The universe of Newton ticked on without a hitch for about two hundred years. If his ghost had come to Switzerland any time before 1900, all the clocks would have chimed hallelujah in unison. And yet, just after 1900 in Berne, not two hundred yards from the ancient clocktower, a young man came to live who was going to set them all by the ears: Albert Einstein. Time and light first began to go awry just about this time. It was in 1881 that Albert Michelson carried out an experiment (which he repeated with Edward Morley six years later) in which he fired light in different directions, and was taken aback to find that however the apparatus moved, always he came out with the same speed of light. That was quite out of keeping with Newton's laws. And it was that small murmur at the heart of physics which first set scientists agog and questioning, about 1900.

It is not certain that the young Einstein was quite up-to-date about this. He had not been a very attentive university student. But it is certain that by the time he went to Berne he had already asked himself, years earlier as a boy in his teens, what our experience would look like seen from the point of view of light.

The answer to the question is full of paradox, and that makes it hard. And yet, as with all paradox, the hardest part is not to .answer but to conceive the question. The genius of men like Newton and Einstein lies in that: they ask transparent, innocent questions which turn out to have catastrophic answers. The poet William Cowper called Newton a 'childlike sage' for that quality, and the description perfectly hits the air of surprise at the world that Einstein carried in his face. Whether he talked about riding a beam of light or falling through space, Einstein was always full of beautiful, simple illustrations of such principles, and I shall take a leaf out of his book. I go to the bottom of the clocktower, and get into the tram he used to take every day on his way to work as a clerk in the Swiss Patent Office.

The thought that Einstein had had in his teens was this: 'What would the world look like if I rode on a beam of light?' Suppose this tram were moving away from that clock on the very beam with which we see what the clock says. Then, of course, the clock would be frozen. I, the tram, this box riding on the beam of light would be fixed in time. Time would have a stop.

Let me spell that out. Suppose the clock behind me says 'noon' when I leave. I now travel 186,000 miles away from it at the speed of light; that ought to take me one second. But the time on the clock, as I see it, still says 'noon', because it takes the beam of light from the clock exactly as long as it has taken me. So far as the clock as I see it, so far as the universe inside the tram is concerned, in keeping up with the speed of light I have cut myself off from the passage of time.
That is an extraordinary paradox. I will not go into its implications, or others that Einstein was concerned with. I will just concentrate on this point: that if I rode on a beam of light, time would suddenly come to an end for me. And that must mean that, as I approach the speed of light (which is what I am going to simulate in this tram), I am alone in my box of time and space, which is more and more departing from the norms round me.

Such paradoxes make two things clear. An obvious one: there is no universal time. But a more subtle one: that experience runs very differently for the traveller and the stay-at-home - and so for each of us on his own path. My experiences within the tram are consistent: I discover the same laws, the same relations between time, distance, speed, mass and force, that every other observer discovers. But the actual values that I get for time, distance, and so on, are not the same that the man on the pavement gets.

That is the core of the Principle of Relativity. But the obvious question is 'Well, what holds his box and mine together?' The passage of light: light is the carrier of information that binds us. And that is why the crucial experimental fact is the one that puzzled people since 1881 : that when we exchange signals, then we discover that information passes between us always at the same pace. We always get the same value for the speed of light. And then naturally time and space and mass must be different for each of us, because they have to give the same laws for me here in the tram and for the man outside, consistently - yet the same value for the speed of light.

Light and the other radiations are signals that spread out from an event like ripples through the universe, and there is no way in which news of the event can move outwards faster than they do. The light or the radio wave or the X-ray is the ultimate carrier of news or messages, and forms a basic network of information which links the material universe together. Even if the message that we want to send is simply the time, we cannot get it from one place to another faster than the light or the radio wave that carries it. There is no universal time for the world, no signal from Greenwich by which we can set our watches without getting the speed of light inextricably tied up in it.

In this dichotomy, something has to give. For the path of a ray of light (like the path of a bullet) does not look the same to a casual bystander as to the man who fired it on the move. The path looks longer to the bystander; and therefore the time that the light takes on its path must seem longer to him, if he is to get the same value for its speed.
Is that real? Yes. We know enough now about cosmic and atomic processes to see that at high speeds that is true. If I were really travelling at, say, half the speed of light, then what I have been making three minutes and a little on my watch, Einstein's tram-ride, would be half a minute longer for the man on the pavement.

We will take the tram up towards the speed of light to see what the appearances look like. The relativity effect is that things change shape. (There are also changes in colour, but they are not due to relativity.) The tops of the buildings seem to bend inwards and forwards. The buildings also seem crowded together. I am travelling horizontally, so horizontal distances seem shorter; but the heights remain the same. Cars and people are distorted in the same way: thin and tall. And what is true for me looking out is true for the man outside looking in. The Alice in Wonderland world of relativity is symmetrical. The observer sees the tram crushed together: thin and tall.

Evidently this is an altogether different picture of the world from that which Newton had. For Newton, time and space formed an absolute framework, within which the material events of the world ran their course in imperturbable order. His is a God's eye view of the world: it looks the same to every observer, wherever he is and however he travels. By contrast, Einstein's is a man's eye view, in which what you see and what I see is relative to each of us, that is, to our place and speed. And this relativity cannot be removed. We cannot know what the world is like in itself, we can only compare what it looks like to each of us, by the practical procedure of exchanging messages. I in my tram and you in your chair can share no divine and instant view of events - we can only communicate our own views to one another. And communication is not instant; we cannot remove from it the basic time-lag of all signals, which is set by the speed of light.

The tram did not reach the speed of light. It stopped, very decently, near the Patent Office. Einstein got off, did a day's work, and often of an evening stopped at the Cafe Bollwerk. The work at the Patent OfEce was not very taxing. To tell the truth, most of the applications now look pretty idiotic: an application for an improved form of pop gun; an application for the control of alternating current, of which Einstein wrote succinctly, 'It is incorrect, inaccurate, and unclear'.

In the evenings at the Cafe Bollwerk he would talk a little physics with his colleagues. He would smoke cigars and drink coffee. But he was a man who thought for himself. He went to the heart of the question, which is 'How in fact do, not physicists but human beings, communicate with one another ? What signals do we send from one to another? How do we reach knowledge?'And that is the crux of all his papers, this unfolding of the heart of knowledge, almost petal by petal.

So the great paper of 1905- is not just about light or, as its title says, The Electrodynamics of Moving Bodies. It goes on in the same year to a postscript saying energy and mass are equivalent, E=mc2. To us, it is remarkable that the first account of relativity should instantly entail a practical and devastating prediction for atomic physics. To Einstein, it is simply a part of drawing the world together; like Newton and all scientific thinkers, he was in a deep sense a Unitarian. That comes from a profound insight into the processes of nature herself, but particularly into the relations between man, knowledge, nature. Physics is not events but observations. Relativity is the understanding of the world not as events but as relations.

Einstein looked back to those years with pleasure. He said to my friend Leo Szilard many years after, 'They were the happiest years of my life. Nobody expected me to lay golden eggs'. Of course, he did go on laying golden eggs: quantum effects, general relativity, field theory. With them came the confirmation of Einstein's early work, and the harvest of his predictions. In 1 915" he predicted, in the General Theory of Relativity, that the gravitational .field near the sun would cause a glancing ray of light to bend inwards - like a distortion of space. Two expeditions sent by the Royal Society to Brazil and the west coast of Africa tested the prediction during the eclipse on 29 May 191 9. To Arthur Eddington, who was in charge of the African expedition, his first measurement of the photographs taken there always stayed in his memory as the greatest moment in his life. Fellows of the Royal Society rushed the news to one another; Eddington by telegram to the mathematician Littlewood, and Littlewood in a hasty note to Bertrand Russell,

Dear Russell:
Einstein's theory is completely confirmed. The predicted displacement was
Yours,
J.E.L.
 

Relativity was a fact, in the special theory and the general. E=mc3 was confirmed in time, of course. Even the point about clocks running slow was singled out at last by an inexorable fate. In 1905 Einstein had written a slightly comic prescription for an ideal experiment to test it. If there are two synchronised clocks at A and if one of these is moved along a closed curve with constant velocity v until it returns to A, which we suppose to take t seconds, then the latter clock on arriving at A will have lost \t (v/c)2 seconds by comparison with the clock which has remained stationary. We conclude from this that a clock fixed at the Earth's equator will run slower by a very small amount than an identical clock fixed at one of the Earth's poles.

Einstein died in 1955. fifty years after the great 1905- paper. But by then one could measure time to a thousand millionth of a second. And therefore it was possible to look at that odd proposal to 'think of two men on earth, one at the North Pole and one at the Equator. The one at the Equator is going round faster than the one at the North Pole; therefore his watch will lose'. And that is just how it turned out.

The experiment was done by a young man called H. J. Hay at Harwell. He imagined the earth squashed flat into a plate, so that the North Pole is at the centre and the equator runs round the rim. He put a radio-active clock on the rim and another at the centre of the plate and let it turn. The clocks measure time statistically by counting the number of radio-active atoms that decay. And sure enough, the clock at the rim of Hay's plate keeps time more slowly than the clock at the centre. That goes on in every spinning plate, on every turntable. At this moment, in every revolving gramophone disc, the centre is ageing faster than the rim with every turn.

Einstein was the creator of a philosophical more than a mathematical system. He had a genius for finding philosophical ideas that gave a new view of practical experience. He did not look at nature like a God but like a pathfinder, that is, a man inside the chaos of her phenomena who believed that there is a common pattern visible in them all if we look with fresh eyes. He wrote in The World as I See It:

We have forgotten what features in the world of experience caused us to frame (pre-scientific) concepts, and we have great difficulty in representing the world of experience to ourselves without the spectacles of the old-established
conceptual interpretation. There is the further difficulty that our language is compelled to work with words which are inseparably connected with those primitive concepts. These are the obstacles which confront us when we try to describe the essential nature of the pre-scientific concept of space.

So in a lifetime Einstein joined light to time, and time to space; energy to matter, matter to space, and space to gravitation. At the end of his life, he was still working to seek a unity between gravitation and the forces of electricity and magnetism. That is how I remember him, lecturing in the Senate House at Cambridge in an old sweater and carpet slippers with no socks, to tell us what kind of a link he was trying to find there, and what difficulties he was running his head against.

The sweater, the carpet slippers, the dislike of braces and socks, were not affectations. Einstein seemed to express, when one saw him, an article of faith from William Blake: 'Damn braces: Bless relaxes'. He was quite unconcerned about worldly success, or respectability, or conformity; most of the time he had no notion of what was expected of a man of his eminence. He hated war, and cruelty, and hypocrisy, and above all he hated dogma - except that hate is not the right word for the sense of sad revulsion that he felt; he thought hate itself a kind of dogma. He refused to become president of the state of Israel because (he explained) he had no head for human problems. It was a modest criterion, which other presidents might adopt; there would not be many survivors.