In Our Time - The Neutrino

Episode Date: April 14, 2011

Melvyn Bragg and his guests discuss the neutrino.In 1930 the physicist Wolfgang Pauli proposed the existence of an as-yet undiscovered subatomic particle. He also bet his colleagues a case of champagne that it would never be detected. He lost his bet when in 1956 the particle, now known as the neutrino, was first observed in an American nuclear reactor. Neutrinos are some of the most mysterious particles in the Universe. The Sun produces trillions of them every second, and they constantly bombard the Earth and everything on it. Neutrinos can pass through solid rock, and even stars, at almost the speed of light without being impeded, and are almost impossible to detect. Today, experiments involving neutrinos are providing insights into the nature of matter, the contents of the Universe and the processes deep inside stars.With:Frank CloseProfessor of Physics at Exeter College at the University of OxfordSusan CartwrightSenior Lecturer in Particle Physics and Astrophysics at the University of SheffieldDavid WarkProfessor of Particle Physics at Imperial College, London, and the Rutherford Appleton Laboratory. Producer: Thomas Morris.

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 program. Hello, about 93 million miles above our heads is a star we call the sun. In the heart of this enormous ball of plasma, nuclear reactions are producing vast amounts of energy,

energy which reaches the earth in the form of heat and light. But in addition to warming our planet and bathing it in light, the sun is also bombarding us with a storm of objects we don't even notice. They're called neutrinos, and billions of them pass straight through our bodies every second. Neutrinos are some of the strangest and most mysterious objects in the universe. They're invisibly tiny, and can travel through solid rock, and even stars as easily as they do through space. Scientists first proved their existence half a century ago. Today, neutrinos are offering us new insights into the nature of the universe and of matter itself. With me to discuss the neutrino are Frank Close, Professor of Physics at Exeter

College at the University of Oxford, Susan Cartwright, Senior Lecturer in Particle Physics and Astrophysics at the University of Sheffield, and David Walk, Professor of Particle Physics at Imperial College London and the Rutherford-Appleton Laboratory. Frank Close, I've hinted at the sheer strangeness of these particles, you think they're the weirdest objects in the universe. Could you give us a bit more detail about them, what they are, how they behave, so on? Well, they're probably as near to nothing as anything that we know. They've got no electrical charge. Until very recently, we thought they got no mass at all. If they have got any mass, it's too small to measure. If you put 100,000 of them on one side of a set of scales, they wouldn't even outweigh a single electron,

which is the lightest of the particle that we know. So they're very weird. As you alluded, they've been said that they pass through the earth as easily as a bullet through a bank of fog. So the sun putting out all these neutrinos that are shining on us at the moment. Half a second later they've passed out the other side of the earth and are shining up through the beds of people in New Zealand. So if we could see with neutrino eyes, night would be as bright as day. They're very strange. They're passing through the universe like mere spectators.

In fact, there's probably more neutrinos in the universe than any other particle that we know. And yet, paradoxically, we know less about them than anything. else almost. When they pass through, do they do anything to us to the earth? Is that what you're trying to find out, I suppose? That's what we're trying to find out. I mean, 60 million of them are passing through our eyeballs every second without a seeing them. And they are totally harmless because they don't really interact with anything very much. The difficulty is trying to capture one or two. It's a bit like the National Lottery. You know, enough people enter, somebody is lucky. If there's enough neutrinos around and you've got a big enough net, you might occasionally

capture one and I'm sure we'll talk about how you do that later on. But they're very difficult to capture and hence is difficult to study and that's why we know so little about them. Can you just beggar our imaginations a little more by what these neutrinos do? They go through nine trillion lead. Just tell people what goes on with these things a little bit more. Well, neutrinos, they don't interact with things like normal things do. They don't feel electrical forces, they don't feel magnetic forces. Gravity is so feeble you can't really make much use of that. They interact only by a force called the weak force, which by its very name, weak means it's hardly likely to do very much. They will, as you said, travel through, I think,

a light year of lead without bumping into anything, 50-50 chance that an individual neutrino will bump into something in a light year of lead. That's about as good as it gets. But if you've got a nuclear reactor, for example, which is pouring out millions and millions and millions of these things every second, then it is possible to capture one or two, and as we will hear later. And this began, noticing them began in the 1930s with a phenomenon known as beta decay. Can you bring us up to speed on beta decay? Beta decay is one form of radioactivity where one atomic element changes into another, and in doing so releases energy in the form of a particle, the electron. and if that was the whole story that one element turns into another emitting an electron,

each and every time you do the experiment, the electron would appear exactly the same, the same speed, the same direction, the same energy. But that's not what happened. They found that sometimes the electron had quite a lot of energy, other times almost none at all. In fact, a whole spectrum of energies, from nothing up to a maximum. And this just didn't fit. And so what was the answer to this?

what was it proving? Well, maybe energy wasn't conserved in nuclear reactions. That was one suggestion that was made in those days. The other idea, which we now know was the correct one, is that there was an unseen member of the feast, the neutrino. Can we talk about that with David Walk? The Australian physicist Wolfgang Paoli proposed his solution to this problem. Can you develop that?

Well, yeah, Wolfgang Powley, who's actually Austrian. He's from... But he was at ETH in Zurich, when he proposed this. He was baffled by this fact that the electrons emitted in beta decay didn't always have the energy they should. And as Frank pointed out, there was some

suggestions around at the time that perhaps energy conservation, conservation of angular momentum, which was also seemed to be, you know, seemed to disappear in beta decays, were just statistical things. They didn't happen in every single decay that only on average were they conserved. And Pally was horrified by this because he thought that it did horrible things to the mathematics.

And so he came up with this idea that in addition to the things we see emitted, something else was emitted that carried away the excess energy. And at the time, that was quite a dramatic thing to propose because of only two known particles at the time, the proton and the electron. And he called this third particle the neutron. It was later dubbed the neutrino after Chadwick discovered the thing we now called the neutron. And this was a very dramatic proposal. At the time, so you would imagine that a dramatic proposal like that would be made. Why did you say dramatic, sorry?

Well, because scientists, I would say they were more conservative about such things back then. They took a lot of convincing that another particle existed. As time has gone on, theorists have been more and more ready to propose the existence of new particles to try to explain deviations from our predictions. Back then, it took a lot of effort to convince somebody there was a new particle in the world. Like I said, there were only two known particles. So proposing a 50% increase in the number of particles in the world, just to cover up the fact that a little energy seemed to be disappearing in this one obscure radioactive decay, seemed quite a dramatic proposal.

So, was he taking seriously? He's predicting a particle which it had been impossible to detect and he's saying that, but he's saying that according to his theory, it ought to be there. Now, briefly, what was his theory? And secondly, did people take this on trust?

Well, he originally, you know, just proposed that this particle would be emitted. It wasn't really turned into a fully mathematical model until Fermi got his hands on it. And he produced a theory of the weak interaction, which would mediate this beta decay and produce these neutrinos. I think people did take it seriously

because there was really no other way to understand what was happening in beta decay. But as you say, Pally apologized for having predicted the existence of a particle that could never be observed. I only wish theorists these days would apologize when they predict the existence of particles that can't be observed.

But luckily for Pauley, Fermi did the calculation and worked out that you ought to be able to observe them. Can we just take a step back? He offered a box of champagne to, is that right? Does anybody who created champagne, anybody who could detect these things? Can we just fundamentally, why are they so difficult to detect? There are billions and billions of them, as we now said, I think sufficient time, so we can leave that aside.

It's impossible to imagine anyway. and why are they so difficult to detect? Well, it just has to do with the interactions that they feel. There's sort of four forces of nature that we know of. There's gravity. There's the strong nuclear force that binds together the nuclei of atoms. Then there's the weak nuclear force. Okay.

And then there's electromagnetism. Now, everything feels gravity, but gravity is so weak in particle terms. It's almost impossible to observe any effect. gravity on particles. The only reason that we think of gravity is strong is because gravity always adds up with the same sign. And so we feel the pull from all the particles in the earth. And it's only by having that huge number of particles that it's strong enough you can feel it. The other forces act at much shorter ranges or rather the strong force acts in a much shorter range. The weak force acts a much shorter range. And the electromagnetic force, you have half plus and

half minus. So the effect of this is that the strong force which binds together the nuclei of atoms is very strong. The electromagnetic force is also has quite a bit of strength. The weak force

has an extremely small effect upon particles. If you think about glass and you think about light, you can see through glass because the light doesn't interact very much with the glass. It goes right through it. However, if you had a piece of glass that was a foot thick, it would start to look dark. And if you had a piece of glass that was 100 feet

thick, you couldn't see through it. So the light does interact with a glass, just not very much. It's the same effect, but carried to a much greater extent with a neutrino. The earth is far more transparent than any piece of glass is. And as Frank says, you need light years of lead to have the effect of an inch of glass. So we only know it when it reacts with the, when we see a reaction and then there's a way of spotting it then. And they began to get underway doing that, thinking about it properly in the 1930s.

But Susan Cartwright, in 1956, the eureka moment, the neutrinos were detected for the first time, and how was that achieved? Well, that was achieved using the reverse. of the reaction that first caused Pauly to believe that the neutrino existed. In beta decay, a neutron turns into a proton and it emits an electron and this mysterious neutrino. And in the 1950s, Fred Rhinus in Los Alamos realized that the inverse process was also possible. If you hit a neutron with a neutrino, you can get a proton out and, or rather, a proton.

proton and an electron, or if you hit a proton with a neutrino, you can get a neutron and a positron. And the key fact is that a positron, being the antiparticle of an electron, will annihilating matter, producing radiation which you can detect. And the neutron, a little bit later, will be captured and will also produce radiation that you can detect. So he figured if he could get a sufficiently intense source of nutrient. of anti-neutrinos, he could detect this reaction that converts a proton to a neutron.

At first, being at Los Alamos, he thought he might use a bomb. The difficulty with using a bomb is having your experiment survive the experience. So although he did get quite far in planning an experiment using a bomb, he had plans to drop it down a shaft so that it wouldn't be vaporized by the shockway, and then have feathers and things at the bottom to prevent it from smashing into bits. Eventually he realised that the next most intense source of... Is this a scientist we can really trust?

He won the Nobel Prize. A shamefully long time later, but he did won the Nobel Prize. I just... It was the feathers that got me. Sorry, I interrupted your flow, right. So he realised the bomb idea, having obvious drawbacks. he realized that the next most intense source of neutrinos was a nuclear power reactor that the fission fragments are not stable

and they decay by a relative of beta decay and produce vast numbers of anti-neutrinos. So he figured he could use a reactor. He wrote a letter to Enrico Fermi saying that he'd had this idea. He previously discussed the bomb idea with Fermi as well. And Fermi wrote a lovely... letter back saying, I think this is a much better idea. For a start, you'll be able to

repeat the experiment. So, he built what, for the time in the 1950s when experiments were literally tabletop instruments, he built what was then considered a very large detector. In these days of LHC, large detectors are the size of a small office block. But in those days, his detector was probably the of this room, not even, maybe the size of this table. It's a perfectly ordinary table. Something, yeah, about a cubic metre. You could just about get a person inside it

because one of the things he did when testing it was measure the radioactivity of a person caused by potassium 40 in your cells. And this was, by the standards of the time, an enormous experiment. He had difficulty persuading people it would work. But that's what you need to detect neutrinos, very large detectors by the standards of your time.

So he was on the track of the neutrinos, but by then scientists had already worked out. As I understand it, that the sun produced far and away the largest number of neutrinos. Why is that the case? Well, the sun, as Frank said, is a nuclear furnace, and the way it generates energy is it converts hydrogen into helium. Now, hydrogen is pure protons.

helium is two protons plus two neutrons. So to convert hydrogen into helium, you must convert two protons into two neutrons. And this is in fact exactly the same reaction that Fred Rhinos used to discover it in the neutrino in the first place. And every time you turn a proton into a neutron, you get a neutrino out as a necessary byproduct. and the sun is converting protons into neutrons at the rate of about 10 to the power 38,

that's one followed by 38 zeros every seconds, and every time a neutrino is spat out, and whereas the light, the energy that is produced in the core of the sun, takes about a million years to make it to the surface, because as we just discussed, neutrinos don't actually notice matter much, the neutrinos get right from the centre to the surface in a couple of seconds and eight minutes later

they're at your detector right I'm going to pause to try to think on after that one Frank Frank close we're now moving on to the discovery of the neutrino which was a quest and it's begun and David and Susanna taken a stunt

let's move on into the 1960s there's a sign that's called Ray Davis and he set up this detector. They're trying to catch these neutrons. So, for the list of benefit, for mine, if I've got this wrong, please tell me. There's a storm of neutrons coming out of way. They interact only with weak energy.

And there's so many of them that it's only very occasioned they do. So you don't know they're there unless you have a very elaborate system to catch. Even though there are billions and billions of them, you have to have an elaborate system to catch just a few of them. So that's the quest, isn't it? Now Ray Davis set up a first...

He's got a detector too So what did he do in the 60s? Right. So Ray Davis is trying to detect neutrinos, the little neutrons, the neutrino. What Susan said about the sun, we now know is true. But in fact, we didn't know that years ago. It was just a theory originally

that the sun is a fusion reactor producing all of this stuff. And Ray Davis set out an experiment to try and prove it because if indeed the sun, is that, then it will be producing neutrinos. The question then is, how do you capture them? And the idea originated with a brilliant Italian

called Bruno Ponticovo, who ruined his scientific career by disappearing to the Soviet Union in the 1950s, which is an interesting story in its own right, but not for today. That you could detect neutrinos if you used a lot of chlorine. Because if a neutrino bumped into chlorine, it would turn it into a form of argon, which would be quite easy to detect. And chlorine is cheap if you get it in cleaning fluid.

So Ray Davis's idea was to get a lot of cleaning fluid. In fact, it ended up being 400,000 litres of the stuff. So we're now, we're no longer talking a cubic metre. We're talking, well, a tank, 400,000 litres of cleaning fluid, which he had to take a mile underground into a disused mine. Why did he do that? Because he wanted to shield his exercise. experiment from cosmic rays. I mean, we're being bombarded from outer space all the time by cosmic rays, which hit the atmosphere and make showers of particles. And if they pass through your detector, you might mistake one of them for one of these rare neutrinos. So go deep underground where all the cosmic rays have been absorbed away. And if you're lucky, what's left will be neutrinos from the sun. So he had all this cleaning fluid shipped across the states in a little on railroad cars. It took them about five weeks to

unload the cars, ship them all down to the bottom, put it all in the tank, and then he had the thing there, and you wait. And occasionally, in theory, a neutrino from the sun will bump into a chlorine atom in your big tank, and every month or so you purge out the tank and hope to find one or two atoms of argon to prove that you've done it. Just a second. That sentence alone, every now and then you purge out the tank to find one or two atoms of argon, and you've done it. that for most of us needs a little bit more explanation. Right, and in fact nobody believed he could do it. Yes, I mean, you've got this huge tank of chlorine

and you're trying to find one or two atoms of something else, argon, that have been made. How do you find you go about it? Shall I move over to David now? In a minute, but it's... You want to finish it? The argon gives off a radioactivity, so it is... For a radiochemist like Davis, it was straightforward. for me as a theorist it's a miracle.

And indeed, he found so few that for years people didn't believe he could do the experiment at all. He had to convince people he could even do it, let alone that his results that he was getting were real. Now over today to explain how he really does it. David. Well, I don't want to wax poetic

on the subject of detecting a few atoms too long. We did a very similar experiment subsequently where we detected a few atoms of, radioactive germanium in a tank of 60 tons of metallic gallium. Basically, you take advantage of the fact that when an atom,

when one of these atoms decays, it has a very particular signature. It's a very particular energy electron. And you can purify. It's a trick of the chemistry. It turns out it's easy to purify these things down to extreme levels of purity.

So you can put them in very, very tiny detectors which have very little background radiation, you can actually find a few atoms, and this has been confirmed experimentally now that using radioactive sources that produce enough neutrinos that you can actually

detect them. So what did Ray Davis get out of what was called a homestake experiment a mile beneath the earth with this cleaning fluid? Well, he started counting his neutrinos, or rather he started counting his atoms of radioactive Argon.

And over time, it became clear there weren't enough. And there was something like a quarter to a third, the number that was expected. Who'd expected? Well, when you mention Ray, you have to mention John Bacall. There was a theorist who was a postdoc at Caltech at the time and later became a long-serving professor in Princeton, who did the very tedious calculation. of how exactly how many neutrinos you would expect to come from the core of the sun.

The actual detail of it is a bit complicated because it's not a single nuclear reaction in the core of the sun. There are many different nuclear reactions. They produce neutrinos of different energies. So it's a very complicated calculation to know exactly how many neutrinos Ray should observe. And when he did these calculations, he consistently got numbers three, four times higher than what Ray saw. And as Frank says, at first people simply suspected that Ray had done the experiment wrong. But Ray's a very nothing, if not patient, and he eventually convinced pretty much everyone that it looked like he had done things properly. Then they just suspected that John had calculated things wrong. In particular, the Dave's experiment sees very high energy solar neutrinos, and the calculation is particularly difficult with those.

and so at the time people just assumed that John had got his calculation wrong. Just to cut it on Dave's the experimentalist, to be fair, it was the theorists like myself who said Ray Davis is doing his experiment wrong, and the experimental said, Barclall can't do the calculations. Now then, Susan Cardwright, so there's more experiments being set up, before we bring this part together, and more experiments to count these solar neutrinos, what were they, and what did they?

find? Well, there were a number of different experiments, as Dave said, the problem with Ray's experiment was that it was only sensitive to neutrinos that were emitted by a small side branch of the Sun's output. And those people who were convinced that John Bacall had got his sums wrong were not claiming that he'd got the vast majority of the Sun's energy production wrong. They were just saying that this small side branch, instead of being 1% of the sun's output was maybe a quarter of a percent of the sun's output. So it was a small byproduct. So there were two issues that were addressed by the next generation experiments. One was that this idea of producing a few atoms of radioactive argon, flushing them out of the tank

and counting them, doesn't actually prove you're detecting neutrinos from the sun at all. because you have no direction sensitivity and you don't even know exactly when those organ atoms were produced. So if you were a real skeptic, you could claim that, in fact, Ray Davis had detected no solar neutrinos. He was detecting some former background

that we hadn't thought of. So what you want is something that can tell you where the neutrinos came from. And that's something is called the Cherenkov effect. Now, I'm going to give you a nasty shock. Everybody knows that nothing can travel faster than light. It's not true. Nothing can travel faster than the speed of light in the vacuum of space. But when light travels

through a transparent medium like water or your glasses or my contact lenses, then it is slowed down and that's how your glasses focus and correct your sight problems. And the particles are not slowed down. So light in water is travelling only about three quarters of the magic speed limit, whereas a particle travelling at 99% of that speed limit is still travelling at 99% of that speed limit in water, which is faster than the speed of light in water. And when an aircraft travels faster than the speed of sound, you get a sonic boom. When a particle travelling in water travels faster than the speed of light in water, you get a light boom. You get a cone of blue light, and that is called Cherenkov radiation after powerful Cherenkov. And you can detect that blue light, and because

it's a cone that goes forward at 40 degrees from the path of the particle, you can therefore find out where the particle was coming from. And an experiment down a mine in Japan called Cameo Kamiokandi, which was actually designed to look for the decay of the proton, but that's another program, was able to detect neutrinos from the sun when they reacted in its water tank and made electrons that travelled faster than the speed of light in water. And for the first time, they were able to prove that the neutrinos that they detected were actually coming from the sun. They could take a photograph of the sun in neutrinos. Or a neutrinograph of the sun, exactly.

So we know where they're coming from, but David Wark, we've been talking about neutrinos as if it's one, but a development was that and which helped massively with the mathematics and this mathematician who'd been laboured because some's weren't right, was proved to be right, that there were three types of neutrinos which people

arrived, they arrived at that conclusion. You call them three flavors. I can't understand why you call them three flavors, but that's up to you. Can you tell us about these three flavors? Yeah, we've been talking about the electron, but in fact we now know that the electron has two heavier cousins.

one we call the muon and one we call the tau. And these particles, the muon and the tau, are pretty much just like electrons, except they're more massive, they're heavy. And because they're heavy, they decay, because there are lighter things they can decay to. So the neutrinos emitted, say, by the reactors that have been detected are electron-type neutrinos. And the neutrinos emitted from the fusion reactions in the core of the sun are also electron-type neutrinos, or they should be.

However, along with the muon and the tau leptons, the things that are like electrons but are heavier, there are also more neutrinos. There's a neutrino that goes with a muon, which we call the muon neutrino, and a muon that goes with a tau, which we call the tau neutrino. And these extra neutrinos are not emitted by the sort of reactions that we've talked about up to now, but they can be observed via the weak interaction

because if a new mu interacts it produces a muon, which you can see, and it has different properties from an electron, so you can tell them apart, and if a new tau interacts, it produces a tau. So we have these three different flavors of neutrino, which makes the whole situation more complicated. So we're drawing to a particular conclusion of this episode, aren't we, Frank, close. They're now three flavors, and they're distinct.

And where does that take us? Well, the processes in the sun are making just one type of these, the electron type of neutrino. And Ray Davis's experiment has been set up to detect the electron type of neutrino. And John Borkle's calculation says he should have been finding more. And so there's a shortfall of electron type neutrinos. Now, the irony is that the realization that there are more than one type of neutrino was around for 10, 15 years. years before this was all sorted out, and Ponticovo, again, of all people, had come up with an idea that if there's more than one type of neutrino, it is possible that in the 150 million

kilometres journey from the sun to here, what started out as the electron type had changed its spots in a strange way and turned into a muon type or a tau type, or maybe it had stayed the same. And by the time you got here, everything has sort of evened out so that only one third remained electron types and the other two thirds Davis was blind to. And that would explain why there was a shortfall. Now the great ironers of these things is that nobody took any notice of this, not least in part because he'd written this in Russian, in a Russian journal and it wasn't translated for a couple of years and so on. But the real problem was that the laws of physics said that could not happen because

everybody knew that neutrinos travel at the speed of light without any mass and it turns out that Ponticorvo's idea won't work if neutrinos have no mass. We now know that neutrinos do have a mass and that Ponte Corvo's idea is right and that in that

150 million kilometreoumeta journey the neutrinos are changing from one type to another it's called oscillating. Susan Cartwright so we're on the way to solving the solar neutrino problem at this stage. We are indeed. The original argument of the problem was either there is something wrong with the Sun or with Bacor's calculations, or there is something wrong with Davis's experiment, or there is something wrong with the assumptions about the neutrino.

At this point, there is more than one experiment confirming the solar neutrino deficit, it, so we can absolve Davis's experiment of blame. And McCall's calculations have been checked by looking at the interior of the sun as revealed by a science called helioseismology, basically sunquakes. So the only thing that can be wrong is that there is something wrong with the neutrino. And the framework of what are called neutrino oscillations, the changing of neutrinos from, one type to another, could provide a solution. So what you need

in order to solve the solar neutrino mystery is an experiment that can detect neutrinos from the sun whatever flavor they come in. And this is an experiment that took place in Canada which Dave was part of, so I think he should take on this story

from there. Yeah, as Susan says... On the neutrino face. The the difficulty is detecting the neutrinos that aren't electron neutrinos. The problem is these neutrinos coming from the sun don't have enough energy to make a mu or a tau. So you can't observe the sort of reactions we've been talking about up to now. They just don't happen.

And so instead you have to build a detector that can see a much subtler type of reaction, much harder to observe type of reaction, which had come along, in the meantime, something called a neutral current. It's another type of weak interaction, which nobody even knew existed when this story started. And so, myself and, you know, 400 of my closest friends, actually, I joined after it was already well on the way, although I was then part of it for 20 years,

which gives you some sort of feeling for how long these things take, build a detector called the Sudbury Neutrino Observatory. it consisted of 1,000 tons of heavy water in an acrylic bubble, 12 meters in diameter, suspended inside a giant soccer ball, 17 meter soccer ball, on which were mounted 10,000 very sensitive light detectors,

all of which was 2 kilometers underground in a nickel mine in lovely crate in Ontario. And using this device, we could independently measure the type of reactions we still talked about at the start, which told us the number of electron neutrinos. And we could see another reaction which broke the Deuteron up

and we could detect that. And using that we could count the total neutrino flux. And what we saw was that the total neutrino flux, the sum of the new muse and the new taos and the new Ease was exactly what John Bacall had been saying it was all along. For how many years had he been in the

doghouse for supposedly I hope he was alive when it was pretty well. Goodness for that. Oh, there's a lovely story about that. John, in fact, one of the great pleasures in my scientific career was when we gave the first talks announcing these results, I had a slide, and it simply said John McCall was right all along. And when we finished the talk, I went back to my office and emailed it to John. And John later said in the New York Times, and then subsequently in an interview, that was shown on the BBC, in fact, that he felt like,

like a criminal who had been convicted of a heinous crime. He didn't commit. And then 30 years later DNA evidence is found that absolves him of all guilt. He said he felt like dancing. If you knew John, that was a hard thing to picture. But no, it's, it was

a tremendous feeling of satisfaction to have resolved this tremendous problem, but also opened up neutrino oscillations, this whole new phenomenon of nature now for study. So, Frank, Close, that study is the turn branch of astronomy, isn't it really? And what does a neutrino telescope look like?

Well, that's probably in its simplest form, the thing that Susan was telling us about, that when a neutrino hits in water, by emitting this Cherenkov light, you can tell where the neutrino came from. So you can see its direction, you can measure its energy, and you can tell the time that it hits. So from that, you can work out, did it come from the sun, or did it come from someplace else? So that's the basic idea.

The most exciting thing perhaps, really, to add to this story, though, is that in 1987, the only other astronomical object ever detected in neutrinos was seen, and that was a supernova. The only supernova that has happened in our lifetime took place in 1987. In fact, it didn't. It took place 170,000 years ago in the large Magellanic cloud, and the burst of light, a supernova is a star that's collapsed. And the theory was that when it collapses, it produces neutrons and neutrinos.

And the theory said that although a supernova shines brighter than an entire galaxy to your eyes than can even be visible in daylight, in neutrinos, it's 99% of the energies in neutrinos. I mean, that's vast. So 170,000 years ago, a star collapses in the large Magellanic clashes in the Large Magellanic Climbabes. cloud and neutrinos set out, travelling across space at almost the speed of light. I'm not sure quite what we were doing here in the BBC 170,000 years ago, or what was going on anywhere, but these neutrinos are flying across. And 169,000 years of travelling later, we've got to the Norman conquest, and 169,000 doing the sums, 950 years later, power comes up with the idea

that maybe there's things called neutrinos. And still this wave of neutrinos and the supernova's travelling on and after 169,998 years totally by chance the Japanese and some others have got this big tank of water deep underground which is now a neutrino telescope beginning to work and at breakfast time in English time through my cornflakes and you pass this little wave of neutrinos from that supernova which also swept through the tanks there and about a couple of dozen of these neutrinos bumped into atoms of water in the tanks and revealed themselves. And this is remarkable when you think about this, that the rate that these things detected neutrinos from the sun

is like a few a day, and here you've detected a couple of dozen in a few seconds that have been travelling for 170,000 years. That alone gives you an idea of how much power took place in the supernova explosion. Well, throughout this programme, I'm exhausted trying to make sense of these numbers, but you're playing.

on. No, you're going on. I'm sort of plowing on here. So what are we learning from this neutrino astronomy season? Well, from Supernova 1987A, where there must have been at least 20 papers generated for every neutrino that Cameo Candy and IMB saw, we learned that the energy does indeed go into neutrinos, because in order to detect a couple of dozen from a supernova 170,000 light years away, you can calculate back as to how many must have been emitted by that supernova. And that turns out to be within the experimental errors, precisely the number that you would expect to be emitted when a supernova explodes. So that detection, minor though it was, a couple of dozen neutrinos, already confirmed much of our theory about how massive star supernovae

happen. The astronomical community is feeling very short-changed on the supernova front. In the late 16th century, there were two naked-eye supernovae only 30 years apart. Tico Brahas in 1572 and Johannes Kepler's in 1604. And there hasn't been one that we saw in our galaxy since that time. So there were two 30 years apart, cunningly time just before the invention of the telescope. in the 400 years of astronomical history since that time. So they're really hoping for another one. If we had a supernova go off anywhere in our galaxy,

which is only about a quarter as far away, even at the far reaches as the large Magellanic cloud, with the neutrino detectors we have now, thousands of neutrinos will be detected. David Wark, what's the latest neutrino experiment going on? Again, we're in Japan, aren't we? Well, yeah, we're building experiments all over the world now, and

Susan and I are involved in one in Japan, to try to probe this phenomenon of neutrino oscillations, it's potentially a hint to one of the biggest unsolved mysteries in fundamental physics, which is, where did the matter in the universe come from?

We started with a big bang, which is essentially energy and radiation, and as that cools, it produces matter. But according to the known laws of physics, it should have produced almost identical quantities of matter and antimatter. But we don't live in a universe that looks like that. We live in a universe that has matter in it.

So if the known laws of physics won't produce more matter than antimatter, there must be unknown laws of physics. And neutrino oscillations is a possible place where you could look for that. So what we're trying to do is build an experiment where we can measure these neutrino oscillations with incredible precision and then compare the oscillations of neutrinos and anti-nutrinos and see if they're the same. And we suspect they won't be, but we'll have to see.

So we've built an experiment in. Japan called T to K. We make a beam of neutrinos on the east coast of Japan, a place called J-Park by using a big particle accelerator. Then we fire that beam of neutrinos for 300 kilometers underneath Japan to a huge detector on near the west coast of Japan, which is Cameo Kanda's bigger offspring, which is called Super Kameokanda, 50,000 tons of water. And by firing these neutrinos under Japan, we look for a tiny branch of neutrino oscillations which we think will open up

the route to doing experiments to look for the difference in oscillation between matter and antimatter. Finally, Frank. What, for listeners, what is this going to lead to? Not necessarily in the utilitarian sense, but we've gone first down the track

since the 1930s. Well, one possibility is that we might discover why we live in a universe dominated by matter, not antimatter. Of course, the real excitement is we don't know. That's the excitement of science. But that's a sort of a bit of a flip answer. I think what we have already learned is by using neutrinos,

we have looked inside the sun. We have looked inside a supernova. And I think John Barcoll is the guy who really had the nice statement. He said, the history of astronomy shows it's very likely that what you discover will not be what you are looking for. Thank you very much, Susan Cartwright, David Walk and Frank Close. And next week we'll be talking about the Pelagian controversy in the 5th century following the sac of Rome. Thanks for listening.

If you've enjoyed this Radio 4 podcast, why not try others, such as Thinking Aloud, where Laurie Taylor discusses the latest social science research. To find out more, visit bbc.co.uk forward slash radio 4.