He went first in his twenties to work with J. J. Thomson, and his one-time student Ernest Rutherford who, round about 1910, was the outstanding experimental physicist in the world. (Thomson and Rutherford had both been turned to science by the interest of their widowed mothers, as Mendeleev had been.) Rutherford was then a professor at Manchester University. And in 1911 he had proposed a new model for the atom. He had said that the bulk of the atom is in a heavy nucleus or core at the centre, and the electrons circle it on orbiting paths, the way that the planets circle the sun. It was a brilliant conception – and a nice irony of history, that in three hundred years the outrageous image of Copernicus and Galileo and Newton had become the most natural model for every scientist. As often in science, the incredible theory of one age had become the everyday image for its successors.
Nevertheless, there was something wrong with Rutherford’s model. If the atom is really a little machine, how can its structure account for the fact that it does not run down – that it is a little perpetual motion machine, and the only perpetual motion machine that we have? The planets as they move in their orbits lose energy continuously, so that year by year their orbits get smaller – a very little smaller, but in time they will fall into the sun. If the electrons are exactly like the planets, then they will fall into the nucleus. There must be something to stop the electrons from losing energy continuously. That required a new principle in physics, so as to limit the energy an electron can give out to fixed values. Only so can there be a yardstick, a definite unit which holds the electrons to orbits of fixed sizes.
Niels Bohr discovered the unit he was looking for in the work that Max Planck had published in Germany in 1900. What Planck had shown, a dozen years earlier, is that in a world in which matter comes in lumps, energy must come in lumps, or quanta, also. By hindsight that does not seem so strange. But Planck knew how revolutionary the idea was the day he had it, because on that day he took his little boy for one of those professorial walks that academics take after lunch all over the world, and said to him, ‘I have had a conception today as revolutionary and as great as the kind of thought that Newton had’. And so it was.
Now in a sense, of course, Bohr’s task was easy. He had the Rutherford atom in one hand, he had the quantum in the other. What was there so wonderful about a young man of twenty-seven in 1913 putting the two together and making the modern image of the atom? Nothing but the wonderful, visible thought-process: nothing but the effort of synthesis. And the idea of seeking support for it in the one place where it could be found: the fingerprint of the atom, namely the spectrum in which its behaviour becomes visible to us, looking at it from outside.
That was Bohr’s marvellous idea. The inside of the atom is invisible, but there is a window in it, a stained-glass window: the spectrum of the atom. Each element has its own spectrum, which is not continuous like that which Newton got from white light, but has a number of bright lines which characterise that element. For example, hydrogen has three rather vivid lines in its visible spectrum: a red line, a blue-green line, and a blue line. Bohr explained them each as a release of energy when the single electron in the hydrogen atom jumps from one of the outer orbits to one of the inner orbits.
As long as the electron in a hydrogen atom remains in one orbit, it emits no energy. Whenever it jumps from an outer orbit to an inner orbit, the energy difference between the two is emitted as a light quantum. These emissions from many billions of atoms simultaneously are what we see as a characteristic hydrogen line. The red line is when the electron jumps from the third orbit to the second; the blue-green line when the electron jumps from the fourth orbit to the second.
Bohr’s paper On the Constitution of Atoms and Molecules became a classic at once. The structure of the atom was now as mathematical as Newton’s universe. But it contained the additional principle of the quantum. Niels Bohr had built a world inside the atom by going beyond the laws of physics as they had stood for two centuries after Newton. He returned to Copenhagen in triumph. Denmark was home for him again, a new place to work. In 1920 they built the Niels Bohr Institute in Copenhagen for him. Young men came there to discuss quantum physics from Europe, America, and the Far East. Werner Heisenberg came often from Germany and was goaded into conceiving some of his crucial ideas there: Bohr would never allow anyone to stop at a half-formed idea.
It is interesting to trace the steps of confirmation of Bohr’s model of the atom, because in a way they recapitulate the life-cycle of every scientific theory. First comes the paper. In that, known results are used to support the model. That is to say, the spectrum of hydrogen in particular is shown to have lines, long known, whose positions correspond to quantum transitions of the electron from one orbit to another.
The next step is to extend that kind of confirmation to a new phenomenon: in this case, lines in the higher energy X-ray spectrum, which is not visible to the eye but which is formed in just the same way by electron leaps. That work was going on in Rutherford’s laboratory in 1913, and yielded beautiful results exactly confirming what Bohr had predicted. The man who did the work was Harry Moseley, twenty-seven years old, who did no more brilliant work because he died in the forlorn British attack at Gallipoli in 1915 – a campaign which cost, indirectly, the lives of other young men of high promise, among them that of the poet Rupert Brooke. Moseley’s work, like Mendeleev’s, suggested some missing elements, and one of them was discovered in Bohr’s laboratory and named hafnium, after the Latin name for Copenhagen. Bohr announced the discovery incidentally in the speech he made when accepting the Nobel Prize for Physics in 1922. The theme of the speech is memorable, for Bohr described in detail what he summarised almost poetically in another speech: how the concept of the quantum had
led gradually to a systematic classification of the types of stationary binding of any electron in an atom, offering a complete explanation of the remarkable relationships between the physical and chemical properties of the elements, as expressed in the famous periodic table of Mendeleev. Such an interpretation of the properties of matter appeared as a realisation, even surpassing the dreams of the Pythagoreans, of the ancient ideal of reducing the formulation of the laws of nature to considerations of pure numbers.
And just at this moment, when everything seems to be going so swimmingly, we suddenly begin to realise that Bohr’s theory, like every theory sooner or later, is reaching the limits of what it can do. It begins to develop little cranky weaknesses, a kind of rheumatic pain. And then comes the crucial realisation that we have not cracked the real problem of atomic structure at all. We have cracked the shell. But within that shell the atom is an egg with a yolk, the nucleus; and we have not begun to understand the nucleus.
Niels Bohr was a man with a taste for contemplation and leisure. When he won the Nobel Prize he spent the money on buying a house in the country. His taste for the arts also ran to poetry. He said to Heisenberg, ‘When it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images’. That is an unexpected thought: when it comes to atoms, language is not describing facts but creating images. But it is so. What lies below the visible world is always imaginary, in the literal sense: a play of images. There is no other way to talk about the invisible – in nature, in art, or in science.
When we step through the gateway of the atom, we are in a world which our senses cannot experience. There is a new architecture there, a way that things are put together which we cannot know: we only try to picture it by analogy, a new act of imagination. The architectural images come from the concrete world of our senses, because that is the only world that words describe. But all our ways of picturing the invisible are metaphors, likenesses that we snatch from the larger world of eye and ear and touch.
Once we have discovered that the atoms are not the ultimate building blocks of matter, we can only try to make models of how the building blocks link and act together. The models are meant to show, by analogy, how
matter is built up. So, to test the models, we have to take matter to pieces, like the diamond cleaver feeling for the structure of the crystal.
The ascent of man is a richer and richer synthesis, but each step is an effort of analysis: of deeper analysis, world within world. When the atom was found to be divisible it seemed that it might have an indivisible centre, the nucleus. And then it turned out, around 1930, that the model needed a new refinement. The nucleus at the centre of the atom is not the ultimate fragment of reality either.
At twilight on the sixth day of Creation, so say the Hebrew commentators to the Old Testament, God made for man a number of tools that give him also the gift of creation. If the commentators were alive today, they would write ‘God made the neutron’. Here it is, at Oak Ridge in Tennessee, the blue glow that is the trace of neutrons: the visible finger of God touching Adam in Michelangelo’s painting, not with breath but with power.
I must not start quite so early. Let me begin the story about 1930. At that time the nucleus of the atom still seemed as invulnerable as the atom itself had once seemed. The trouble was that there was no way it could come apart into electrical pieces: the numbers simply would not fit. The nucleus has a positive charge (to balance the electrons in the atom) equal to the atomic number. But the mass of the nucleus is not a constant multiple of the charge: it ranges from being equal to the charge (in hydrogen) to much over twice the charge in the heavy elements. That was inexplicable, so long as everyone remained convinced that all matter must be built up from electricity.
It was James Chadwick who broke with that deeply rooted idea, and proved in 1932 that the nucleus consists of two kinds of particles: not only of the electrical positive proton, but of a nonelectrical particle, the neutron. The two particles are almost equal in mass, namely equal (roughly) to the atomic weight of hydrogen. Only the simplest nucleus of hydrogen contains no neutrons, and consists of a single proton.
The neutron was therefore a new kind of probe, a sort of alchemist’s flame, because, having no electric charge, it could be fired into the nuclei of atoms without suffering electrical disturbance, and change them. The modern alchemist, the man who more than anyone took advantage of that new tool, was Enrico Fermi in Rome.
Enrico Fermi was a strange creature. I did not know him until much later, because in 1934 Rome was in the hands of Mussolini, Berlin was in the hands of Hitler, and men like me did not travel there. But when I saw him in New York, later, he struck me as the cleverest man I had ever set eyes on – well, perhaps the cleverest man with one exception. He was compact, small, powerful, penetrating, very sporty, and always with the direction in which he was going as clear in his mind as if he could see to the very bottom of things.
Fermi set about shooting neutrons at every element in turn, and the fable of transmutation came true in his hands. The neutrons he used you can see streaming out of a reactor because it is what is lightly called a ‘swimming pool’ reactor, meaning that the neutrons are slowed down by water. I should give it its proper name: it is a High Flux Isotope Reactor, which has been developed at Oak Ridge, Tennessee.
Transmutation was, of course, an age-old dream. But to men like me, with a theoretical bent of mind, what was most exciting about the 1930s was that there began to open up the evolution of nature. I must explain that phrase. I began here by talking about the day of Creation, and I will do that again. Where shall I start? Archbishop James Ussher of Armagh, a long time ago, about 1650, said that the universe was created in 4004 BC. Armed as he was with dogma and ignorance, he brooked no rebuttal. He or another cleric knew the year, the date, the day of the week, the hour, which fortunately I have forgotten. But the puzzle of the age of the world remained, and remained a paradox, well into the 1900s: because, while it was then clear that the earth was many, many millions of years old, we could not conceive where the energy came from in the sun and the stars to keep them going so long. By then we had Einstein’s equations, of course, which showed that the loss of matter would produce energy. But how was the matter rearranged?
Very well: that is really the crux of energy and the door of understanding that Chadwick’s discovery opened. In 1939 Hans Bethe, working at Cornell University, for the first time explained in very precise terms the transformation of hydrogen to helium in the sun, by which a loss of mass streams out to us as this proud gift of energy. I speak of these matters with a kind of passion, because of course to me they have the quality, not of memory, but of experience. Hans Bethe’s explanation is as vivid to me as my own wedding day, and the subsequent steps that followed as the birth of my own children. Because what was revealed in the years that followed (and finally scaled in what I suppose to be the definitive analysis in 1957) is that in all the stars there are going on processes which build up the atoms one by one into more and more complex structures. Matter itself evolves. The word comes from Darwin and biology, but it is the word that changed physics in my lifetime.
The first step in the evolution of the elements takes place in young stars, such as the sun. It is the step from hydrogen to helium, and it needs the great heat of the interior; what we see on the surface of the sun are only storms produced by that action. (Helium was first identified by a spectrum line during the eclipse of the sun in 1868; that is why it was called helium, for it was not known on earth then.) What happens in effect is that from time to time a pair of nuclei of heavy hydrogen collide and fuse to make a nucleus of helium.
In time the sun will become mostly helium. And then it will become a hotter star in which helium nuclei collide to make heavier atoms in turn. Carbon, for instance, is formed in a star whenever three helium nuclei collide at one spot within less than a millionth of a millionth of a second. Every carbon atom in every living creature has been formed by such a wildly improbable collision. Beyond carbon, oxygen is formed, silicon, sulphur and heavier elements. The most stable elements are in the middle of Mendeleev’s table, roughly between iron and silver. But the process of building the elements overshoots well beyond them.
If the elements are built up one by one, why does nature stop? Why do we find only ninety-two elements, of which the last is uranium? To answer that question, we have, evidently, to build elements beyond it, and to confirm that as the elements become bigger, they become more complex and tend to fall apart into pieces. When we do that, however, we are not only making new elements but are making something that is potentially explosive. The element plutonium, which Fermi made in the first historic Graphite Reactor (we called it a ‘Pile’ in those old colloquial days) was the man-made element that demonstrated this to the world at large. In part it is a monument to the genius of Fermi; but I think of it as a tribute to the god Pluto of the underworld who gave his name to the element, for forty thousand people died at Nagasaki of the plutonium bomb there. It is one more time in the history of the world when a monument commemorates a great man and many dead, together.
The first historic graphite reactor.
Experimental graphite-uranium pile designed by the group under Enrico Fermi, which went into operation for the first time on 2 December 1942 on the squash court, West Stands, Stagg Field, University of Chicago.
I must return briefly to the mine at Wieliczka because there is a historical contradiction to be explained here. The elements are being built up in the stars constantly, and yet we used to think that the universe is running down. Why? Or how? The idea that the universe is running down comes from a simple observation about machines. Every machine consumes more energy than it renders. Some of it is wasted in friction, some of it is wasted in wear. And in some more sophisticated machines than the ancient wooden capstans at Wieliczka, it is wasted in other necessary ways – for example, in a shock-absorber or a radiator. These are all ways in which the energy is degraded. There is a pool of inaccessible energy into which some of the energy that we put in always runs, and from which it cannot be recovered.
In 1850 Rudolf Clausius put that thought into a basic principle. He said that there is energy which
is available, and there is also a residue of energy which is not accessible. This inaccessible energy he called entropy, and he formulated the famous Second Law of Thermodynamics: entropy is always increasing. In the universe, heat is draining into a sort of lake of equality in which it is no longer accessible.
That was a nice idea a hundred years ago, because then heat could still be thought of as a fluid. But heat is not material any more than fire is, or any more than life is. Heat is a random motion of the atoms. And it was Ludwig Boltzmann in Austria who brilliantly seized on that idea to give a new interpretation to what happens in a machine, or a steam engine, or the universe.
When energy is degraded, said Boltzmann, it is the atoms that assume a more disorderly state. And entropy is a measure of disorder: that is the profound conception that came from Boltzmann’s new interpretation. Strangely enough, a measure of disorder can be made; it is the probability of the particular state – defined here as the number of ways it can be assembled from its atoms. He put that quite precisely,
S = K log W;
S, the entropy, is to be represented as proportional to the logarithm of W, the probability of the given state (K being the constant of proportionality which is now called Boltzmann’s constant).
Of course, disorderly states are much more probable than orderly states, since almost every assembly of the atoms at random will be disorderly; so by and large any orderly arrangement will run down. But ‘by and large’ is not ‘always’. It is not true that orderly states constantly run down to disorder. It is a statistical law, which means that order will tend to vanish. But statistics do not say ‘always’. Statistics allow order to be built up in some islands of the universe (here on earth, in you, in me, in the stars, in all sorts of places) while disorder takes over in others.