In Our Time - Nuclear Physics

Episode Date: January 10, 2002

Melvyn Bragg examines one of the greatest scientific breakthroughs of the 20th century, and certainly the most controversial; the development of nuclear physics. Harnessing the enigmatic qualities of the atom’s tiny core brought us nuclear power and gave us The Bomb, a breakthrough with such far-reaching consequences that it moved the physicist Albert Einstein to say, “Had I known, I should have become a watch maker”.How can such outlandish power be released from such infinitesimal amounts of matter and what does the science of the nucleus tell us about how our universe is built? Nuclear technology provokes strong emotional and political reactions, but what are the plain facts behind its development as a science? With Jim Al-Khalili, Senior Lecturer in Physics at the University of Surrey; Christine Sutton, Particle Physicist and Lecturer in Physics at St Catherine’s College Oxford; John Gribbin, Visiting Fellow in Astronomy at the University of Sussex.

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 4. I hope you enjoy the programme. Hello, one of the greatest scientific breakthroughs of the 20th century and certainly the most controversial was the development of nuclear physics. Harnessing the enigmatic qualities of the atom's tiny core brought us nuclear power

and gave us the bomb, a breakthrough with such far-reaching consequences that it moved the physicist Albert Einstein to say, had I known, I should have become a watchmaker. How can such outlandish power be released from such infinitesimal amounts of matter? And what does the science of the nucleus tell us about how our universe is built?

Nuclear technology provokes strong, emotional and political reactions, but what are the plain facts behind its development as a science? With me to discuss the development of nuclear physics is Jim Alcalili, Senior Lecturer in Physics at the University of Surrey, and author of Nucleus, A Trip into the Heart of Matter. Christine Sutton, particle physicist and lecturer in physics at St. Catherine's College Oxford, author of Spaceship Neutrino, and John Gribbin, visiting fellow in astronomy at the University of Sussex,

and author of Q's for Quantum and In Search of Schrodinger's Cat. John Grubbin, can we begin with a brief outline of how an atom is arranged? What do we know about its constituent parts? We know that it's mostly empty space. I think that's the most staggering thing about it. There's a very, very tiny nucleus that contains virtually all of its mass, and that's surrounded by a cloud of electrons, which have very little mass indeed, with a lot of space between them.

So something that seems really solid and physically sound like the table that we're sitting at is mostly empty space, tiny particles held together by electric forces. The Greek philosophers, like Democritus, that Epicure has talked about Atta, but they had an idea of atoms being rather like ping-pong balls. Yes, they did. Now, that's much more like a solid object.

They would have imagined an atom as something solid that was indestructible, couldn't be subdivided. And the great difference that the 20th century physicists brought out from the study of atoms and the study of the nucleus is this idea that it's divisible, that there are pieces within atoms that can be chipped off and broken away, and also this idea that it's far from, being solid, that it is an empty space containing forces.

Can it, would you say that the modern interest in our knowledge of nuclear physics could reasonably be said to start with the British physicist JJ Thompson in the late 90th century in the 1890s? Well, sitting here in London, we can reasonably say that. I think there are people on the continent who would point the finger at Philip Lerard, who did similar work at the same time. But yes, during the 1890s, it was. realised that pieces could be chipped off atoms and those pieces were electrons and that's what

Thompson is credited with discovering is the electron and the astonishing discovery then when he made the announcement to to the scientific community that it was possible to divide the atom people didn't believe him people who believed in atoms believed that they were indestructible according to the Greek model and it's very hard for us to appreciate now what a dramatic breakthrough it was to find that you could knock bits off atoms so from being as it were a Greek ping-pong the atom begat a different image, didn't it, through Thompson? Yes, I mean, the image... A very Victorian image, is it.

It changed over the years. I mean, at first people thought that it was still a kind of a solid object, you know, with the electrons embedded in it, and an analogy that was sometimes made is like with a plum pudding or with a raisin pudding, and that these little pieces could get knocked out. And then over a period of 20 years or so, it became appreciated that, as we said earlier, that there is this tiny central nucleus and the electrons outside.

But let's say with the plum pudding, which is very sort of hobbitish and Tolkienish and so on, very Victorian that they should say it's like a plum pudding. So that's what Thompson arrived at with electrons. Yes. There was this spongy material and with the raisins. Little bits inside. Christine Sutton, the plum pudding model of the item

was turned round by Ernest Rutherford's experiments at the end of the first decade of the 20th century. Can you tell us how he approached it and how he, as it were, developed at? It was something that was discovered partly through the available technology and just through pursuing curiosity. I mean, Rutherford was at Manchester. He had been earlier at Cambridge when radioactivity was first discovered. And radioactivity is a natural phenomenon that emits various kinds of, as we know now, bits of atom.

But, of course, if you didn't know that you could make bits of atom, nobody knew that at the end of the 19th century. That's what was going on with radioactivity. And there were a particular kind of a bit of the atom that's called an alpha particle. And Rutherford really made these alpha particles his own in the sense, that as soon as he'd discovered them, he started using them as tools. So he did the exciting thing of turning the alpha particles back on the atoms. What's an alpha particle? Well, we now know an alpha particle is the nucleus, the core of an atom of helium,

the light was next to the lightest element of matter. hydrogen is the lightest element, helium's the next lightest. And this core, this nucleus happens to be a very, very stable form of nucleus that holds together and can be thrown out of heavier materials like uranium or thorium or these exotic things that people started to use it. How did he discover that it could be useful? I think because he realised that he discovered that these things could travel through a certain amount of matter

that they had quite a lot of energy and so it became interesting to see what happened when they went through matter and it was in trying to look at what happened when they went through matter that he made his dramatic discoveries in Manchester But how do you see all this in 1911? I mean you've got the...

How do you say, so you tell me how do you track an alpha particle? He knows about alpha particles presumably he has to kind of make it up because he can't see them but what happens next? Well, what happens is when an alpha particle hits a material, a particular kind of material, the material will emit a little flash of light.

And what these guys had to do was to sit in a pitch black room with their source of alpha particles pointing at something, and then they were looking at what happened, say, when it went through the thing, like very thin gold foil, or when it was bounced back, say, so, you know, like hitting a tennis ball at a wall and seeing it come back. And when it comes back and hits your material, which is called scintillator, which emits the little flashes of light, you see the flash of light. So you count it. And they were literally counting the flashes of light on their detector.

I'm sorry, I'm not quite clear about this. So how do they know, they're sitting in this dark and wrong Rutherford and his colleagues. How do they know this stuff's coming off as alpha particle? What's it coming off? It's coming off some kind of radium. But they know it's coming up. And they've got this gold forth. Now, what does I understand it took them by surprise that some of the particles

went right through. Some of them were deflected slightly, but some most significantly, for this brief history, bounced straight back. Now what did that signify, and why was that a surprise? Well, it was a surprise because the foil

they knew that most of the particles would go through it, and that's what they expected because the foil was very, very, very thin. I don't know if you've ever, most of us haven't actually touched gold foil. It's a very sort of funny, sticky, weird stuff, which is very, very thin.

You know, you're talking about something that's, you know, not many atoms thick, that the alpha particles are going through. Most of them would go through. Some were slightly deflected. Now, he knew that these alpha particles were really quite energetic. And, you know, like you can imagine, firing tennis balls from Pete Sampras at tissue paper hanging up.

You'd expect these things just to go whacking through. And the fact that he knew that they were, were deflected backwards, meant that there had to be some powerful forces at work inside the atom, much more powerful than he'd imagined. And to be fair, it took him a year or so to go away and think about what is actually happening here. And the reason why this led to the idea of a different concept of the atom was in trying to understand how these forces were at work inside the atom. only force people knew about was the electric force that John mentioned earlier. So they had to

understand what was going on inside the atom in terms of the electric force there. Jim Alcalili, can you take up this history now? So what does that signify? We're talking about 1912 and where do we go from there? Can you summarize that stage? Tell us what that signifies and then move us on from that. Well, until then, as John mentioned, there was this Thompson plum pudding model of the atom where it was assumed the electrons were distributed. throughout the whole space of the atom. Like the raisins and plums, sorry. But the alpha particles that were hitting the atoms

were thousands of times more massive than the electrons. So Rutherford knew these alpha particles weren't going to bounce back after hitting the electrons. The only possible way any of the alpha particles could bounce back was if most of the mass of the atom was concentrated in a very, very tiny volume. So he suggested that maybe the electrons, which you can completely ignore in terms of the alpha particle experiment

were floating around the outside. But most of the positive charge, which he knew, had to balance the negative charge of the electrons to make an atom electrically neutral. All that positive charge had to be concentrated in a very tiny volume, and that then became the atomic nucleus. So he discovered the atomic nucleus. Yes, so his model was then this solar system model of the atom,

where the nucleus was like the sun and the electrons were like the orbiting planets. So what did that signify? Where did that take you, physicists, in terms of what this opened up as a subject in terms of John Grimmons' idea of pure knowledge, but also in terms of what it might lead to? Well, we have to remember at the same time, another theory was being developed called quantum theory, which suggested that down at this very tiny length scale, things behaved very, very differently. And, of course, when people thought about Rutherford's model of the atom, the big problem, the big problem. problem was how did these negative electrons orbit around the nucleus without being sucked in due to the attractive positive force? Yes, the nucleus is positive and they're negative.

They're supposed to attract and therefore it should collapse in. They should spiral in very much. They remember that much of form physics. Yes. They didn't. So why didn't they? Well, at this point, the Danish physicist, Niels Bohr came in. He'd gone to work with Rutherford.

He was interested in Rutherford's model of the atom. and he applied the then very new rules of quantum theory to show how an atom could remain stable. So he suggested that these electrons followed fixed orbits around the atom. There were certain quantum rules that stopped them from jumping, from spiraling to the atom. They would only change from one orbit to the other according to certain very strict quantum rules. And so atom, he gave atom stability.

What I'm really fascinated by it comes back, John Gribbens remark at the very top of the program about the development of this subject being associated with the development of technology. How did they find out about this? How did they track it? How did they see? What instruments are they using for measurement? Well, they...

I mean, we're talking about, you know, the beginning of the 20th century. First two or three decades. It was very, very crude experiments. As Christine mentioned, they had to sit in dark and rooms for hours upon hours, getting their eyes adjusted to the dark just to see the tiny flashes of light on scintillation screens. They developed special counters, Geiger counters,

which would give off a click every time a subatomic particle, like an alpha particle, entered in them. But a lot of these ideas were theoretical ideas that they knew an atom had to be stable. Atoms exist. And if atoms had this structure of electrons orbiting around the nucleus, there had to be certain rules that allowed them to be stable. I mean, we're talking about imagination in a way, aren't we?

Absolutely, yes. Imagination, illustrating your imagination. Yes. And all the models, as John said in one of his articles, about these models and ideas and analogies we have, which are very useful but never can be accurate. Well, of course, the other thing is that because the atom and its nucleus are subject to the rules of quantum mechanics,

these are very, very strange concepts, and we can't picture. We learn at school that the atom is this nucleus with electrons buzzing around the outside, but of course that's not an accurate picture of what an atom is really like. We can't imagine what an atom is really like because it's something so far from our... everyday experience. And so we can really only describe it using mathematics. All physicists have pictures of what things look like.

Can I move on to talk about, or can you move on, to talk about the power of fission here. Jim Alcalidi, iron has the most stable nucleus of all, I've read, of all the elements, with a total of 56 protons and neutrons. That seems to be very significant, iron 56. Can you tell us why that's significant in this story of nucleoph? efficient. Well, protons and neutrons are the two types of particles that make up a nucleus. And protons are positively charged. Neutrons are neutral. So they don't attract each other via electrical forces. They attract each other via this strong nuclear force, a force that only acts

within the confines of the nucleus. And so a strong enough nuclear force will hold the protons and neutrons together. But if you get a nucleus that's too big, then protons on either side of the nucleus will only feel their mutual repulsive electrical force because the strong nuclear force has the property that it only acts over a very short range, not even across the whole span of a large nucleus.

So strong nuclear force is really, this nuclear strong force is a law to itself so far, isn't it? It's one of the four fundamental forces. But where does iron come in? I am supposed to come in significantly. Well, iron happens to have a certain structure. Protons and neutrons hold together according to certain rules. People developed models of the nucleus.

For instance, one of the early models was the liquid drop model where a nucleus was likened to a drop of water, the way it wobbles about. Later, they developed the idea that protons and neutrons follow the same rules that electrons follow in the way they arrange themselves in orbits and shells around in the atom. protons and neutrons also seem to fit together in shells within the nucleus. And so you have to understand how these shells get filled up

according to certain quantum rules to explain why it is that iron with a certain number of protons and neutrons happens to have just the right arrangement to make the most of the strong nuclear force that holds them together. But something has to, Jim. I mean, it doesn't really matter that it's iron or what the rules are. You've got a trade-off between one force that's trying to hold things together and the electric force that's trying to blow things apart.

And if you put enough particles together, if it's big, the electric force is going to win. If it's small, the strong force is going to win. And iron is just in the middle. So something has to be in the middle, and it happens to be iron. But it wins better, iron wins better than a small nucleus like carbon, say, because in carbon, a lot of the protons are near the surface of the nucleus, so they don't have other things trying to hold them in. So as John said, it is a trade-off, but you can imagine it almost like a valley

where you have a sort of stable bottom with a deepest point in that valley, which is where iron is, the most stable thing. And all the other nuclei that we know about are like on the hills of the valley, and they're all wanting to sort of tumble down into that most stable point. John Greby, nuclear fission was discovered in the 1930s. How did scientists manage to get nuclei to split? And what happened when they did? It's really, it's a development of the original thing that Rutherford and his colleagues were doing firing alpha particles at atoms and gold foil to see what happened.

And as the technology improved and it became possible to accelerate these particles more effectively, bombard other nuclei more effectively, and you had better detectors to measure what was going on, people did more and more experiments until they reached the point where they were firing particles at nuclei, at targets, and what was coming out wasn't those same particles being bounced off or reflected. They were getting debris from having smashed apart the nuclei that the particles were colliding with. It's technology again, more energy going in, so you're able to do more interesting things. So what did that result in? What happened in the 1930s? They could split the nuclei, so that meant what?

Well, it meant that the nucleus itself is indivisible. Now, we've gone from the stage at the end of the 19th century. It's divisible. Sorry, that at the end of the 19th century, this sort of shattering revelation that the atom is divisible, and then you have the idea that there's a solid nucleus inside the atom, and at first that was then thought of as being a fundamental entity that couldn't be divided.

Now you've reached the stage where you're dividing even the nucleus up into pieces. Now that tells you, in terms of pure knowledge, you can study the pieces and find out how they join together and then eventually learn how to make use of the energy that's released. But again, it's a great conceptual advice, You know, where does this process stop? It does matter carry on being divided forever and ever. So people were absolutely fascinated by the discovery.

Christine Sutton, why is uranium always using nuclear fission? Well, the big thing with fission is that we talk about actually breaking a big nucleus up into two smaller lumps, just two lumps. And that goes back to my sort of valley analogy, that uranium is at one end of this valley sort of near the beginning of the valley, where the walls are steep, and that's the heavy uranium nucleus. When you break it up into two fragments,

they're lower down in the bottom of the valley. By releasing, going down to the bottom of the valley, you've released a lot of energy. That's one reason why you use uranium. The other reason why you use uranium is that, in fact, to trigger fission, you don't use alpha particles, as John mentioned. You use these neutrons, the things that have no electric charge,

and are actually rather tricky to handle. But the neutrons, because they have no electric charge, they can sort of infiltrate into a nucleus. There's nothing to sort of repel them and say, go away. You know, you can't come in here. The neutrons just sort of creep in like invisible secret agents or something. And the addition of one neutron upsets that delicate balance between these strong forces and these electric forces that we've talked about.

The whole thing about nuclear physics is this competition that's going on between the electric forces that we're familiar with or more familiar with and the strong forces that even the physicists feel a bit sort of uncertain about. In uranium, once this little neutron comes in, it can easily trigger the fission reaction and release the energy that we've talked about.

The power of nuclear fission comes from a chain reaction, as I understand it, which releases energy trapped in the nuclear. How is that nuclei? How is that chain reaction set off? Well, what happens when a uranium nucleus breaks in two by absorbing a neutron is that it also releases several neutrons from within it that existed within it in the first place. Those neutrons then fly off, and if one or more of those is captured by surrounding uranium nucleus, they might prompt that to undergo fission as well.

And so all the time a neutron causing one uranium nucleus to fission, allows another neutron coming out to cause another uranium to fission, you'll get a sustained chain reaction. If more than one of the neutrons within the uranium nucleus causes fission, then you get a runaway chain reaction where more and more explanation more and more uranium nuclei will fission. And that's, of course, what you get in a bomb. Why does this, Christine, son, where does this trapped energy come from?

That's the hard question I was hoping nobody was going to ask. No, I mean, I think the simplest way, thinking about this is that if you broke a uranium nucleus up into all its 296, 8, 200 whatever neutrons and protons and particles, individual pieces

and you weighed each of the protons and neutrons you would get a total mass for all the building bricks. But when you weigh find out the mass of a uranium nucleus that mass is different from all those bits added together. and we know from Einstein's theory

that mass and energy have an equivalence E equals MC squared, the only equation in physics that most of us have heard anything about. So E energy, MC squared the mass. When the uranium breaks up, its mass is changing and that change in mass comes out as the energy.

It's energy locked up holding the uranium together, But because the uranium is unstable, it very much would prefer to break up into two more stable pieces. Going back to the valley analogy, you know, when you know about hydroelectric power, if water runs down a valley, you get energy coming out because the water's changed its position in the valley. Uranium is higher up. Uranium has changed its position and you're releasing energy. But it is related to the mass difference as well. In fusion, the nucleus of one atom is fused the nucleus of another, as I understand.

and energy is released. Under what circumstances can nuclei be forced to fuse, be made to fuse? Under what circumstances can they be made to fuse? Well, in that case, what we're trying to do is to bring, remember, our nuclei have positive charges on them. They have to overcome that natural repulsion there is, likes repel and likes attract. So you can either have very sort of energetic nuclei

that come together with quite a lot of energy, so the energy that they have overcome, the natural repulsion. As long as they can get close together, this strong force that Jim introduced, and he mentioned that the strong force is very short range. If you can get the nuclei close enough together so that that short range force locks in.

You know, it's a bit like Velcro. Imagine, you know, most of the time when your garments with Velcro and, like, your waterproofs, the Velcro is far apart. There's no problem, but you just get it to touch slightly and it snaps in. Well, the strong force is a bit like that. And if you can get that to happen with these nuclei that are lightweight,

because now we're talking about going the other side of iron, we're talking about lightweight nuclei that want to get to this bottom of the valley. And the way they do that is by joining together. And again, it has to do this competition of forces. How do they do that, Jim? Well, it happens around us. This is the reason why the sun shines. what's going on inside the sun is a thermonuclear reaction.

So you have light element hydrogen or protons, which are the nuclei of hydrogen, fusing together to make heavier nuclei. First of all, they make heavy hydrogen, deuterium, and then deuterium fuse together to make helium and so on. This is how the very light elements are produced, the very light elements are produced inside stars. What we'd want to do is try and mimic that reaction in the laboratory on Earth. But what we don't have are the conditions in the centre of the sun,

100 million degrees centigrade. We'd like to have those conditions in the laboratory because that's the temperature, the energy that we need, to push these things closely enough together in the first place. What are the benefits of nuclear fusion going to be? I mean, at the moment what you're saying is that we can't do it well enough. We're working on it. But then people working on nuclear fusion have been saying that for a very long time.

But if and when they succeed, it has several benefits over nuclear fission, which is the way we produce nuclear power at the moment. First of all, it produces more energy. Secondly, we don't have to rely on a limited supply of uranium. We can make use of water, hopefully, sea water, which is unlimited pretty much. And thirdly, it has the benefits, in fact that it doesn't have, it's a cleaner form of energy. It doesn't have the radioactive waste of the level of nuclear fission. But there are no nuclear fusion reactors at work.

in the world at the moment, are they? Not yet. People working in the field have said for many years now that nuclear fusion is about 40 years away. They keep saying that for at least 40 years. They've been saying 40 years. They say now it's 40 years away, but what's different now is that they feel they have a roadmap.

They know exactly what they should have achieved 10 years from now, 20 years from now. People have actually achieved fusion, but it takes more energy to make things fused together at the moment than you get out from the reaction, so it's not a very efficient So they had just about gone beyond the break even.

But do you think Einstein, we've only got a few minutes left, do you think Einstein is right, and he would have been better of being a watchmaker, and this whole adventure in nuclear physics hadn't started? Oh, absolutely not. I mean, it's two reasons. I mean, someone else would have done it. But I think the benefits do outweigh the costs. We've managed to avoid nuclear war,

and we've got a lot of benefit out of this stuff. Such as, can you instance of you? Well, I think nuclear power, though that's an emotive subject. I think in the long term, we will have clean fusion power, and it will be good for society, and the benefits from medicine. People very seldom appreciate how much benefit comes from radioactive materials in medicine. Well, dealing with tumours, cancer and so on, radiation therapy, things like the imaging that's done to look inside the body without invasive surgery.

All of that comes from nuclear physics. Magnetic resonance imaging, MRI scanners, used to be called nuclear magnetic resonance resonance, because it's a nuclear physics concept, but people drop the word nuclear because they saw it had these negative connotations. But of course, that's all nuclear physics. I think the other thing is that it has helped us to understand the universe, and we describe ourselves as carbon-based life forms. The carbon that's in all of us was created originally in the heart of a star.

And I think that's a very sort of powerful concept and something we became aware of in the 20th century with nuclear physics. So it's this way we can go back to the idea of origins, can we, John Grimmond? Absolutely. I mean, this is very much my own pet area of interest, the relationship between ourselves, humankind and the universe at large, and that we are products of the stars. We are absolutely literally stardust.

I can't think of anything to say after that. To tell us, I feel quite exhausted. I've been concentrated on that for all three quarters of an hour. Anyway, thank you all very much. Thanks, John Grimmin. Thanks, Jim Alcalilely. Thank you to Christine Sutton. Next week, we're taking on the medieval heresy, the Cathars.

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