In Our Time - Maxwell

Episode Date: October 2, 2003

Melvyn Bragg and guests discusses the life and ideas of James Clerk Maxwell whose work is not widely known, but whose genius and contribution to the age in which we live is enormous.He took the first colour photograph, defined the nature of gases and with a few mathematical equations expressed all the fundamental laws of light, electricity and magnetism - and in doing so he provided the tools to create the technological age, from radar to radio and televisions to mobile phones. He is credited with fundamentally changing our view of reality, so much so that Albert Einstein said, “One scientific epoch ended and another began with James Clerk Maxwell”. But who was James Clerk Maxwell? What were his ideas, and does this nineteenth century ‘natural philosopher’ deserve a place alongside Newton and Einstein in the pantheon of science? With Simon Schaffer, Reader in History and Philosophy of Science at the University of Cambridge; Peter Harman, Professor of the History of Science at Lancaster University and editor of The Scientific Letters and Papers of James Clerk Maxwell; Joanna Haigh, Professor of Atmospheric Physics at Imperial College London.

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the programme. Hello, he took the first colour photograph, defined the nature of gases, and with a few elegant mathematical equations, expressed all the fundamental laws of light, electricity and magnetism.

And in doing so, he provided the tools to create the technological age, from radar to radio and television to mobile phones. He's credited with changing fundamentally our view of reality, so much so that Albert Einstein said, one scientific epoch ended and another began, with James Clark Maxwell. But who was James Clark Maxwell? What were his ideas,

and does this 19th century natural philosopher deserve a place alongside Newton and Einstein in the pantheon of science? His relative lack of fame is puzzling. With me to discuss his legacy, is P.M. Harmon, editor of the scientific letters and papers of James Clark Maxwell, and Professor of the History of Science at Lancaster University. Simon Schaffer, reader in history and philosophy of science at the University of Cambridge,

and Joanna Hay, Professor of Atmospheric Physics at Imperial College London. Simon Schaffer, Maxwell was born at the beginning of the 19th century. Can we just briefly say born parents' education up to Edinburgh? So Maxwell was born in the great city of Edinburgh, the Athens of the North, in 1831. was the son of a relatively wealthy and slightly eccentric Scottish lawyer and he was brought up in that rather characteristic Scott's way marrying together urban life with the rural estate. His father was indeed a laird in rural Galloway. And Maxwell had charmed early life.

His mother died when he was very young, and it's been argued that that had a profound effect on him. He was a boy of extraordinary ingenuity, clarity and curiosity. He went to school in Edinburgh, to the school in Edinburgh, in fact, which had been founded to turn Scotsman into English gentlemen, and founded by Walter Scott to do that. He went up to Edinburgh University, as was common for his class, and was exposed there to really a very advanced and profoundly influential course in natural philosophy and mathematics and in general philosophy. I think it's the exposure to a wide and deep philosophical education,

which begins to mark Maxwell out as a natural philosopher of genius. Two more things, that was terrific. He started to write scientific papers very young, so when he was 14, as I understand it, he wrote something of serious significance. Yes, he had that mathematical precocity, which I think distinguishes people of the kind of mind that Maxwell shows us. It was well known that if you stick two pins into a piece of paper

and tie a string round the pins and then move a pencil with the string, you can make an ellipse, you can make an oval on the sheet of paper. But what Maxwell began to do, and I think this is absolutely typical of his kind of intelligence, was to explore the different kind of geometrical figures, the different kind of patterns in space. You can make, if you move the pins, if you wind the string more than once, and so on. It's pushing at the boundaries, taking apparently simple ideas, and exploring the way in which they show really deep structure that I think is important.

important there, and also the fundamentally spatial, topological intelligence that Maxwell has. So this at 14, at the same time as he'd been, as I understand it, because this might be relevant later on in the discussion, he'd been obliged to learn the Bible by heart? Yes, he was brought up as an evangelical Protestant, and that kind of faith stays with him for the whole of his life. It's an intensely private faith. He was very resistant to the spectacular public display of religious emotion, which some of his, in fact, many of his contemporaries indulged in, just as he was extremely skeptical of those who saw a fundamental conflict between science and religion. For Maxwell, there was no such conflict, but one simply didn't make a song and dance about it.

At the time, before we moved on, at the time that he went to Edinburgh, and was at the university in the middle of the 19th century, was the great figure in science still Newton? Was it Newton and his ideas which still dominated, or had other figures surged up? What was the, broadly, and I know this is broad brush-struck, Simon, but you're being very helpful. Broadly, what was the primary colours on the intellectual palette?

I think it was still absolutely the case in the 1840s that if you wish to show that your science, your physical science, made sense, you showed the way that it came, out of Newton's ideas. And there were many different Newton's, as it were, that there were many different ways of making sense of what Isaac Newton had done more than 150 years earlier. But in the Scottish university curriculum,

Newtonian natural philosophy remained, it seems to me, at the core of one key aspect of the physical sciences. The idea that what there is in the world is matter and motion, the idea that the mathematical analysis of force, acting between bodies held the key to understanding phenomena here on Earth and in the heavens. The idea that mathematical astronomy was the key, the queen of sciences, the pattern science, and the idea that by building up from what Newton had done in the theory of light and color,

as well as the theory of gravitation, you could bring more and more phenomena under the remit of physical explanation. Those were still some of the dominant themes. Peter Harmon, which of the... Did Maxwell consciously take on the Newton inheritance? If so, in which areas? We're still talking about Maxwell as quite a young man at Edinburgh or when he begins to move to Cambridge. It's around then.

No, I don't think one can talk of him directly taking on a Newtonian inheritance. The sort of patterns of thought that Simon has been describing functioned as kind of general vectors, general modes of thinking about the world. Maxwell inevitably located himself within the problem area of his own day, which of course wasn't directly necessarily concerned with the Newtonian, the classic Newtonian problems, or certainly not in the way in which Newton had been. So take, for example, the third of the papers he wrote when he was still an undergraduate at Edinburgh University. A really quite remarkable piece on a subject, the equilibrium of elastic solids it was called.

But it involves a sort of vast familiarity with a lot of difficult current mathematics developed both in France and indeed in this country in Cambridge. It involved experimental investigation of a quite sophisticated kind, and it involved also familiarity with the latest work, experimental and theoretical, on rather technical issues within that structure. And all of this then, even before he'd gone as an undergraduate onto Cambridge, to study the mathematical tripos there, which gave him the kind of basic structural grounding that he was going to need, he'd already moved well within the world of current scientific theorising.

Newton used the term action at a distance to describe how gravitational and magnetic forces act instantly from one object to another without passing through any third medium necessarily. Can you explain what Newton meant by that and why Maxwell thought that wasn't a satisfactory explanation in what developed from that? Well, Newton didn't actually think it was entirely satisfactory either,

which is a bit of a problem. But in Principia Mathematica, his great work of 1687, Newton, among other things, sets out this law of gravity, which is, as you say, a law of instantaneous action, supposing a gravitational force acting between bodies such as the Earth and the Moon. And the great achievement of that really is to show how the fall of an apple is to be explained in the same way. way, say, as the path of the moon in its orbit. Now, Newton was unable to explain what this force, he calls it a force or power, what this was. And he was reluctant to draw upon the kinds of modelling which other of his older contemporaries had engaged in, which would have been to explain this force in terms of the motions of particles. Newton didn't think that ultimately the rudiments of nature could be totally explained in that way.

Why not? Well, if you did that, he thought, you would be explaining nature in a way, he called it, the mechanical way. And this would somehow... He wanted to leave room for God. He wanted to leave room for divine agency. And so as time went on then, as he was pressed to say, well, what exactly is this force of gravity? he was unwilling to invoke some sort of medium, mechanical medium or ether, although ultimately he does suggest such a hypothetical substance.

At the same time, he was cautious about saying, well, it's all due to divine agency because somehow one didn't quite do that. So he said the reader of this thesis should make their own mind up, whether it's material or immaterial? In a private letter, he said that. He said it. Indeed, he said it. He said it in a private letter, and somehow the Newtonian message came through that... So why did Maxwell think this was unsatisfactory, and what did you do about it? Well, what was unsatisfactory about it, is that you were left with a purely mathematical equation. Newton himself said, okay, well, I've given you the equation, I've given you the law. That's really all you need to ask me. Don't bother me with any of this other stuff.

But really, that didn't seem satisfactory in Newton himself in the way that. I've been indicating, and nor that it seems satisfactory to men of Maxwell's generation. And what spurred them on, really, what spurred Maxwell particularly on, were the experimental investigations of Michael Faraday from the 1830s onwards in electrostatics, or the theory of static electricity initially, and then on into magnetism in particular, which was particularly fecund in this area, in the 1840s and into the early 1850s. And that seemed to suggest that these forces of electricity and magnetism were in some sense mediated.

And the word magnetic field came to be used by Faraday, for example, to describe what was going on. Faraday was able to show that certain kinds of magnetic effects could only be explained if there was something sort of mediating the forces. And this then provided the spur to move away from an actual. to distance model towards some other kind of model which would focus on mediation. Joanna Hagen, the substance through which this forces passed

was called at the time the ether. Maxwell accepted that and used that. We know now, well, people like you know now, that the ether doesn't exist, but it helped him greatly in his experiments and moves forward. Could you tell us how he dealt with this ether? Well, Maxwell did believe in the ether,

but actually he didn't necessarily need to use it in setting up his theory of electromagnetism. What he did was he used the results of existing experiments such as those of Faraday and Gauss and Ampair and put them all together in a sort of unified theory with the mathematics, and solving the mathematics, he showed that there would be this electromagnetic field.

Now he thought that there would have to be an ether through which the electromagnetic field would pass. and throughout his life he maintained that there must be an ether. However, he realised he couldn't... Why did you think there had to be an ether? So we talked about, say, the light coming from the sun to the earth must pass through something called an ether.

Well, if you think of light as waves, so it would be a bit like you'd imagine that there would have to be something within which the waves would carry. So that's what I think, you know, he thought. Did he think of light as waves? Well, he produced this theory of electromagnetic waves. Then he showed that, in fact, if you looked at the speed of propagation,

of electromagnetic waves, that is the speed of light, which had been determined experimentally. Thus he concluded that light was an electromagnetic wave. And then if you think of it as a wave, you might think perhaps, well, there must be a medium through which the wave has to be transmitted. Because waves can only go through a medium. Yes, exactly.

But, well, no, not, I mean, not, but that's what he thought. If you think about sound waves, you need a medium or waves on the surface of an ocean, you need a medium. Actually, electromagnetic waves can pass through a vacuum. But Maxwell had a large enough mind to realise that he couldn't understand the ether. And so he sort of put it to one side and continued with his theory whether or not the ether existed and therefore was able to develop it as far as he did. So how did he come to this idea that light is an electromagnetic wave?

And what significance did that have in terms of the thinking of the time? It's the beginning of a unifying theory, isn't it? Yes. Because at that time, in the mid-century, as I understand him, gravity, electricity, magnetism, heat and light were, those core things were understood, but they seemed to be separate. They were all considered to be separate aspects of science. And the key development that Maxwell did was, as you say, produced a unified theory of electricity and magnetism. And he was able to do that by taking the experimental observations, writing down mathematical equations, representing them. but the key aspect that he also had to do was introduce a new idea

which hadn't been put forward by anybody else which was an extra sort of fictitious current that you need to explain how a magnetic field can be produced in certain regions for example around a capacitor when you're charging up a capacitor as a magnetic field ampere would say that the magnetic field would come from a current now clearly there's no current passing across the capacitor so he came up with this new

current which is called the displacement current, which isn't a passage of electrons, but it is a changing electroelectric field. When he inserted that into the equations, he could then produce this whole theory in a mathematical sense that you can then solve the equations and produce the electromagnetic field. And you could, using the constants in the equations, which were derived from experiments, and then produce the speed of the propagation of the electrical. of magnetic waves. How original was this then? Entirely original. I mean

the components, the observations that currents call magnetic fields or changing magnetic fields induced currents were all established. But the actual representative representation of these theories in terms of vector calculus

and then actually combining them together into this one theory was completely new. So Simon Scherper, where does that put this in the history of physics? Where does that put this this discovery theory of Maxwell. Well, I think its significance is overwhelming, and you started by quoting Einstein.

And certainly Einstein looked back to the Maxwellian theory, as he understood it, as the foundation stone of an entirely new view, not only of what physics might be able to do, but in a certain sense what the relationship between the most fundamental components of nature are the components of space, time and matter. So, in retrospect, looking back at Maxwell's electromagnetic theory,

it's clearly decisive. It's important, I think, also to remember that it was pretty controversial. Some of Maxwell's closest friends, closest scientific friends, like William Thompson, Lord Kelvin, the head of physics at Glasgow, remained pretty skeptical about many of the most important elements of what Maxwell was saying here

about ether, electromagnetic waves, and the identity between electromagnetic radiation and light. I know you've all got masses more to say about electromagnetism. We're now going to move on to gases, I'm afraid, because the man did a lot, and this is an introduction, and here we go. I'm going to start with you this time, Joanna. The first great achievement is to do electromagneticism,

and that later on I hope will say what that led to and so on. But his second concern gases, and it's fundamental in an area in which you work, which is the Earth's atmosphere, can you explain, I'm sorry about this, here we go, can you explain Maxwell's work on the kinetic theory of gases? Can you explain it in terms of cigar smoke to start with, which might give us a bit of a hang of it?

Well, yes, cigar smoke, you see battering around in the air in a way that's referred to as Brownian motion, which was first observed by somebody called, brown and this led people to believe that in fact the air was composed. Because, you're saying, because when you puff out of cigar, the smoke goes, seems to go at different speeds to different parts of it. If it's drifting westward in the room, it seems to go at different speeds. Yes, and it's also sort of clitoring because it's moving around within its own body. Yeah. So that was a puzzle which then led to.

To the idea that gases were composed of lots of small particles. Now that wasn't Maxwell's idea, but what Maxwell did was he thought about how the interactions of the particles would influence the large-scale properties of the gas. So, for example, that the temperature of the gas depends only on the speed of motions of the particles. Now, other people had thought that, but they thought that all the particles must be moving at the same speed, just hitting off each other.

what Maxwell did and simultaneously this was done by Ludwig Boltzmann was deduce that in fact there would be change of momentum between all the particles as they hit each other and that there therefore must be a distribution of speeds and he was able to show mathematically what this distribution of speeds would be

so you'd have such a proportion going slow such a proportion going fast. When you have to show mathematically what did you do just sat down and did you do experiments to get to this? Did he sit down? No, you can actually do it mathematically, you can consider you have a certain function that you're trying to derive,

and then mathematically in three-dimensional calculus, you can derive the form that this equation must have. Just like mathematically, he worked out what Saturn's rings would be made of, and 100 years later when they got there, he was proved right, but it was just mathematically. Yes. That was made. Not so much.

That was much. People listening, I just hope you're hanging on. So he worked out. Now, what does that signify? Well, I ask Peter, what does that signify, having worked out? Well, the development of the distribution function, the distribution of velocities, came from very specific technical questions in the theory of gases. And by introducing it, he was able to answer a number of quite practical experimental issues to do with the measurements of gas viscosity and other kinds of features which experimentalists encounter.

in work on gases. But once you take that, put that aside for a moment, you then find that he's developing the insights and implications of this notion in a very grand way, entirely characteristic of his way of thinking. And he starts to think about the way in which nature itself is in some sense fundamentally statistical and can only be understood in a statistic.

way. And he argues that one of the two great principles of the new physics emergent from the 1850s, which became known as the second law of thermodynamics, the first law being the law of energy conservation, the second law being a way of theoretically understanding the way in which heat always flows from hot to cold. And Maxwell was able to show how this law, this fact, if you like, that heat does flow macroscopically from hot to cold could only be understood as a kind of phenomenon of statistical motion. And that even though individual molecules within a gas, which would, because of the distribution of velocities, could have varying velocities,

could indeed go the other way. could go from cold to hot, i.e. particles could move against the current, so to speak, that overall, in the mass, the distribution function showed that the gas particles would flow from areas of high velocity, i.e., where you have heat, to low velocity, where you have relative cold, and that this fundamental feature of nature, the second rule of thermodynamics, could only be understood statistically. And this creates then the whole basis, which is really fundamental, stands at the foundations

of a great deal of modern theorising, of understanding nature in statistical terms. There was something, a figure was introduced, called Maxwell's Demon, Sipan, Schaffer, in this respect of... What's the significance of the demon? And then I want to try to get to grips with what we really mean here by statistical, because most people think it's the number of unemployed. Well, Maxwell actually thought it meant the number of unemployed as well.

And the introducing of the idea demon just puts up a tiny flag if they've got time to interrupt the scientific flow of this conversation that he was a joking man and set questions in verse and exams and played with catch and shame. We'll have to leave that aside. Let's bang on back with the significance of the demon in the second-law thermodynamics. Well, as usual with the history of science,

it's even more complicated than that, because Maxwell actually didn't like the fact that it was called. or the demon and it was his friend William Thompson who called him that. Maxwell rather thought of a railway points man which is all too actual image for us these days. Imagine that you have two boxes

and imagine that they're connected by a door and imagine that there's a very small being sitting at the door who can open and shut the door and this being is so smart and so clever that when a fast molecule

comes from one compartment towards the door, he can let it through. And when a slow molecule comes from the other side of the door, he can let that through, keeping the same number of molecules on either side of the door. If he keeps on opening and shutting the door in this incredibly clever way, then he can get heat to flow from a colder to a hotter body. He can get the part of the box which has slower molecules to look as though it's heating up the part of the box that has as Hodder molecules. Now that's, as Peter just explained, a violation of the second law of thermodynamics or the principle of laziness, as William Thompson called it, in contrast with the principle of greed, which is the first law of thermodynamics. And that was a way...

We're talking these terms from the start. Well, exactly. And that was a way Maxwell showed, and I think it's a beautiful way of showing that what we might think is a determinist, mechanical principle, that the universe is. running down that available work tends to dissipate. That's why it's called the principle of laziness. Well, that principle, although it's certainly a physical law, is not in the end a determinist law. It's a probable statistical result. On the whole, that's the way things go.

When he was pressed by one of his friends to really make this clear for people who had a hard time with natural philosophy, he said, yeah, it's exactly as if you poured a glass of water into the ocean and then dipped that glass into the ocean and got exactly the same water molecules back. It's not impossible, but it's really, really unlikely. Exceedingly improbable. Exceedingly improbable. I'm going to come back to you in a moment to ask what the practical result consequences,

two or three generations, even a hundred years later, of these theories were. But I just want to hammer on for one more round with you, Joanna, on the difference between dynamical and statistical. Why is it so important that he looked at things from a statistical with a statistical eye from a statistical view? Because that word is now kind of ruined

in a way, and I'm trying to flailing around, to try to sort of give it a different sort of energy for this discussion. What does it mean as far as you're saying? Why has it got such weight in the discussion of Maxwell's ideas? Well, in a mechanical idea, you would have to describe or predict the individual positions of each of the molecules in the gas and thereby no properties.

So you'd have to put it out precisely, seeing that every single... If you wanted a perfect description of the system, that's what you'd need to do. Like certain people think that that's what's going to end up by how we understand the brain. We know every single thing that happens, therefore we will understand the brain. That would be a parallel, yes. Just a second. I wanted it out. Honestly, it's difficult enough, Peter. Right, here we go, Joanna. So clearly, in that sense, you need some sort of representation of this whole body of molecules.

And then you say, well, what is the average property? You're beginning to do statistics when you talk about averages. And then you say, well, can it deviate from this? And you get more statistics. And when you started thinking about this, he's laying down the foundations for the whole statistical physics in a sense, which is then able to lead on to things like semiconductors and low-temperature physics and all these sort of things,

you need to understand the probabilities of certain things happening or not happening, and that way you can explain the behaviour of some strange... Why is statistical, so why do you think it's... Simon certainly does. Why do you think it's superior to dynamic or mechanical? What virtues does it have that the other does, what possibilities to reach at the other. When you get on to sort of quantum mechanics,

it's quite impossible to explain things without using statistics. You're talking only in terms of probabilities that, certain things will or won't happen. So in a sense you can see it as a move from classical physics towards more modern quantum mechanical ideas that we have today. Again, and again, as when we were talking about the electromagnetism and the originality of that, we're talking again about an original theory, original to Maxwell here.

Yes, I think perhaps, I mean, he did an awful lot of work in thermodynamics and it's all contributing to the ongoing understanding that was sort of progressing at the time. I think perhaps this development can't be seen quite so much as a key development as you can in electromagnetism. Although the Maxwell Demon idea is quite fun. Peter, can you... Sorry. It took them several 70-odd years before anybody could explain

why the demon couldn't actually exist. But it still worked even though it couldn't exist. It doesn't... You can't have it happening. It doesn't work. It's a thought experiment. It's not intended to suggest that molecules... There could be such a finite being, as he jokingly called it. I didn't think of that. I didn't think it was a finite being, but the idea of cold moving to hot is a probability.

Well, it happens all the time spontaneously. There always are these spontaneous fluctuations. No. But you said it didn't work about three sentences again. No. What I meant was that the supposition of the finite being, to use his words for it, was not intended to suggest that there could be operating such a little creature, observing in this kind of way.

It was going to be a metaphor, wasn't it? It was a metaphor, but it's a metaphor that. describes processes that on the micro level are happening all the time. So all the time one can allow the fact, because one is conceiving molecules along this error curve, their velocities are situated along this error curve. Because of that, all the time there are going to be these fluctuations, and so some molecules are indeed going to go cold to hot. But the important point is that that is not a violation of the second law of thermodeone,

because the second law of thermodynamics does not apply to the operations of individual molecules. Okay, can I just ask, before we come to the end of this section, what this has led to? First of all, what did Electra briefly solve? And what did you say his unifying theory, what did that lead to, that people listening can sort of say, oh, it led to that, and that, and that? Because at the top I said radar, mobile phones, just that and the other. Can you just elaborate that?

One rather obvious thing that it led to was the fact that there can be people listening to us. In 1887, which is but eight years after Maxwell's tragically early death, Heinrich Hertz, in Germany, produces the first convincing experimental demonstration that there are electromagnetic waves and that they do behave the way light behaves. And within another decade to 15 years, that experimental technology had been turned into commercial technology by Marconi and his Royal Navy sponsors. Was he using Maxwell's ideas then, Hertz? Absolutely. In fact, Hertz is, I think, a fascinating man for us to think about

because it was Hertz probably more than most other late 19th century physicists, who, as it were, turned Maxwell's often, as we've just realized, complex, rather deep, philosophical program into a series of tools lots of people could use. It was Hertz who said of the electromagnetic project, Maxwell's theory is Maxwell's equations. In other words, men and coming after Maxwell in the next generation were able to turn this extraordinarily rich bag of philosophy, mathematics, techniques, equipment, textbooks, into a series of fantastically powerful technologies, not just industrial technologies,

though those matter in the light and power system and in the radio system, but also what we might call theoretical technologies. In other words, the entire program that Einstein began to work on after 1900 is in many ways based on some of the most important puzzles that Maxwell's work leaves us with and some of the most important techniques

that he taught people how to use. Peter, what would you say in terms of these work on kinetic gases? Where can we see that? And then Joanna, because you work with that, don't you? Well, in 1905, Einstein began one of his most important papers of that year, the paper on the so-called light quantum hypothesis, which argued that light was to be considered in some kind of particulate way

using the quantum theory of energy which had recently been introduced. He pointed out what he called a profound formal dichotomy between on the one hand particle physics, particle theory, and on the other hand, field theory. And he traced the delineation of...

As in the theory of the... I have used the word field earlier on, but that is the Maxwellian theory of the electromagnetic field, yes, out of which comes the electromagnetic theory of light of Maxwell. And so Einstein, for his own purposes, constructed this kind of problem area as this disjunction on the one hand between particles arising from the kinetic theory of gases, a kind of physics based on particles arising from that, and on the other hand, the field approach, which again he finds in Maxwell.

Both of these elements of theorizing he sees as coming from Maxwell's work. Of course, Maxwell himself didn't see this as a disjunction in the way in which Einstein sets it out for his own purposes in 1905. And this then, this kind of characterization really constructs the basis upon which modern physics was to further develop towards general relativity and elaboration of the field approach. and on the other hand towards quantum theory and elaboration of the particulate and statistical and bases this then, grounds this,

upon the achievements of James Clark Maxwell. Thank you very much, Joanna. What about in your field, do you find that this is the kinetic theory of gases? Is that still at work? Has that worked its way through? Well, of course, it's very relevant to the way that we represent gases in the atmosphere

or anywhere else. And what I work on is interaction of electromagnetic radiation in the atmosphere So in fact, I'm really sort of using the Maxwell's results both from the electromagnetic end and from the thermodynamic end. And so you need to understand the spectrum of radiation that's coming through, the properties of the gases that are doing the absorbing or emitting or scattering, and all of this takes into account thermodynamics and electromagnetism.

One of the few things in his day he got a little bit of public reclam for was showing the tartan at the Royal Institutions, in three colours. If you could briefly tell us about that, and then I'd like to spend the few minutes you have left on discussing why he's so little known. I mean, you have very succinctly and brilliantly, I think,

shown how powerful he was and how powerful he still is, and he's very little known compared with Faraday Newton. But first of all, this demonstration of the Royal Institution, the Tartan on the screen. Yes. Maxwell was so broad as well as deep in his interests that it's been very hard to sort of summarise all the interventions

he made one of the areas in which he really worked very hard for a considerable period of time in the first half of his life was on the very way in which we see colour and he began by identifying as some of his contemporaries in Scotland were keen to do real puzzles about the old Newtonian theory of what colour is for example, how colours mix, the difference between mixing paints

and mixing coloured lights. Maxwell did brilliant work, some of it inspired by his Edom Professors, most of it off his own bat, in which he would spin tops or coloured discs to show the way in which we see psychophysiologically colours, which are composed.

And he concluded pretty rapidly that we see the colours that we do because we have three different kinds of colour receptor, say red, green and blue, and that all the colours we see can be made up of different combinations of those. So, with his characteristic ingenuity, which I think always takes him to some material device,

to some cunning schemes, we might now say, he worked out that one could, and he got a photographer friend to do this, take a photograph of, and of course the choice of object was inevitable, a piece of tartan, first with a red filter in front of the camera, then with a green, and then with a blue. And then if you project all three of those images simultaneously onto a screen,

which he arranged to do at his lecture at the Royal Institution, you can recompose the original colours of the tartan. And for a very long time indeed, it was very puzzling to people how he'd managed to do this, because, for example, one of the filters shouldn't have worked. and the reason it did work was that it was picking up infrared, not that it was picking up the red. So he was, and I think this is also one of the characteristic features of a really great scientist, he often had luck on his side.

He seemed to commit mistakes in even numbers so that the errors often cancelled out. And the photo that he showed certainly achieved great Eclay at the time, even though it stayed a bit puzzling for people afterwards. Briefly, Peter Harmon, he died young, he died of cancer young, in his forties. And many of his theories were proven way after his death. Even so they were proven, even so Einstein spoke of him in the same breath as Newton.

And here we are talking and flagging it all the way as someone that the general listener, a general intelligent person in this country will think, well, who is James Clark Maxwell? Why has he achieved that position of rather puzzling anonymity? Not in Scotland, of course. Not in Scotland, please, not in Scotland. Well, partly because he did die young and he did not hold any important official office. In fact, he rather stood apart from such public engagement. But I think also really because of the great abstraction of his idea, something we've been really struggling with, I think, in our discussion today.

You talk about a field or you talk about situation. statistical distribution of molecules, which then explains the second law of thermodynamics. It's very difficult to get a sort of handle on such notions. Everybody can imagine the fall of an apple. They can see the moon in the sky. Everybody can see a rainbow, after all, which exhibits the prismatic colors of Newton. Even notions like relativity have a somewhat mixed fortune, but people think they know what relativity might well mean.

Or perhaps with Einstein's theory of gravity, the idea that space is bent and that's causing gravity. That's an very complicated idea, hard to understand in its full profundity, but can nevertheless be grasped at in an intuitive way. You can't really say that about the Maxwellian contributions. And therefore, I think that's part of the reason. They're just the abstraction of the ideas themselves. Would you agree with that, John?

I would agree with that. I think it's just that the ideas are quite difficult. I mean, your average physics undergraduate is struggling with Maxwell's equations. So, you know, what can the general public really expect to know about it? But from a point of view of relativity, I think it's interesting that you mention that because Einstein's development of the theory of special relativity actually developed straight on from his consideration of the Maxwell electromagnetic equations and what would happen if you put them in a moving frame of reference

from which he deduced the fact that light must be constant speed in a vacuum, however fast the frame of reference is moving, just by looking at Maxwell's equations. So the predatory of Maxwell is amazing, really. I read somewhere, not for this program, a few years ago, someone saying that Maxwell's equations would stand with Mozart's music as beautiful examples of the Maya. What is it about these equations?

Are they, in scientific terms, particularly elegant, are they? Well, the thing about them is that it was the first unification theory. We've already mentioned that, but it unified electricity and magnetism. and an essential goal of theoretical physics today is to provide more unification. Abdu Salama, Imperial College, a unified electromagnetism with a weak nuclear force, and now theoretical physicists are working on how you could then unify that theory of gravity, unify it with strong nuclear forces, so that all the fundamental forces of nature are in some sort of grand unified theory.

And Maxwell was the first person to produce a unification theory. And I suppose the unification theory, if had we time, we could relate to his religious beliefs, but that's another program. But it could be interesting that, couldn't it? Anyway, thank you all very much indeed for going the course on that one. Next week we'll be talking about bohemianism with Virginia Nicholson, Graham, Robin Hermione Le,

but three of you, thank you very much, and thank you for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programs about history, science and philosophy at BBC.com.com.com. Radio 4.