The Ascent of Man (31 page)

These forecasts made Mendeleev famous everywhere – except in Russia: he was not a prophet there, because the Tsar did not like his liberal politics. The later discovery in England of a whole new row of elements, beginning with helium, neon, argon, enlarged his triumph. He was not elected to the Russian Academy
of Sciences, but in the rest of the world his name was magic.

The underlying pattern of the atoms is numerical, that was clear. And yet that cannot be the whole story; we must be missing something. It simply does not make sense to believe that all the properties of the elements are contained in one number, the atomic weight: which hides – what? The weight of an atom might be a measure of its
complexity. If so, it must hide some internal structure, some way the atom is physically put together, which generates those properties. But, of course, as an idea that was inconceivable so long as it was believed that the atom is indivisible.

And that is why the turning-point comes in 1897, when J. J. Thomson in Cambridge discovers the electron. Yes, the atom has constituent parts; it is not
indivisible, as its Greek name had implied. The electron is a tiny part of its mass or weight, but a real part, and it carries a single electric charge. Each element is characterised by the number of electrons in its atoms. And their number is exactly equal to the number of the place in Mendeleev’s table that that element occupies when hydrogen and helium are included in first and second place. That
is, lithium has three electrons, beryllium has four electrons, boron has five, and so on steadily all through the table. The place in the table that an element occupies is called its atomic number, and now that turned out to stand for a physical reality within its atom – the number of electrons there. The picture has shifted from atomic weight to atomic number, and that means, essentially, to
atomic structure.

That is the intellectual breakthrough with which modern physics begins. Here the great age opens. Physics becomes in those years the greatest collective work of science – no, more than that, the great collective work of art of the twentieth century.

I say ‘work of art’, because the notion that there is an underlying structure, a world within the world of the atom, captured
the imagination of artists at once. Art from the year 1900 on is different from the art before it, as can be seen in any original painter of the time: Umberto Boccioni, for instance, in
The Forces of a Street
, or his
Dynamism of a Cyclist
. Modern art begins at the same time as modern physics because it begins in the same ideas.

Since the time of Newton’s
Opticks
, painters had been entranced by
the coloured surface of things. The twentieth century changed that. Like the X-ray pictures of Röntgen, it looked for the bone beneath the skin, and for the deeper, solid structure that builds up from inside the total form of an object or a body. A painter like Juan
Gris is engaged in the analysis of structure, whether he is looking at natural forms in
Still Life
or at the human form in
Pierrot
.

The Cubist painters, for example, are obviously inspired by the families of crystals. They see in them the shape of a village on a hillside, as Georges Braque did in his
Houses at L’Estaque
, or a group of women as Picasso painted them in
Les Demoiselles d’Avignon
. In Pablo Picasso’s famous beginning to Cubist painting – a single face, the
Portrait of Daniel-Henry Kohnweiler
– the interest has
shifted from the skin and the features to the underlying geometry. The head has been taken apart into mathematical shapes and then put together as a reconstruction, a re-creation, from the inside out.

This new search for the hidden structure is striking in the painters of Northern Europe: Franz Marc, for example, looking at the natural landscape in
Deer in a Forest
; and (a favourite with scientists)
the Cubist Jean Metzinger, whose
Woman on a Horse
was owned by Niels Bohr, who collected pictures in his house in Copenhagen.

There are two clear differences between a work of art and a scientific paper. One is that in the work of art the painter is visibly taking the world to pieces and putting it together on the same canvas. And the other is that you can watch him thinking while he is doing
it. (For example, Georges Seurat putting one coloured dot beside another of a different colour to get the total effect in
Young Woman with a Powder Puff
and
Le Bec
.) In both those respects the scientific paper is often deficient. It often is only analytic; and it almost always hides the process of thought in its impersonal language.

I have chosen to talk about one of the founder fathers of twentieth-century
physics, Niels Bohr, because in both these respects he was a consummate artist. He had no ready-made answers. He used to begin his lecture courses by saying to his students, ‘Every sentence that I utter should be regarded by you not as an assertion but as a question’. What he questioned was the structure of the world. And the people that he worked with, when young and old (he was still
penetrating in his seventies), were others who were taking the world to pieces, thinking it out, and putting it together.

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.