The Ascent of Man (32 page)

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.

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
, 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.

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?

People who ask that question always picture it thus. Think of all the atoms that make up my body at this moment. How madly improbable that they should come to this place at this instant and form me. Yes, indeed, if that was how it happened, it would not only be improbable – I would be virtually impossible.

But, of course,
that is not how nature works. 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 material 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. Evolution is the climbing of a ladder from simple to complex by steps, each of which is stable in itself.

Since this is very much my subject, I have a name for it: I call it
Stratified Stability
. That
is what has brought life by slow steps but constantly up a ladder of increasing complexity – which is the central progress and problem in evolution. And now we know that that is true not only of life but of matter. If the stars had to build a heavy element like iron, or a super-heavy element like uranium, by the instant assembly of all the parts, it would be virtually impossible. No. A star builds
hydrogen to helium; then at another stage in a different star helium is assembled to carbon, to oxygen, to heavy elements; and so step by step up the whole ladder to make the ninety-two elements in nature.

We cannot copy the processes in the stars as a whole, because we do not command the immense temperatures that are needed to fuse most elements. But we have begun to put our foot on the ladder:
to copy the first step, from hydrogen to helium. In another part of Oak Ridge the fusion of hydrogen is attempted.

It is hard to recreate the temperature within the sun, of course – over ten million degrees centigrade. And it is still harder to make any kind of container that will survive that temperature and trap it for even a fraction of a second. There are no materials that will do; a container
for a gas in this violent state can only have the form of a magnetic trap. This is a new kind of physics: plasma-physics. Its excitement, yes, and its importance, is that it is the physics of nature. For once, the rearrangements that man makes run, not against the direction of nature, but along the same steps which nature herself takes in the sun and in the stars.

Immortality and mortality is
the contrast on which I end this essay. Physics in the twentieth century is an immortal work. The human imagination working communally has produced no monuments to equal it, not the pyramids, not the
Iliad
, 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 that 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.