In Our Time - The Photon

Episode Date: February 12, 2015

Melvyn Bragg and his guests discuss the photon, one of the most enigmatic objects in the Universe. Generations of scientists have struggled to understand the nature of light. In the late nineteenth century it seemed clear that light was an electromagnetic wave. But the work of physicists including Planck and Einstein shed doubt on this theory. Today scientists accept that light can behave both as a wave and a particle, the latter known as the photon. Understanding light in terms of photons has enabled the development of some of the most important technology of the last fifty years.With:Frank Close Professor Emeritus of Physics at the University of OxfordWendy Flavell Professor of Surface Physics at the University of ManchesterSusan Cartwright Senior Lecturer in Physics and Astronomy at the University of Sheffield.Producer: Thomas Morris.

Thank you for downloading this episode of In Our Time, for more details about In Our Time, and for our terms of use, please go to BBC.co.com.uk slash Radio 4. I hope you enjoy the program. Hello, what is light? This is a question which has perplexed the greatest thinkers for hundreds of years. In the late 19th century, scientists thought they'd finally solved the problem. Light they thought it was an electromagnetic wave, a form of radiation closely related to electricity and magnetism. But in 1905, Albert Einstein showed that it can also be thought of as a stream of tiny particles, later named photons. Photons are all around us. Every second, the light bulb above my head is producing a million times more photons than there are cells in my body.

Photons are radiating in their trillions from the BBC transmitters to bring you this radio programme. And x-ray machines and microwave ovens are both clever ways of making use of them. But the photons still remains mysterious. Towards the only of his life, Einstein said that after five, 50 years of thought, he still didn't understand what a photon was. Of course, today he said, every rascal thinks he knows the answer, but he is deluding himself. Win me to discuss the photo now, Frank Close, Professor Emeritus of Physics at the University of Oxford.

Wendy Flavall, Professor of Surface Physics at the University of Manchester, and Susan Cartwright, Senior Lecturer in Physics and Astronomy at the University of Sheffield. Frank Close, I cited this programme by saying, what is light? Could you begin to tell us how a scientist in the 19th century might have answered that question? Well, in the 19th century, the scientist would have said it was a wave, an electromagnetic wave, where at one moment you've got an electric field filling space, but that electric field is dying away and a magnetic field is building up to replace it.

And then the magnetic dies away and the electric comes back. So up and down, up and down, electric to magnetic and back again. And as this oscillation takes place, the energy of the wave travels, through space at 300,000 kilometres every second, which is what we call the speed of light, because light is an example of an electromagnetic wave. Now, although all these waves travel at this universal speed, the frequency that the electric and magnetic flip backwards and forwards

can be anything you like. So what we perceive as colour, for example, is our eyes response to different frequencies of this oscillation. So violet light at the blue end of the rainbow, its frequency is about twice as fast as that at the red end. And go beyond the violet, the ultraviolet and x-rays and gamma rays, you've got very high frequency oscillations. And at the other end, below the red, the infrared, which we feel as heat,

and the radio waves that you referred to are very low frequency oscillations. So the visible rainbow that our eye is familiar with is just a single octave in a vast spectrum of electromagnetic waves. So in the 19th century, they had a very small entry hole, as they were, into light, as it could probably be described. The wave, how did they come to the idea of the wave? Well, one way is that, of course, there's a great theoretical advance in the middle of the 19th century

with James Clark Maxwell, who came up with his theory of light, and the discoveries in electricity and magnetism that have been building up in the early parts of that century led him to write down equations, as to how electric phenomena behave and how magnetic phenomena behave. And these equations, when you play with them,

reveal that there should be electromagnetic waves. And by putting in numbers into the equations from measurements that have been made of other things, it was possible to work out the speed of these waves. And when you put the numbers in, he found the speed was about 300,000 kilometres a second, which was quite astonishing because that was a number that had been measured

for the speed of life. And so at that moment you have a choice. Is this a coincidence or is it something very profound? And in science we tend not to believe in coincidences, and indeed it was profound. This was a theoretical suggestion that light is nothing more or less than an electromagnetic wave. So now take that theory and ask what are its consequences.

Well, the theory said you can have waves of any frequency. And so the theory predicted there should be waves beyond the visible ray and the discovery later of infrared and ultraviolet and radio waves confirmed that a theoretical way. To demonstrate the wave nature of light, next time you're with your kids blowing soap bubbles and you see these beautiful little colours on the soap bubbles. Ask where do those colours come from? Or when some dump truck has spilled oil on the top of water on the road and you see these little rings of coloured light, what is happening there is that if you've got a very very very very,

thin layer, the light wave is reflecting from the front side back to your eye, but some of it goes through the layer and reflects from the back side to your eye. And so those two reflections have gone a little different distance. And if that distance is similar to the wavelength of light, there's a mismatch between the reflected wave coming from the rear and that from the front. And depending if that mismatch is in phase or out of phase, makes the colours be red or green or not. at all. So these little coloured rings called interference phenomena is the interferences of waves. So light as a wave gives rise to

these effects of refraction and diffraction and colours on soapholes. But it began to Susan Cartwright, around the turn of the 20th century, it began to emerge that there might be something else going on and it might be more complicated than Frank has disclosed to us.

Yes, indeed. One of the problems of 19th century science was that they could not understand a phenomenon called black body radiation. Now this sounds complicated, but it's not. If you've seen a poker or an old-fashioned electric bar fire, as you heat it up, it starts by glowing dull red, then it becomes brighter orange and then still brighter yellow.

And if you are from Sheffield as I am, if you consider steel foundries, then that will go even brighter towards white-hot. And this is not dependent on the material. that you are heating. It's something that happens for all materials, so it's not related to the intrinsic colour of the material, and this is why it's called black body radiation. And this radiation has a very particular spectrum. The amount of it that comes from each of the frequencies that Frank was discussing depends on the temperature of the body in a very well-defined and well-measured way.

And it turned out that nobody could understand how that spectrum came to. be. Maxwell and Boltzmann in the 19th century had understood the similar spectrum of the speeds of gas particles. That was fine. But nobody could understand how the spectrum of light waves came to have the form that it did. And in 1901, Max Planck demonstrated that you could understand this spectrum and you could understand it perfectly if you assume that instead of being able to produce arbitrary amounts of energy of each colour, you insisted that the energy came in little packets proportional, an amount proportional to the frequency of the wave. So that blue light, the little packet had contained twice the energy that red light did. And the size of the

packet was the frequency of the wave multiplied by constant, which we now call Planck's constant. And if you did that, it all worked beautifully and you could understand, the spectrum of black body radiation. Then what did Einstein bring to the table in 1905? Well, Plank, although he had constructed these little packets, he really didn't think that they were real. He thought that they were properties essentially of the atoms that were emitting the light rather than properties of the light itself.

And what Einstein did was understand a mysterious phenomenon, mysterious at that time, called the photoelectric effect. If you take the surface of metal and you shine light on it, the energy of the light can liberate electrons from the surface of metal. The mysterious thing was that for every metal, there was a minimum frequency that you needed to liberate electrons. In some metals, if you shone blue light on them, electrons came off. If you shone red light, however brighter red light, you've got no electrons.

and Einstein pointed out that if these little packets of energy that Max Planck had invoked to understand black body radiation if they were real, then you could understand this because there was a minimum amount of energy you needed to liberate an electron and if that minimum amount of energy was greater than Planck's constant times the frequency of the light

you didn't get an electron. If it was less than Planck's constant times, constant times the frequency of the light. You got an electron liberated and its kinetic energy was the difference between H times the frequency and the minimum energy you needed to liberate the light.

And this explained the properties of the photoelectric effect in a way that the wave theory just couldn't do. Wendy Flaville, giving the veneration in which we now hold Einstein, it seems strange that this was not widely believed at the time. In fact, it was thought of as unbelievable, described as reckless, and it was not confirmed for quite a few years. Why was there so much opposition to it? Well, that's perfectly true. He was regarded as a bit of a lunatic, I suppose, initially for this idea. It wasn't taken seriously for quite some time. But eventually people began to realize that there were more experiments that required this idea of Quanta to explain them.

One of them was... The little packets of energy that Susan's described given by H Planck's constant times the frequency of the light. So one of these experiments was done by Compton, who was working in 1923 in Washington University, and he used a graphite target and fired x-rays at the target. And he discovered that... But the x-rays that were deflected from the target had a slightly lower wavelength, longer wavelength, than the x-rays that had gone into the target.

So we would call that, I guess, a red shift. That wasn't explained by the scattering theories of light, which used light as an electromagnetic wave at the time. He also noticed that he could still see this effect at very low intensities. and he gradually realized that if he assumed this idea of Einstein that we have these light quanta little packages of energy in the x-rays and he assumed that if an electron scattered an x-ray, the x-ray gave up its energy to the electron in these little packets, then he could understand the process of scattering.

It obeyed the laws of conservation of. energy, but also it obeyed the laws of conservation of momentum. So there was defined geometry to his scattering. The x-rays were scattered in a particular direction. And he concluded that, in his own words, that a radiation quantum comes with it, directed momentum as well as energy. And it was key in the acceptance of the idea that these little quanta exist. He won the Nobel Prize in 1920.

in fact, which was quite soon after those experiments because they were so fundamental. And so he proved, in effect, what Einstein had been saying. He did, but slightly before that in 1913, Bohr had also used the idea to explain the spectrum of the hydrogen atom. So we come to uncertainty that, don't we? Well, Compton's experiments, I think, showed us quite clearly that electrons interact with light. and that led to a series of thought experiments.

So, for example, if you were imagining that you could see an electron, how would you see an electron? Well, light would have to bounce off the electron into your eye for you to be able to see it. And of course, he'd already shown that that would produce a change in the momentum of the electron. So in other words, by trying to measure the position of the electron, you change its momentum. and fundamental to the new theory of quantum mechanics

was an idea that there's an intrinsic uncertainty with which you can measure certain properties together. A pair of them is momentum and position for an electron. The equivalent relationship for light is actually the number of photons and the phase that Frank referred to the way in which the light beams are offset. Okay, Frank close. Can you, let's go into photons. Now, what are the basic properties of photons?

Well, photons are effectively like little particles, like little bullets that come staccato-like, which explains the phenomena that... And they're all around us, the trillions... Trillions hitting your eyes at this moment. These unimaginable numbers, whenever we talk to people like you, we just have to take it for granted.

Okay, there are trillions of them. And each one of them has got no mass at all? That baffled me. I've read it several times. I'm sorry to be such an ignoramus, but can you explain what no mass means? Do you know what mass means? I do.

So if you haven't got it, that's no mass. How can you get hold of no mass? You can have energy without having mass. And photons of light, they have energy, but they have no mass. And the consequence of this mysterious combination is that it means that they travel at the speed of light. This sounds like tautology, but it is actually indeed.

Well, how do we know that there? have got no mass? Because they will transmit momentum and energy from one place to another. So we know them by their effect? We know them by their effects. And indeed, as Wendy just said, when Compton did his experiment, he measured not directly the photons hitting, but by the way that wavelengths change from before to after can do inferences, and you do the experiment over and over again, you find certain correlations happen, you build up an experience of how nature works. So the photons have no mass but they have energy. They can have a variety of energies, as Susan said, that the high frequency has high energy, the low frequency has low energy.

If you want to use very high energy photons, like in particle physics, because of the complementarity, the uncertainty that Wendy mentioned, high energy, high frequency correlates with measuring things on very short times. So if you want to take instantary instantaneous images of nature or very short distance images of things, you need to have high energy, high momentum photons. Another property that photons have is that they are bosons. Now, everybody's heard of the Higgs boson, but they know who Higgs is, but what boson is. Well, particle physicists classify all particles into two families.

They're fermions or bosons. Roughly speaking, fermions are like cuckus and bosons like penguins. More than one cuckoo in the same nest is one, two, many, whereas penguins, it's the more the merrier. So, electrons are... I've never seen a merry penguin with plants. Not enough photons, that's why. Electrons are fermions. Now, that's why, if you put one electron somewhere in an atom,

you can't put another one in the same place, and that's what gives rise to structure. It's ultimately the reason why I'm not sinking through my seat at this moment. Photons, however, are bosons the more than merrier. So you can put as many photons as you like together, and that's what. is, if you like, the principle behind laser beams. So that property of photons is what gives rise to that. Can we come back to this perplexing question for me anyway, obviously for none of you, but the no mass of photons, and what implications that has?

Well, as Frank said, as Frank said, photons travel at the speed of light, and he also said that sounds like a totology. And it's not. In Einstein's relativity, the key speed, the speed that we call the speed of light, the speed denoted by C, is actually the speed of massless particles. So anything that has no mass travels at the speed of light,

and conversely, anything that has mass cannot travel at the speed C in relativity, the speed of massless particles. C as in MC squared. C as in MC squared, the very one. So if the photon did have a mass, then paradoxically, light would not travel at the speed of light. And in fact, the other thing that masslessness produces is that because they have no mass,

then when you spread out the field, the electric field or the magnetic field of from a point source, you get an inverse square law, that is to say the strength of the force, decreases like one over the square of the distance. If you go twice as far away, you feel one quarter of the force. And that turns out to be very deeply related to having a massless force carrier. The weak interaction has force carriers which have mass, the W and the Z, and it has an incredibly short range.

And we have tested this using the magnetic fields in the solar system. And you can set an upper limit on the mass of the photon. and it is 10 to the minus 54 kilograms. That's zero followed by 53 zeros, followed by one, and the mass of the photon is less than that. Frank, you want to come in? Yes, I should probably have included, to lead in for Susan, that one of the properties of the photon is that electrically charged particles,

like electrons, can emit photons and absorb photons. So they are emitted by electrically charged particles, and as they exchange between one electric charge and another, whether they transmit the electromagnetic force between them. Can I turn to you again, wonderful farewell. We know now, we've been told anyway, that light exists in many different wavelengths, correspond with different visible colours

and other types of electromagnetic radiation. How is this described in the photon model? Well, in the photon model, of course, the energy of the photon, as Susan described, is just a constant times the frequency. So we can translate all of those frequencies into energies. and we find that the photons therefore have different energies across the spectrum. So very hard x-rays, for example, have very high energies.

They might be perhaps 100 kilo-electron volts or 10-kiloelectron volts. If we go into the visible part of the spectrum, the photons have much lower energy. So yellow light from the sun has got an energy of two electron volts. So an electron volt is a convenient unit that we use because it's the amount of energy that a single electron would get in going through a vault. But if we go down to BBC Radio 4, which we're currently communicating on, at 92 to 95 gigahertz, the wavelength there is about three metres, just over three metres.

The corresponding energy is only about a millionth of an electron volt. So Radio 4 won't hurt us, but ultraviolets and x-ray radiation might well. Frank, although photons have famously, as we know now, no mass, they're affected by gravity. How do we know this? Well, I thought the first question you were going to probably say paradoxically if they've got no mass. How can they be affected by gravity? I was rather you say to me because you accept with a certain authority. Right, having bought myself time, let's address that one.

Well, I mean, the law of gravity as we first meet it going back, to the days of Isaac Newton is that the force between two bodies is proportional to their masses and inversely proportional to the square of the distance between them, but it's proportional to their masses. And that is indeed true for massive bodies, like the earth and the sun and so on. A particle that has got no mass, such as a photon,

you might think, oh, so there shouldn't be any force of gravity at all in it. Well, Newton's law is what we learn for massive bodies, We now know since Einstein that that is an approximation to a richer description. And roughly speaking, it's not the masses that are important, it's the energy that's important. So for a conventional mass sitting at rest where his energy is m c squared, that's Newton's law proportional to the masses. But in general it's proportional to the energies. So even a massless photon, because it has energy, can feel a gravitational force.

and how do we know this in practice? I think it's 1919 or there about the famous experiment that Eddington did during a total solar eclipse that the stars, you can see where the stars are in the sky, and when the sun is in the direct line of sight of you, of course, you can't normally see the stars except during a total solar eclipse.

And the starlight, coming from a distant star, comes near the sun, grazing the edge of the sun, is deflected by the gravitational attracted. and so the star appears to be in a slightly different position in the sky than you would expect it to be. And so it was the experiment that was done now nearly a century ago, measuring the deflection of the stars in the near line of sight of the sun during the total eclipse that confirmed that. I actually find it very difficult to believe that he had the sang foire to do an experiment during the total solar eclipse.

If you've ever seen one, it's so mind-blowing, I couldn't believe anything. But the claim is he did, and that was the proof. You want to say something here? I just wanted to pause my new thing. A couple of things. One is that there were in fact two 1919 expeditions to two different places, both organised by Eddington, but they did actually allow for the possibility that somebody might be too overwhelmed to take decent photos. And it was a very, very tiny effect. If you look at the image of a star on a photographic plate, it's blurred by the effect of the atmosphere.

and that gets worse during a total solar eclipse because you've just switched off the sunlight so the atmosphere is very turbulent. So your images are very poor and it was incredibly difficult to make these measurements and there's been a certain amount of controversy ever since as to whether the experimental errors on those measurements really justified Eddington's claim.

He was a great supporter of relativity and really seriously wanted it to be right. However, nowadays, when you look out in the... into the wider galaxy and other more distant galaxies, this so-called gravitational lensing phenomenon is really very, very clear. You can look at distant clusters of galaxies and you see background galaxies warped into arcs of a circle

by this gravitational lensing effect. So it was a very tiny effect in 1919. But nowadays, if you look at Hubble Space Telescope images, it's a huge effect. You can't possibly believe it's a huge effect. you can't possibly believe it's not there. Wendy, just to take up this point about the Hubble telescope or to work back from it,

what's the way you've been talking about, this happened and that happened and the other happened? Can you give us some idea of the development of the technical ability to do this over the last century? What was invented that made this more and more possible? I think a lot of these experiments were conjectured as thought experiments initially because we just couldn't do the relevant experiments. So one of the classic experiments,

which perhaps illustrates both the wave and the particle nature of light, is Young Slit experiments, which is an interference experiment. So Thomas Young was an English polymath who did this experiment early in the 1800s, with a bright light source. You shine a light source at two slits. and you see an interference. In some sort of card or something.

Two slits in a card, yeah. And then a screen behind that, and you see some kind of interference pattern appearing on the screen. So as Frank explained, when light, if we think of it as a wave hits the slits, it kind of spreads out and interferes behind the slits. And the phase differences between the light

that's come from different positions, gives us this interference pattern between the waves. And one of the key experiments that people wanted to do for many years was to try and do that experiment with single photons. Okay, so that was a key experiment which was always taken as demonstrating on ambiguously the wave nature of light. What happens when you do it with single photons? And of course, then we have to develop single photon counters

and we have to develop very bright, coherent light sources that we can attenuate a lot to very low intensities. When we do that, if we think of light as a particle going through a slit, we might naively imagine that it goes straight through the slit and you'd only see a bright bit on the screen reflecting exactly the slit. And that's not what happens at all. If we start counting individual photons going through a slit, gradually an interference pattern builds up on the slit.

So initially we see a bit of a mess. but it's definitely not just located going directly through the slit so something strange is going on and if you count and count it's not behaving as a classical particle at all because we see this almost random pattern

and eventually the statistics build up and we start to see something that is actually an interference pattern so we have this idea that the probability of a particle being found at a particular point on the screen is determined by the intensity of the wave.

So it's expressing both the wave and the particle properties simultaneously. And then even more strange things go on. So if we try to do experiments where we actually measure which slit the single photon goes through, which we can now do, the interference pattern disappears. But if we don't measure which slit the single photon goes through, we have an interference pattern. If we actually do what are called quantum eraser experiments where we do measure which way the photon goes through the slits

and then we just erase that information, the interference pattern comes back. And of course physicists are pushing at this and pushing at this. It brings us right up to date really because there's now a concerted effort to try to see whether we can actually measure the properties of a single photon and still have the interference. So Heisenberg's principle of complementarity, which has been referred to a few times,

tells us we should be able to measure the wave property or the particle property, but not both. And so the 2012 Nobel Prize was awarded for the kind of experiments which make what are called very weak measurements on these kind of systems. Can we measure these systems in such a way that we don't disturb them very much? and we can test the limits of the uncertainty principle. Susan Goddrey. It just occurred to me that some of our listeners may not know what Wendy and Frank mean by an interference pattern. And what it means is that if you have these two slits and this screen,

what you see on the screen is not two bars of light corresponding to the two slits, but a pattern of light and dark stripes, somewhat like a zebra crossing. and the light bits are where the waves arrive at the screen in phase, that is to say, crests and crests and troughs and troughs, and the dark bits are where they arrive out of phase, that is to say, a crest from one slit, meeting a trough from the other and cancelling out. And I would also like to say,

just to demonstrate the interchangeability of particles and waves, that you can do exactly the same thing with electrons, and get exactly the same pattern. It's one of the great wonders of the Nobel Prize that J.J. Thompson got the Nobel Prize for discovering the electron as a particle and his son, GP Thompson, got the Nobel Prize for demonstrating that it was a wave.

Satisfaction all around the time on that one. But I wanted to ask you, Frank. I know you wanted to say something much more important than my question, but still, can you... Have we said everything that you want to say about the particle wave duality? Or is there something missing? If there isn't, we'll move on. No, I just think the wonder of nature that you could ask a child,

is it possible to shine two pieces of light at a spot and get darkness? And the obvious answer is, of course you can't, but the interference pattern shows indeed you can. And that's one of these great paradoxes that comes out of the wave nature of light. Right, let's talk about something called the standard model. What role does that play in all this? Well, the standard model is the description of the way that the fundamental particles build up the universe as we know it and the forces that bind them together to give the structures of the universe.

One of these forces, the electromagnetic force, is the one that is nearest to the photon that we've been talking about today. I think as Susan said earlier, the photon is transmitted between electrically charged particles. So one electron can emit a photon, which can be absorbed by another electron far away and transmit its energy and momentum to that electron, and it will recoil as a result. So a force is transmitted by the exchange of that photon. So a fundamental principle in the standard model is that forces are transmitted by the exchange of particles such as photons. The weak and strong nuclear forces, the weak force, rise to certain forms of radioactivity,

and the strong force is what holds nuclei together in the first place. They are also, we now are sure, described by theories, very similar to the theory that has been developed to describe the electromagnetic force. The role of the photon, in the case of the strong force, is called the gluon. We're not very good at names, right? But you can see what it does. It glues the fundamental bits together.

The gluon has no mass also. the weak force of radioactivity, which Susan also has alluded to, is carried by what we call W for weak bosons, and Z because they've got zero charge. Again, not the greatest of names, but that's how it is. Whereas the photon and the gluon have no mass at all, which you've already confessed is a mind-blowing thought, the W and the Z have a large amount of mass,

about 80 times or 90 times as much mass as the hydrogen atom has. So whereas it is relatively easy, to emit photons and let them transmit forces around, it's pretty difficult to suddenly irradiate that amount of energy locked up in a W or Z boson, and that is part of the reason why the effects are so feeble, so weak. So the modern picture is that the theory of the electromagnetic force, which combines relativity and quantum mechanics,

is called quantum electrodynamics. It describes how photons are exchanged between one, particle and another. And the calculations in this, in some cases, are remarkable that you can theoretically compute a property of, say, the electron to one part in a billion and the experiment agrees with it. That's like saying you can measure the width of the Atlantic to the accuracy of a human hair. I mean, that really says that you've got the theory right. And that is now the paradigm for the other theories of the weak and strong forces. Maybe this is rather late in the program I've asked this question, Susan, but how are photons produced?

Well, photons, as Frank said, are the carriers of the electromagnetic force. So the electromagnetic force relates charged particles. So anything that has an electric charge can emit or absorb a photon. And as we said at the very beginning, if you heat an object up, then its component atoms jiggle about and the electrons get raised to higher levels and when they fall back down to their lowest energy level, they emit photons as a consequence, carrying away the energy that the electron has lost.

And so basically any charged particle that has energy can lose some part of its energy by emitting photons. and in fact, we think that charged particles are constantly surrounded by a cloud of photons which are constantly being emitted and reabsorbed. And what essentially happens is that sometimes some of those photons manage to escape out to long distances. Can we move on, Wendy Playbaud, understanding the particle behaviour of photons

led in the 1950s to the invention of lasers. Can you explain the connection of lasers? Can you explain the connection there and how they work? One of the key processes behind how a laser works is a process called stimulated emission of radiation, which was originally proposed by Einstein. He was investigating the transitions between the quantized levels in an atom, the emission and absorption of photons in the way that Susan's just described.

He decided that there must be actually two processes by which, something that was excited could emit a photon. One was just spontaneously and the other was stimulated by which... How do you know it's spontaneous? Well, that's a very interesting question actually because it later turned out that it's not actually spontaneous

it's encapsulated in the standard model of particle physics. At the time to him though, it appeared that he couldn't make his sums ad up in a thermodynamic sense unless he assumed that in a light field, the light could cause the emission of a photon, but also emission of a photon could occur independently of a light field. But in fact, it's the stimulated emission caused by the photon that's important to lasers.

So what we have is in a light field, the light field essentially jiggles the electrons about, as Susan has described, and causes them to emit another photon. But it's emitted in, in a way that we would say is coherent, which means that the phases of the waves are all lined up. So all the troughs and all the peaks line up.

And that means that if we can add those waves together, we get a large amplitude wave and an enormous intensity. So that's what gives us this very directional and strong high power laser light emission. We have to do a few tricks with that because obviously if we're getting a lot of photons emitted, to send electrons back up to higher levels to re-emit photons. So we have to produce what's called a population inversion.

So we have to produce more excited atoms than we have unexcited atoms, which we do via a kind of ladder of energy levels. But essentially that can give us the laser light radiation. It was originally demonstrated, as you say, in the 1950s in the microwave region. and then in the 1960s for a red laser operating on Ruby, actually, transitions in the chromium atoms in Ruby. Frank, close. The Einstein break that year in 1905 where it produced these four papers,

which seemed to have changed your world, or therefore our world, how did it lead to the development of electronics and how far did it reach? Wow, that's a good question. I'm probably not the right person to ask that, but the concept of the photon that came out, as Susan said, the photoelectric effect,

where a photon will kick out electrons and create electric current. So being able to convert light into electric current is one immediate application of that. Other ways that photons, as particle bullets have been manifesting themselves. If they pass through a gas, then those bullets will hit electrons in the atoms

and knock them out of the atoms, leaving ions behind, and ejecting electrons, which in turn will radiate more photons and eject more electrons. You can get a shower of electric charge, and that is part of the principle in like the Geiger counter, making clicks as a result of that.

And in the world of particle physics, by making photons of very high energies, which means probing very short distances. 45 or thereabouts years ago, the resolution of these photons was such that they were able to probe inside an individual proton or neutron and reveal the existence of the quark layer of reality. Can you have almost fine this isn't cartwright?

What are the other areas in which understanding photon theory could lead to technological breakthroughs? Well, one example that, uses some of the classical ideas of photons and combines them with quantum, is the idea of what's called quantum entanglement. At the beginning of the program, Frank talked about the oscillations of electric fields and magnetic fields, and this occurs in a particular direction. So your electric field may be oscillating vertically or horizontally.

And this is called polarization of the photon, and it's how polaroid spectacles work. When you scatter light, you get preferentially one polarization and your Polaroid glasses cut out that polarization and therefore cut out the scattered light, which is why they work. And there are some quantum processes that produce two photons and although you do not know what their polarizations are,

you know that they are related, that they must be opposite. it. And this actually enables you to use those photons to produce a cryptography that is in principle unbreakable, because the people on either end can tell if you are trying to listen into their transmission. That's quantum technology, isn't it really? That's quantum technology. Can I just finish with the question which started the programme? Einstein said that he didn't really grasp the nature of the photon. And as he said at that time, it's all changed.

Of course he didn't think anybody else did. Has it all changed? Do you? I'm with Einstein. Nonetheless, one can still use them and if I can calculate to one part in a billion and it works, that's good enough for me. I'm with Einstein as well, and I'd like to quote actually from John Wheeler

on this subject who said, behind it all is surely an idea so simple, so beautiful that when we grasp it in a decade, a century or a millennium, we'll all say to each other, how could it have been otherwise, how could we have been so stupid? Are you with Einstein?

I am also with Einstein. I think Niels Bohr also said that anybody who thought they understood quantum mechanics had demonstrated that they did not understand quantum mechanics. Thank you very much, Susan Cartwright, Frank Clers. Wendy Flavall next week we'll be talking about Adam Smith's The Wealth of Nations. Thanks for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests.

What was it you said to John Humphers this morning about Electra Week? I didn't hear it, I was told. Did I get it wrong? I've no idea. I just rhymed off some of the things. I didn't know how to do. When John, and John comes on,

I started listening at 5.30. When John comes on at 6, I'm both delighted and I groan. It's like a wave and a particle. I did, oh, way, oh, it's John, particle. Oh, God, I've got to think of something. And I couldn't think of much I had to go at something

but Tom and I really wasn't so good at all so I just picked out all the things that seemed to me to be rather difficult and said to John why don't we swap jobs He agreed he'd come up and help

but he never showed face, difficult promises, promises really Shall I show you some Now we're after the program Shall I show you some quantum mechanics In action Can you describe to people listeners who are hanging on what you're doing.

In your right hand, you have two very small, like very small perfume bottles. I've got two very small bottles which look rather on prepossessing sort of browny coloured, very uninteresting. They actually contain solutions of quantum dots which are small clusters of semiconductor material. We're interested in them for their lighting, emitting properties and also for their light absorbing properties in solar cells.

So I'm illuminating them here with a torch. a UV and blue torch. And even though the two bottles contain the same material, one is glowing yellow and one is glowing red, which is telling us that different frequency of quanta are being emitted

from the two dots. And that's essentially because the dots are a different size. So the energy levels which can be accommodated in each dot are different. It's like saying that a different size of wave can be accommodated in each dot.

And so we can tune the properties of the dots purely by the size of the dot, which would allow us in principle to make a solar cell that absorbs right across the solar spectrum, which is one of the things that we're actually working on. Better than television, I think, did you? Well, actually, I recently heard a description of papering the wall on radio four a couple of weeks ago, which was very entertaining, a demonstration in the studio. Actually, while you're here, I could ask you, I was beforehand trying to think of analogies between sound and lights

and decided not to use any of them. But I'm then wondering how this studio, which is sort of like dead, isn't it, cuts out reflections, and there's all these different size of things on the wall. I mean, are there interference effects being used in deadening off the echoes? I believe. Yeah.

And then we bring our own cardboard to block out the next studio. This is in our time cardboard. There are many more Radio 4, Arts and Discussion programmes, to download for free. Find these on the website at BBC.com.ukuk slash radio four.