In Our Time - Cosmic Rays

Episode Date: May 16, 2013

Melvyn Bragg and his guests discuss cosmic rays. In 1912 the physicist Victor Hess discovered that the Earth is under constant bombardment from radiation coming from outside our atmosphere. These so-called cosmic rays have been known to cause damage to satellites and electronic devices on Earth, but most are absorbed by our atmosphere. The study of cosmic rays and their effects has led to major breakthroughs in particle physics. But today physicists are still trying to establish where these highly energetic subatomic particles come from.With:Carolin Crawford Gresham Professor of Astronomy and a member of the Institute of Astronomy at the University of Cambridge Alan Watson Emeritus Professor of Physics at the University of Leeds Tim Greenshaw Professor of Physics at the University of Liverpool.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, one of the world's largest and most unusual astronomical observatories can be found on a vast empty plane in western Argentina. The Pierre-Oge Observatory covers an area larger than Luxembourg.

Instead of telescopes, it uses 1,600 massive tanks of water to look at the heavens. The scientists who work there aren't looking for light from the stars or even radio waves. Instead, they're studying cosmic rays. First identified a century ago, cosmic rays are subatomic particles which constantly bombard the Earth from space. The discovery of high-energy radiation coming from far beyond our solar system led to the emergence of particle physics as a new scientific discipline. Today, more scientists than ever, are dedicated to the study.

of cosmic rays, but many questions remain unanswered, including, most importantly, where they come from. With me to discuss cosmic rays are Carolyn Crawford, Gresham Professor of Astronomy and a member of the Institute of Astronomy at the University of Cambridge, Alan Watson, Emeritus Professor of Physics at the University of Leeds, and Tim Greenshaw Professor of Physics at the University of Liverpool. Carolyn Crawford, would you begin by giving us a slightly fuller explanation of what cosmic rays are? Well, to start with, I think it's just reiterating what you said right at the beginning. despite the name, we're not talking about rays of light.

We're talking about matter. Energetic particles that are impacting the top of the Earth's atmosphere. They're coming from all directions in outer space. And some we even think are coming from outside our galaxy. And these are pieces of atoms. And in some ways, there are only direct samples of matter outside the solar system coming towards us. They're electrically charged and they're travelling at huge speeds, sometimes close to the speed of light. and that means they pack an enormous amount of energy behind them.

They come with a range of energies, but to try and express it, one of the ways of putting it is you have the energy of a well-hit tennis ball, but instead of packing that into a tennis ball, you're packing into something like a proton, a subatonic particle. Colossal energies, sometimes a far in excess of any energy we can launch a particle with in a particle accelerator here on Earth. They are, some are electrons, but most of them are atomic nuclei.

So you have an atom and you strip the electrons off and it's what's left at the core. And most of these very simple atomic nuclei is just protons from hydrogen atoms. We get some helium nuclei. So again, very simple. And then just a tiny smattering of heavier nuclei from elements right up to iron. And then there's a minuscule amount of even antimatter particles. so a full range. And the number of them arriving at Earth is phenomenal.

You've got probably tens of them zipping through your body every second. Tens? Tens, yes, 10 to 100, yeah. And so there's this constant flux raining down on the earth. Just give us a broad idea, because we'll go into this as the programme develops, where the cosmic rays are thought to come from. Well, there's a range of sources, because where the cosmic grey comes from really depends on its energy

because only certain sources or cosmic objects can generate enough energy to far particles at different speeds. The very lowest energy cosmic rays are relatively local. We think they come from processes in the solar winds. So this is a stream of charge particles that come off the sun the whole time. And middling energies, so you might call these sort of mildly relativistic speeds, we're looking at processes within our galaxy, and most likely something originating from supernova explosions,

not the explosion itself, but just the way that the blast wave from that explosion propagates through its surrounding space between the stars. But the biggest challenge are the very highest energy cosmic rays that impact the Earth. These are incredibly rare, and they require hugely energetic events to launch them at those speeds. and there's some suggestion they could be associated with relatively nearby galaxies.

It's also possible they could be associated with exotic events such as Gamerae Burst or even they could give us clues to the nature of this dark matter, this gravitational glue that holds structures in the universe together. So our sense of what the origin is varies according to the kind of cosmic ray we're talking about. Alan Watson, the first evidence for cosmic rays was observed a lot. long before anybody knew they existed in the 18th century. What was the earliest evidence? Well, the earliest evidence was that if you put an electrical charge,

a static charge that we're familiar with if we walk across a carpet and our bodies get charged up, if you put that on an object, the charge leaked away in a way that people didn't understand. And it took more than a century from the work of Cullum in the late 18th century to realize what was happening. And this really came about because in the last decade of the 19th century, x-rays were discovered by Ronkin,

radioactivity by Beccarell, and J.G. Thompson discovered the electron. And that allowed people to understand what the basic constituents of matter were like at that time. Can you just... Sorry to interrupt, because it's so fascinating. When you say discovered, for the rest of us, how on earth did they discover this stuff, which is...

You can't see it, you can't... And so how did they get to it in the first place? These first steps. When you just said, they discovered, what did they do? What do they do? You mean X-rays? Well, yes. Well, just give people an idea of how they discovered them.

Well, X-rays were discovered because Becherell was looking at discharges in gases. Where you put a high voltage across a gas, you get a spark. And he discovered that a photographic plate, which he'd wrapped in black paper, actually became blackened with the radiation that came out of this X-ray tube. Then, in the next year, Becherell discovered radioactivity in a very similar way. And suddenly, physicists knew that there were effects that could, as we say, ionise the particles, ionise the molecules or the atoms. An atom, as Carl was explaining, has a central nucleus surrounded by electrons.

The scale that's usually given is the nucleus is the centre of St. Paul's Cathedral, and the electrons are round at the dome. Is a person in the centre of St. Paul's Cathedral? Okay, you're a person there. And you rip off one of these electrons. It usually attaches itself to another atom. So you have two, what we call ions. And these have the property of making air conducting.

In a wire, in a copper wire, you can pass a current because there are electrons in the wire. In air, there is conductivity. It's much, much less than it is in the conductivity of copper. But the big question, which really the best brains of that time were actually trying to tackle, even people like Rutherford, was what causes this ionisation. They knew some of it came from radioactivity, from rocks and in the air.

But when they did the calculations, they didn't actually get the right answer. There seemed to be more ionisation produced than they expected. So there were lots of ingenious ideas. C.T.R. Wilson, a Scottish scientist, he thought maybe there was radiation coming from outer space. This was the first suggestion in 1901 that might be radiation coming from outside our atmosphere. And he took a device called an electromagnetor invented by an Englishman in the 18th century into a railway tunnel near Peebles south of Edinburgh. And he expected to see the rate of discharge decrease, but in fact was the same. And that was because the radioactivity in the rock, which we didn't know about at that time, was actually quite high,

and it compensated for the shielding. So he got a bit fed up, and it wasn't until later on in developments in electrometers made particularly by a man called Theodore Wolfe. who was a Jesuit priest. He took one of these up the Eiffel Tower before it was opened to 330 metres. In his paper, he thanks Monsieur Eiffel for permission to go up the tower. He saw a decrease in the rate of ionisation,

but it wasn't as big as he expected. And then in 1912, an amazing Austrian, Victor Hess, flew in a balloon up to 5,000 metres and found that the intensity, the rate of production of ions, was three times what it was at ground level. That was confirmed shortly afterwards by German Kohlhuster and then the First World War happened.

But it was Hess who really nailed the ionisation change as something coming from out of space. Did he publish and did it have an impact? First World War is a great intervention in a lot of studies, but a lot of physicists around then, did he publish and did people say, yes, this is where we're going now? It was accepted certainly with a lot of questioning in Germany, and this is why Coalhurst had made amazing flights up to 9,000 metres with oxygen, but manned flights.

The UK hardly noticed it. It wasn't a subject that was of great interest. They were aware of it, and after the war certainly Rutherford, and his people became very interested in this. But it had a very big impact in Germany, but not so much elsewhere. Tim Greenshaw, for some years after this is going, there was some disagreement about what cosmic rays really were. Carolyn said at the beginning of the programme that we talked about rays,

we're talking about particles. Why do we keep calling them a rays then? Initially, Milliken, who was a very powerful figure, very significant figure in the time, thought that cosmic rays were probably photons because they were the most penetrating particles, not at the time. And this was eventually disproved

when, for example, Clay undertook voyages from Amsterdam to Java, in 1927, I believe, and while he was travelling, he made measurements with his electromagneters and discovered that the flux of cosmic rays, the rate per unit area, decreased as he was getting closer towards the equator, and it dropped by something like 15%.

And he deduced that this was due to the cosmic rays being influenced by the magnetic field of the Earth, because of course neutral particles aren't, that implied they must be child. particles. And at that point, he thought they could be positive, negative, that wasn't known, and further investigations were needed to discover what the charge of the particles was. At this time, I think, something I'd like to say anyway, it's important that we know that this is pure research, isn't it? They're finding out for the sake of finding out. Nobody's saying

at the end of this, there's Silicon Valley, or at the end of this, something's going to happen. It's just because they want to find out. Quite true, yes, yes. Some interesting applications are now being suggested for cosmic grace. That's always the kickback, I'm 100 years old. The whole civilization is richer for it, but still at that time they're just trying to track this down. And there's another important figure in this story, his name was in the introduction.

Pierre Ogre Ogey, what significant discovery did he make? And what did he bring to the table? Well, he was using particle detectors in pairs, up in the Alps on the Youngfrau York, and what he observed was that his particle detectors would be indicating the presence of cosmic rays but far more often than he expected he'd see particles being detected simultaneously in the two separate detectors even if he was separated by several metres and what he eventually suggested was that these simultaneous events were being caused by high energy cosmic rays

coming into the atmosphere, interacting and producing a shower of particles, one of which was entering his right-hand detector and the other, the left, therefore causing the simultaneous signals. What data we're talking about now, Tim, 1930s? And we have, you passed over Milliken, and you said he was a powerful man who won a Nobel Prize. And he was wrong, wasn't he? He had this argument with Compton. Can you just tell the listeners what all that was about? Well, as I said, Milliken thought that Cosmicius,

rays were photons. It was discovered that they must be charged because the effects of the Earth magnetic field on cosmic rays. And Compton actually initiated a study in the 30s where cosmic ray directions

were measured very carefully and rather the flux was measured very carefully and he discovered that the rate was associated with the geomagnetic position, not the geogrammedic position, not the geographical, so really tying down the effect as being due to the magnetic field and therefore

cosmic rays being charged. In around about 33, Rossi suggested that in fact you could deduce whether the particles were positive or negative by investigating asymmetries in the direction in which particles entered the, were detected, the so-called East-West effect, east-west effect. and he and others actually measured particle directions and discovered they came primarily from the West, which implied that they were positive particles. So at this stage, we have,

Alan Woodson was saying that at the First World War, the Germans were very interested, and certainly we weren't all that interested, and maybe many others weren't. But in the 30s, we've got the American, and America are on it, we've got, by the sign of it, Italy on it, we've still got Germany.

Are the Brits taking any interest in it at this stage? Alan? Yes, a very strong interest, particularly through people like PMS Blackett who set up a cloud chamber which is a device for visualising the tracks of particles with Geiger counters and he was able to see showers of particles in the tracks

and actually saw positrons more or less at the same time as Anderson saw the positron in 1932 Anderson being C.D. Anderson and Carl Anderson I'm just interested in how many countries are involved. He was an American. It'd become pretty international at that stage. And can you just explain that cloud effect, but it sounds fascinating, but I didn't quite get it. Well, open your mouth and breathe in your hand.

You mean now? No, yes. All right, then here we go. But it's felt hot. Now, if you push your lips and breathe in your hand, it's cold. And that's because the air is expanding. And when you have a moist gas and you expand it very rapidly, as you just did, it cools. And it has to condense out.

And if you make it very clean, it doesn't condense out in the dust. It condenses on the ions that I was talking about earlier. And this allows you to see the tracks of the particles. And this beautiful technique was used by people like Blackett, by Carl Anderson, lots of people, to make really fundamental discoveries of particle physics rather than cosmic rays. But we didn't know it was particle physics at the time. This was all done in the 30s and just after the war.

So there's a sense of intellectual groping, isn't it? Groping towards a solution, as it were. Oh, very much. Stumbling towards the time. Karen Cropett, they can have a range of energies. So when did it begin to be... Let's follow this historical path for a little while. When it began to be resolved so they started to move on to other matters?

Was there a sense in the 40s that people said, oh yes, this is what it's about? I think it's actually much later than that. So you get this sort of divorce between the astrophysics and the particle physics when you start with the particle physicists, start having their own accelerators on Earth, and they don't necessarily need cosmic rays to study the particles. Then you start worrying about where these cosmic rays are actually coming from.

And I believe one of the first sources of cosmic rays identified was a crab nebula, which is one of these supernova remnants that we think account for a lot of the sort of middling energy cosmic rays. So can we have a range of energies which you alluded to in your, well, you referred to in your opening remarks, can you just emphasize again the different types and different energy levels? There seem to be generally speaking three types. Yeah, that's just my great simplification. They come with a huge range of energies.

You get lots of the lower energy ones. They're the local ones, mainly from the solar wind. Many middling energy ones, which we think originate from our galaxy for various reasons. And then you get the incredibly rare, very high energy events that are the most delusive to track down where they're from. What's the crucial distinction in the impact that each of those has? How do you mean? Well, low energy does what?

Yeah, atmosphere, our planet. The low energy does what? The middle energy does what? The high energy does what? Well, it's the very, the high energy particles are the ones that create these air showers that Tim was describing. They impact on the top of the atmosphere. They collide with the molecules.

They ionise the air. And they create the shower of secondary particles. So the very high energy particles have an effect on our atmosphere, and we don't detect them directly on the ground. We just see their side effects of these showers that we receive at the detectors. So in terms of energy and how you detect them, if you get one of these ultra-high energy electrons, they are incredibly rare.

Each one is an exciting event when it happens. You know, you can... Each particle? Yeah, each arrival of an ultra-high-energy cosmic array, you can number them their events. And certainly the very first ones that arrived were spectacular events that, you know, amazed the scientific society.

Alan Watson, would you explain what happens when a cosmic grey particle enters the Earth atmosphere? Well, I think the first thing to say is the same thing happens right across the energy spectrum. Whether it's the low-energy particles or the very, very highest-energy ones, it's only a question of what happens to the secondaries because we don't see the primary particles

down at ground level. They're all absorbed in the atmosphere. So a particle comes in, let's say it's a proton. It hits an atom, typically of nitrogen, because there's more nitrogen than anything else in the air. And it's actually interaction between the quarks in the proton and the quarks in the nucleus of the nitrogen. This produces lots of secondary particles, mainly pyons, pymesons, some of which are very, very unstable,

decay into gamma rays which produce lots of electrons. The charged pylons produce particles called mons and this whole cascade multiplies up. So that if it 10 to the 19 electron volts which is about a jewel, if you drop that microphone onto the floor and that's about a jewel of energy will be released. That creates something like 10 to the power of 10 particles at sea level. Now they're not concentrated in a column.

They're spread out because of the angles at which they produce because of the scattering of the electrons in the air. And you can think of the shower as being like a giant dinner plate moving through the atmosphere at the speed of light. And the dimensions of the dinner plate depend on the energy. At 10 to 19 electron volts,

it will be about 20 square kilometres. So there's a vast dinner plate hitting detectors at different times, and from the times they hit the detectors we can reconstruct the direction. But in these interactions at the very, very highest energies, all of the processes that take place at the LHC occur

with probably higher probabilities because the energies are higher. So it's a micro-accelerator collision that you're looking at. Tim Greenshaw, lower-energy cosmic rays are responsible for one of the great spectacles, the northern and the southern Aurora Borealis. Why there and what's happening? These are really low energy,

the lowest energy. at the end of the spectrum they're talking about. They put on a big display. They do, yes. And this is probably the first place that cosmic rays were detected in the sense that people could see the interactions in the atmosphere. So what's happening is that particles

streaming from the sun, charged particles, are trapped in the Earth's magnetic field and they can then be accelerated along the field lines. So they enter the Earth's atmosphere where those field lines go into the Earth, namely the North and South, poles and as the charged particles spiral along the field lines into the atmosphere they interact with

the nitrogen and oxygen atoms exciting them raising electrons to higher energy levels which then fall back to their ground states and as that occurs as the fall back occurs they emit light and with the typical colours that you see in the aurora so typically sort of greens and reds depending on whether it's coming from nitrogen or oxygen why does it happen at those two points because the earth's magnetic field is like a dipole. So you've got field lines coming out of the North Pole going around to the South Pole, and the charged particles that produce the aurora

are constrained to move around those field lines. So they enter the atmosphere at the north and south pole. Are they very predictable? I mean, you almost get holidays. Come to the north of Scotland and see the Aurora Morales, aren't you? Join our Orrubyla's tours, yes. Look at the solar weather, and try and deduce from the solar weather

well, they're likely to have a lot of charged particles in the trapped in the Earth. Going back to weather forecasting, right? Yes. But this is for the sun, not for the Earth, which is equally difficult at least. But no, just, I mean, it's a fairly serious question. Can you predict them? And if so, how? And what's the significance of that?

Well, I mean, there is interest, for example, in predicting solar weather because extreme flares can cause interruptions to communications or to power networks on the earth because they generate such strong electromagnetic pulses on the earth, they can cause problems for our power supplies. And so there are satellites that observe the Earth's weather, sort of rather the sun's weather, so we can predict such events and try and react

and prevent serious damage happening. Caroline Crawford, various attempts have been made, as I understand it, to measure cosmic rays in space. What are the advantage of doing that and how far all now people in doing it? Well, the obvious advantage is you're out of the Earth's atmosphere, which, you know, as described, it absorbs the high energy rays and it slows them down.

So you're out of that shielding, and so you can get the full range of energies arriving to your detector. And there have been instruments flown on satellites before. The most relevant one currently is this giant cosmic ray detector, which is being flown on the International Space Station. This is a huge thing. It weighs seven tons. It's like 27 cubic meters, and they've kind of bolted it onto the side of the space station. And it's been there since 2011. And it detects something like a thousand cosmic rays a second. It's called the AMS, which stands for alpha magnetic spectrometer. And the idea is it uses magnets to sort of deflect the particles onto the detector. And they're particularly interested in the sort of antimatter component. these positrons which are like the positive electrons and whether there's an excess of them

and what that might tell us about dark matter particles. But that's still in the very early stages. They've published results in something like the first 6.8 million detections or so. Obviously there's going to be far more information because this is expected to run for 10 years or so. How is it possible? And I'm asking to detect cosmic rays on Earth. Is it possible?

Oh yes. the cosmic rays can be detected. When you're in bed at night, about a million go through your body as you're sleeping. But these are from the very low-energy cosmic rays. As you go higher and higher in energy, you make use of this extensive air-sharp phenomenon that Tim introduced the work that Pierre-Oge did. And what you do is spread detectors out over as large an area as you can.

In your introduction, you referred to the Pierre-Oge Observatory being the size of Luxembourg, inside the M-25 is another analogy that I like to use. and you do this because the rate at the highest energies is very low. At 10 to the 20 electron volts, which is towards the highest energy we've ever seen, the rate is less than one on a square kilometre every century. So you have to build something that is really enormous.

A long wait then? A long wait or build something pretty big. So what was decided to do for the Peer-Oge Observatory was to build something really big. And that was something that I started with, the Bell Laureate Jim Cronin more than 20 years ago now and we built an international collaboration

to raise the money to build our detector in Western Argentina. Can you explain that in detail because it does sound like an extraordinary project? 1600 tanks? 1600 tanks of water reached with 12 tonnes of very, very clean water in them and they're overlooked and it connects to what Tim

was talking about with the Aurora. It's overlooked by a very sensitive telescope which are fixed in position because when the shower comes through the atmosphere it's the same particles as Tim was explaining about the aurora it lights up the sky but it's a very very weak amount of light so you need a very sensitive detector to see it

and we pick up this light and this light actually gives us a direct measurement of the energy of the particles at the same time because you can only do that when the light when the sky is dark at the same time we use the water cherncoff detectors to pick up particles and to measure the direct

and they're running all of the time. So the two detectors work together in a combination to measure the energy and to measure the high rate of events and measure the direction that the particles come from to do astronomy. But is the principal purpose of this extraordinary setup? Is the principal purpose to wait for that one event a century? No, no, no. Well, we'll get more than...

Actually, because it's 300,000 square kilometres, we get a few a year, the very highest energies. But those are the ones that make us really excited. Lower down, of course, there's lots of physics, lots of astrophysics, lots of particle physics we can do. Well, where does this one a century come from then? Well, I wish I knew.

The problem is that the particles are charged, and it turns out that they may actually not be protons, they might be a lot of iron nuclei there, and they get bent by the magnetic field. If you can imagine being in your house with snow around about it, and you see a drunk man arriving, You'll see the steps of his footprints, but you don't know what pubbies come from.

He's been scattered on his way to you. In the same way the cosmic rays are scattered as they come through the galaxy, come through extraterrestrial space. We're trying to track them down. Some of them, the protons, may come from very energetic objects called active galactic nuclei,

which have black holes at their centre. And that's what we're working at, trying to get more of these. So can you take this on, Tim Greenshaw? Where they come from, That seems to be the question we're on now. What other techniques can scientists use to try to work this out? Well, what you want to do is to find a particle

that comes directly from the source of these cosmic rays in a straight line, so you can point back and identify where they're coming from. And the obvious choice is the photon, because that's neutral and not influenced by the magnetic fields that Alan was talking about. So what we're also trying to do is identify, is measure very high-energy photons coming from the cosmos, and use those to point back to what we think of the sources of cosmic rays

because these photons are produced in conjunction with the cosmic rays either if those were to be electrons by radiation from those electrons or in the case of protons if those protons interact with gas molecules near the source and then produce pi zeros which decay to photons and to actually detect these things the way that What that's done is, again, we use the interactions of the photons in the atmosphere. They enter the atmosphere and produce electron-positron pairs, which themselves produce more photons, more electron-positron pairs, so again you get a shower of particles, but this one contains

specifically only electrons and positrons, and not large numbers of hadronic particles like protons and so on as well. And these electrons and positrons are actually travelling faster than the speed of light in air, they emit Charenkov radiation as a result which travels through the atmosphere and you can detect using telescopes on the ground. There's two things. Faster than the speed of light will give us pause and Charenko effect will give us a second pause

so shall we do one at a time? Okay, so faster than the speed of light in air. Not fast than the speed of light full stop. So when light enters air, it slows down. It travels more slowly in air than it does in the vacuum or in water than it does in the vacuum. but charged particles which are travelling at very nearly the speed of light

won't slow down until the end of going interactions and so in air they can be travelling faster than the speed of light and that's what allows this churinco radiation to be generated what is the churenko radiation I feel as if I'm just serving lobs here

I'll tell you what it's absolutely fine by me but what's happening there is that the electromagnetic fields associated with the charged particle because they can't travel away from the charged particle at a speed higher than its own rate of travel through the air,

they sort of stack up, they get built up in front of the particle, and that produces this sort of shockwave of ultraviolet light, a little bit like when a supersonic aeroplane travels through air fast than the speed of sound, you get shock waves generated in the air, which you can

hear, and you can detect that UV light coming from these fast-moving particles. Caroline Crawford, Tim mentioned this earlier. at the possible dangers of cosmic rays for humans and electronic devices. Can you give us some idea what they are? Well, they're particularly relevant the higher you get in the atmosphere. So at ground level, this cosmic rays provide probably about 10% of the general background radiation.

Of course, it gets more at higher altitude. So by the time you're talking long-distance plane flights, maybe your crew might experience twice the sort of, general background radiation than they would at sea level. Still not at dangerous levels, but it is increased. And as soon as you get out to space, you're away from the sort of sheltering effect of both Earth's atmosphere and the magnetic field lines.

And both electronics and people are much more vulnerable out in space. So this is true for satellites. The electronics can be affected by cosmic rays. It can cause transient errors. It can corrupt data. It can affect memory. it can cause all kinds of effects and that's very important for satellites

you know that their safety and very vital components probably have to have some kind of magnetic shielding nowadays just to protect them from cosmic rays it of course comes much more important when you're talking about people, astronauts in space and the dangers that they might be exposed to

from these cosmic rays for a start there is an increased you know, it's sounding very serious now but there is an increased cancer risk cosmic rays can cause genetic mutations, they can affect the DNA, they can make you much more vulnerable to cancers and of course the longer you spend in space, the more exposure you get to cosmic rays and the higher your risk.

And this is a very serious consideration, both in terms of the electronics and the effects and people, if we are going to do long-distance space travel within our solar system because as soon as you move away from that protective influence of the earth, you're really out in the full blast of space. It could be very serious. Alan Watson, archaeologists today often date out. In fact, using it, excuse me, a technique called carbon dating.

How is that technique reliant on the existence of cosmic rays? Oh, totally. When I was talking to you about how the cosmic ray interacts in the atmosphere, I didn't give you quite the full story, as well as producing the pylums and so on, the cosmic ray essentially heats up the nucleus and some of the consequences of this are the production of neutrons and these neutrons interact with the nitrogen of the atmosphere

to form carbon 14. Carbon normally is carbon 12 but carbon 14 has two extra neutrons in it and this makes it radioactive. It has a lifetime, half-lifetime of about 6,000 years. Now it forms carbon dioxide And that is absorbed into our bodies Into any living thing like a tree Or any material that is alive

In the same way as carbon dioxide With carbon 12 as the carbon there It's something like one in a billion parts Of the carbon dioxide has carbon 14 Now when this organism dies If the tree dies Then the carbon 14 is not replenished So by measuring the ratio of carbon 14 to

carbon 12, you can date the specimen. And this was used to date things like the Turin Shroud, which was discovered to being a 13th century object, and also the Dead Sea Scrolls, which were something like 300 years BC. And that is an indirect consequence of cosmic rays, which was proposed by a man called Libby back in the late 1940s. He won the Nobel Prize for that work in 1960. Tim Greenshaw, what influence has the development of that research,

had on the discipline of particle physics? Cosmic arrays were the sort of first source of high-energy particles that particle physicists could use and they've led to a range of discoveries. So the first was really particle physics, I think was Anderson in 1932 who used a cloud chamber to study interactions but also placed in a magnetic field

so you could look at the curvature of particles induced by the interactions between the charges, of the particle and the field. And he saw actually an upward going track in one of his cloud chambers that passed through a layer of lead and emerged out

at the top of that layer of lead, obviously traveling more slowly. And because you could see that it was moving more slowly at the top section of the chamber, he knew it moved upwards, and from that he could deduce that the particle and the direction of the curvature that he could then infer,

he knew that the particle positive. Working out as well it's its mass charge to mass ratio he knew it was like an electron but with the wrong sign some wrong sign of charge so it was a positron. That was the first antimatter particle that was discovered and then subsequently the muon was the next one that was found that was again intermediate mass between protons and electrons and was therefore initially called the mesotron from the Greek meso for in-between. And that was eventually, after a fairly complex story,

that was discovered to be the muon rather than the strongly interacting particle which was initially thought to be. Caroline Crawford, what importance does cosmic ray research out for astronomers? Well, and usually for astronomy, we're not getting an idea of what an object looks like, you know, from the cosmic rays it admits,

but we're learning something about the physical processes within it. And the kind of process, these are the most energetic processes, you know, to try and fire up particles to those phenomenal speeds, they are some of the most, perhaps, most violent, most energetic regions

of our universe. So you have this... So these are neighbouring galaxies, perhaps. But yes, some of them in which enormous things are happening. Can you just give us some idea of enormous things that are happening, which will send these things off the high energy at high speeds? Okay, well, if you... It's not just neighbouring galaxies, it's perhaps neighbouring galaxies with, as Alan said, they've got a supermassive black hole, something, you know, tens of millions perhaps times the mass of our sun right at the core.

Now, it's not necessarily the black hole, but the twin jets of plasma. It spues out in two directions, these blasts through the galaxies, you get these enormous lobes of radio plasma. Here you've got all the right ingredients. You've got charged particles travelling at relativistic speeds. you've got large-scale magnetic fields. You've got all the combination you need, and perhaps there are shocks happening within these jets. The actual details are hard to pin down,

but it looks promising that it's that kind of energetic event that can produce some of these particles. Alan Watson, do you want to take that on? Well, I think what Carlin said is totally correct. One of the reasons for studying these particles is to try and understand the magnetic field between us and the source. that's very important for astrophysics, but also to try and understand the acceleration processes.

And if indeed there are particles coming from objects with black holes there, then studying the jets will be of real interest to astronomers. And in fact, particle physics will also benefit by studying these very high-energy particles because we're operating at energies way beyond the energy of the large Hadron Collider at CERN. And we don't know how to extrapolate, so we have to try and study the particle physics, as well. And we have had some success in that and measuring some

important parameter which relates to the probability of collisions. So there's a lot of things still to come from studying. So in a sense, this is terribly crude to me, but if it's far more impactful than the large hadron collider, you're able to the way you do

your research to be ahead of the way they're doing. No, no, I didn't say it was more impactful in terms of impact on physics. I mean there was higher energies involved. Cosmic rays will always be at the energy frontier because nature produces much higher energies than we're ever going to be able to produce on Earth. But the number of particles involved in an accelerator

allow you to make much more detailed studies but at lower energies. What about the connection with lightning and thunderclaps with cosmic rays by hand? Well, that actually is interesting because it goes back to the very early days when cosmic rays were first established. And CTR Wilson, who I mentioned already, He was actually more interested in atmospheric electricity than cosmic rays,

but he did speculate that cosmic rays might be accelerated in the thunder clouds. Now, probably it's not recognized that most of the thunder and lightning takes place actually in the cloud, only about 25% of the strikes are to ground. And people still don't understand how you get the very high voltages that you need to get these discharges. And it may very well be that the cosmic rays in the clouds cause little showers to take place

which ionise the gas, back to the start of the talk, they ionise the gas, and this allows a discharge to take place between voltages which are set up by movements of ice particles. It's a very, very complicated area of physics. And recently it's been identified that these cosmic rays processes

may take place because they've seen radio signals coming out of the clouds. It's quite complicated to explain how that happens. I'm just as well be conroying to a close. I mean, it's finally full stretch. I put it mildly. What do you hope future research briefly will reveal about cosmic rays? Well, I hope we managed to solve this puzzle,

this puzzle of where these ultra-high objects come from, and we may have cracked it. They could be active galaxies. We can't rule out even more exotic interpretations. And so there is also the potential that it may give us clues towards stuff we know even less about, like the dark matter. What about you, Jim? Why do you think it's... What do you hope it's going?

I think Carolyn's mentioning the dark matter. The Turingcove Telescope are rare, using the technique I've been discussing for detecting photons, may be able to identify dark matter, which would be fantastic. We may also be able to identify axon-like particles, which travel much longer distances through the universe than we expect. And we may also be able to use similar ideas to actually measure the integrated density of light that's been produced by all the stars ever since.

stars were first formed using the same kind of ideas. And if you're giving another 10 minutes, I'll tell you about it. That's an agenda. That's another 17 programmes. And we hope we're around to talk about more. Thank you very, very much. Carolyn Crawford, Alan Watson and Tim Greenshaw. And next week we'll be talking about the French anthropologist Claude Levi Strauss. Thank you for listening.

There are many more Radio for Arts and Discussion programs to download for free. Find these on the website at BBC.com.ukh slash radio four.