Philosophy

possible orbits, it may sooner or later jump suddenly to a smaller orbit, either the minimum or one of the intermediate possible orbits. So long as the electron does not change its orbit, the atom does not radiate energy, but when the electron jumps to a smaller orbit, the atom loses energy, which is radiated out in the form of a light-wave. This light-wave is always such that its energy divided by its frequency is exactly h. The atom may absorb energy from without, and it does so by the electron jumping to a larger orbit. It may then afterwards, when the external source of energy is removed, jump back to the smaller orbit; this is the cause of fluorescence, since, in doing so, the atom gives out energy in the form of light. The same principles, with greater mathematical complications, apply to the other elements. There is, however, with some of the latest elements, a phenomenon which cannot have any analogue in hydrogen, and that is radio-activity. When an atom is radio-active, it emits rays of three kinds, called respectively α-rays, β-rays, and γ-rays. Of these, the γ-rays are analogous to light, but of much higher frequencies, or shorter wave-lengths; we need not further concern ourselves with them. The α-rays and β-rays, on the contrary, are important as our chief source of knowledge concerning the nuclei of atoms. It is found that the α-rays consist of helium nuclei, while the β-rays consist of electrons. Both come out of the nucleus, since the atom after radio-activity disruption is a different element from what it was before. But no one knows just why the nucleus disintegrates when it does, nor why, in a piece of radium, for example, some atoms break down while others do not. The three principal sources of our knowledge concerning atoms have been the light they emit, X-rays and radio-activity. As everyone knows, when the light emitted by a glowing gas is passed through a prism, it is found to consist of well-defined lines of different colours, which are characteristic for each element, and constitute what is called its “spectrum”. The spectrum extends beyond the range of visible light, both into the infra-red and into the ultra-violet. In the latter direction, it extends right into the region of X-rays, which are only ultra-ultra-violet light. By means of crystals, it has been found possible to study X-ray spectra as exactly as those of ordinary light. The great merit of Bohr’s theory was that it explained why elements have the spectra they do have, which had, before, been a complete mystery. In the cases of hydrogen and positively electrified helium, the explanation, particularly as extended by the German physicist Sommerfeld, gave the most minute numerical agreement between theory and observation; in other cases, mathematical difficulties made this completeness impossible, but there was every reason to think that the same principles were adequate. This was the main reason for accepting Bohr’s theory; and certainly it was a very strong one. It was found that visible light enabled us to study the outer rings of planetary electrons, X-rays enabled us to study the inner rings, and radio-activity enabled us to study the nucleus. For the latter purpose, there are also other methods, more particularly Rutherford’s “bombardment”, which aims at breaking up nuclei by firing projectiles at them, and sometimes succeeds in making a hit in spite of the smallness of the target. The theory of atomic structure that has just been outlined, like everything in theoretical physics, is capable of expression in mathematical formulæ; but like many things in theoretical physics, it is also capable of expression in the form of an imaginative picture. But here, as always, it is necessary to distinguish sharply between the mathematical symbols and the pictorial words. The symbols are pretty sure to be right, or nearly so; the imaginative picture, on the other hand, should not be taken too seriously. When we consider the nature of the evidence upon which the above theory of the atom is based, we can see that the attempt to make a picture of what goes on has led us to be far more concrete than we have any right to be. If we want to assert only what we have good reason to believe, we shall have to abandon the attempt to be concrete about what goes on in the atom, and say merely something like this: An atom with its electrons is a system characterised by certain integers, all small, and all capable of changing independently. These integers are the multiples of h involved. When any of them changes to a smaller integer, energy of a definite amount is emitted, and its frequency will be obtained by dividing the energy of h. When any of the integers concerned changes to a larger integer, energy is absorbed, and again the amount absorbed is definite. But we cannot know what goes on when the atom is neither absorbing nor radiating energy, since then it has no effects in surrounding regions; consequently all evidence as to atoms is as to their changes, not as to their steady states. The point is not that the facts do not fit with the hypothesis of the atom as a planetary system. There are, it is true, certain difficulties which afford empirical grounds for the newer theory which has superseded Bohr’s, and which we shall shortly consider. But even if no such grounds existed, it would be obvious that Bohr’s theory states more than we have a right to infer from what we can observe. Of theories that state so much, there must be an infinite number that are compatible with what is known, and it is only what all of these have in common that we are really entitled to assert. Suppose your knowledge of Great Britain were entirely confined to observing the people and goods that enter and leave the ports; you could, in that case, invent many theories as to the interior of Great Britain, all of which would agree with all known facts. This is an exact analogy. If you delimit in the physical universe any region, large or small, not containing a scientific observer, all scientific observers will have exactly the same experiences whatever happens inside this region, provided it does not affect the flow of energy across the boundary of the region. And so, if the region contains one atom, any two theories which give the same results as to the energy that the atom radiates or absorbs are empirically indistinguishable, and there can be no reason except simplicity for preferring one of them to the other. On this ground, even if on no other, prudence compels us to seek a more abstract theory of the atom than that which we owe to Rutherford and Bohr. The newer theory has been put forward mainly by two physicists already mentioned, Heisenberg and Schrödinger, in forms which look different, but are in fact mathematically equivalent. It is as yet an impossible task to describe this theory in simple language, but something can be said to show its philosophical bearing. Broadly speaking, it describes the atom by means of the radiations that come out of it. In Bohr’s theory, the planetary electrons are supposed to describe orbits over and over again while the atom is not radiating; in the newer theory, we say nothing at all as to what happens at these times. The aim is to confine the theory to what is empirically verifiable, namely radiations; as to what there is where the radiations come from, we cannot tell, and it is scientifically unnecessary to speculate. The theory requires modifications in our conception of space, of a sort not yet quite clear. It also has the consequence that we cannot identify an electron at one time with an electron at another, if in the interval, the atom has radiated energy. The electron ceases altogether to have the properties of a “thing” as conceived by common sense; it is merely a region from which energy may radiate. On the subject of discontinuity, there is disagreement between Schrödinger and other physicists. Most of them maintain that quantum changes—i.e. the changes that occur in an atom when it radiates or absorbs energy—must be discontinuous. Schrödinger thinks otherwise. This is a matter in debate among experts, as to which it would be rash to venture an opinion. Probably it will be decided one way or other before very long. The main point for the philosopher in the modern theory is the disappearance of matter as a “thing”. It has been replaced by emanations from a locality—the sort of influences that characterise haunted rooms in ghost stories. As we shall see in the next chapter, the theory of relativity leads to a similar destruction of the solidity of matter, by a different line of argument. All sorts of events happen in the physical world, but tables and chairs, the sun and moon, and even our daily bread, have become pale abstractions, mere laws exhibited in the successions of events which radiate from certain regions.