amplification. The initial photon merely
• 1:28:34
  Speaker 1: triggers the process. Modern experiments also use avalanche photodiodes. These
• 1:28:40
  Speaker 1: are semiconductor devices in which an absorbed photon can free
• 1:28:44
  Speaker 1: a charge carrier. A strong electric field accelerates that charge
• 1:28:48
  Speaker 1: through the material, allowing it to create additional charge carriers
• 1:28:52
  Speaker 1: through collisions. The process can develop into an avalanche of
• 1:28:56
  Speaker 1: electrical charge large enough to detect. Again, an extremely small
• 1:29:01
  Speaker 1: quantum interaction has been converted into a macroscopic electrical signal.
• 1:29:07
  Speaker 1: Semiconductor image sensors work through related principles. When light strikes
• 1:29:12
  Speaker 1: the semiconductor, photons can transfer energy to electrons and create
• 1:29:16
  Speaker 1: mobile charge. The accumulated charge can then be read electronically
• 1:29:21
  Speaker 1: and turned into the pixels of an image. The photograph
• 1:29:24
  Speaker 1: on a digital camera then ultimately depends on countless individual
• 1:29:29
  Speaker 1: interactions between light and matter. At the more sensitive end
• 1:29:33
  Speaker 1: of modern photon detection are superconducting detectors designed to register
• 1:29:37
  Speaker 1: individual photons with very high efficiency. Some superconducting single photon
• 1:29:43
  Speaker 1: detectors use extremely thin wires, cooled to very low temperatures.
• 1:29:48
  Speaker 1: The material is kept in a superconducting state, allowing electrical
• 1:29:52
  Speaker 1: current to flow with essentially no resistance. When a photon
• 1:29:56
  Speaker 1: is absorbed, it can disturb a tiny region of the
• 1:29:59
  Speaker 1: superconducting wire. That disturbance briefly produces electrical resistance. The resulting
• 1:30:06
  Speaker 1: voltage pulse reveals that an absorption event has occurred. Devices
• 1:30:10
  Speaker 1: of this sort are used in areas including quantum optics,
• 1:30:14
  Speaker 1: astronomy and quantum communication because they can detect exceptionally faint
• 1:30:19
  Speaker 1: light while producing very little background noise. Across these different technologies,
• 1:30:25
  Speaker 1: a common feature appears. Detection is discrete. A detector does
• 1:30:31
  Speaker 1: not normally report that one photon has deposited 23% of
• 1:30:35
  Speaker 1: itself in one detector, 41% somewhere else and the rest
• 1:30:39
  Speaker 1: across several unrelated devices. Instead, an individual photon detection appears
• 1:30:45
  Speaker 1: as a particular event, a click, a pulse, a changed
• 1:30:49
  Speaker 1: electronic state, a localised transfer of energy. This discrete character
• 1:30:54
  Speaker 1: is one reason the particle language remains so useful. But
• 1:30:59
  Speaker 1: it does not mean the photon must have been a
• 1:31:01
  Speaker 1: tiny hard object occupying one sharply defined point throughout its
• 1:31:05
  Speaker 1: entire journey. The detector tells us where the interaction happened.
• 1:31:11
  Speaker 1: It does not provide a photograph of a little luminous
• 1:31:14
  Speaker 1: sphere that had travelled along a perfectly definite hidden track
• 1:31:17
  Speaker 1: from source to detector. This distinction becomes especially important in
• 1:31:22
  Speaker 1: interference experiments. A photon can be described before detection by
• 1:31:28
  Speaker 1: a quantum state spread across several possible regions or paths.
• 1:31:32
  Speaker 1: Yet when detection occurs, the energy transfer appears at one
• 1:31:35
  Speaker 1: particular place. The spatially extended quantum description and the localised
• 1:31:41
  Speaker 1: detection event are both genuine parts of the physics. Experiments
• 1:31:45
  Speaker 1: can examine this discreteness in even more revealing ways by
• 1:31:49
  Speaker 1: studying photon statistics. One important phenomenon is called photon antibunching.
• 1:31:55
  Speaker 1: Imagine observing the arrival times of photons from a carefully
• 1:31:58
  Speaker 1: prepared light source. Ordinary classical light can sometimes produce photons
• 1:32:03
  Speaker 1: in clusters, with several detections occurring relatively close together. A
• 1:32:08
  Speaker 1: genuine single photon emitter can behave differently. If the system
• 1:32:13
  Speaker 1: emits one photon, and must then return through its physical
• 1:32:16
  Speaker 1: cycle before another can be emitted, the probability of detecting
• 1:32:20
  Speaker 1: two photons at precisely the same moment can become strongly suppressed.
• 1:32:25
  Speaker 1: The photons appear anti-bunched. This behaviour cannot be reproduced by
• 1:32:30
  Speaker 1: ordinary classical electromagnetic waves. It is a distinctly quantum optical signature.
• 1:32:37
  Speaker 1: Measurements of this kind help physicists determine whether a source
• 1:32:41
  Speaker 1: is truly producing light with strong single-photon characteristics, rather than
• 1:32:46
  Speaker 1: merely producing an extremely dim classical beam. Experimental work by
• 1:32:51
  Speaker 1: researchers including John Clauser, Philippe Granger, Gérard Roger and Alain
• 1:32:57
  Speaker 1: Aspect helped establish increasingly sophisticated tests of the non-classical behaviour
• 1:33:03
  Speaker 1: of light. Experiments involving beam splitters and carefully controlled photon
• 1:33:08
  Speaker 1: sources demonstrated that individual quanta do not behave like classical
• 1:33:14
  Speaker 1: waves whose energy is simply divided continuously between two outputs.
• 1:33:19
  Speaker 1: The statistics of the resulting detections reveal genuinely quantum behaviour.
• 1:33:24
  Speaker 1: This brings us to one of the deepest questions in
• 1:33:26
  Speaker 1: quantum mechanics. What exactly happens when the photon is measured?
• 1:33:32
  Speaker 1: Quantum theory provides remarkably precise rules for calculating the probabilities
• 1:33:36
  Speaker 1: of different outcomes. Given the quantum state and the arrangement
• 1:33:40
  Speaker 1: of the apparatus, Physicists can predict how likely a photon
• 1:33:44
  Speaker 1: is to be detected in one place rather than another.
• 1:33:47
  Speaker 1: These predictions have been tested again and again with extraordinary success.
• 1:33:52
  Speaker 1: But there remains a deeper conceptual puzzle. Before detection, the
• 1:33:56
  Speaker 1: photon may be described by a state containing several possible outcomes.
• 1:34:01
  Speaker 1: After detection, the experimental record contains one definite result. Why
• 1:34:07
  Speaker 1: does this particular outcome occur? How does one definite event
• 1:34:11
  Speaker 1: emerge from a quantum state containing several possibilities? That family
• 1:34:15
  Speaker 1: of questions is known as the measurement problem. Different interpretations
• 1:34:19
  Speaker 1: of quantum mechanics approach it differently. Some formulations speak of
• 1:34:24
  Speaker 1: wave-function collapse, in which the quantum state changes from a
• 1:34:28
  Speaker 1: spread of possibilities to the state associated with the observed result.
• 1:34:33
  Speaker 1: Other interpretations attempt to understand the same experimental facts without
• 1:34:37
  Speaker 1: treating collapse as a fundamental physical process. There is no
• 1:34:42
  Speaker 1: universally agreed philosophical interpretation of what the mathematical formalism ultimately means.
• 1:34:48
  Speaker 1: What should not be confused with this debate is the
• 1:34:51
  Speaker 1: practical success of the theory. Physicists know extremely well how
• 1:34:56
  Speaker 1: to calculate detection probabilities, they know how to build photon detectors,
• 1:35:01
  Speaker 1: they can predict interference patterns, photon statistics and correlations with
• 1:35:06
  Speaker 1: exquisite precision. The unresolved part is the deeper interpretation of
• 1:35:11
  Speaker 1: how definite outcomes fit into the underlying quantum description. And
• 1:35:15
  Speaker 1: despite what some popular accounts suggest, there is no need
• 1:35:19
  Speaker 1: to place human consciousness at the centre of the process.
• 1:35:22
  Speaker 1: A photodetector does not need a person staring at it
• 1:35:26
  Speaker 1: before it can absorb a photon. An atom can interact
• 1:35:29
  Speaker 1: with light. A semiconductor can change its electrical state. A
• 1:35:34
  Speaker 1: superconducting wire can produce a measurable pulse. These are physical
• 1:35:39
  Speaker 1: interactions governed by quantum mechanics, whether or not anyone happens
• 1:35:44
  Speaker 1: to be watching at that moment. The word observer in
• 1:35:48
  Speaker 1: quantum theory often refers to a measurement interaction, or physical
• 1:35:52
  Speaker 1: system capable of becoming correlated with an outcome. It does
• 1:35:56
  Speaker 1: not automatically mean a conscious mind. So when a detector
• 1:36:00
  Speaker 1: finally clicks, something very specific has happened. Energy and momentum
• 1:36:04
  Speaker 1: have been transferred from the electromagnetic field into matter. A
• 1:36:09
  Speaker 1: quantum interaction has produced a definite physical record. That click
• 1:36:14
  Speaker 1: tells us where the interaction occurred. It may tell us
• 1:36:17
  Speaker 1: something about the photon's energy, polarisation, arrival time or direction,
• 1:36:23
  Speaker 1: depending on the apparatus. But it doesn't reveal a hidden
• 1:36:27
  Speaker 1: photograph of the photon's entire previous existence. The photon arrives
• 1:36:32
  Speaker 1: as a discrete event. the quantum state tells us the
• 1:36:35
  Speaker 1: probabilities of where that event may occur. And between those
• 1:36:39
  Speaker 1: two statements lies one of the most successful, and still
• 1:36:43
  Speaker 1: one of the most philosophically unsettling, features of quantum physics.
• 1:36:48
  Speaker 1: Polarisation is one of those words that tends to appear
• 1:36:51
  Speaker 1: first in connection with sunglasses. A pair of polarised sunglasses
• 1:36:56
  Speaker 1: can reduce glare from roads, water and glass by blocking
• 1:37:00
  Speaker 1: light with certain orientations. Camera filters can use the same principle,
• 1:37:05
  Speaker 1: but polarisation is not merely a useful trick for making
• 1:37:08
  Speaker 1: reflections less annoying. It is a fundamental property of electromagnetic radiation.
• 1:37:15
  Speaker 1: In the classical picture developed from James Clerk Maxwell's electromagnetism,
• 1:37:20
  Speaker 1: a light wave consists of oscillating electric and magnetic fields
• 1:37:24
  Speaker 1: travelling through space. Imagine a beam moving forwards. The electric
• 1:37:28
  Speaker 1: field does not point forwards along the beam. Instead, it
• 1:37:32
  Speaker 1: oscillates sideways, perpendicular to the direction in which the light
• 1:37:36
  Speaker 1: is travelling. The magnetic field also oscillates perpendicular to the
• 1:37:40
  Speaker 1: direction of travel, and perpendicular to the electric field. The
• 1:37:45
  Speaker 1: pattern traced by that electric field defines the polarisation. The
• 1:37:49
  Speaker 1: simplest case is linear polarisation. If the electric field repeatedly
• 1:37:55
  Speaker 1: oscillates along one fixed direction, perhaps vertically, the light is
• 1:37:59
  Speaker 1: vertically polarised. If it oscillates horizontally, the light is horizontally polarized.
• 1:38:06
  Speaker 1: The orientation can lie at any angle between those simple examples.
• 1:38:11
  Speaker 1: But the field does not always remain confined to one
• 1:38:14
  Speaker 1: straight line. If two perpendicular components of the electromagnetic field
• 1:38:19
  Speaker 1: are combined with the right relationship between their phases, the
• 1:38:22
  Speaker 1: direction of the electric field can rotate as the wave
• 1:38:25
  Speaker 1: moves forward. In one special case, its tip traces a circle.
• 1:38:31
  Speaker 1: This is circular polarisation. Depending on the direction of rotation,
• 1:38:36
  Speaker 1: the light can be assigned one of two circular polarisations.
• 1:38:40
  Speaker 1: More generally, the tip of the electric field can trace
• 1:38:43
  Speaker 1: an ellipse. That is elliptical polarisation. Linear and circular polarisation
• 1:38:48
  Speaker 1: are therefore not completely unrelated categories. There are different possible
• 1:38:53
  Speaker 1: ways of describing the same underlying electromagnetic field. Quantum mechanics
• 1:38:58
  Speaker 1: preserves this structure, but now it applies to individual photons
• 1:39:01
  Speaker 1: as well. A single photon can be prepared in a
• 1:39:04
  Speaker 1: particular polarisation state. It can be horizontally polarised. It can
• 1:39:09
  Speaker 1: be vertically polarised. It can be diagonally polarised. It can
• 1:39:14
  Speaker 1: be circularly polarised. And, importantly, quantum mechanics allows these different
• 1:39:20
  Speaker 1: descriptions to be related through superposition. A photon prepared in
• 1:39:24
  Speaker 1: one polarisation basis can be described as a combination of
• 1:39:28
  Speaker 1: possibilities in another basis. Suppose a photon is horizontally polarised.
• 1:39:34
  Speaker 1: If it is measured with an apparatus designed to distinguish
• 1:39:37
  Speaker 1: horizontal from vertical polarisation the result is predictable. But if
• 1:39:42
  Speaker 1: the same photon is measured using an analyser turned at
• 1:39:45
  Speaker 1: 45 degrees, quantum mechanics does not say that the photon
• 1:39:50
  Speaker 1: secretly contained a tiny arrow that the analyser merely discovered. Instead,
• 1:39:56
  Speaker 1: the photon's state can be expressed as a superposition of
• 1:39:59
  Speaker 1: the two possible outcomes associated with that new measurement basis.
• 1:40:04
  Speaker 1: The measurement produces one definite result. With probabilities determined by
• 1:40:09
  Speaker 1: the relative angle between the prepared polarisation and the analyser,
• 1:40:14
  Speaker 1: this can be demonstrated with something as simple as a
• 1:40:17
  Speaker 1: polarising filter. Classically, the behaviour of polarised light passing through
• 1:40:22
  Speaker 1: such a filter is described by a relationship known as
• 1:40:25
  Speaker 1: Malus's law, named after Étienne-Louis Malus. If linearly polarised light
• 1:40:31
  Speaker 1: passes through an analyser, The transmitted intensity depends on the
• 1:40:35
  Speaker 1: angle between the incoming polarisation and the transmission direction of
• 1:40:40
  Speaker 1: the analyser. When the two directions are aligned, the transmission
• 1:40:44
  Speaker 1: can be greatest. Turn the analyser gradually and the transmitted
• 1:40:49
  Speaker 1: intensity falls. Rotate it by 90 degrees relative to the
• 1:40:53
  Speaker 1: incoming polarisation and, for an ideal system, essentially no light
• 1:40:58
  Speaker 1: is transmitted. With individual photons, the same pattern appears statistically.
• 1:41:03
  Speaker 1: A single photon either passes through the analyser and is detected,
• 1:41:08
  Speaker 1: or it does not. It does not emerge with 37%
• 1:41:12
  Speaker 1: of itself missing. But repeat the experiment with many identically
• 1:41:16
  Speaker 1: prepared photons, and the fraction that passes reproduces the same
• 1:41:21
  Speaker 1: angular relationship described classically by Malus's law. The smooth classical
• 1:41:26
  Speaker 1: intensity emerges from many discrete quantum outcomes. Polarisation is also
• 1:41:32
  Speaker 1: closely related to another intrinsic photon property, spin. The photon
• 1:41:38
  Speaker 1: is described as a spin-one particle. Once again, the word
• 1:41:42
  Speaker 1: spin should not be taken too literally. A photon is
• 1:41:46
  Speaker 1: not a tiny sphere rotating around itself like a microscopic planet.
• 1:41:51
  Speaker 1: Quantum spin is an intrinsic form of angular momentum built
• 1:41:55
  Speaker 1: into the particle's quantum description. For massive spin, one particle's
• 1:42:00
  Speaker 1: It is natural to speak mathematically of three possible spin
• 1:42:03
  Speaker 1: projections along a chosen axis. That can make the photon
• 1:42:07
  Speaker 1: seem puzzling. If the photon is spin one, why do
• 1:42:11
  Speaker 1: freely propagating photons have only two physical polarisation states? The
• 1:42:16
  Speaker 1: answer involves the fact that photons are massless. A massive
• 1:42:20
  Speaker 1: particle can have a valid rest frame. In principle, one
• 1:42:23
  Speaker 1: can imagine travelling alongside it and considering its spin relative
• 1:42:27
  Speaker 1: to different spatial directions. A photon has no rest frame.
• 1:42:32
  Speaker 1: It always propagates through vacuum at the invariant speed of light.
• 1:42:36
  Speaker 1: For massless particles such as photons, the most useful quantity
• 1:42:40
  Speaker 1: is helicity. Helicity describes the projection of intrinsic angular momentum
• 1:42:46
  Speaker 1: along the direction in which the particle is moving. A
• 1:42:49
  Speaker 1: freely propagating photon has two physical helicity states. Very loosely,
• 1:42:55
  Speaker 1: these correspond to the two possible circular polarizations. One helicity
• 1:43:00
  Speaker 1: is associated with one sense of rotation around the direction
• 1:43:03
  Speaker 1: of travel, and the other with the opposite sense. There
• 1:43:06
  Speaker 1: is no third freely propagating longitudinal polarisation in ordinary electromagnetism.
• 1:43:13
  Speaker 1: That missing state is not an experimental accident. It is
• 1:43:17
  Speaker 1: tied deeply to the structure of the electromagnetic field. The
• 1:43:21
  Speaker 1: photon is the gauge poisson of electromagnetism, and the gauge
• 1:43:26
  Speaker 1: symmetry of the theory removes degrees of freedom that would
• 1:43:29
  Speaker 1: otherwise appear in a less constrained description of a generic
• 1:43:32
  Speaker 1: spin-1 field. The result is that the physical photon has
• 1:43:37
  Speaker 1: two independent polarisation states. This is another example of how
• 1:43:41
  Speaker 1: the phrase spin-1 can become misleading if we try to
• 1:43:44
  Speaker 1: translate it too quickly into everyday mechanical imagery. The photon's masslessness,
• 1:43:51
  Speaker 1: its gauge symmetry, its helicity and its polarisation are all linked.
• 1:43:56
  Speaker 1: Polarisation is especially useful because it is relatively easy to prepare,
• 1:44:01
  Speaker 1: manipulate and measure. A crystal, filter, waveplate or other optical
• 1:44:07
  Speaker 1: element can change the polarisation state of light in a
• 1:44:10
  Speaker 1: controlled way. That makes polarisation an ideal tool for quantum experiments.
• 1:44:17
  Speaker 1: Photons with carefully chosen polarisation states can be used to
• 1:44:20
  Speaker 1: encode quantum information Pairs of photons can be prepared with
• 1:44:24
  Speaker 1: entangled polarisations, creating correlations that have become central to experimental
• 1:44:30
  Speaker 1: tests of quantum mechanics. Quantum communication protocols can use polarisation
• 1:44:35
  Speaker 1: to represent different logical possibilities. Experiments probing the foundations of
• 1:44:41
  Speaker 1: quantum physics often rely on polarisation because the measurement settings
• 1:44:46
  Speaker 1: can be changed cleanly and the possible outcomes can be
• 1:44:49
  Speaker 1: recorded precisely. So this apparently simple property seen through a
• 1:44:54
  Speaker 1: pair of sunglasses reaches surprisingly deep. Polarisation describes how the
• 1:44:59
  Speaker 1: electromagnetic field is structured. For individual photons, it becomes part
• 1:45:05
  Speaker 1: of the quantum state. It can exist in superpositions. It
• 1:45:09
  Speaker 1: can be measured in different bases. It is connected to
• 1:45:12
  Speaker 1: intrinsic angular momentum and to the two physical helicity states
• 1:45:16
  Speaker 1: available to a massless photon. And once again, The photon
• 1:45:21
  Speaker 1: refuses to behave like a tiny classical object, carrying a
• 1:45:25
  Speaker 1: simple little arrow inside it. Its polarisation is not painted
• 1:45:29
  Speaker 1: onto its surface. There is no surface to paint. It
• 1:45:33
  Speaker 1: is part of the quantum structure of the electromagnetic field itself.
• 1:45:39
  Speaker 1: A single photon can produce one tiny click in a detector.
• 1:45:43
  Speaker 1: And yet when you switch on a lamp, stand in sunlight,
• 1:45:47
  Speaker 1: or look into the beam of a laser, Light does
• 1:45:49
  Speaker 1: not usually seem grainy at all. It appears smooth. A
• 1:45:53
  Speaker 1: lamp fills a room continuously. Sunlight seems to pour through
• 1:45:57
  Speaker 1: a window. A laser produces a clean beam that can
• 1:46:01
  Speaker 1: remain remarkably stable over time. So how does a world
• 1:46:05
  Speaker 1: built from quantized electromagnetic interactions produce something that looks so continuous?
• 1:46:11
  Speaker 1: The answer begins with scale. A visible photon carries an
• 1:46:15
  Speaker 1: extraordinarily small amount of energy by everyday standards. Depending on
• 1:46:20
  Speaker 1: its colour, the energy is typically only a few electron volts.
• 1:46:24
  Speaker 1: That means even a very modest beam of visible light
• 1:46:26
  Speaker 1: can contain an enormous flow of photons. Consider a green
• 1:46:30
  Speaker 1: laser producing only about 1 milliwatt of optical power. That
• 1:46:35
  Speaker 1: is a thousandth of a watt. Depending on the precise wavelength,
• 1:46:39
  Speaker 1: such a beam can involve roughly a few quadrillion photons
• 1:46:42
  Speaker 1: passing a given point every second. A quadrillion is a
• 1:46:46
  Speaker 1: thousand trillion. So even what feels like an extremely weak
• 1:46:50
  Speaker 1: beam can represent an astonishing number of quantum excitations. At
• 1:46:55
  Speaker 1: that point, individual photon events become rather like individual raindrops
• 1:46:59
  Speaker 1: in a heavy downpour. The drops remain discrete, but from
• 1:47:03
  Speaker 1: a distance the rainfall can appear almost continuous. The analogy
• 1:47:07
  Speaker 1: is imperfect because photons are quantum objects rather than tiny droplets,
• 1:47:12
  Speaker 1: but it captures the basic reason ordinary light rarely looks grainy.
• 1:47:16
  Speaker 1: deeper explanation belongs to quantum optics. One of the central
• 1:47:21
  Speaker 1: figures in developing the modern quantum theory of optical coherence
• 1:47:24
  Speaker 1: was Roy Glauber. During the 20th century, Glauber showed how
• 1:47:29
  Speaker 1: quantum theory could describe not merely whether photons existed, but
• 1:47:33
  Speaker 1: the statistical organisation of entire light fields. This matters because
• 1:47:38
  Speaker 1: two beams can contain photons with similar energies while being
• 1:47:43
  Speaker 1: very different quantum states. The number of photons alone does
• 1:47:47
  Speaker 1: not tell the whole story. One possible state is a
• 1:47:50
  Speaker 1: photon number state. In a photon number state, also called
• 1:47:54
  Speaker 1: a Fock state, a particular electromagnetic mode contains a definite
• 1:47:58
  Speaker 1: number of photons. It might contain exactly one photon, or
• 1:48:02
  Speaker 1: exactly ten, or exactly one million. The number is fixed,
• 1:48:06
  Speaker 1: but ordinary laser light is not usually described by a
• 1:48:09
  Speaker 1: state containing one perfectly definite photon number. Instead, An idealised
• 1:48:14
  Speaker 1: laser is closely associated with what is called a coherent state.
• 1:48:19
  Speaker 1: A coherent state is one of the quantum states that
• 1:48:22
  Speaker 1: most closely resembles a classical electromagnetic wave. Its field behaves
• 1:48:28
  Speaker 1: in a remarkably orderly way. The phase is comparatively well defined,
• 1:48:33
  Speaker 1: and the expectation values of the electric and magnetic fields
• 1:48:36
  Speaker 1: can oscillate in the smooth fashion familiar from Maxwell's classical theory.
• 1:48:42
  Speaker 1: But the number of photons is not exactly fixed. Instead,
• 1:48:46
  Speaker 1: a coherent state contains a distribution of possible photon numbers.
• 1:48:51
  Speaker 1: If the light is measured repeatedly, one trial might contain
• 1:48:54
  Speaker 1: slightly more photons and another slightly fewer. For an ideal
• 1:48:59
  Speaker 1: coherent state, these number fluctuations follow what is called a
• 1:49:03
  Speaker 1: Poisson distribution. This is why saying that a laser beam
• 1:49:07
  Speaker 1: contains exactly a particular number of photons can sometimes be
• 1:49:10
  Speaker 1: too simplistic. One can calculate an average photon number or
• 1:49:15
  Speaker 1: an average photon flux. But the quantum state may contain
• 1:49:18
  Speaker 1: a spread of possible photon numbers around that average. The
• 1:49:22
  Speaker 1: more intense the coherent state becomes, the smaller those relative
• 1:49:26
  Speaker 1: fluctuations usually appear. If the average number of photons is enormous,
• 1:49:31
  Speaker 1: A variation of a few thousand or even a few
• 1:49:34
  Speaker 1: million photons may be negligible compared with the total. The
• 1:49:38
  Speaker 1: field then begins to look increasingly smooth. This is one
• 1:49:42
  Speaker 1: reason classical electromagnetism works so beautifully for ordinary optical systems.
• 1:49:48
  Speaker 1: The quantum nature has not disappeared. It has simply become
• 1:49:51
  Speaker 1: difficult to notice in the collective behaviour of enormous numbers
• 1:49:55
  Speaker 1: of field excitations. Laser light provides an especially clean example.
• 1:50:00
  Speaker 1: A well-stabilised laser can possess strong coherence. Coherence means, roughly speaking,
• 1:50:06
  Speaker 1: that the phase relationships within the electromagnetic field remain highly organised.
• 1:50:12
  Speaker 1: This allows laser light to produce very clear interference effects
• 1:50:16
  Speaker 1: and to remain tightly controlled in frequency and direction. In
• 1:50:20
  Speaker 1: an ideal coherent state, photon detections occur with approximately Poissonian statistics.
• 1:50:27
  Speaker 1: The photons do not normally arrive at perfectly regular intervals
• 1:50:30
  Speaker 1: like carriages in a very disciplined train. There are fluctuations,
• 1:50:35
  Speaker 1: but those fluctuations have a characteristic quantum statistical structure. Thermal
• 1:50:40
  Speaker 1: light behaves differently. Light from a hot object, such as
• 1:50:44
  Speaker 1: a glowing filament, a stove element, or the surface of
• 1:50:47
  Speaker 1: a star, is generated through vast numbers of microscopic interactions,
• 1:50:52
  Speaker 1: occurring with comparatively little phase coordination. The resulting radiation can
• 1:50:58
  Speaker 1: still contain enormous numbers of photons, but its statistics are
• 1:51:02
  Speaker 1: different from those of ideal coherent laser light. Thermal photons
• 1:51:06
  Speaker 1: tend to show a phenomenon called bunching. In suitable measurements,
• 1:51:11
  Speaker 1: detections are more likely to occur close together than they
• 1:51:14
  Speaker 1: would be for an ideal coherent state. That difference is
• 1:51:18
  Speaker 1: not usually obvious to the human eye. A room illuminated
• 1:51:21
  Speaker 1: by a warm lamp does not visibly shimmer because its
• 1:51:24
  Speaker 1: photons possess thermal statistics. But sensitive experiments can measure these differences.
• 1:51:31
  Speaker 1: The statistical structure of the light reveals something about the
• 1:51:34
  Speaker 1: physical process that produced it. Sunlight is another important example.
• 1:51:39
  Speaker 1: It is sometimes tempting to imagine the sun as an
• 1:51:41
  Speaker 1: unimaginably powerful laser. It is not. The visible surface of
• 1:51:46
  Speaker 1: the sun behaves approximately like a hot thermal emitter. The
• 1:51:50
  Speaker 1: photons escaping from it have undergone complicated interactions within the
• 1:51:54
  Speaker 1: solar plasma, and the light arriving at Earth has statistical
• 1:51:58
  Speaker 1: properties much closer to thermal radiation than to the highly
• 1:52:02
  Speaker 1: coherent state associated with an ideal laser. A red photon
• 1:52:06
  Speaker 1: from sunlight and a red photon from a laser may
• 1:52:09
  Speaker 1: carry essentially the same energy if their frequencies are the same.
• 1:52:12
  Speaker 1: They are not different species of photon. But the larger
• 1:52:16
  Speaker 1: quantum states containing those photons can be very different. One
• 1:52:20
  Speaker 1: field may be highly coherent. Another may be thermal and
• 1:52:24
  Speaker 1: comparatively disordered. So when physicists describe light, they often need
• 1:52:28
  Speaker 1: to ask more than how many photons are present. They
• 1:52:32
  Speaker 1: may also ask how the photons are distributed among modes.
• 1:52:35
  Speaker 1: They may examine phase relationships. They may study correlations between
• 1:52:39
  Speaker 1: detection events. They may ask whether the photons are bunched, anti-bunched, coherent, entangled,
• 1:52:46
  Speaker 1: or organized into a definite number state. This is where
• 1:52:49
  Speaker 1: the simple phrase, a beam of photons, begins to reveal
• 1:52:52
  Speaker 1: its limitations. A beam is not merely a bag containing
• 1:52:56
  Speaker 1: identical little particles. It is a quantum state of the
• 1:53:00
  Speaker 1: electromagnetic field, and different quantum states can behave very differently,
• 1:53:04
  Speaker 1: even when their average energies look similar. Roy Glauber's work
• 1:53:09
  Speaker 1: gave physicists the tools to describe these distinctions systematically. Quantum
• 1:53:14
  Speaker 1: optical coherence could now be treated using measurable correlation functions
• 1:53:18
  Speaker 1: and photon statistics, rather than relying entirely on classical wave ideas.
• 1:53:25
  Speaker 1: That helped explain why some forms of light look almost
• 1:53:28
  Speaker 1: perfectly classical, while others reveal unmistakably quantum behaviour. The connection
• 1:53:34
  Speaker 1: back to James Clerk Maxwell is especially important. Quantum theory
• 1:53:38
  Speaker 1: did not overthrow the classical electromagnetic field and leave it behind.
• 1:53:43
  Speaker 1: In suitable quantum states containing enormous numbers of photons, the
• 1:53:47
  Speaker 1: average behaviour of the electromagnetic field approaches the smooth waves
• 1:53:51
  Speaker 1: described by Maxwell's equations. Electric and magnetic fields propagate through space.
• 1:53:57
  Speaker 1: They interfere. They diffract. They reflect from mirrors and refract
• 1:54:01
  Speaker 1: through glass. The classical equations remain astonishingly accurate. What quantum
• 1:54:07
  Speaker 1: theory adds is the deeper structure underneath. It explains why
• 1:54:12
  Speaker 1: energy can be exchanged in discrete amounts. It describes photon
• 1:54:16
  Speaker 1: counting statistics. It predicts fluctuations that have no classical counterpart.
• 1:54:22
  Speaker 1: And it tells us what kinds of quantum states can
• 1:54:24
  Speaker 1: produce something that looks almost exactly like an ordinary electromagnetic wave.
• 1:54:30
  Speaker 1: The classical field and the photon are therefore not rival
• 1:54:33
  Speaker 1: explanations from which one must be chosen. They apply at
• 1:54:37
  Speaker 1: different descriptive levels. When billions upon billions of photons occupy
• 1:54:42
  Speaker 1: suitable quantum states, the collective electromagnetic field can become so
• 1:54:47
  Speaker 1: smooth and predictable that the classical wave description is practically perfect.
• 1:54:52
  Speaker 1: Zoom in far enough, however, and individual interactions still occur
• 1:54:56
  Speaker 1: in discrete quantum events. A lamp can look continuous while
• 1:55:00
  Speaker 1: its light is detected photon by photon. A laser can
• 1:55:04
  Speaker 1: behave like an elegant classical wave, while its underlying photon
• 1:55:08
  Speaker 1: number fluctuates quantum mechanically. Sunlight can warm your skin with
• 1:55:12
  Speaker 1: a seemingly steady flow, while arriving through an astronomical number
• 1:55:17
  Speaker 1: of individual electromagnetic interactions. The smooth world of ordinary light
• 1:55:22
  Speaker 1: is not separate from the quantum world. It is what
• 1:55:25
  Speaker 1: the quantum world can look like when enough photons act together.
• 1:55:29
  Speaker 1: Entanglement is one of the clearest reminders that quantum physics
• 1:55:33
  Speaker 1: does not always allow us to think of particles as
• 1:55:36
  Speaker 1: separate little objects carrying their own private sets of instructions.
• 1:55:41
  Speaker 1: Two photons can be produced in such a way that
• 1:55:44
  Speaker 1: their properties are described by one shared quantum state. This
• 1:55:48
  Speaker 1: is quantum entanglement. The important idea is not merely that
• 1:55:52
  Speaker 1: the photons are strongly correlated. Ordinary objects can be correlated too.
• 1:55:57
  Speaker 1: Imagine placing one red card and one blue card into
• 1:56:00
  Speaker 1: separate envelopes, mixing them up and sending the envelopes to
• 1:56:04
  Speaker 1: opposite sides of the world. If someone opens one envelope
• 1:56:08
  Speaker 1: and finds the red card, they immediately know the other
• 1:56:10
  Speaker 1: envelope contains the blue one. There is nothing quantum about that.
• 1:56:14
  Speaker 1: The colours were already fixed. The observers were simply ignorant
• 1:56:18
  Speaker 1: of which envelope contained which card. Entanglement is deeper because
• 1:56:22
  Speaker 1: the correlations predicted by quantum mechanics cannot, in general, be
• 1:56:27
  Speaker 1: reproduced by giving each photon a complete pre-existing list of
• 1:56:31
  Speaker 1: local properties and then sending the pair apart. The useful
• 1:56:36
  Speaker 1: way to see this is through polarisation. Suppose two photons
• 1:56:40
  Speaker 1: are created in an entangled state involving their polarisations. Instead
• 1:56:45
  Speaker 1: of each photon being assigned one independent polarisation that it
• 1:56:50
  Speaker 1: quietly carries away, the pair can be described by a
• 1:56:53
  Speaker 1: joint state. For example, the state might be arranged so
• 1:56:57
  Speaker 1: that when both photons are measured using the same polarisation basis,
• 1:57:01
  Speaker 1: their results show a particular pattern of correlation. The crucial
• 1:57:05
  Speaker 1: point is that the full description belongs to the pair.
• 1:57:09
  Speaker 1: Neither photon necessarily possesses a complete individual set of predetermined
• 1:57:14
  Speaker 1: answers for every possible polarisation measurement. Experiments can produce such
• 1:57:20
  Speaker 1: pairs using a process called spontaneous parametric downconversion. The name
• 1:57:25
  Speaker 1: is more intimidating than the basic picture. A laser is
• 1:57:29
  Speaker 1: directed into a non-linear optical crystal. Under suitable conditions, the
• 1:57:34
  Speaker 1: interaction between the electromagnetic field and the crystal allows one
• 1:57:38
  Speaker 1: higher energy photon from the pump light to be converted
• 1:57:41
  Speaker 1: into two lower energy photons. Energy and momentum are conserved.
• 1:57:47
  Speaker 1: The two outgoing photons can emerge along different directions, and
• 1:57:50
  Speaker 1: their properties can be linked in carefully controlled ways. Depending
• 1:57:55
  Speaker 1: on the experimental arrangement, the pair may be entangled in polarisation, momentum, energy.
• 1:58:01
  Speaker 1: time of emission, or combinations of these properties. The crystal
• 1:58:06
  Speaker 1: is not simply snapping one photon into two smaller classical pieces.
• 1:58:11
  Speaker 1: The process creates a new quantum state of the electromagnetic field,
• 1:58:16
  Speaker 1: containing two photons whose properties can be correlated in a
• 1:58:20
  Speaker 1: genuinely quantum way. Once produced, the photons can be sent
• 1:58:24
  Speaker 1: far apart. One may travel toward one detector while the
• 1:58:28
  Speaker 1: other travels toward another. At each detector physicists can choose
• 1:58:32
  Speaker 1: how to measure the photon's polarisation. Perhaps one apparatus distinguishes
• 1:58:37
  Speaker 1: vertical from horizontal polarisation. Another setting might be rotated to
• 1:58:42
  Speaker 1: measure along a diagonal basis. The strange part appears when
• 1:58:46
  Speaker 1: the measurement results from many photon pairs are compared. Quantum
• 1:58:50
  Speaker 1: mechanics predicts correlations that can be stronger than those allowed
• 1:58:54
  Speaker 1: by a broad class of theories in which each photon
• 1:58:58
  Speaker 1: simply carries local hidden instructions, determining all its possible answers
• 1:59:03
  Speaker 1: in advance. This question was transformed from philosophy into experiment
• 1:59:08
  Speaker 1: by John Stuart Bell. In the 1960s, Bell developed what
• 1:59:13
  Speaker 1: became known as Bell's theorem. He showed that certain theories
• 1:59:17
  Speaker 1: based on local hidden variables must obey mathematical limits on
• 1:59:21
  Speaker 1: the correlations that can appear between distant measurements. these limits
• 1:59:26
  Speaker 1: are expressed through Bell inequalities. Quantum mechanics predicts that entangled
• 1:59:31
  Speaker 1: systems can violate those inequalities. That made the disagreement testable.
• 1:59:37
  Speaker 1: One of the early important experiments was carried out by
• 1:59:40
  Speaker 1: John Clauser and Stuart Friedman in the early 1970s. Using
• 1:59:45
  Speaker 1: pairs of photons and polarisation measurements, they found correlations consistent
• 1:59:50
  Speaker 1: with quantum mechanics and incompatible with the particular Bell inequality
• 1:59:55
  Speaker 1: being tested. Later experiments became increasingly sophisticated. Alain Aspect and
• 2:00:02
  Speaker 1: his collaborators performed influential tests in the early 1980s, using
• 2:00:07
  Speaker 1: rapidly changing analyzer settings and improved experimental control. Again, the
• 2:00:13
  Speaker 1: observed correlations agreed with quantum predictions and violated the relevant
• 2:00:17
  Speaker 1: Bell inequalities. Over the following decades, Experiments became more precise
• 2:00:23
  Speaker 1: and closed more of the possible loopholes that could, in principle,
• 2:00:28
  Speaker 1: have allowed alternative explanations. Modern Bell tests have separated measurement stations,
• 2:00:34
  Speaker 1: improved detector efficiencies, randomised measurement choices and addressed combinations of
• 2:00:39
  Speaker 1: experimental weaknesses at the same time. The broad conclusion has
• 2:00:44
  Speaker 1: remained remarkably consistent. The correlations predicted by quantum mechanics are real.
• 2:00:50
  Speaker 1: They cannot be explained by a simple picture in which
• 2:00:53
  Speaker 1: each photon carries a complete set of local predetermined properties
• 2:00:57
  Speaker 1: independent of what measurement is performed. This does not mean
• 2:01:01
  Speaker 1: that entanglement allows messages to be sent instantaneously across space.
• 2:01:06
  Speaker 1: That point is essential. Suppose two observers share a pair
• 2:01:10
  Speaker 1: of entangled photons. One observer measures their photon and receives
• 2:01:15
  Speaker 1: one of the possible outcomes. The result is individually random.
• 2:01:19
  Speaker 1: they cannot choose which outcome appears. The distant observer also
• 2:01:23
  Speaker 1: receives a locally random result. Only when the two sets
• 2:01:27
  Speaker 1: of data are later compared do the quantum correlations become visible.
• 2:01:32
  Speaker 1: That comparison still requires ordinary communication, which cannot travel faster
• 2:01:36
  Speaker 1: than light. Entanglement therefore produces correlations that defy classical local,
• 2:01:42
  Speaker 1: hidden variable explanations, but it does not provide a faster-than-light telephone.
• 2:01:48
  Speaker 1: Relativity remains safe from that particular inconvenience. The experimental possibilities
• 2:01:53
  Speaker 1: created by entangled photons have been developed extensively by researchers,
• 2:01:58
  Speaker 1: including Anton Zielinger. Zielinger and his collaborators helped turn entanglement
• 2:02:04
  Speaker 1: into a practical laboratory resource. They demonstrated increasingly sophisticated experiments
• 2:02:10
  Speaker 1: involving entangled photons, quantum teleportation and long-distance quantum communication. Quantum
• 2:02:18
  Speaker 1: teleportation does not transport matter from one location to another
• 2:02:22
  Speaker 1: in the science fiction sense. Instead, it transfers the quantum
• 2:02:27
  Speaker 1: state of one system onto another distant system, using shared
• 2:02:31
  Speaker 1: entanglement together with ordinary classical communication. The original state is
• 2:02:36
  Speaker 1: not copied in the process. The protocol relies on the
• 2:02:39
  Speaker 1: peculiar structure of quantum information and the correlations provided by entanglement.
• 2:02:45
  Speaker 1: Entangled photons have also been distributed across increasingly large distances
• 2:02:50
  Speaker 1: using optical fibres, free space links and satellites. The purpose
• 2:02:55
  Speaker 1: is not merely to perform dramatic demonstrations. Entanglement is becoming
• 2:03:00
  Speaker 1: an important resource in quantum communication, quantum cryptography, precision measurement
• 2:03:06
  Speaker 1: and emerging quantum networks. Yet its conceptual importance remains just
• 2:03:11
  Speaker 1: as striking. Entanglement tells us that the quantum state of
• 2:03:16
  Speaker 1: a pair of photons can contain information that cannot be
• 2:03:19
  Speaker 1: divided neatly into one complete state for photon 1 and
• 2:03:23
  Speaker 1: another complete state for photon 2. The pair is not
• 2:03:27
  Speaker 1: always merely the sum of two independent parts. That is
• 2:03:31
  Speaker 1: a profound departure from the classical picture. If two ordinary
• 2:03:35
  Speaker 1: marbles are separated, each marble possesses its own position, velocity,
• 2:03:39
  Speaker 1: colour and shape. we can describe one without needing the other.
• 2:03:44
  Speaker 1: For entangled photons, the most complete quantum description may belong
• 2:03:48
  Speaker 1: to the combined system. The individual photons can still be
• 2:03:52
  Speaker 1: detected separately. Each detector registers one localised event. But the
• 2:03:58
  Speaker 1: pattern connecting those outcomes emerges only when the joint state
• 2:04:01
  Speaker 1: is considered. This is especially important for understanding what a
• 2:04:06
  Speaker 1: photon actually is. A photon is not simply a miniature
• 2:04:10
  Speaker 1: object carrying a little packet of properties through space. Its
• 2:04:14
  Speaker 1: physical description depends on its quantum state, and sometimes that
• 2:04:18
  Speaker 1: state cannot be assigned to one photon alone. The relevant
• 2:04:22
  Speaker 1: excitation of the electromagnetic field can be part of a
• 2:04:26
  Speaker 1: larger entangled state extending across widely separated regions. Two photons
• 2:04:31
  Speaker 1: can travel kilometres apart and still belong to one joint
• 2:04:35
  Speaker 1: quantum description. That does not mean there is a hidden
• 2:04:38
  Speaker 1: signal rushing between them. It does not mean one photon
• 2:04:41
  Speaker 1: sends an instruction to the other at the moment of measurement.
• 2:04:45
  Speaker 1: And it does not mean relativity has been bypassed. It
• 2:04:49
  Speaker 1: means that quantum theory organises physical possibilities in a way
• 2:04:53
  Speaker 1: that classical separability cannot fully reproduce. The strange part is
• 2:04:58
  Speaker 1: not that two photons communicate unusually quickly. The strange part
• 2:05:03
  Speaker 1: is that, at the quantum level, The assumption that every
• 2:05:06
  Speaker 1: object must always carry a completely independent set of local
• 2:05:10
  Speaker 1: properties is too simple. Sometimes the physics belongs to the
• 2:05:14
  Speaker 1: relationship itself, and for entangled photons that relationship can stretch
• 2:05:18
  Speaker 1: across space while remaining one shared quantum state. Where is
• 2:05:23
  Speaker 1: a photon before you detect it? For an ordinary object,
• 2:05:27
  Speaker 1: this sounds like one of the easiest questions imaginable. A
• 2:05:30
  Speaker 1: book is on the table. A train is somewhere along
• 2:05:33
  Speaker 1: the track. a planet occupies a particular region of space
• 2:05:37
  Speaker 1: as it travels around its star. Even when we do
• 2:05:41
  Speaker 1: not know exactly where something is, we usually assume that
• 2:05:44
  Speaker 1: it has one definite location all the same. With a photon,
• 2:05:48
  Speaker 1: the question becomes much more subtle. Quantum physics does not
• 2:05:52
  Speaker 1: generally describe a freely travelling photon as a tiny bead
• 2:05:56
  Speaker 1: possessing one perfectly definite position that moves continuously along an
• 2:06:00
  Speaker 1: invisible line from source to detector. Instead, The theory describes
• 2:06:05
  Speaker 1: a quantum state. That state can tell us where a
• 2:06:09
  Speaker 1: photon might be detected, how its momentum is distributed, what
• 2:06:13
  Speaker 1: frequencies are present, and how different possibilities can interfere. But
• 2:06:18
  Speaker 1: those quantities do not fit together in quite the way
• 2:06:21
  Speaker 1: our everyday picture of a moving object suggests. Consider momentum first.
• 2:06:27
  Speaker 1: It is possible to describe a photon in a state
• 2:06:30
  Speaker 1: with an extremely well-defined momentum. Because the photon's momentum is
• 2:06:35
  Speaker 1: connected with its wavelength, this also corresponds to a very
• 2:06:39
  Speaker 1: well-defined wavelength and frequency. The price of making the momentum
• 2:06:43
  Speaker 1: so precise is that the state becomes spread broadly through space.
• 2:06:48
  Speaker 1: In the ideal mathematical limit of a perfectly definite momentum,
• 2:06:52
  Speaker 1: the corresponding wave extends indefinitely rather than occupying one neat
• 2:06:56
  Speaker 1: little region. That is hardly the image of a microscopic
• 2:07:00
  Speaker 1: pellet sitting at one particular coordinate. If we instead want
• 2:07:04
  Speaker 1: a photon that is more strongly localised, we can construct
• 2:07:07
  Speaker 1: what is called a wave packet. A wave packet combines
• 2:07:11
  Speaker 1: a range of wavelengths, frequencies and momenta. Those different components
• 2:07:16
  Speaker 1: interfere in such a way that the resulting state becomes
• 2:07:19
  Speaker 1: concentrated within a more limited region of space. The photon
• 2:07:23
  Speaker 1: can then be described as having a greater probability of
• 2:07:26
  Speaker 1: being detected within that region. But localisation comes with a trade-off.
• 2:07:31
  Speaker 1: Because the wave packet contains a range of momentum components,
• 2:07:35
  Speaker 1: the momentum is no longer perfectly definite. This reflects the
• 2:07:38
  Speaker 1: broader uncertainty structure of quantum mechanics associated with Werner Heisenberg.
• 2:07:45
  Speaker 1: The uncertainty principle is sometimes explained as though it was
• 2:07:49
  Speaker 1: simply a problem with experimental equipment. Perhaps, the story goes,
• 2:07:54
  Speaker 1: the photon really does possess an exact position and an
• 2:07:57
  Speaker 1: exact momentum, but our instruments are too clumsy to measure both.
• 2:08:02
  Speaker 1: That misses the deeper point. Quantum uncertainty concerns the structure
• 2:08:08
  Speaker 1: of the state itself. Certain pairs of physical quantities cannot
• 2:08:12
  Speaker 1: simultaneously possess arbitrarily narrow distributions in the same quantum state.
• 2:08:17
  Speaker 1: A highly localised state requires a spread of momenta. A
• 2:08:21
  Speaker 1: state with extremely precise momentum becomes spatially extended. This does
• 2:08:26
  Speaker 1: not mean physicists are unable to say anything useful about
• 2:08:29
  Speaker 1: photon location. Quite the opposite. Quantum electrodynamics and quantum optics
• 2:08:35
  Speaker 1: allow extremely precise predictions about where photon detections are likely
• 2:08:40
  Speaker 1: to occur. Physicists can describe localised pulses of electromagnetic radiation.
• 2:08:47
  Speaker 1: They can calculate how those pulses propagate through optical systems.
• 2:08:51
  Speaker 1: They can predict diffraction patterns, interference effects and the probability
• 2:08:56
  Speaker 1: that a particular detector will register an event. What becomes
• 2:09:00
  Speaker 1: difficult is assigning the photon a position in exactly the
• 2:09:05
  Speaker 1: same mathematical way used for a simple, massive particle in
• 2:09:10
  Speaker 1: non-relativistic quantum mechanics. For particles such as electrons treated in
• 2:09:15
  Speaker 1: ordinary non-relativistic quantum mechanics, a familiar position operator can be
• 2:09:20
  Speaker 1: introduced with properties that support the usual idea of a
• 2:09:24
  Speaker 1: position measurement. For photons, constructing an equivalent position operator with
• 2:09:30
  Speaker 1: all the same desirable properties runs into difficulties connected with
• 2:09:34
  Speaker 1: their masslessness, their spin, their two physical polarisation states. and
• 2:09:40
  Speaker 1: the relativistic nature of the electromagnetic field. This does not
• 2:09:45
  Speaker 1: prevent photons from being detected at particular places. It means
• 2:09:49
  Speaker 1: that the phrase, the position of the photon needs to
• 2:09:52
  Speaker 1: be handled carefully. Several questions can otherwise become mixed together.
• 2:09:57
  Speaker 1: Where is the electromagnetic energy concentrated? Where does the photon's
• 2:10:02
  Speaker 1: quantum state have significant amplitude? Where could a detector register
• 2:10:06
  Speaker 1: the photon? Where does the eventual absorption event actually happen?
• 2:10:12
  Speaker 1: Those questions are related, but they are not automatically identical.
• 2:10:16
  Speaker 1: A wave packet can distribute electromagnetic energy in quantum amplitude
• 2:10:20
  Speaker 1: across a region of space. A detector placed within that
• 2:10:24
  Speaker 1: region may have some probability of registering the photon. When
• 2:10:28
  Speaker 1: a detection finally occurs, however, the interaction is localised. Energy
• 2:10:34
  Speaker 1: is transferred at a particular place. detector produces one event there.
• 2:10:40
  Speaker 1: It would be a mistake to work backwards from that
• 2:10:42
  Speaker 1: final click and conclude that the photon must therefore have
• 2:10:45
  Speaker 1: been a tiny point-like bead occupying one definite classical position
• 2:10:51
  Speaker 1: at every instant before detection. The quantum state can have
• 2:10:55
  Speaker 1: been much more widely distributed. This becomes particularly striking when
• 2:11:00
  Speaker 1: we think about astronomy. Imagine a photon associated with light
• 2:11:05
  Speaker 1: from a galaxy millions of light years away. A familiar
• 2:11:09
  Speaker 1: picture might show a tiny glowing dot leaving one star,
• 2:11:13
  Speaker 1: travelling along a razor-thin invisible line through intergalactic space, entering
• 2:11:18
  Speaker 1: Earth's atmosphere and finally landing on a telescope mirror. That
• 2:11:22
  Speaker 1: picture can be useful for tracing the direction from which
• 2:11:25
  Speaker 1: the light arrives, but it should not be mistaken for
• 2:11:28
  Speaker 1: a complete quantum description. Light propagating across enormous distances possesses
• 2:11:34
  Speaker 1: wave-like spatial structure. Its quantum state can spread across regions
• 2:11:40
  Speaker 1: much larger than the eventual point at which one detector
• 2:11:43
  Speaker 1: records an absorption. A telescope does not simply sit at
• 2:11:47
  Speaker 1: the end of a microscopic tunnel, waiting for one tiny
• 2:11:50
  Speaker 1: bead to arrive. Its finite aperture collects an electromagnetic mode
• 2:11:56
  Speaker 1: over an area. The telescope mirror or lens then redirects
• 2:12:00
  Speaker 1: that light. Light diffraction places a fundamental limit on how
• 2:12:04
  Speaker 1: sharply a finite aperture can focus radiation. A larger telescope
• 2:12:09
  Speaker 1: generally produces finer angular resolution, partly because a wider aperture
• 2:12:14
  Speaker 1: can collect and combine light across a larger region. Even
• 2:12:18
  Speaker 1: when the incoming radiation becomes so faint that photons are
• 2:12:22
  Speaker 1: detected individually, the wave-like spatial structure still matters. A single
• 2:12:28
  Speaker 1: photon may ultimately trigger one pixel in an astronomical detector.
• 2:12:32
  Speaker 1: Yet the probabilities for where that detection occurs are governed
• 2:12:36
  Speaker 1: by diffraction and interference across the optical system. Repeated photons
• 2:12:42
  Speaker 1: from the same distant source build up the familiar diffraction
• 2:12:46
  Speaker 1: pattern associated with the telescope. The localised detection does not
• 2:12:51
  Speaker 1: erase the extended quantum description that determined its probability. This
• 2:12:56
  Speaker 1: is one reason asking where the photon really was can
• 2:12:59
  Speaker 1: become misleading if the word really is taken to demand
• 2:13:02
  Speaker 1: an ordinary classical answer. Quantum mechanics is not unable to
• 2:13:07
  Speaker 1: describe reality because it refuses to provide one hidden trajectory. Instead,
• 2:13:13
  Speaker 1: it describes the quantities experiments can actually probe using a
• 2:13:17
  Speaker 1: different framework. We can prepare a photon in a wave packet.
• 2:13:22
  Speaker 1: We can calculate how that state evolves. We can predict
• 2:13:26
  Speaker 1: how likely different detectors are to respond. We can manipulate
• 2:13:30
  Speaker 1: its spatial mode with mirrors, lenses and beam splitters. We
• 2:13:35
  Speaker 1: can allow alternative paths to interfere. And we can test
• 2:13:39
  Speaker 1: those predictions with extraordinary accuracy. What we cannot always do
• 2:13:44
  Speaker 1: is add an extra classical story in which a tiny
• 2:13:47
  Speaker 1: object secretly followed one perfectly defined line throughout the entire experiment.
• 2:13:53
  Speaker 1: In many quantum situations, that additional trajectory is not part
• 2:13:58
  Speaker 1: of the physical description required by the theory. The same
• 2:14:01
  Speaker 1: point applies whether the photon crosses a laboratory bench or
• 2:14:05
  Speaker 1: travels for billions of years through the universe. Its eventual
• 2:14:09
  Speaker 1: detection may be very sharply localised. Its quantum state beforehand
• 2:14:14
  Speaker 1: need not be. So where is a photon before you
• 2:14:17
  Speaker 1: detect it? Sometimes the best answer is not one coordinate,
• 2:14:22
  Speaker 1: It is a quantum state spread through space, carrying amplitudes
• 2:14:26
  Speaker 1: for where an interaction might occur. That state can be
• 2:14:29
  Speaker 1: tightly concentrated or broadly extended. It can diffract. It can interfere.
• 2:14:35
  Speaker 1: It can contain a range of momenta. And when the
• 2:14:39
  Speaker 1: photon finally interacts strongly enough with matter to be detected,
• 2:14:43
  Speaker 1: one particular location becomes part of the experimental record. The
• 2:14:47
  Speaker 1: click is local. The state that led to it may
• 2:14:50
  Speaker 1: not have been. Once again, the photon behaves perfectly well.
• 2:14:55
  Speaker 1: It is our classical picture of a tiny object following
• 2:14:59
  Speaker 1: one invisible line that has trouble keeping up. It is
• 2:15:02
  Speaker 1: often said that a photon does not experience time. The
• 2:15:06
  Speaker 1: phrase is memorable, and there is a piece of genuine
• 2:15:09
  Speaker 1: relativity hidden inside it. But taken too literally, it becomes misleading.
• 2:15:16
  Speaker 1: To understand why, we need to separate two different ideas.
• 2:15:20
  Speaker 1: One concerns the amount of proper time along the path
• 2:15:23
  Speaker 1: followed by light. The other concerns whether a photon can
• 2:15:27
  Speaker 1: have a valid point of view or rest frame of
• 2:15:29
  Speaker 1: its own. Those are not the same thing. Albert Einstein's
• 2:15:34
  Speaker 1: special theory of relativity tells us that the speed of
• 2:15:37
  Speaker 1: light in vacuum is invariant. Every inertial observer, regardless of
• 2:15:42
  Speaker 1: their own constant velocity, measures freely propagating light in vacuum
• 2:15:47
  Speaker 1: to move at the same invariant speed. That speed is
• 2:15:51
  Speaker 1: approximately 300,000 kilometres per second. This is one of the
• 2:15:55
  Speaker 1: strangest and most important facts in modern physics. If you
• 2:16:00
  Speaker 1: run toward an ordinary moving object, you measure it approaching faster.
• 2:16:05
  Speaker 1: If you chase after it, you measure it moving away
• 2:16:07
  Speaker 1: more slowly. Light does not behave that way. No matter
• 2:16:12
  Speaker 1: how quickly an inertial observer moves, a photon in vacuum
• 2:16:16
  Speaker 1: is still measured to travel at the invariant speed of light,
• 2:16:20
  Speaker 1: This is not because the photon adjusts its speed, to
• 2:16:23
  Speaker 1: be polite. It is built into the geometry of spacetime.
• 2:16:27
  Speaker 1: Special relativity replaces the old idea of absolute space and
• 2:16:31
  Speaker 1: absolute time with spacetime, in which distances and time intervals
• 2:16:36
  Speaker 1: depend on the state of motion of the observer. Different
• 2:16:39
  Speaker 1: observers can disagree about how much time passes between events.
• 2:16:44
  Speaker 1: They can disagree about distances. They can even disagree about
• 2:16:47
  Speaker 1: whether two separated events happened at the same time, but
• 2:16:51
  Speaker 1: they agree on the underlying relativistic structure. One useful quantity
• 2:16:56
  Speaker 1: in this structure is the space-time interval. For an ordinary
• 2:17:00
  Speaker 1: massive object following what is called a time-like path, the
• 2:17:04
  Speaker 1: space-time interval along its journey is connected with the proper
• 2:17:07
  Speaker 1: time measured by a clock travelling with that object. Proper
• 2:17:12
  Speaker 1: time is, in effect, the time measured along that object's
• 2:17:16
  Speaker 1: own worldline. Light is different. A photon's path through spacetime
• 2:17:21
  Speaker 1: is called a null worldline, or a light-like worldline. Along
• 2:17:26
  Speaker 1: an ideal light-like path, the spacetime interval is zero. And
• 2:17:30
  Speaker 1: because proper time is linked to that interval, the proper
• 2:17:33
  Speaker 1: time along a light-like trajectory is also zero. This mathematical
• 2:17:38
  Speaker 1: fact is the source of the popular statement that no
• 2:17:41
  Speaker 1: time passes for a photon. but there is a crucial limitation.
• 2:17:45
  Speaker 1: A photon does not have a valid inertial rest frame.
• 2:17:50
  Speaker 1: For a massive object, we can always imagine an observer
• 2:17:53
  Speaker 1: moving alongside it at the same velocity. In that frame,
• 2:17:57
  Speaker 1: the object is at rest. Its clock can tick. Distances
• 2:18:02
  Speaker 1: and times can be discussed from that object's rest frame.
• 2:18:05
  Speaker 1: Trying to do the same thing with light does not work.
• 2:18:09
  Speaker 1: You cannot accelerate an ordinary massive observer until they move
• 2:18:13
  Speaker 1: alongside a photon at the speed of light, and then
• 2:18:16
  Speaker 1: ask what the photon sees. The transformations of special relativity
• 2:18:21
  Speaker 1: do not provide a physically valid inertial frame at the
• 2:18:24
  Speaker 1: speed of light, so statements such as the photon sees
• 2:18:28
  Speaker 1: its entire journey happen instantly, or from the photon's perspective
• 2:18:33
  Speaker 1: the universe is flattened into nothing, are not legitimate conclusions
• 2:18:37
  Speaker 1: of special relativity. There is no standard photon perspective. There
• 2:18:43
  Speaker 1: is no photon rest frame from which such observations can
• 2:18:46
  Speaker 1: be made. The more precise statement is simply this. Light
• 2:18:51
  Speaker 1: follows null trajectories with zero proper time. That is a
• 2:18:55
  Speaker 1: statement about spacetime geometry, not about the subjective experience of
• 2:19:00
  Speaker 1: a photon. Photons, after all, are not tiny observers carrying wristwatches.
• 2:19:06
  Speaker 1: Relativity also tells us something interesting about photon energy. The
• 2:19:10
  Speaker 1: energy of a photon depends on its frequency. Different observers
• 2:19:15
  Speaker 1: moving relative to one another can measure different frequencies for
• 2:19:18
  Speaker 1: the same radiation. This is the relativistic Doppler effect. If
• 2:19:23
  Speaker 1: an observer moves toward a source of light, the measured
• 2:19:27
  Speaker 1: frequency can shift upward. The photons are then measured to
• 2:19:31
  Speaker 1: carry more energy. If the observer moves away, the frequency
• 2:19:35
  Speaker 1: can shift downward and the measured photon energy decreases. The
• 2:19:40
  Speaker 1: same electromagnetic radiation can therefore have different measured frequencies and
• 2:19:46
  Speaker 1: energies for different observers. This does not violate relativity. Energy
• 2:19:51
  Speaker 1: and frequency are observer-dependent quantities. The underlying physical laws remain
• 2:19:56
  Speaker 1: the same. The same basic idea appears in gravity. Albert
• 2:20:01
  Speaker 1: Einstein's general theory of relativity describes gravity not as an
• 2:20:05
  Speaker 1: ordinary force acting across space, but as the curvature of
• 2:20:09
  Speaker 1: spacetime produced by matter and energy. Light follows the geometry
• 2:20:14
  Speaker 1: of that curved spacetime. One consequence is gravitational redshift. Imagine
• 2:20:20
  Speaker 1: a photon travelling upward away from a massive object. An
• 2:20:24
  Speaker 1: observer higher in the gravitational field can measure the photon
• 2:20:28
  Speaker 1: at a lower frequency than an observer deeper down. The
• 2:20:32
  Speaker 1: light is gravitationally redshifted. In the opposite direction, a photon
• 2:20:37
  Speaker 1: falling deeper into a gravitational field can be measured at
• 2:20:41
  Speaker 1: a higher frequency by an appropriate observer. Its energy, as
• 2:20:45
  Speaker 1: measured locally, increases correspondingly. It is sometimes said that the
• 2:20:50
  Speaker 1: photon loses energy while climbing out of gravity. That can
• 2:20:54
  Speaker 1: be a useful shorthand but in general relativity the safest
• 2:20:59
  Speaker 1: description is relational. Different observers at different gravitational potentials can
• 2:21:04
  Speaker 1: assign different frequencies and energies to the same radiation. The
• 2:21:09
  Speaker 1: geometry of spacetime determines how those measurements compare. Curved spacetime
• 2:21:15
  Speaker 1: also changes the path that light follows. This is gravitational lensing.
• 2:21:20
  Speaker 1: A massive object such as a star, galaxy or cluster
• 2:21:23
  Speaker 1: of galaxies can bend the trajectories of light passing nearby.
• 2:21:27
  Speaker 1: The light is not being deflected because the photon possesses
• 2:21:31
  Speaker 1: ordinary rest mass. Its rest mass remains zero. Instead, the
• 2:21:36
  Speaker 1: photon follows the null paths allowed by curved spacetime. This
• 2:21:40
  Speaker 1: can produce spectacular effects. A distant galaxy may appear stretched
• 2:21:45
  Speaker 1: into an arc. Multiple images of the same background object
• 2:21:49
  Speaker 1: can appear around a foreground galaxy. In particularly precise alignments,
• 2:21:54
  Speaker 1: light can form an Einstein ring. Gravity influences light because
• 2:21:59
  Speaker 1: gravity in general relativity is spacetime geometry. The most extreme
• 2:22:05
  Speaker 1: example comes from a black hole. Outside the event horizon,
• 2:22:09
  Speaker 1: photons can still travel in many directions depending on their
• 2:22:12
  Speaker 1: initial path and the curvature of spacetime. Near the black hole,
• 2:22:17
  Speaker 1: those paths can be bent dramatically. At the event horizon, however,
• 2:22:22
  Speaker 1: the causal structure becomes decisive. A photon still moves locally
• 2:22:27
  Speaker 1: at the invariant speed of a light. Nothing special happens
• 2:22:30
  Speaker 1: to its local speed. But inside the event horizon, every
• 2:22:35
  Speaker 1: future-directed light-like path leads towards smaller radial distance. The problem
• 2:22:40
  Speaker 1: is not that light suddenly becomes too slow. The geometry
• 2:22:44
  Speaker 1: of spacetime has changed so drastically. that escaping outward is
• 2:22:47
  Speaker 1: no longer one of the available future directions. Even light
• 2:22:52
  Speaker 1: cannot reach the external universe. This distinction is important. A
• 2:22:56
  Speaker 1: black hole does not trap photons by grabbing hold of
• 2:23:00
  Speaker 1: them and slowing them beneath the speed of light. Locally,
• 2:23:04
  Speaker 1: light continues to behave exactly as relativity requires. What changes
• 2:23:09
  Speaker 1: is the shape of spacetime and the set of possible
• 2:23:12
  Speaker 1: paths through it. So when we ask whether a photon
• 2:23:15
  Speaker 1: experiences time, the most accurate answer is more careful than
• 2:23:20
  Speaker 1: the popular slogan. A freely propagating photon follows a null worldline.
• 2:23:26
  Speaker 1: The proper time along that worldline is zero. But special
• 2:23:30
  Speaker 1: relativity does not allow us to transform into the photon's
• 2:23:33
  Speaker 1: rest frame, because no such inertial frame exists. We therefore
• 2:23:38
  Speaker 1: cannot meaningfully describe what the universe looks like from the
• 2:23:41
  Speaker 1: photon's point of view. We can describe how different observers
• 2:23:45
  Speaker 1: measure the photon's frequency. We can calculate gravitational redshift. We
• 2:23:50
  Speaker 1: can predict gravitational lensing. We can trace light-like paths near
• 2:23:55
  Speaker 1: black holes. And we can say precisely that the interval
• 2:23:59
  Speaker 1: along the photon's ideal trajectory is null. That is already
• 2:24:04
  Speaker 1: strange enough. The photon does not need an imaginary clock
• 2:24:07
  Speaker 1: that has mysteriously stopped ticking. Relativity gives us something cleaner.
• 2:24:13
  Speaker 1: Light has no valid rest frame. Its path through space-time
• 2:24:16
  Speaker 1: carries zero proper time. And those two facts, carefully kept separate,
• 2:24:22
  Speaker 1: tell us far more than the simpler claim that time
• 2:24:24
  Speaker 1: somehow stops for light. The photon sits inside a much
• 2:24:28
  Speaker 1: larger description of nature. That description is called the Standard
• 2:24:32
  Speaker 1: Model of Particle Physics. The Standard Model is the framework
• 2:24:36
  Speaker 1: physicists use to describe the known elementary particles. and three
• 2:24:40
  Speaker 1: of the four fundamental interactions– electromagnetism, the strong interaction, and
• 2:24:47
  Speaker 1: the weak interaction. Gravity is not included in the standard model.
• 2:24:53
  Speaker 1: Within this framework, the photon is the gauge boson associated
• 2:24:56
  Speaker 1: with electromagnetism. That phrase can sound rather forbidding, but the
• 2:25:02
  Speaker 1: underlying idea is manageable. A gauge boson is a quantum
• 2:25:07
  Speaker 1: excitation of a field associated with a particular symmetry of
• 2:25:10
  Speaker 1: the theory. For electromagnetism, that field is the electromagnetic field.
• 2:25:16
  Speaker 1: Its quanta are photons, so the familiar photon that carries
• 2:25:21
  Speaker 1: visible light is also part of the same fundamental field
• 2:25:24
  Speaker 1: responsible for electric and magnetic interactions between charged particles. The
• 2:25:30
  Speaker 1: visible beam crossing a room and the electromagnetic force holding
• 2:25:34
  Speaker 1: electrons within atoms are therefore not unrelated phenomena. They belong
• 2:25:40
  Speaker 1: to the same underlying theory. The word symmetry is important here.
• 2:25:45
  Speaker 1: In physics, a symmetry means that certain changes can be
• 2:25:49
  Speaker 1: made to the mathematical description without changing the observable predictions.
• 2:25:54
  Speaker 1: Electromagnetism possesses what is called a gauge symmetry. The details
• 2:25:59
  Speaker 1: are mathematical. but the physical consequence is profound. The theory
• 2:26:05
  Speaker 1: allows a consistent electromagnetic interaction between charged matter and the
• 2:26:10
  Speaker 1: electromagnetic field, and the photon emerges as the corresponding massless
• 2:26:15
  Speaker 1: gauge boson. This electromagnetic symmetry remains unbroken. That fact is
• 2:26:22
  Speaker 1: closely connected with the photon having zero rest mass. But
• 2:26:26
  Speaker 1: electromagnetism is not entirely separate from the weak interaction. At
• 2:26:31
  Speaker 1: sufficiently high energies, the two are described together by electroweak theory.
• 2:26:37
  Speaker 1: The electroweak theory was developed through the work of physicists,
• 2:26:40
  Speaker 1: including Sheldon Glashow, Abdus Salam and Steven Weinberg. At everyday energies,
• 2:26:48
  Speaker 1: electromagnetism and the weak interaction appear dramatically different. Electromagnetism has
• 2:26:54
  Speaker 1: an effectively infinite range. It holds atoms together, allows electric
• 2:26:59
  Speaker 1: currents to flow, produces magnetic fields and governs the behaviour
• 2:27:04
  Speaker 1: of light. The weak interaction acts only over extremely short
• 2:27:08
  Speaker 1: distances and plays an important role in processes such as
• 2:27:12
  Speaker 1: radioactive beta decay and nuclear reactions inside stars. Yet the
• 2:27:18
  Speaker 1: standard model describes these apparently different interactions as parts of
• 2:27:22
  Speaker 1: a more unified electro-weak structure. The separation we see today
• 2:27:27
  Speaker 1: is connected with electroweak symmetry breaking. At very high energies,
• 2:27:32
  Speaker 1: the underlying theory contains several gauge fields. After electroweak symmetry breaking,
• 2:27:38
  Speaker 1: combinations of these fields appear as the particles observed at
• 2:27:41
  Speaker 1: lower energies. One combination becomes the photon. Another becomes the
• 2:27:47
  Speaker 1: neutral weak force boson known as the Z boson. The
• 2:27:51
  Speaker 1: charged weak force bosons also emerge from the electroweak theory.
• 2:27:55
  Speaker 1: The reason the photon remains massless while the weak force
• 2:27:58
  Speaker 1: bosons become massive is connected with the Higgs field. The
• 2:28:02
  Speaker 1: Higgs field exists throughout space and has a non-zero value
• 2:28:06
  Speaker 1: even in the lowest energy state of the universe. Through
• 2:28:09
  Speaker 1: their interactions with this field, the weak force bosons acquire mass.
• 2:28:14
  Speaker 1: The photon does not acquire mass in the same way
• 2:28:17
  Speaker 1: because the electromagnetic gauge symmetry remains unbroken. This is why
• 2:28:22
  Speaker 1: the electromagnetic interaction can extend across enormous distances while the
• 2:28:27
  Speaker 1: weak interaction has such a short range. A massive force
• 2:28:31
  Speaker 1: carrier generally produces an interaction that falls away very rapidly
• 2:28:35
  Speaker 1: over distance. A massless photon allows electromagnetism to remain long-ranged.
• 2:28:42
  Speaker 1: Experiments have searched very carefully for any sign that the
• 2:28:45
  Speaker 1: photon might possess a tiny non-zero rest mass. So far,
• 2:28:50
  Speaker 1: none has been found. Measurements ranging from laboratory tests to
• 2:28:55
  Speaker 1: observations of magnetic fields on planetary and astronomical scales have
• 2:28:59
  Speaker 1: placed extremely strict limits on any possible photon mass. Everything
• 2:29:05
  Speaker 1: established experimentally remains consistent with the photon having exactly zero
• 2:29:10
  Speaker 1: rest mass. The photon is also frequently described as the
• 2:29:14
  Speaker 1: force carrier of electromagnetism. That phrase is useful, but it
• 2:29:19
  Speaker 1: can create one of the most persistent misleading pictures in
• 2:29:23
  Speaker 1: particle physics. It is easy to imagine two charged particles
• 2:29:28
  Speaker 1: throwing invisible photons back and forth, like people tossing a
• 2:29:32
  Speaker 1: ball across a room. Each throw supposedly pushes or pulls
• 2:29:36
  Speaker 1: the particles. That is not a literal description of what
• 2:29:39
  Speaker 1: the theory says. Charged particles interact through the quantum electromagnetic field.
• 2:29:46
  Speaker 1: When physicists calculate these interactions using perturbation theory, the mathematics
• 2:29:51
  Speaker 1: can be organised into terms that involve photon propagators. These
• 2:29:56
  Speaker 1: are often represented as internal photon lines in Feynman diagrams.
• 2:30:01
  Speaker 1: That is where the language of exchange photons comes from.
• 2:30:05
  Speaker 1: But the internal objects appearing in those calculations are not
• 2:30:08
  Speaker 1: the same as freely propagating photons that can travel across
• 2:30:12
  Speaker 1: a laboratory and strike a detector. They are commonly called
• 2:30:16
  Speaker 1: virtual photons. A real photon can propagate freely. It can
• 2:30:21
  Speaker 1: carry energy and momentum over large distances. It can leave
• 2:30:25
  Speaker 1: a star. cross interstellar space and eventually be absorbed by
• 2:30:29
  Speaker 1: a telescope. A virtual photon is different. It appears as
• 2:30:34
  Speaker 1: part of a mathematical description of an interaction inside a
• 2:30:38
  Speaker 1: perturbative calculation. It is not a directly detectable free particle.
• 2:30:44
  Speaker 1: The distinction matters because popular explanations sometimes say that virtual
• 2:30:49
  Speaker 1: photons briefly appear, violate energy conservation and disappear before nature notices.
• 2:30:56
  Speaker 1: That is not a good description. Energy conservation is not
• 2:30:59
  Speaker 1: casually suspended. Virtual particles are internal elements of a calculation,
• 2:31:05
  Speaker 1: not little detectable objects temporarily breaking the laws of physics.
• 2:31:09
  Speaker 1: The same caution applies to Feynman diagrams themselves. Richard Feynman
• 2:31:15
  Speaker 1: introduced these diagrams as a powerful way of organising terms
• 2:31:18
  Speaker 1: in quantum electrodynamics. They are extraordinarily useful. A line can
• 2:31:23
  Speaker 1: represent an electron. Another can represent a photon. Vertices indicate
• 2:31:29
  Speaker 1: interactions permitted by the theory. But a Feynman diagram is
• 2:31:33
  Speaker 1: not a literal frame from a microscopic film. The lines
• 2:31:36
  Speaker 1: do not necessarily show exact trajectories through space. The diagram
• 2:31:41
  Speaker 1: is a bookkeeping device for a contribution to a quantum amplitude.
• 2:31:45
  Speaker 1: Several diagrams may contribute to the same measurable process, and
• 2:31:49
  Speaker 1: their amplitudes must be combined. The mathematics is the physical prediction.
• 2:31:55
  Speaker 1: The little drawing is a convenient way of keeping track
• 2:31:58
  Speaker 1: of it. This brings the photon into focus from several
• 2:32:01
  Speaker 1: different directions at once. It carries energy determined by frequency.
• 2:32:06
  Speaker 1: It carries momentum despite having no rest mass. It can
• 2:32:10
  Speaker 1: be absorbed in a single localised detector event. It can
• 2:32:14
  Speaker 1: interfere through quantum amplitudes. It can exist in superpositions involving
• 2:32:19
  Speaker 1: different paths. Its polarisation can be prepared and measured. It
• 2:32:24
  Speaker 1: possesses two physical helicity states. It can become entangled with
• 2:32:29
  Speaker 1: another photon so that the pair shares one joint quantum state.
• 2:32:33
  Speaker 1: It can be created when matter transfers energy into the
• 2:32:36
  Speaker 1: electromagnetic field. It can disappear as a photon when that
• 2:32:40
  Speaker 1: energy is absorbed elsewhere. It can travel for billions of
• 2:32:43
  Speaker 1: years across the universe and still be described by the
• 2:32:47
  Speaker 1: same underlying quantum rules. And beneath all of those behaviours
• 2:32:51
  Speaker 1: lies the same central idea. The photon is a quantum
• 2:32:56
  Speaker 1: excitation of the electromagnetic field. Calling it a particle is
• 2:33:00
  Speaker 1: useful because individual interactions occur in discrete amounts. Calling light
• 2:33:05
  Speaker 1: a wave is useful because amplitudes interfere, diffract and propagate
• 2:33:10
  Speaker 1: through space with wave-like structure. Neither classical picture is fundamental
• 2:33:15
  Speaker 1: by itself. The standard model goes deeper. It places the
• 2:33:19
  Speaker 1: photon inside a quantum field theory governed by gauge symmetry. relativity,
• 2:33:25
  Speaker 1: and the rules of quantum measurement. The photon is not
• 2:33:29
  Speaker 1: a microscopic marble. It is not simply a classical electromagnetic
• 2:33:33
  Speaker 1: ripple chopped into little pieces. It is not secretly switching
• 2:33:37
  Speaker 1: between being a particle and being a wave. It is
• 2:33:40
  Speaker 1: a genuinely quantum object. More precisely, it is the massless
• 2:33:45
  Speaker 1: gauge Poisson of electromagnetism, one quantum of excitation of the
• 2:33:50
  Speaker 1: electromagnetic field. That description may be less immediately visual than
• 2:33:55
  Speaker 1: a tiny glowing dot, but it explains enormously more, and
• 2:34:00
  Speaker 1: in modern physics that is what a photon really is.
• 2:34:03
  Speaker 1: So what is a photon, really? At first the question
• 2:34:07
  Speaker 1: seems as though it should have an answer we could draw.
• 2:34:10
  Speaker 1: Perhaps a tiny glowing point. Perhaps a little wave moving
• 2:34:14
  Speaker 1: through space. Perhaps some microscopic object carrying a packet of
• 2:34:18
  Speaker 1: light from one place to another. But after following the
• 2:34:22
  Speaker 1: physics carefully, none of those pictures is quite enough. They
• 2:34:26
  Speaker 1: each capture something real. None captures the whole thing. The
• 2:34:31
  Speaker 1: story began with light behaving beautifully as a wave. Michael
• 2:34:35
  Speaker 1: Faraday found deep connections between light, electricity and magnetism. And
• 2:34:41
  Speaker 1: James Clerk Maxwell showed that changing electric and magnetic fields
• 2:34:45
  Speaker 1: could travel together through space as electromagnetic waves. Visible light
• 2:34:50
  Speaker 1: became one small part of the electromagnetic spectrum. Radio waves, microwaves,
• 2:34:56
  Speaker 1: infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays
• 2:35:03
  Speaker 1: could all be understood as electromagnetic radiation, differing primarily in
• 2:35:07
  Speaker 1: wavelength and frequency. That classical description remains extraordinarily successful. Whenever
• 2:35:15
  Speaker 1: the quantum granularity of light is unimportant, Maxwell's equations can
• 2:35:19
  Speaker 1: describe electromagnetic radiation with remarkable accuracy. Nothing about quantum physics
• 2:35:26
  Speaker 1: made classical electromagnetism useless. It simply revealed that there was
• 2:35:31
  Speaker 1: another layer underneath. That deeper layer began to appear when
• 2:35:35
  Speaker 1: Max Planck studied thermal radiation and found that energy exchange
• 2:35:39
  Speaker 1: could not always be treated as perfectly continuous. Energy came
• 2:35:44
  Speaker 1: in discrete amounts connected with frequency. Albert Einstein then took
• 2:35:49
  Speaker 1: the idea further. In explaining the photoelectric effect, he proposed
• 2:35:54
  Speaker 1: that light itself behaves during emission and absorption as though
• 2:35:59
  Speaker 1: its energy is concentrated into individual light quanta. Arthur Compton
• 2:36:04
  Speaker 1: later showed that X-rays could scatter from electrons in a
• 2:36:07
  Speaker 1: way that was naturally understood if those quanta carried momentum
• 2:36:12
  Speaker 1: as well as energy. And in 1926, Gilbert Newton Lewis
• 2:36:17
  Speaker 1: introduced the word photon. The name survived. The classical picture
• 2:36:23
  Speaker 1: did not. A modern photon has no electric charge. Its
• 2:36:28
  Speaker 1: measured rest mass is zero. It carries energy determined by
• 2:36:32
  Speaker 1: its frequency. It carries momentum even though it has no
• 2:36:37
  Speaker 1: rest mass. It possesses intrinsic angular momentum and is classified
• 2:36:42
  Speaker 1: as a spin-1 boson. freely propagating photon has two physical
• 2:36:47
  Speaker 1: helicity states. And because photons are bosons, many of them
• 2:36:52
  Speaker 1: can occupy the same quantum state. Those are genuine physical properties.
• 2:36:57
  Speaker 1: A rigid surface is not. A miniature diameter is not.
• 2:37:01
  Speaker 1: A hidden classical trajectory running continuously from source to detector
• 2:37:06
  Speaker 1: is not something quantum theory generally requires. This becomes especially
• 2:37:12
  Speaker 1: clear in interference experiments send photons through an appropriate apparatus
• 2:37:17
  Speaker 1: one at a time, and each detection appears at one
• 2:37:20
  Speaker 1: particular place. One photon produces one event, another photon produces another.
• 2:37:26
  Speaker 1: But after enough detections, an interference pattern emerges. The individual
• 2:37:30
  Speaker 1: arrivals look particle-like. The probability distribution governing them behaves wave-like.
• 2:37:37
  Speaker 1: That simple experiment reveals why the old categories become strained.
• 2:37:41
  Speaker 1: The photon does not need to be secretly switching between
• 2:37:44
  Speaker 1: being a wave and being a particle. The deeper description
• 2:37:48
  Speaker 1: is quantum. Its state can contain superpositions. Probability amplitudes can interfere.
• 2:37:56
  Speaker 1: The state can extend across several possible paths. And yet
• 2:38:00
  Speaker 1: when the photon is finally absorbed, the interaction can appear
• 2:38:04
  Speaker 1: as one sharply localised event. Quantum electrodynamics provides the framework
• 2:38:10
  Speaker 1: that brings those behaviours together. In that theory, the electromagnetic
• 2:38:15
  Speaker 1: field is fundamental. A photon... is a quantized excitation of
• 2:38:19
  Speaker 1: that field. That is probably the most precise short answer available.
• 2:38:24
  Speaker 1: A photon is one quantum of the electromagnetic field. This
• 2:38:30
  Speaker 1: also explains why photons can be created and destroyed. There
• 2:38:34
  Speaker 1: is no universal rule requiring the total number of photons
• 2:38:38
  Speaker 1: to remain constant. An excited atom can transfer energy into
• 2:38:42
  Speaker 1: the electromagnetic field and create a photon. A nucleus can
• 2:38:47
  Speaker 1: emit a gamma-ray photon. Accelerating charges can radiate. Particle interactions
• 2:38:54
  Speaker 1: can produce photons. A photon can later be absorbed by matter,
• 2:38:59
  Speaker 1: transferring its energy and momentum into electrons, atoms, molecules or
• 2:39:04
  Speaker 1: other physical systems. The photon that ceases to exist as
• 2:39:08
  Speaker 1: a photon, energy remains conserved, momentum remains accounted for. The
• 2:39:13
  Speaker 1: number of photons simply changes. and photons can occupy quantum
• 2:39:18
  Speaker 1: states with no convincing classical equivalent. A single photon can
• 2:39:23
  Speaker 1: exist in a superposition involving several possible paths. Its polarisation
• 2:39:28
  Speaker 1: can be prepared in different quantum states. Two photons can
• 2:39:32
  Speaker 1: become entangled so that the most complete description belongs to
• 2:39:35
  Speaker 1: the pair rather than to each photon separately. Enormous numbers
• 2:39:40
  Speaker 1: of photons can form coherent states, whose collective behaviour closely
• 2:39:44
  Speaker 1: resembles a smooth classical electromagnetic wave. The familiar light filling
• 2:39:50
  Speaker 1: a room and the strange single photon arriving at a
• 2:39:53
  Speaker 1: detector are therefore not separate kinds of physics. They are
• 2:39:58
  Speaker 1: different regimes of the same quantum electromagnetic field. Relativity adds
• 2:40:03
  Speaker 1: another layer. A photon follows a light-like or null trajectory
• 2:40:08
  Speaker 1: through spacetime. The proper time along an ideal null trajectory
• 2:40:12
  Speaker 1: is zero. but there is no valid inertial rest frame
• 2:40:16
  Speaker 1: for a photon, so it is misleading to ask what
• 2:40:19
  Speaker 1: the universe looks like from the photon's point of view.
• 2:40:22
  Speaker 1: Light also responds to curved spacetime. It can be gravitationally redshifted.
• 2:40:28
  Speaker 1: Its path can be bent by gravitational lensing. Near a
• 2:40:32
  Speaker 1: black hole, light can follow dramatically curved trajectories. Inside an
• 2:40:37
  Speaker 1: event horizon, The geometry of spacetime prevents future-directed light from
• 2:40:42
  Speaker 1: escaping to the external universe. None of this requires the
• 2:40:46
  Speaker 1: photon to possess rest mass. Its path is determined by
• 2:40:50
  Speaker 1: the geometry of spacetime itself. There is one more distinction
• 2:40:54
  Speaker 1: worth keeping clear. Real photons can propagate freely and be detected.
• 2:41:00
  Speaker 1: They can cross a laboratory, travel through interstellar space, or
• 2:41:04
  Speaker 1: arrive at a telescope after billions of years. Virtual photons
• 2:41:08
  Speaker 1: are different. They appear within particular methods of calculating quantum interactions.
• 2:41:14
  Speaker 1: They are not directly detectable free particles secretly being thrown
• 2:41:18
  Speaker 1: between charged objects. Feynman diagrams showing exchange photons are powerful
• 2:41:24
  Speaker 1: mathematical bookkeeping tools, not literal films of tiny particles flying
• 2:41:29
  Speaker 1: back and forth. That distinction helps remove one final classical
• 2:41:34
  Speaker 1: picture that can easily become too concrete. The difficulty in
• 2:41:38
  Speaker 1: answering what a photon really is does not mean physics
• 2:41:41
  Speaker 1: has failed to understand it. Quite the opposite. Photons are
• 2:41:45
  Speaker 1: described with extraordinary precision. Their energies can be calculated. Their
• 2:41:50
  Speaker 1: momentum can be measured. Their polarisation can be controlled. Their
• 2:41:54
  Speaker 1: interference can be predicted. Their correlations can be tested. Their
• 2:41:59
  Speaker 1: interactions with matter can be modelled to astonishing accuracy. Quantum
• 2:42:03
  Speaker 1: electrodynamics is one of the most successful theories ever developed.
• 2:42:08
  Speaker 1: What struggles is not the physics. It is our intuition.
• 2:42:13
  Speaker 1: Human intuition was built around stones, water, falling objects, shadows, sounds,
• 2:42:18
  Speaker 1: and things that occupy obvious places. The photon belongs to
• 2:42:22
  Speaker 1: a level of reality where those familiar categories stop being fundamental.
• 2:42:27
  Speaker 1: It can look wave-like. It can arrive in particle-like events.
• 2:42:31
  Speaker 1: It can exist in superpositions. It can become entangled. It
• 2:42:35
  Speaker 1: can be created and absorbed. It can travel across the
• 2:42:38
  Speaker 1: observable universe without ever becoming a tiny glowing marble. The
• 2:42:42
  Speaker 1: deepest object is the quantum electromagnetic field. And the photon
• 2:42:47
  Speaker 1: is one of its fundamental excitations. That may be harder
• 2:42:51
  Speaker 1: to picture than a little speck of light. But it
• 2:42:54
  Speaker 1: is a far more accurate answer to the question. What
• 2:42:58
  Speaker 1: is a photon, really? Not quite a particle. not quite
• 2:43:01
  Speaker 1: a wave, a quantum of the electromagnetic field.