THE ASCENT OF MAN

 JACOB BROWNOWSKI

MACDONALD FUTURA PUBLISHERS                        1973

PART X

 

Chapter 10: World Within World

• There are 7 basic shapes in nature and a multitude of colours.

Of all the variety of crystals, the most modest is the simple colourless cube of common salt; and yet it is surely one of the most important. Salt has been mined at the great salt mine at Wieliczka near the ancient Polish capital of Cracow for nearly a thousand years, and some of the wooden workings and horse-drawn machinery have been preserved from the 17^th century. The alchemist Paracelsus may have come this way on his eastern travels. He changed the course of alchemy after AD 1500 by insisting that among the elements that constitute man and nature must be counted salt. Salt is essential to life, and it has always had a symbolic quality in all cultures. Like the Roman soldiers, we still say ‘salary’ for what we pay a man, though it means ‘salt money’. In the Middle East a bargain is still sealed with salt in what the Old Testament calls ‘a covenant of salt forever’.

In one respect Paracelsus was wrong; salt is not an element in the modern sense. Salt is a compound of two elements: sodium and chlorine. That is remarkable enough, that a white fizzy metal like sodium, and a yellowish poisonous gas like chlorine should finish up by making a stable structure, common salt. But more remarkable is that sodium and chlorine belong to families. There is an orderly gradation of similar properties within each family: sodium belongs to the family of alkali metals, and chlorine to the active halogens. The crystals remain unchanged, square and transparent, as we change one member of a family for another.

• What makes these family likenesses among the elements? The man who solved the problem most triumphantly was a young Russian called Dmitri Ivanovich Mendeleev, who visited the salt mine at Wieliczka in 1859.
• He was 25 then, a poor, modest, hardworking and brilliant young man. What distinguished Mendeleev was not only genius, but a passion for the elements.
• Mendeleev wrote on his cards the atoms with their atomic weights, and dealt them out in vertical columns in the order of their atomic weight. There is something in the sequence of atomic weights that is not accidental but systematic.
• But he had a problem; only 63 out of the 92 elements were known in 1871. Sooner or later he was bound to come to gaps and the first gap was the 3^rd place in the 3^rd column. He solved the difficulty by interpreting it as a gap.

The conception of gaps or missing elements was a scientific inspiration. It expressed in practical terms what Francis Bacon had proposed in general terms long ago, the belief that new instances of a law of nature can be guessed or induced in advance from old instances. And Mendeleev’s guesses showed that induction is a more subtle process in the hands of a scientist than Bacon and other philosophers supposed. In science we do not simply march along a linear progression of known instances to unknown ones. Rather, we work as in a crossword puzzle, scanning two separate progressions for the points at which they intersect: that is where the unknown instances should lie in hiding. Mendeleev scanned the progression of atomic weights in the columns, and the family likenesses in the rows, to pinpoint the missing elements at the intersections. By doing so, he made practical predictions, and he also made manifest (what is still poorly understood) how scientists actually carry out the process of induction.

• The most famous of Mendeleev’s forecasts, germanium, was found in Germany 20 years later. He had predicted that it would be 5.5 times heavier than water; that was right. He predicted that its oxide would be 4.7 times heavier than water; that was right. And so on with chemical and other properties.

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.
• 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 20^th 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.

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 the 20^th 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 70s), were others who were taking the world to pieces, thinking it out, and putting it together.

• He went first in his 20s 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.

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 center, 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 300 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 of 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.
• 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 27 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 marvelous 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 characterize 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 on 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.

And just at this moment, when everything seems to be going so swimmingly, we suddenly begin to realize 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 realization 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.

• 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 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 indivisible it seemed that it might have an indivisible center, the nucleus. And then it turned out, around 1930, that the model needed a new refinement. The nucleus at the center of the atom is not the ultimate fragment of reality either.

• About 1930 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 mass of the nucleus is not a constant multiple of the charge: it ranges from being equal to the charge to much over twice the charge. 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 non-electrical 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.

• In the 1930s, while it was 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 which showed that the loss of matter would produce energy. But how was the matter rearranged?

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.

• 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.
• 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.

• If the elements are built up one by one, why does nature stop? Why do we find only 92 elements?
• The element plutonium was a man-made element. 40,000 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.
• By and large any orderly arrangement will run down. It is a statistical law, which means that order will tend to vanish.
• 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.

That is a beautiful conception. But there is still one question to be asked. If it is true that probability has brought us here, is not the probability so low that we have no right to be here?

Nature works by steps. The atoms form molecules, the molecules form bases, the bases direct the formation of amino acids, the amino acids form proteins, and proteins work in cells. The cells make up first of all the simple animals, and then sophisticated ones, climbing step by step. The stable units that compose one level or stratum are the raw materials for random encounters which produce higher configurations, some of which will chance to be stable. So long as there remains a potential of stability which has not become actual, there is no other way for chance to go on. Evolution is the climbing of a ladder from simple to complex by steps, each of which is stable in itself.

Immortality and mortality is the contrast on which I end this essay. Physics in the 20^th century is an immortal work. The human imagination working communally has produced no monuments to equal it, not the pyramids, not the Illiad, not the ballads, not the cathedrals. The men who made these conceptions one after another are the pioneering heroes of our age. Mendeleev, shuffling his cards; J. J. Thomson, who overturned the Greek belief that the atom is indivisible; Rutherford, who turned it into a planetary system; and Niels Bohr, who made the model work. Chadwick, who discovered the neutron, and Fermi, who used it to open up and to transform the nucleus. And at the head of them all are the iconoclasts, the first founders of the new conceptions: Max Planck, who gave energy an atomic character like matter; and Ludwig Boltzmann to whom, more than anyone else, we owe the fact that the atom – the world within a world – is as real to us now as our own world.

 I will take a quotation from the poet William Blake, who begins the Auguries of Innocence with four lines:

To see a World in a Grain of Sand

And a Heaven in a Wild Flower

Hold Infinity in the palm of your hand

And Eternity in an hour.

 

Chapter 11: Knowledge or Certainty