In Our Time - Radiation

Episode Date: November 12, 2009

Melvyn Bragg and guests Jim Al-Khalili, Frank Close and Frank James discuss the history of the discovery of radiation.Today the word 'radiation' conjures up images of destruction. But in physics, it simply describes the emission, transmission and absorption of energy, and the discovery of how radiation works has allowed us to identify new chemical elements, treat cancer and work out what the stars are made of.Over the course of the 19th century, physicists from Thomas Young, through Michael Faraday to Henri Becquerel made discovery after discovery, gradually piecing together a radically new picture of reality. They explored the light beyond the visible spectrum, connected electricity and magnetism, and eventually showed that heat, light, radio and mysterious new phenomena like 'X-rays' were all forms of 'electromagnetic wave'. In the early 20th century, with the discovery of radioactivity, scientists like Max Planck and Ernest Rutherford completed the picture of the 'electromagnetic spectrum'. This was a cumulative achievement that transformed our vision of the physical world, and what we could do in it.Jim Al-Khalili is Professor of Theoretical Physics and Chair in the Public Engagement in Science at the University of Surrey; Frank Close is Professor of Physics at Exeter College, University of Oxford; Frank James is Professor of the History of Science at the Royal Institution.

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, since the end of the 19th century, and especially since the dropping of the atomic bomb in 1945, the word radiation has carried a fearful resonance.

But in physics, it doesn't just mean the emissions of radioactive material. radiation is any process by which a body emits energy travels through a medium or no medium and is absorbed by another body. So the history of the discovery of radiation is the story of a process of uncovering the ways that apparently quite different forces of nature like electricity, magnetism and light are connected

and how they in turn are linked to quite new discoveries like x-rays and radioactivity and radio waves. Along the way, the discovery of radiation has allowed us to identify new chemical elements and to work out what the stars are made of. And this is a story of pure science and how it led to enormous social, military and economic projects. Would me to discuss the history of the discovery of radiation

at Jim Al-Khalili, Professor of Theoretical Physics and Chair in the Public Engagement in Science at the University of Surrey, Frank Close, Professor of Physics at Exeter College University of Oxford, and Frank James, Professor of the History of Science at the Royal Institution. Jim Al-Kalili, can you just give us a headline of, what for this discussion, radiation and radioactivity will mean? Yes, you described it well in your introduction, that people, when they hear the word radiation,

they immediately assume it's a bad thing, radiation poisoning, exposure to radiation, because they do tend to confuse it with radioactivity. But radiation, as you say, is just any emanation of energy from an object. So light, sunlight is radiation, heat is radiation. and radioactivity is quite specific. I guess we'll come to that later on in the programme. When we talk about radiation and most of the radiation that we'll be discussing today, we mean by it electromagnetic radiation.

So let's start at that, at the end of that, before we find out I get there. Can you give us a picture of the end result, the electromagnetic spectrum, which we're aiming for in this programme? Yes, as we understand it today, the electromagnetic spectrum, it describes radiation as waves. So if think of a wave, traveling through space, it has a frequency, it has a wavelength. If you think of the spectrum of light, the colors of the rainbow, visible light, at one end is red light, which is the lower frequency, longer wavelength part of the spectrum, and blue light or violet light is at the higher frequency,

squeezed shorter wavelength end of the spectrum. Now imagine that spectrum is on a line, on a grid, where you mark off say, either wavelength or frequency. They're two sides of the same coin. You can either talk about high frequency, short wavelength, or low frequency, long wavelength. So let's stick with wavelengths, so we don't get too confused.

So imagine visible light spectrum. That's Richard of York game battle in vain, isn't it? That one, yeah. So visible light on a grid marking off the wavelength. So you have red lights on one end and blue light, violet light on the other. Then imagine zooming out so we can see beyond the limits of visible light. So when you get to a longer wavelength than red, you move into the infrared region. You get to shorter wavelength than violet.

You go to ultraviolet. And you zoom right out. And we see there are all these other categories of electromagnetic radiation in the spectrum, all the way from the longest wavelengths, which are radio waves, through infrared, then the visible light part is a very tiny little slice through the spectrum, then through ultraviolet and up to the higher energy, very high frequency, short wavelength, part of the spectrum, which is x-rays and gamma rays. So that's the spectrum which we will arrive at by a process of, well, you three, really.

So let's decide to start the story with Sir Isaac Newton, who said that light consisted of particles, and about a century, that was in 70 to 4, about a century later, another British physicist, Englishist, Thomas Young, said that light is waves. Can you take us from particle to waves and tell us how that starts our story? Yes, Newton believed that light was, in fact, made of particles. He called corpuscles of light, so he thought there was tiny little bullets, you know, like little drops of water.

You don't actually see, you see a continuous stream of water, and so we see a continuous stream of light. Thomas Young carried out several key experiments which showed that light behaves like a wave, like the waves that we would encounter at school in ripple tanks. In fact, that was one of Thomas Young's famous experiments. If you have water waves in a ripple tank squeezing through a gap or encountering a barrier, and we see the way water waves spread out,

they diffract or the way they interfere with each other, he then did the same experiment with light and showed that light also behaves in this same way, the same as water waves. So light has these characteristics of waves, which is diffraction, interference, refraction, telling us that it's a spread out thing, it's not particles.

Franklis, did this enable the, although it didn't seem like it at the time, the discussion and the discovery, did this discovery enable it to go, forward? Well, it went forward in a confusing way in that, as Jim says, there was clearly a wave character to light, and yet at the times it appeared to act like particles, to go in straight lines. And so there's a lot of confusion about is light a wave or is a particle, and the whole resolution of that didn't come until 20th century with quantum theory, which we'll probably

get to later on in the programme. Treats to come. Treats to come. But Jim's nice picture of the electromagnetic spectrum, I think there's a beautiful analogy with sound that you have the whole piano keyboard, many octaves that we can hear, and the electromagnetic spectrum is many octaves, but we can only see one of them with our eye. And if you imagine that music was restricted to a single octave, we would lose all of Beethoven. And then you suddenly open up the whole piano spectrum. And it's only in the last 50 years we've had the ability to open up the electromagnetic spectrum and go beyond our eyes and use radio waves, as he mentioned, and then. and other things in astronomy. We're talking about these scientists, one after another, an experimental scientist, just in the pursuit of knowledge. It's a wonderful story.

So, can you tell us, we've established, Newton and Young, can tell us what William Herschel brought to the table? Well, William Herschel, about 1800s, he was bending light through a prism, like the raindrops

make the rainbow, so a triangular piece of glass will split light into its component colours. Richard of York gave battle in vain, or read out your green book in verse, is the other one that I remember. And he was measuring the temperature

while he was doing this, and whether it's by chance or design, I've no idea. But he had a thermometer, which was just beyond the red end of where the light was being split. The last light. The last part of the visible rainbow.

And discovered that the temperature there was warmer than the rest of the room. And then realized that there must be, some radiation beyond the red, we now call it infrared, that was heating things. And if you put your hand on your head, you will feel the warmth. We are emitting infrared radiation. And one of the applications of that today, of course,

is the ability of military to see in the dark. Or if you watch these police chasers on the TV, infrared cameras showing people trying to escape in the darkness and giving themselves away by their body heat. So Herschel was exploring that And at much the same time Joanne Ritter is exploring the other end of the spectrum The violet, the vein thing

And how does he arrive at his conclusions Well that was just one year later, 1801 And Johann Ritter was shining light on silver chloride Now those of us of a certain age I remember my father was an avid photographer Back in the 1950s and the good old days of black and white photographs Where you had silver chloride which was the medium.

And I remember that the picture would appear out of nowhere, very dark where the blue colours were, and rather lighter where the red end of the rainbow was. And Ritter noticed that when he went beyond the blue end of the rainbow, the silver chloride was getting even darker still. And from this he inferred that there must be what we will now call radiation beyond the violet. ultraviolet we now call it. Though the confusion in the history of science,

what's obvious to us today isn't obvious at the time of the discovery, I suppose, because he was using a chemical means and he thought he'd discovered what were chemical rays. The fact that these ultraviolet rays are part of the electromagnetic spectrum of light came on only later.

Frank James, can we just bring to bear in this the idea that these men are, it's an international group, switching from Britain to Germany to France, Italy comes in an American son, but they're working on pure knowledge. There isn't a sense of a project in order to discover things to change the world. So we're talking about independent.

How are they linked to these people? Well, they're seeking to discover the laws of nature that most of them believe that God wrote into the universe at the creation. And Thomas Young is a very good example of that who somebody starts off as a quaker, but then becomes an Anglican does his work. on interference and proposes a wave theory of light that doesn't actually get very far because it's severely criticized because in the early 19th century, Newtonianism was the orthodoxian people who criticized Newton,

even a century after his death, were able to get into serious trouble. But the key figure, the key figure who develops the 19th century wave theory of light that lasts all the way through the end of the century is Augustinell, who's a French physicist who works as an engineer for the French department, apartment of bridges and roads. And clearly he had enough time when he wasn't building bridges and roads to develop a theory of diffraction. And he argued that light waves must travel longitudinally.

And he develops this mathematically and proves it mathematically. The problem of a longitudinal theory of light is it requires an elastic solid ether because even young required an ether because a light wave's got a wave in something and that's something's called the ether. So the ether is very important, although it turns out to be. not important at all, but at this stage it's very important. And the analogy was that sound travels through air, and so waves had to have something to travel through.

And so they invented the notion and the word ether, who invented it and why was it so important for so long? Well, ether goes way back. I mean, Aristotle has the ether. I mean, Aristotle in the 5th century BC, it's the quintessence, the ether. That's the sort of fundamental thingness of everything. But why does it come into this, as it were, argument, this process of towards modern physics.

Why is it useful for them? You need a word to describe this imponderable weightless fluid through which light which transmits light waves. And that the scientist does call the ether because ethers got a very long history through the 17th and 18th century. So people like Young and Fennell simply pick it up and change it out of all recognition.

But it hadn't changed from Aristotle in one respect. There was an act of faith to think it was there. There wasn't any proof of this ether, was there? Well, that was, the ether was criticised by serious natural philosophy. People like David Brewster in Scotland was very unhappy with the notion of ether. Michael Faraday in London was equally unhappy with the notion of ether. It was exactly that reason because there was no experimental proof you were inferring it from observations of light. So we talked about light and then we bring in electricity and magnetism and this is becoming the sort of going to become the Trinity of modern physics.

Can you tell us about how physicists began to connect these things, electricity and magnetism? We bring a Danish person here. Hans Christian Erstead, who worked in Copenhagen in 1820, discovered if you had a wire carrying a kind of electricity, their compass needle, the compass needle was deflected. That's electromagnetism, one of the fundamental discoveries of 19th century physics. And if one wants to have a meta-narrative for the history of physics in 19th century, it's a seeking after the unification of the fundamental forces of nature

into one single scheme of nature. And Erster kicks that off by showing how electricity and magnetism in its new phenomenon that he calls electromagnetism. I'm interested in how, were these people writing to each other? Did they pick up each other's experiments? People who talked about, and everybody knew about Newton. But after that. Yes, I mean, everybody in Europe knew exactly what everybody else was doing.

There were three or four major journals in French, English, and French, and some Italians, which weren't so major, which carried information and translations of everybody else's work and people corresponded. So Faraday correspondent with Ersted, Ursulae Correspond with Arm Pair, everybody knew Thomas Young because he was Secretary of the Board longitude. So it's a sort of very tight, very small, coterie of natural philosophers doing this kind of work.

I was going to bring that, because now it seems massive to us, this development is what it led to, which is it's extraordinary, it's the modern world. But at that time, would it be regarded by, would most even intelligent education, people know that it was going on at all? Oh yes, because science at that point in the early 19th century was seen much more as a part of general culture,

what people should actually know something about. So you would get reviews of Thomas Young's theory in the Edinburgh Review, which is one of the sort of big literary quarterlies at the time. So, yes, the educated population throughout Europe would know about this sort of work. So you've got live, we've got the beginnings of electricity and magnetism of Frank Close. So pre-Faraday, we're into the 19th century. century. Where is the state of play in this development of radiation? Well, the idea that it's electromagnetic radiation still has to come.

There's Robert Bunsen, who discovers what we now call spectroscopy. Bunsen of the Bunsen burner that we all meet in chemistry laboratories found that different elements gave light with characteristic fingerprints, characteristic spectra. Why they did, nobody knew. That was just a phenomenon. the explanation of that would not come until the 20th century. But it was probably really Maxwell. That's going too far ahead for us. Is it? Yeah, yeah, yeah, we've got bags of time and Maxwell comes a bit later.

What was the relevance of this discovery of Bonsons? Well, the phenomenon that different elements shine in different colours, even though they didn't understand why, meant you could turn it around and say, if I see a particular set of colours, I can deduce what element is giving rise to them. Now, that's no big deal. If you're doing it here in the laboratory, if I've got a piece of sodium, I know it's sodium.

But if a star out there is shining its light at us and I put the piece of apparatus called a spectroscope in the way and I split the light up into its characteristic colours and then I go and look and see what these colours are the autographs for, I can work out which elements are out there making the stars in space. and so in a sense, by this you have got a means of getting messages from the stars in a sort of literal way. That was a very important discovery because 10 years before Bunsen and also Kirchhoved, Huddlberg, did that experiment. Auguste Comte, the French positive philosopher, said as an example of something that could never ever possibly be known in the future

that we would never know the composition of the stars, and then 10 years later, Vincent and Kirchhoff show how you can do that. Stay with you, Frank James. Let's talk about Michael Faraday, so we're pulling the story back of it. who arrived at the field theory of electromagnetism, which was a huge stage, and it's a huge platform in this staging post in this story. How did he arrive at it? What does it mean, and how did he arrive at it?

Faraday, in 1845, did two crucial experiments. The first was he discovered that light electricity and magnetism could be linked through something called the magneto-optical effect. And secondly, he showed that if you hung a piece of glass or indeed any matter between the poles of a very powerful electro-magnet. And in his basement at the barotry at one institution,

he had an electromagnetic made up half an anchor ring, wearing more than the tum, and hung a piece of glass between the poles. It aligned itself along the lines of force, as he called them, of the electromagnetic. So he showed that magnetism was a universal property of matter, and this allowed him to get rid of the notion of chemical atoms, because Faraday never liked atoms.

He didn't actually like matter that much as an analytical category. Instead of atoms wavering in an ether, which has been the orthodoxy since for now, what you have are points of force distributed throughout space and linked by lines of force. What you see as a chemical molecule is a combination of particular lines of force meeting at a particular point in space. What you see as a wave of light or gravitational traction is a vibration along those lines of force. And this faraday called the field theory of electromagnetism and became and remains one of the cornerstones of modern theoretical physics.

Can we, as this is such an important part, Jim Alkalili, can we just talk more about it about what this, as it were, take on from what Frank James has been saying? Yes, of course, Faraday was an experimental scientist, he wasn't a theorist, so he didn't work out the maths. That was something that came much later, or Frank James is shaking his head, oh dear. Faraday certainly wasn't a mathematician,

but he was a very important theorist, as both Maxwell and Einstein fully acknowledged. Well, okay. I'll retract and I'll say that Maxwell developed the mathematics of the electromagnetic theory, which Faraday had demonstrated experimentally. Can you just talk about the demonstration? Because it's so simple. Yes. I'm going down to that little laboratory, which is still there in the Royal Institute.

It's the most extraordinary museum in London, really. And you do this simple experiment. You think like this, in a little porridge bowl, change the world. Yes, it's the sort of experiments that kids do at school all around the world to this day, which is what Eustard had shown is that an electric current, when you switch an electric current on and off, changing an electric current, it creates temporarily this magnetic field which deflects a compass needle. Faraday was exploring that, and he showed that changing an electric current produces a magnetic field,

but changing a magnetic field can produce an electric current. So if he moves a wire through past a magnet, as it moves, an electric current flows through the wire. A current is induced in the wire. And the opposite is true that if you switch an electric current on and off, a steady current won't produce a magnetic field, but switching on and off or having an alternating current, a current that goes back because of forwards, which we sort of use commonly today, will generate a magnetic field. So he knew there was a connection between electric fields and magnetic fields. One could create the other.

And that's the key for electric technology. Because Faraday, in that work, it essentially invented the electric generator and the electric transformer, which from the 1930s on was completely altered the practice of engineering. And we wouldn't be here today in this studio without those discoveries.

So we're going from a tiny laboratory in the middle of London to changing the world, really? Changing the technological we were living, yes. Faraday is one of these people that we always hold up as an example. We always float from this little porridge bowl and a bit of wire in the mercury? Well, in terms of theoretical physics, it was the cornerstone,

that and then the work of people like Maxwell later in the 19th century altered the whole course of physics. As Frank James mentioned, it's the electromagnetic force is one of the four fundamental forces of nature. And understanding the nature of light, magnetism, electricity,

is one of the cornerstones of modern physics. Just to sort of wet the abidates of this, What did this bring about? Everything. That's easy. I mean, much of modern technology is ultimately rooted in our understanding of the nature of electricity and magnetism and light. And Maxwell's equations, which you can develop the T-shirt industry. Let there be light and Maxwell's equations are written underneath it.

We've got to Maxwell now, have we? No, not quite. There's a nice thing. You're very often. You're very reluctant, everything. You give a hold of modern technology. You're turning it to generalists. Very often, certainly...

Facts machines, do you want me to do. Physicists like Frank and me, often, you know, are told if you do blue sky research, they say, well, you know, what applications does it have? That's what I'm trying to get at. So if you're a particle physicist, you know, why are you looking for the Higgs boson? Why are you doing research into sort of nuclear theory?

And you say, well, it's not going to lead to a nonstick frying pan next week. We don't know what it's going to lead to. Take the example of Faraday. Of Gordon. It's always the... It's always the example. Take the example of Faraday. When he was messing around with coils and magnets and electromagnetic fields,

he didn't know he was going to invent the television. Well, he didn't. He didn't. Okay, he didn't go. Sorry, okay. He could go again. Correcting me. He didn't know the television was going to be invented much, much later.

Based on what he'd done. Would you give us that, Frank Joe? Yeah, absolutely, yes. That's just one of so many different examples. Anything that uses electricity, magnetic fields, anything that generates electric power relies on these experiments. So we've got Faraday with electricity and magnetism. We've started with the idea of light.

It's the bringing of the three of them together. Shall we go there first before we turn our guns on? I'm sorry about this, Frank Lewis. Before we turn to Clark Maxwell, you just have to ride a little longer. Can you say what part Light was playing in Faraday's work? Frank James. Faraday once described light as a great tool for exploring the world, and it's very, very important to Faraday.

And Faraday's major aim was to unify all the forces of nature into one scheme. Light, electricity, dynamic motion, heat, gravitation. He didn't succeed with gravitation. That wasn't been solved, so Farad is forgiven for that, I suppose. And he realized that by bringing those forces together, he would be able to develop a completely new view of how the physical world operated, and this is encapsulated in the field theory of electromagnetism. And that had an immediate effect technologically, because field theory allowed people like William

Thompson, later Lord Kelvin, very distinguished Professor Natural Phosphate Glasgow, to develop the theories that allowed long-distance telegraph signalling. And by 1856, using Faradies field theory, the first Atlantic telegraph cable was laid from Ireland to Newfoundland, and that provided the enormous Philip to show that field theory could describe electromagnetic phenomena in the way that other... previous existing mathematical theories, one has to say, simply could not have described that process.

Faraday conducted these experiments. Franklis, your time to shine. And James Maxwell got hold of them and turned them into mathematics. Incidentally, which rather distressed Faraday because he didn't see why they couldn't be expressed in prose and he chided in a letter. he well there's a letter to black maxwell saying when you talk to me about them I understood them

why do you have to put them in these mathematical formulations nevertheless he did take it on james clott maxwell can you tell us what he did with the ideas well as you said he encoded the fact that as jim has said electric effects can give rise to magnetic effects and magnetic effects can give rise to electric effects back and forth back and forth and his equations showed there was a sort of a wave electric to magnetic and back again electromagnetic waves waves travel they have a speed and hidden inside his equations the maths showed there was a speed to these waves

something which poetry alone would not have revealed he then took measurements that Faraday and others had made put them into his equations to work out what this speed is and he found it was a huge number it was about in old units 186,000 miles every second or about 300,000 kilometres every second, which turned out to be the speed of light, which had been measured sometime before.

And I think that is the moment of generalisation, that he has these equations, he's put the information in, he finds a speed for electromagnetic waves, and it's identical to another measurement of light waves speed. And from that makes the connection that light is electromagnetic radiative. electromagnetic waves. That is the first great thing that comes out of the

Maxwell's equations. The second thing is that his waves have a frequency, an oscillation or a wavelength. As Jim has said, the visible spectrum that we know varies between one wavelength and another. But Maxwell's equations say there are

waves with wavelengths shorter than these and bigger than these. And indeed, infrared and ultraviolet have already. been discovered by Herschel and Ritter about 70, 80 years before. This is the first hint that Maxwell's really onto something. So what about the rest of the spectrum? And of course, later, as we will come to,

all the rest of the electromagnetic spectrum gets revealed, in particular the one that we're using right now, radio waves. Do you want to add, Donovan? Yeah, I mean, Frank James. Maxwell and Faraday knew each other extremely well. When Maxwell was made redundant from Aberdeen University, he came to London to teach at King's College and we'll trot along to one institution to talk to Faraday.

Faraday is a bit ambivalent towards Maxwell's work. I mean, he does, as you say, so write to Maxwell saying, do you have to put your equations in these high glyphs, as Faraday puts it. But on the other hand, he writes to one of his nieces saying, Professor Maxwell has done wonderful things with my work. He's mathematicised my field theory, or worse, that general effect.

And Maxwell, in his turn, acknowledges Faraday's contribution. Maxwell actually says in his biographical memoir, sorry, Maxwell says in the biographical memoir of Faraday that Faraday had the electromagnetic theory of light in 1846, but the one thing that Faraday didn't have was the velocity of electricity and of light, and that's the key to Maxwell's work, and that's enormously generous, I think,

on Maxwell's part to acknowledge Faraday in that way. Jim, Jim, I'd just wanted to add that Maxwell's equations, as Frank Cloose explained, describe how electric and magnetic fields are connected to each. other. It's something that all physics undergraduate students have to learn to go through, this going from these Maxwell's equations involving fields through to this, what's called the wave equation. And I always remember this was the highlight of my undergraduate physics career in lectures on electromagneticism when they go through the algebraic derivation, they end

with this equation that has nothing to do with light and suddenly out pops this constant and say, and there is a speed of light proof that electric and magnetic fields travel at the speed of light. And it was a shiver down the spine moment for me. I remember looking around at fellow students and no one else seemed to be as impressed. I was a bit worried. I was being rather too keen. I remember somebody telling me that Maxwell's equations

were the equivalent for him of music by Mozart. And he said they would last longer. Frank close. No, Jim said it beautifully. I remember we all spend a whole term learning electricity. And then we turn another term learning magnetism. And then the wonder, when you discover that they are unified, into electromagnetism.

And I think that in the modern language, what Maxwell did in unifying two areas of science, electricity and magnetism, is a symbol of what we're trying to do today, which is to unify all the forces into some united things. So Maxwell was the start of that.

Frank James, Maxwell came up with something called the displacement current, for which you got quite a hammering. But what was he saying there? Well, basically, he was saying that mathematics is primary, that even though you have these sort of equations, there are terms in those equations that had no obvious physical meaning at that point. And his fellow Scott, William Thompson, next to Lord Kelvin,

was severely critical of Maxwell for putting some of these terms in his equations. And Maxwell said, well, look, the equations work. The fact that a term doesn't have a physical meaning at the moment doesn't mean it's not going to have physical meaning at some point in the future. A German physicist Frank Close, Hertz, Heinrich Hertz, managed to make something of the displacement current, he managed to make it a reality. Why is that important?

Well, what Hertz did was he is credited with being the first person to discover what we now call radio waves. And that's why Hertz is such a common word, killer Hertz and so on for the frequency of the radio waves as you turn across the radio dial. What's not widely known, though, is that completely independently in Britain, Oliver Lodge, independently discovered the same phenomenon

at about the same time and instead of announcing the fact and writing it off went off on holiday and while he was on holiday he read about what Hertz had done and with great generosity he never sort of made any grand claims of his own he always mentioned Hertz as the discoverer but it's generally agreed that Lodge

completely independently discovered the things and then at the British Association meeting in September that year he gave a demonstration of the radio wave phenomenon in Oxford and I'm told it in the Pitts River's Museum and here I am from Oxford saying I've never actually been in to see if there's a plaque

and if what I'm saying is correct there ought to be and if I'm not saying the correct thing this is the problems of live radio the benefits of what Hertz has given us but that is how more than one person often discovers something but only one person ends up in history being credited with it can we talk about

x-rays now Jim and they come into the I'm not in strict historical order but let's talk about those now. Who gets onto that and what part do they play in this building picture? Well, now we're reaching

the last decade of the 19th century, by which point, I should say, that physicists were pretty convinced they had discovered everything that needs to be discovered. Newtonian mechanics together with Maxwell's electromagnetic theory covered all phenomena in nature.

And then in the 1890s, there are these several discoveries, one of which is the discovery of x-rays by Rensken. And they're called x-rays in his experiment because he knew there was this emanation, these rays radiation of a type that he didn't know.

So X for the unknown, that's entirely where the name comes from. And it was only much later that it was appreciated that these were connected with electromagnetism. At the time, they had no idea what they were. Frank James, can you tell us the significance of that discovery?

Well, x-rays and then radioactivity discovered by Beckerwold, the following year, caused real problems for classical physics. Because although Maxwell had argued for the primacy of mathematics, he still believed in the ether, and he developed extraordinarily complicated mechanical models of the ether. But when you start getting x-rays and then radioactivity, trying to reconcile that into classical physics becomes very, very difficult indeed.

And so somebody like... So what they're saying, just to start to keep the listeners, to keep us all, me, as one of the listeners, the ether is now being challenged, the idea that there should be something for... Not quite, not yet. Not being challenged yet? Not even with the x-ray.

When he does the x-ray at the hand and shows that there's no ether involved because he shows that x-ray goes around. As Jim says, they didn't know what it was. So it only starts challenging the ether when people tried to explain it in terms of the ether. And people like William Bragg in Adelaide and South Australia, does some pretty fundamental work,

both on nature of x-rays and nature of radioactivity, trying to reconcile the phenomena into classical physics. William Thompson does the same, G.C. Stokes does the same. All these Cambridge classical physicists try and bring x-rays and radioactivity into the ether, and they cannot do it. And that's the point at which people start to realise where perhaps ether has problems

and phenomena that simply cannot be explained in those terms. Well, when is the ether finally discredited? And when do people, when does the company of physicists say it doesn't... It's Einstein. And it never did... 1905. It's 1905. So that's Einstein.

A lot of other physicists had laid the groundwork for Einstein's special theory of relativity in 1905, but he's the person who makes the leap. So a lot of the mathematics has been developed by people like Lorentz and Poincare, a mathematical physicist. But Einstein's the person that makes the leap. And his leap is that the ether doesn't exist at all. electromagnetic waves, such as visible light, don't need a medium to travel through. They're not like sound waves that needs the air to travel through.

Water waves wouldn't exist if there wasn't the water. The medium that oscillates to allow the way to travel. So light electromagnetic radiation is not the same. It doesn't need an ether. It can just travel through the vacuum of empty space. And the other thing that Einstein does is to show you do not need a special frame of reference. and that's also very important in getting rid of the ether because the one thing that ether still does

up until 1905 is it acts as a special frame of reference for determining where forces and bodies move and Einstein gets rid of that as well. But Faraday, he had decided many years before that there wasn't that ether, hadn't he? Anyway, Frank, you were going to, Frank, close, you were going to say something else. Yes, I was just going to say,

I mean, Maxwell's equations, of course, which had this speed in them, which turned out to be the speed of light, turned out to be what we now call relativistically invariant. The speed of light is the same whether you're travelling this way or that. It doesn't matter what the relative speed

of the light emitter is relative to you. It always comes out the same. And that was one of the apparent paradoxes that led Einstein to his new view that space and time must somehow be changing to take this into account. And then, of course, afterwards, it was realised that Maxwell's equations

contained this within them. I think a lot of the debate Frank James will know perhaps better than I, is to what extent Einstein was aware of this when he was creating relativity or whether this was only realised after the event? Einstein certainly knew that Maxwell believed in the ether,

and he tended to lump Faraday and Maxwell together. So he refers to Faraday's Faraday Maxwell theory of the ether, which is grossly unfair on Faraday because Faraday did not believe in the ether. So when the ether was dispersed by Einstein, with others, did that change the way? people attack this subject? Jim? It took a while, but of course we have to remember that at this time

there's another development in physics that's taking place at the beginning of the 20th century, namely quantum theory. And that overlaps with some of these ideas on a number of fronts. So people like Max Planck, the German physicist in 1900, is the first person to say that radiation, I think he was talking particularly about a particular type of radiation. heat radiation, comes ultimately in tiny bundles and packets of energy called quantum. And Einstein takes up that idea and applies it to light,

which leads to this notion that light actually need not only be thought of as waves, but you can think of light as particles, bundles of energy. Can I jump forward? Frank, you say what you want to say. Frank close. I'm actually going to sort of jump slightly back because the quantum, of course, leads into the whole 20th century, understanding of the nature of light and why there are spectra. But of course the radiation question, I suppose, really, is Rutherford, who appears on the sea. I was going to say to Rutherford, so you're going forward. Let's go sideways.

Thank goodness that we're at last travelling in the same direction. Or in the same frame of reference. There's a lot going on at this time. I know, but still, we can't get away. We must address radioactivity. And we will do it through Rutherford, New Zealander who came over here and spent most of his created discoveries here. Right.

What's his significance, you Mark, Malcolili? Well, Rutherford took on the study of radioactivity after it was first discovered by Bacarell, and he's working on the same subjects as people like the Curies. And in a sense, this is a bit like the x-rays. It's energy, it's radiation that they can see having an effect, but not knowing what it's made up of. So Rutherford comes up with the names, for a shorthand name, called alpha, beta and gamma radiation.

just like x-rays. These are just letters to denote different types of radiation that he doesn't quite understand the nature of and what they're made of. And it only turns out subsequently that only one of these three types, namely the gamma part, is actually electromagnetic radiation, very high energy electromagnetic radiation, and very closely related to x-rays, which turn out to be electromagnetic radiation.

The other two, the alpha and beta types of radioactive emanations, are in fact particles of matter. One of the great iron is that Rutherford arrived from New Zealand, as you said, in 1895, the same year that Runcan had discovered X-rays, and J.J. Thompson, in charge of the Cavendys Lab, wanted Rutherford to work on these new radiations. But Rutherford, as a student in New Zealand, had been experimenting with radio. In fact, he was one of the most advanced people in radio at that time,

and he wanted to work on radio waves. and so Thompson consulted Lord Kelvin, who we've also mentioned earlier in this programme, and Kelvin opined that there was no future in radio, as a result of which Rutherford was put to work and the rest, as they say, is history, and here we are. Kelvin was getting old by that point. Can we have the rest of history briefly, right?

What did Rutherford do that, again, help to change, make the, have to change the world we live in? Well, one of the things Butherford did was to show the notion of chemical elements is not a stable one. There isn't a particular type of atom, that is associated with a particular chemical element

that you can have different types, different arrangements of atoms. And Jim can talk about this a lot better than I can, but that's one thing he certainly did that made a significant difference. But this dread word radioactivity comes out, well, the word that can be using. Can you just explain how that came out of what Rutherford was doing?

Yes, Rutherford was studying the nucleus of the atom. In fact, he was the person who first looked inside atoms to realize they have some internal structure, this tiny, dense, positively charged nucleus. And the radiation that comes out of the nucleus is what he called alpha-beta and gamma radiation. And so that led to the whole field of nuclear physics, understanding how atomic nuclei can fuse, can break up,

confusion, leading to things like the chain reaction and ultimately the bomb. But because that's such a powerful image, the mushroom cloud, for instance, and nuclear radioactivity is seen as such a menace, so evil, that somehow there's this link that radioactivity, even though, of course, in nuclear medicine, I mean, it's a wonderful discovery we can make use of these particles to cure disease.

It's such a powerful image that radiation and radioactivity have somehow been mixed up. But only one part of radioactivity is electromagnetic radiation. And that's gamma rays. So the distinction between, now that we understand, the distinction between gamma rays and x-rays is that x-rays is the electromagnetic radiation given off by electrons orbiting around the atomic nucleus. Gamma rays are the electromagnetic radiation given from within the atomic nucleus. That's the distinction between them.

Very close. No, I think that's a beautiful demonstration. And, of course, the use of radiation, the mushroom cloud is the image that people have. and of course we all know the damaging effects that that has had on people. But at the same time, if you put your radiation in the right place, in medicine, for example, you can kill the very cells in cancer that you want to kill. So, I mean, for me, I would say my aphorism about radiation to students is,

in the wrong place it's a killer, and in the right place it saves lives. Well, thank you all very much. I'm sorry I rush you, but there was so much it was interesting there. And I wanted to know about it, and I hope that everybody else did who was listening. Thanks very much to Frank James, Frank Clace, Jim Al Kalili. Next week, we'll be talking about the history of Sparta. Thank you for listening.

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