In Our Time - Antimatter

Episode Date: October 4, 2007

Melvyn Bragg and guests discuss Antimatter, a type of particle predicted by the British physicist, Paul Dirac. Dirac once declared that “The laws of nature should be expressed in beautiful equations”. True to his word, he is responsible for one of the most beautiful. Formulated in 1928, it describes the behaviour of electrons and is called the Dirac equation. But the Dirac equation is strange. For every question it gives two answers – one positive and one negative. From this its author concluded that for every electron there is an equal and opposite twin. He called this twin the anti-electron and so the concept of antimatter was born.Despite its popularity with Science Fiction writers, antimatter is relatively mundane in physics – we have created antimatter in the laboratory and we even use it in our hospitals. But one fundamental question remains – why isn’t there more antimatter in the universe. Answering that question will involve developing new physics and may take us closer to understanding events at the origin of the universe. With Val Gibson, Reader in High Energy Physics at the University of Cambridge; Frank Close, Professor of Physics at Exeter College, University of Oxford; Ruth Gregory, Professor of Mathematics and Physics at the University of Durham

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio four. I hope you enjoy the programme. Hello, the Nobel Prize-winning British physicist Paul Dirac declared that the laws of nature should be expressed in beautiful equations. True to his word, he is responsible for one of the most beautiful.

Formulated in 1928, it describes the behaviour of electrons and is called the DRAC equation. But the DRAC equation is strange. For every question, he gives two answers, one positive and one negative. From this, its author concluded that for every electron, there's an equal and opposite twin.

He called this twin the anti-electron, and so the concept of antimatter was born. Since then, physicists have created antimatter in the laboratory, and we even use it in our hospitals, but antimatter remains fundamentally mysterious. There should be much more of it around, but there isn't. and to understand why may bring us closer to understanding events at the origin of the universe.

With me to discuss antimatter, Ruth Gregory, Professor of Mathematics and Physics at the University of Durham, Frank Close, Professor of Physics at Exeter College University of Oxford, and Val Gibson, read in high-energy physics at the University of Cambridge. Val Gibson, let's establish some basic definitions. Before we talk about antimatter, can you outline us the picture of matter and explain how that breaks down? Yes, of course. Matter is the stuff that we and everything around us is made off.

So if you had the most powerful microscope in the world, you could look inside yourself and you would see that you're made of atoms. And in the atoms, you have a nucleus, and surrounding the nucleus is a cloud of electrons. And the nucleus is made of what we call protons and neutrons. And then if you could peer a thousand times deeper inside the nucleus, you would discover that the protons and neutrons were made of quarks.

And these are the limit of our resolution and that they're smallest particles that we know about. And that's why, along with the electrons, we call the quarks the most fundamental particles. Now, in ordinary matter, you'll find that there's two sorts of quarks. There's up and down quarks, and each of them have a different charge.

The up quark has plus two-thirds the electron charge, and the down quark has minus one-third the electron charge. And they just combine together the two up quarks and a down quark to give us the proton and two down quarks and an up quark give us the neutron and that's what matter is. And that's tables and everything you see around and so and so forth. Can you give us some idea of the sizes involved here? Just start with the atom because after that it gets to be

issue your imagination as far as I'm concerned. Yeah I mean the atom is of the order of what we call one angstrom. So it's one over one and ten zeros meters. I'm sorry to reduce this as a nursery. Please forgive me. I read in the notes 1 million atoms would go across one strand of hair.

Yes, something like that. That's more to my kind of... I mean, I can't imagine that either, but it gets a bit nearer. So we're going to be talking about the most fundamental things in nature based on the most minute things that we know about. So this is a rather nice... Yes. Right, given the picture of matter,

which you've been very clear about, down to the building blocks, the quarks, which is a wonderful word, what's antimatter? Well, anti-matter, although it's beloved of science fiction, right, it's not extraordinary in any way. For every matter particle, it has a partner which we call an antimatter particle. And in most respects, they're identical. If you could see it, it would look the same as matter. But you can't see it.

It would look the same. It has the same mass. It has the same size. It has the same amount of electric charge. but in one respect they are very different and that is for example the electron with minus electric charge the antimatter equivalent of the electron which we call the positive electron or positron

has plus electric charge so that's the main difference between them there's also another subtle symmetry between them and that is as an electron travels through matter it likes to spin and it spins with a left-handed corkscrew. Whereas the positron, when it travels through matter, it spins with a right-handed corkscrew.

It likes to travel right-handed. And there's a, it seems to be a perfect symmetry between the two. From my reading, antimatter is something that you find the most mysterious and that's what we're going to get towards in this programme, most mysterious entity or non-entity.

Well, it's mysterious in the sense that we don't know where it is, but it's not mysterious in the sense that we can, and there's lots of antimatter being made all the time, for example, when high-energy cosmic rays travel through the atmosphere produces lots of antimatter, we can make antimatter in particle accelerators.

But the mystery is, is why there is more matter than antimatter. Frank Close, I met an introduction that antimatter was first conceived by Paul Dirac in what's now known as a Derrick equation. Can you explain in a little more detail what that equation is? Well, it was back in 1928, Dyrrach was a mathematician working at Cambridge, and quantum mechanics had just been created. And Dierak came up with the idea of trying to combine quantum mechanics with the other great pillar of 20th century physics, which was Einstein's theory of relativity, and to apply it to the simplest thing then known, the electron. And the surprise was that he found that he couldn't do it, at least not just by writing a single equation. He set out to write a single equation to describe the energy of the electron.

And the equation insisted upon splitting into four parts. In mathematical jargon, he had to use matrices. But for our purposes, there were four equations, whereas he only wanted one. And they all had to mean something. And the question was what? Now, he quickly realized what two of them meant, as Val had said. The electron has a sort of corkscrew. Think of it as a spinning top going clockwise or anticlockwise.

And that spin had been recognized must exist because people knew from the way that atoms behaved that when electrons were in magnetic fields, they would spin one way or the other. For the first time, DERAC's equation was saying, aha, that is why there's a doubling up these two spin possibilities. But what about the other two? And that was the great puzzle,

because as he looked at the equations, they seemed to be saying the electron can exist with negative energy. At this point, I imagine Derax's thoughts were probably like the listeners. What's going on here? I mean, negative with respect to what? What on earth can this mean? And clearly it's a nonsense phrase that way. And then he had what to me was the great insight,

which was there was another way of interpreting this doubling up. And it was that the negatively charged electron with negative energy that it seemed to be saying the equations wanted could also be read as saying a positively charged electron with positive energy. So now at least one had got something sensible, positive energy appeared, which makes sense. But a positively charged electron,

there was no such thing. Nobody knew of any such thing. The only positively charged particle that was then known was the proton, and there's been a lot of debate about whether DERAC actually thought that he had explained the proton, the fact that it's 2,000 times heavier.

He thought might just be an incidental problem to be solved later. But within four years, the positron was discovered in cosmic rays by Carl, Anderson in the States. The way he detected these things, just like aircraft flying across the sky,

leave a vapour trail behind them. So charged particles passing through what was called a cloud chamber would leave a little trail of drops where they'd gone through. And with a magnetic field around the chamber, you could make the trails bend left or right depending upon whether their charge was negative or positive.

So Anderson discovered a trail in his cloud chamber with all the characteristics. This is in America, in America. Yes, in America. with all the characteristics of an electron, except it bent the wrong way, a positively charged electron. So that's four years after Dierak had written his equations,

and to me it's remarkable. I find it in a strange way quite uncomfortable that Dierak is writing, scribbling things on a piece of paper, and the equations say, you can't just have an electron, or you've got to have a positive version as well. And the equations know about nature, and then we go out and do an experiment and discover that's how it is.

It's a very profound, in some ways disturbing thing. Ruth Gregg, what did it signify this theory of Dyrgy? How did it take the argument forward? Dirac, if you like, was one of the pioneers of taking quite a sophisticated mathematical equation and yet extracting physics from it. And it's begun a dialogue between mathematicians and physicists that continues to this day, where people trying to find out the very fundamental theories of nature,

you know, do turn to mathematics to try and get tools to help them. And also, actually, it's not just a one-way street. People working on fundamental theories of physics have come up with mathematical advances, which then the mathematicians have gone off and taken and worked out and found to have some very interesting and deep mathematical worth as well. So I think this was, you know, if you like, at the start of a beautiful friendship, so to speak, between mathematicians and physicists,

which has led to a lot of interesting new ideas. What antimatter is, how it was arrived at, I think it's been very well described. One of the big points for myself, what happens when matter and antimatter meet? Now, can you describe that graphically? Graphically. Well, I think you've now got to the reason

why antimatter is so beloved of science fiction writers and of course the reason that it's used for the propulsion system of the enterprise. So what we've found out is that matter and antimatter appear the same from afar, but once you get in closer, you see that what matter does, antimatter does the opposite. Electron spins left, positron spins right, electrons negative, positrons positive. It's as if the two are mirror images of one another, but the only thing they have in common is they both have a positive. mass. So what happens when an electron

meet is they completely annihilate. So they destroy each other in a burst of energy of radiation. And so when matter and antimatter meet the simplest way of putting it is that they totally wipe each other out. So why is this spectacular? Why does science fiction writers like this idea? Well if you think now of a nuclear bomb, say the original Trinity test, In that explosion, only about, say, a gram of matter there is annihilated.

So one gram of matter, or annihilation of matter in a nuclear test is what gives you that huge mushroom cloud. So imagine what a kilogram of antimatter meeting with a kilogram of matter would do. It would be like having a thousand nuclear bombs going off all at once. So that's pretty dramatic, if you can do it. The good news and the bad news The good news is Do not worry about the idea of an antimatter bomb The good news and the great paradox

Which is confronting us all is Antimatter doesn't exist out there You just don't dig it up You can't dig up antimatter and make your antimatter bomb You have to make anti-atoms one at a time And the fastest we can do that today Even if you could do it a thousand times faster To make even a gram

Would take you as long as the universe So that's the good news. Don't worry about antimatter bombs. Of course, the bad news is don't think very much about antimatter being a source of power. It costs you more to make it than you get back at the end.

But let's come to this question now, Franklis. Why there isn't more antimatter in the universe? There's plenty of matter, but very little antimatter. This is an asymmetry. Can you develop that? Well, the experiments that we've done at CERN by using positrons and annihilating them with electrons

at very high... Can you just tell... Do you remind a bit of what CERN is? CERN is the particle physics laboratory in Geneva. And for many years at the end of the 20th century, they were able to accelerate positrons and electrons around a 27-kilometer ring to almost the speed of light, smashing them into which you had the head on. And from the annihilation energy, for a brief moment in a very small region of space,

you were recreating the sort of conditions that the universe itself had about a billionth of a second after the original Big Bang. and so at the individual particle level we can see what was going on then and what we find is that particles of matter and antimatter electrons and positrons quarks and anti-quarks protons and antiprotons emerge from this in perfect balance which is what our understanding of the symmetries between matter and antimatter that were buried in Dirac's equation

and every experiment done since pretty well says should be the case which is fine up to a point. We understand in pretty good ways how the particles of matter ended up making atoms and stars and galaxies and you and me and pretty well everything we see out there today.

But the antimatter, we don't find any at all pretty well in bulk. Mark Gibson, can I come back to you? What evidence is for thinking that there was an equal amount of matter and antimatter what is a billionth of a second after the Big Bang? Well, if you take yourself back towards a big bank, you may ask yourself what was there, right? And what the common belief from the cosmology side of things is that it was just energy there.

And in the same way that Ruth says when you've got matter and antimatter, you bring them together, produce energy, then you can go in the reverse process and you have energy to start with, and that will give you matter and antimatter in equal amounts. All right. So at the beginning of the universe, all that energy turns into matter and antimatter. And at that point, then the matter and antimatter can annihilate again and become energy and so on. But we found that there must be some flaw, some flaw in the laws of nature. To jump in for a second here, we've been told categorically that they annihilated each other.

So why didn't they annihilate each other before they got started? Good question. Good question. coming from the water. If the temperature is high enough, so if we're in the very early universe when conditions are very different from today, in some sense, we can't just say, oh, it'll all annihilate, we'll all go back to having energy. There's a lot of jostling, a lot of going one way and going the other way. And so it's where that takes you to. That's a real question. There's an analogy that you can do at the beginning of the universe. We've got all

this matter and antimatter around. And you can consider them, if you like, as two armies, right, that meet. And if you had an army, say, in the Napoleonic times, where you've got the ranks and you've got muskets and they are firing at each other, then, and the ranks, as it works, you fire, you kneel down, you reload and so on. So you get waves which go through the matter and antimatter armies. And you just find that the matter is just slightly quicker, right? It's just slightly quicker than the antimatter army. And the net effect of that is, you get, and the matter effect of that is, is it soon annihilates all the antimatter and you're just left with matter. And that happens in the minutest fraction of a second, which actually would give you the imbalance

of matter and antimatter at the beginning of the universe. The whole universe, as we perceive it now, is but one billionth part of an even grander creation. We're the little bit left over after that huge annihilation took place. So you're saying that antimatter might be somewhere else? That is one possibility. We suspect that there is something subtly different between matter and antimatter, and exactly what it is is what we're trying to find out. I mean, we're talking here about the early universe,

and the biggest observation we can have is that we are made of matter and not antimatter. However, we have now, we can recreate the conditions of the early universe in the particle physics experiments that we've done, and we can recreate this matter-antimetry in the experiments themselves. And in fact, one of the greatest breakthroughs,

I think, of recent times was the discovery of the matter-antimetry, matter asymmetry in the 1960s. And that showed us that we could explain this flaw in the symmetries of nature. Unfortunately, the floor's not big enough to explain the universe completely made of matter. So there's something missing. Is it taking this on, Ruth Gregory, to talk about the process of bariogenesis? Can you tell us what a barion is first and then bariogenesis and why this fits into the process

we're going through now? Well, I mean, I suppose just put it simplistically. Barions are the stuff that are nuclei made of. And so in a sense, you could think of, when cosmologists use barons, we're often using it as a shortcut to saying matter in the context of this discussion. And so when we talk about bariogenesis,

then we are talking about the generation of matter. It's perhaps worth pointing out that we have, we have so many interesting questions, cosmology, but one thing we should remember is the matter that we are made of is actually a small component of the universe that we live in. There's an awful lot more out there. Mark Gibson, just to say where you are in finding out about this vital asymmetry. The first discovery of this asymmetry in the laboratory was in 1964, right? And that was with something called the K-on, with this very strange quark. Now, we now know from the experiments that we did at the

end of the last century, that we have what we call three families, three generations of quarks. We have the up and down quarks that all matter is made of. We have the second generation which includes this strange quark and we have a third generation which has what we call the top and the bottom quark. There is no other quarks around. So we've seen the matter-antimetry with the second family of quarks and we now know that we would expect a bigger effect in the third generation of quarks. And we indeed have done experiments at the end of the last century, which have shown that we get bigger matter-antimiter asymmetry. It's a factor of 10 bigger than the second generation using this bottom quark. So that has now been discovered. So we've seen it two places

in the laboratory. Unfortunately, both of those places, the amount of matter-antimatter asymmetry that we observe is still not enough to describe the overall matter-antimatter asymmetry in the universe. So we need some new phenomena which is going to produce a bigger effect. So you need more evidence? It's got to be some new physics out there. It may have a label of something like supersymmetry, extra dimensions, multi-higs models, you name it, right? There's lots of theories. This is a terrible anarchic thought, but it kind of reminds me of version Russell saying if you woke up dead and met God, what would he say? And he would say, sir, yes, I'd call him, sir, you should have given me more evidence. That's the solution we're looking for, right?

Mr Gregory, can we come back up, if not back, in a slightly different direction, can you give us any sense of the challenge involved in trying to understand the behaviour of matter at high temperatures? Val has said that in the colliders, the conditions of the early universe are recreated. In the certain colliders, yes, that's true, but it's in a highly specialised and very small sense.

I think when we study high energy physics, we often tend to take for granted these, you know, at the very beginning you said, all the zeros. And I think I'm not sure whether we truly understand it or whether we just become used to it. But in the very early universe, if you wind the clock back, our universe we know is expanding now. So you wind the clock back, it contracts.

So the very early universe was a very compact, small, dense, an extremely hot place. So as you squash things together, they tend to heat up. And one of the things that we know about matter in general, and certainly about the theories of physics, is that as you heat things up, they tend to change in nature. So we have ice, we warm it up and get water,

warm that up, it boils, and we have a gas, steam. If you had someone coming from outside with no means of measuring the temperature, they may think those were three different states, you know, three different things, three different compounds. But in the early universe, the same sort of thing happens.

As you heat it up, our whole idea of physics changes. So the matter that we see and are made up of now is not necessarily the same, we wouldn't see the same things as we go up to higher temperatures. And I think the real difference between the early universe and the colliders where we try and recreate those conditions is the fact that it's not just at some single point

under the tunnel under Geneva, but it's everywhere. And this idea of having something where, everywhere is at that temperature is at least one possibility for accessing new phenomena. We believe, because we've worked very hard at trying to think about how the physics we know would behave, we believe that there isn't a way of creating this asymmetry of matter with the physics that we know. But of course, you know, there's always the possibility of someone coming in with a brand new idea. Can we come back to anemite in this way, I'll let you said, I think, at the beginning,

that you can create antimatter and it is in use now. Can you just tell us what those ways are? Well, there are useful, if you like, ways that antimatter can be used. And I'll just give you one as a spin-off of all these studies, really, is that if we go back to our beloved positron, that's being used in hospitals all over the country, in pet scanners, positron emission tomography scanners. And that is a pure use of antimatter.

What does it do? Well, a pet scanner, you have to inject the pet scanners. patient with a small radioisotope that emits positrons. And you can detect the positrons, for example, in the brain. When they collide with the matter in the brain, you get two photons, some light coming out, and it comes out back to back 180 degrees apart, and you can just... But isn't annihilation going on now, then? Yes, that's right, yes.

So it's not harmful. It's not harmful. You put things in your brain and annihilating things and this is not harmful. It's not harmful. Look, you've got a million, million, million, million. of these things in there. You don't mind if you lose a few and indeed it is... It depends who you are really.

We'll let that pass. That is one of the... I would say one of the best uses of antimatter but we just do it for the pure research of trying to understand what nature is and in order to do that we can make

anti-atoms at CERN. We can't contain them very well right. We don't have huge bottles sitting in a laboratory somewhere, we have a few thousand at maximum Well, if they went in bottles, they'd antimatter, they'd bomb the bottles, isn't that?

Absolutely, yes. You've got to try and keep it away from matter. So that's a very, very difficult. So what you can find it in? In what we call a magnetic bottle. What's a magnetic bottle? So there's antimatter around the place now, but it's in magnetic

bottles. If we let it out, it would... The problem is it always collides with the matter, and then you lose it. I think you mentioned the phrase, pure research, though. You are talking about going into this for the sake of going into it, in a sort of way, hoping that the law of unexpected consequences kicks in. Yeah, the thing that drives me and I hope my colleagues, is we do it really just to try and understand the laws of nature.

And so the biggest project is that's going to happen to start next year, and that's the Large Hadron Collider. Physicists, thousands of physicists across the world. This is kilometres, kilometres long. Yeah, thousands of physicists, engineers, students, all over the world have been working on this for 15 years, and it's now going to come to fruition next year. We had a 27-kilometer tunnel,

which we collided electrons and positrons in at the end of the last century. We've now replaced all the magnets and things in there to enable us to collide protons on protons. And that means we can go to the highest energy ever achievable in order to understand, to look for the new phenomenon. out there. Now when I say the highest energy, it's not actually that high. I mean, the energies we're talking about, if you had one proton of energy, it's just like having a little elastic band and a piece of paper, you just flick it across the room. It does nothing, right? It just

falls to the floor. But when you put 100,000 million protons in bunches and you accelerate them to the speed of light, and you collide them together, and it's in such a small area, right, such a 110th of a millimeter or something area, then you've got such a concentrated energy, then all the energy you can turn through equals MC squared into mass. Experiments at CERN will be looking next year for new forms of mass, new forms of matter, which will hopefully give us some indication of the new phenomena around. Is this a sense in which evidence is overtaking theory because DRAC had a theory

he didn't have a collider and later this was proved to be true so is the sense in which you're saying that we've come to the end of theory we've got to have some more evidence before we can construct any more theories? If you go through history you find that theory sometimes leads the way as in the case of DRAC

writing equations which predicted things which were then found and other occasions you have experiment leads the way and the best example perhaps of that was the origins of quantum mechanics. It was the fact that experimental phenomena seen in the end of the Victorian era led to the invention of quantum mechanics. So sometimes it's an experimental discovery that demands an explanation that points us the way to go. And that is in part where the LHC, I think, comes in on both sides.

On the one hand, there are fundamental ideas in theory, which say that there are phenomena which we expect to manifest themselves under the sort of conditions that the LHC will access, for example, discovering the Higgs boson of which you talked about many. The Large Hedron Collider should be able to discover the Higgs boson. But the thing that I am sure of is, you know, only nature knows what's really going on. The answer is out there somewhere and the LHC is the only way we know that we can go and answer such things within the technology that we currently have. I just want to say, I hope I haven't given the impression that experiment is leading the way. I think the problem is theorists are perhaps getting too clever.

We have several very good ideas about what may lie beyond. The problem is that we as theorists have no way of discriminating between them at the current time. And this is what we're waiting for. Well, from three theorists, you've enlightened me massively, and I hope that I can remember it for long enough. Thank you very much, Ruth Gregory, Val Gibson. and Frank Close, and thank you for listening. Next week I'll be looking at the divine right of kings

in the 16th and 17th centuries. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.uk forward slash radio 4.