The Mystery of Matter
I
THE doctrine of Materialism, although often regarded as incredible, was not considered to be ambiguous. The statement that mind is a product of matter, although objected to as unlikely, was not objected to as indefinite. The two terms, mind and matter, were accepted as referring to two distinct entities. Indeed, it was the great distinction between the two entities which made the doctrine that one is a form of the other seem so very unlikely. We knew mind and we knew matter, and it is precisely because we knew them so well that we had the greatest difficulty in seeing any connection between them.
Nowadays the position is rather different. Mind is still, of course, the thing of which we have the most direct and intimate knowledge, but matter has become mysterious. The statement that mind is a product of matter is not the clear statement it once was. It may be true, but it is also very obscure. To say that something is ‘material’ is to say very little about it — it is not to explain it in terms of something more familiar than itself. For what, in the light of modern science, do we mean by ‘material’? When we have explained everything in terms of matter, what precisely have we done? What is matter?
The scientific answer to this question is of very great interest, not only for what it tells us about matter, but for what it tells us about science.
It has always been recognized, of course, that there are different kinds of matter. The most cursory survey of the external world is sufficient to convince us of this. But the first step toward the modern scientific analysis of matter came with the discovery that all matter, of every kind, is atomic in constitution. A piece of matter is not continuous; it cannot be divided indefinitely. It is composed of a host of separate little ultimate particles. This fact about matter is curious and interesting, but it is not in the least disturbing. It does nothing to alter our fundamental conception of that substantial familiar thing called matter. For the tiny ultimate particles were supposed to be, in all essentials, just like the pieces of matter we know. They were very much smaller, that is all. It is true that some scientific men surmised that this analysis was not final. It was surmised that atoms themselves were complicated bodies, composed, probably, of still smaller bodies. But there was nothing at all revolutionary in this speculation. The new idea was on exactly the same lines as the old one.
The experimental confirmation of this idea did, however, introduce a disturbing factor. The constituents of the atom, when discovered, were found to be electrified. They were very small and light — about two thousand times lighter than the lightest known atom — and they all carried an electric charge.
This was somewhat unexpected, but further investigation led to a far more unexpected result. These little particles were found to consist of nothing but electricity! Since all atoms are constituted of these electric charges, it follows that all matter is electricity. This discovery was a real shock to one’s habitual conception of matter. For one thing, the cardinal property of matter, its substantiality, seemed to be dissolved away. Electricity had been classed as one of the ‘imponderables,’ like light and heat. How could such an entity form the familiar hard, resistant matter of experience? If it was indeed true that matter is merely electricity, then it would seem that there must be something illusory about our ordinary perceptions of matter. Thus our common-sense notions of matter received their first shock. It became evident that matter is not the simple, straightforward thing we had always supposed it to be. We saw that it was something more mysterious, more elusive, and we could only accommodate these new ideas by making our notion of matter more abstract.
The ordinary human mind advances only very slowly along the path of abstraction. It always dowers the new abstraction with as many concrete elements as it can possibly preserve. Thus the electric constituents of matter, which experiment had revealed, were conceived, as far as possible, in terms of familiar things. They were conceived, for instance, as particles — that is to say, they were supposed to occupy definite positions in space and to move from one definite position to another by definite paths. The mathematicians had shown that such particles, although composed purely of electricity, would be able to play the part of material particles in a sufficiently convincing way. Thus it was still possible to ‘ picture,’ more or less, these ultimate constituents. No intolerable strain had yet been placed on the pictorial imagination, so that the theory was still what is called ‘intelligible.’
When the mathematicians came to work out this theory in more detail, however, the strain on the pictorial imagination was greatly increased. For it was found that every now and again these electric particles moved from one point to another without passing over the intermediate space. We could not suppose that they were annihilated at one point and re-created at another. We had to admit that these electric particles did not quite fit into the conceptions of space and time which man had hitherto developed. This meant, of course, that they were strictly non-picturable, since we can form no picture of anything except within a space-time framework. Certain picturable elements still remained, however. The two kinds of electric constituents of the atom (electrons and protons) were still conceived as particles. They were still supposed to have definite locations and to describe definite paths, although their singular manner of vanishing from one path and immediately appearing on another was admitted to be baffling. This picture of the atom — Bohr’s picture — was not a perfectly clear and definite thing. Nevertheless, a good deal of successful work was done on the basis of it. It evidently corresponded, in certain important respects, to reality.
About ten years ago, however, the defects of this way of picturing the atom became very apparent. The experimentalists had obtained certain results which could not be accommodated, as it were, by the atomic model that Bohr had imagined. Now when a scientific theory, which has had a successful career, encounters recalcitrant facts, scientific men do not, as a rule, immediately abandon the theory, lock, stock, and barrel. They proceed to modify the theory, preserving its main lines as far as possible. That procedure was followed in this case. The effect was tried of complicating the Bohr theory, while still preserving the main conception of the atom as a collection of particles. Singularly enough, this procedure is hardly ever successful. No amount of tinkering with Newton’s law of gravitation will give us Einstein’s law. The ether theory of light, after its first brilliant successes, encountered difficulties. It was modified and complicated for generations, but it never became successful and is now abandoned. In these cases, as in others, it was found that the fundamental assumptions on which the theory rested were inadequate. No amount of tinkering with the superstructure was of any avail; it was the foundations that were at fault. And in this case of the atom we now realize that the pure particle conception will not do.
II
The new theory is described by the title, ‘The Wave Theory of Matter.’ The ultimate constituents of matter can, it appears, be represented as groups of waves. It might be thought that this change, although doubtless of great importance scientifically, does nothing to lessen the picturability of these ultimate constituents. We are familiar with waves as we are familiar with particles. Our pictorial imagination can form images of both these things. But can we unite these images? Can we picture an entity w hich is both a wave and a particle? It is here that the new experimental analysis of matter becomes so baffling.
If we take a sheet of glass and lightly powder it with zinc sulphide crystals and then bombard it with electrons, we shall find that scintillations appear irregularly all over it. Each electron, on striking the screen, causes a faint spark which can be seen in the dark by the help of a magnifying lens. The stream of electrons behaves like a shower of rain falling on the screen, each impact giving a tiny spark. Here electrons quite certainly behave as if they were little particles. Now consider another experiment. We know that X-rays are waves. They are of the same nature as light waves except that they are very much smaller. This fact is proved by passing X-rays through crystals and examining the patterns produced on a photographic plate. We get alternate dark and bright bands, the dark bands being produced by the crest of one wave coinciding with the trough of another, and the bright bands by the coincidence of two crests. Nothing but a wave motion can produce this phenomenon. It occurred to Professor G. P. Thomson to perform a similar experiment with electrons. Electrons have nothing like the penetrating power of X-rays, but Thomson succeeded in preparing sheets of gold leaf only about one millionth of an inch thick, and the electrons were fired through these. Circular bright and dark bands were produced, just as in the X-ray experiments. Here is a conclusive proof, therefore, that electrons are waves.
The fact that an electron behaves both as a particle and as a group of waves takes it right outside the pictorial imagination. We have seen that matter has shown a baffling, elusive tendency ever since the atom was first disintegrated. The electrons and protons that constitute it have never been really domesticated, as it were, by the scientific imagination. There has always been something odd and paradoxical about them. But for the purpose of forming a picture these oddities could be ignored, and they could be likened, more or less, to objects of our ordinary experience. We can do that no longer. The new discoveries show that the ultimate constituents of matter are like nothing we have ever known. As Eddington says, we can combine the words ‘wave’ and ‘particle’ in a word, — say ‘wavicle,’ — but we cannot combine the ideas corresponding to them.
We have encountered similar difficulties in the phenomena of light. The wave theory of light has been very successful, and it is quite true that light does sometimes indubitably behave as a system of waves. At other times it as indubitably behaves as a flight of little bullets. We cannot picture to ourselves the sort of entity that can behave in this way. The theory that light exists in atomic form explains many things, yet here again questions arise which have no satisfactory answer. If we make experiments to determine the size of an atom of light, — its spatial dimensions, — we find that it is both big enough to fill the lens of the great Mount Wilson telescope and small enough to enter an atom. We can only conclude that the notion of spatial size does not apply to such an entity. It somehow transcends space. If this is so, then it follows, of course, that it is forever unpicturable.
III
Scientific men arc pragmatists in practice, whatever they may think they are in theory, and they are quite prepared to work with conceptions they know to be partial provided they get results. Recently some very interesting and important experimental work has been done on the atom, although the precise nature of the natural processes involved is not yet fully understood. But whatever it may be that is really going on in the atom, these experimental results can be fairly well described on the basis of the ‘ particle’ conception.
Until quite recently the atom was supposed to consist of two kinds of electric particles—the ‘proton’ and the ‘electron.’ The proton is a charge of what is called positive electricity and the electron is a charge of negative electricity. In any atom these charges are so arranged that they balance, which is why an atom normally seems to have no electric charge. All the protons of an atom are concentrated in its ‘nucleus,’ together with a smaller number of electrons. Thus the nucleus of an atom always has a resultant positive charge of electricity, since it contains more protons than electrons. This resultant charge is compensated for by electrons which circulate round the nucleus, very much as the planets circulate round the sun. This is the celebrated ‘solar system’ model of the atom. As we see, it assumes the ‘particle’ conception of the electron and proton. This picture is inadequate and, if taken too seriously, can land us in difficulties. But, if we are not content merely with mathematical symbols, it is perhaps as good a picture as we can form. In its main lines it certainly accounts for a large number of experimental results. The main features of radium disintegration, for example, can be explained by it.
In a radium atom the nucleus is actually breaking up. It shoots out electrons and also a much heavier kind of particle called the Alpha particle. This particle consists of a combination of four protons and two electrons, bound together in some exceptionally stable way. As a proton weighs nearly two thousand times as much as an electron, we see that an Alpha particle is, comparatively speaking, a very massive affair. It occurred to Lord Rutherford, some years ago, to use these particles to bombard atoms. The idea was that every now and then an Alpha particle might score a direct hit on the nucleus of an atom and possibly disrupt it. Since all the chemical properties of an atom depend on the constitution of its nucleus, we should thus be able to change an atom of one substance into an atom of another substance. We should have achieved, in fact, the transmutation of the elements. The experiments were successful. Certain elements were transmuted into others.
Nowadays we are not confined, for our projectiles, to the Alpha particles shot out by radium. A hydrogen atom consists of one proton with one electron circulating round it. It is a comparatively easy matter, by passing an electric discharge through hydrogen gas at low pressure, to strip the hydrogen atoms of their circulating electrons, and thus to obtain a supply of protons. If these protons are now subjected to an intense electric field, — something on the order of a million volts, — they can be made to acquire very great velocities. They can thus be used as artificially produced projectiles for atom bombardment. It is found that these projectiles disrupt atoms on which Alpha particles have no effect. It is a usual rule that, under Alpha particle bombardment, atoms release protons, whereas under proton bombardment they release Alpha particles. It is not a question of the bombarding particle knocking something out of the atom. It is rather that it sets up a disturbance in the atom and releases intense innate forces, for the energy of the particle emitted by the atom is altogether greater than the energy of the bombarding particle.
This is, in fact, a way of tapping the huge inner stores of atomic energy. But it is not in the least a practical method at present. In these bombardment experiments only one particle in many thousands, or millions, scores a direct hit. The energy obtained in this way is only a very minute percentage of the energy expended. Nevertheless, the point has been made that the atom can be artificially disintegrated and its inner store of energy partially released.
IV
These results by no means exhaust the experimental discoveries which have been recently made in this field. We have said that an atom is built up of two sorts of entities, protons and electrons. We now know that these are not the only entities concerned. A positively charged electric particle has been found which is altogether different from the proton. It has the same electric charge as a proton, but it is very much lighter, being, in fact, of the same weight as an electron. It is called a ‘positron.’ It is a very evanescent entity, for its life is about one thousand-millionth of a second. At the end of that time it combines with an electron and the two vanish in a flash of radiation.
In modern experiments a positron has been caused to travel about a yard during its short life and to register its passage on a photographic plate. Years before its discovery, mathematical reasoning indicated its existence, but on such queer grounds that very few people could take it quite seriously. The mathematical researches of Dirac, who is the greatest of the British mathematical physicists, led to the conception that there are ‘holes’ in the universe destined for the reception of electrons. Most of such holes are already occupied, and the electrons occupying them cannot reveal their presence in any way. In time all the holes will be occupied, and that will mean the end of the material universe. In the meantime the unoccupied holes, surrounded by electrons already placed, manifest as positive charges, since they attract electrons. And these positive charges could not be of the dimensions of a proton, the only elementary positive charge known to exist; they must be of the dimensions of an electron.
It might have been supposed that this queer theory was merely a wild flight of fancy. But the actual discovery of the positron shows that it must be taken seriously. Indeed, the scientific analysis of matter has revealed it as so mysterious an entity that only the greatest efforts of the creative imagination, the greatest feats of abstraction, are now able to deal with it.
Another entity which has recently been discovered is the ‘neutron.’ When the element beryllium was bombarded by Alpha particles it was found to shoot out extremely penetrating rays. It was at first thought that these rays must be a sort of wave motion, like X-rays, as their penetrating power was much greater than that of any known type of particle. But calculations made on this supposition led to very unsatisfactory results. The nature of these rays was finally elucidated by Dr. James Chadwick, at Cambridge. He showed that they consist of a stream of ‘neutrons,’ each neutron being formed by a proton and an electron in very close combination. The electron and the proton are sufficiently close together to mask one another’s electrical charge, so that the neutron does not appear to be electrically charged at all. From this fact comes its great penetrating power. It can pass through the atoms of matter without being deflected by their electric charges. This is not possible for such electric bodies as protons, for example, which can only penetrate a mere film of lead or a few centimetres of air. The neutrons from beryllium can penetrate several feet of lead and a mile of air.
The existence of these bodies raises various interesting speculations. Since they manifest no electric charge, it would be possible for them to come in actual contact with one another. Now the diameter of a neutron is only one hundred-thousandth part of that of an atom, and its mass is the same as that of hydrogen. Thus the density of a compact mass of neutrons would be enormous. A quart pot full of the stuff would weigh about one million million tons. We know of no material as dense as this anywhere in the universe. There are certain stars whose density is estimated to be one hundred thousand times that of water, and it may be that they contain some neutron material.
The existence of neutrons also makes it easier to conceive how complex atoms may have been built up from simpler ones. The nucleus of any complex atom contains many protons, and it is difficult to understand how random encounters could ever have brought them into such close proximity, considering the intense repulsive forces between them. But we can more easily imagine a number of neutrons collecting together, and their constituent protons and electrons, under the influence of some shock, forming the sort of combination we find in the nuclei of atoms. The existence of the neutron also throws light on the now wellknown fact that a chemical element may have atoms of different weights. We used to be taught that the atoms of any chemical element were all of the same weight. That has been experimentally demonstrated to be untrue. In fact, the atoms of one and the same element may have quite a large range of different weights. Now the chemical properties of an atom, as we have said, depend on the electrical charge of its nucleus. Evidently the addition of a neutron to the nucleus does not affect the electric charge, since the neutron is electrically neutral. But it affects its weight. Thus those atoms which have the same chemical properties but are of different weights probably differ in the number of neutrons they contain.
Still another entity that has been recently discovered is ‘heavy hydrogen,’ or ‘diplogen.’ This is a form of hydrogen whose atoms are twice the weight of those of ordinary hydrogen. This discovery has been called the greatest American contribution to physical science since the MichelsonMorley experiment — the basis of the Theory of Relativity — of fifty years ago. The nucleus of an atom of this heavy hydrogen consists, not of one proton, as is the case with ordinary hydrogen, but of two protons and one electron. In any average specimen of hydrogen it is estimated that one atom out of every four or five thousand is of this heavy kind.
These heavy hydrogen atoms combine with oxygen, just as ordinary hydrogen atoms do, to form water. But the water so formed is rather heavier than ordinary water and has a higher freezing point and a higher boiling point. Also, it seems to be fatal to some forms of life that flourish in ordinary water. Seeds of the tobacco plant will not germinate in it, for example, although they develop well enough in ordinary water. Tadpoles of the green frog cannot live in it for more than an hour, and it kills the common aquarium fish in two hours. Researches on its influence on various forms of life are now being actively prosecuted. Various ‘natural waters’ are being analyzed to see if their properties depend at all on the amount of heavy water they may happen to contain. By the discovery of heavy hydrogen, therefore, a new field of inquiry has been opened up for the science of living things.
An even heavier form of hydrogen has been recently discovered, its atom having three protons and two electrons in its nucleus. It is possible, also, that there exist still other atomic entities. It seems likely, for instance, that there is an ‘opposite’ of the proton, just as the electron is the opposite of the positron.
V
So far we have been dealing with the experimental work that has been recently done on the atom, but the theoretical developments are not less interesting. In fact, they are even more interesting, for they seem to indicate a fundamental change in the attitude of science toward the problem of the external world. Our scientific knowledge, it is now realized, is subjective to an extent that we had not before suspected. We have already said that the ‘particle’ conception of matter is inadequate. It used to be thought that scientific analysis had shown that the material universe consisted of waves of radiation and of vast multitudes of electric particles. But we now know that to say the world is constituted in this way is to go beyond the evidence. This hypothesis does not enable us to account for the actual phenomena we observe. We should only be justified in saying that the world is constituted in this way if we could show that these particles, by obeying certain laws, would produce the facts of experience. And this is just what we cannot do. We have not succeeded in formulating a satisfactory set of laws on the basis of the particle conception. Our knowledge is not sufficiently definite, and there is good reason to suppose that it never can be sufficiently definite.
An electron, if it is conceived as a particle, must have at every moment a definite position and a definite velocity. But it appears to be impossible, in the nature of things, for us ever to have precise knowledge on these points. This is not due to the defects of our measuring apparatus. It is not an inability that could conceivably be overcome by technical improvements. It is due to the fact that nature is not the kind of thing that can be analyzed in that way. This doctrine, perhaps the most remarkable and far-reaching of all scientific doctrines, is called the Principle of Indeterminacy, or, sometimes, the Principle of Uncertainty. It states, in effect, that exact prediction in science is impossible. All our knowledge is knowledge of probabilities. We cannot say what must be, but only what the chances are that any one thing rather than any other shall happen.
What, then, becomes of the doctrine of strict cause and effect? Science has always assumed that all material processes are strictly determined, that all natural — or, at any rate, material — phenomena exemplify unambiguously the Reign of Law. We may not always know these laws, but it has not been doubted that the laws exist. The future, it has always been supposed, is the inevitable outcome of the present.
What the Last Dawn of Reckoning shall read.
What is now the status of that belief? There is at present a great difference of opinion in the scientific world on this point. All are agreed that our present scientific knowledge is, in fact, a knowledge only of probabilities. But whether this is due to the limitations of science, or whether it is due to the fact that nature itself does not obey the law of strict cause and effect, is a question that is being hotly debated. Those who oppose the principle of determinism ask what reason, other than scientific evidence, we have for believing in it. We have no intuitive knowledge of it. Indeed, the thing we know most intimately, ourself, leads us to believe in the existence of free will. The idea that all our actions are strictly fated from the cradle to the grave has never been acceptable to the great bulk of mankind. We gave a grudging assent to its possibility because we thought that science had proved that determinism was triumphant in the external world, and we thought that what applied to matter might conceivably apply to man. But now that we know that science does not use the principle, and finds no evidence for it, why should we continue to believe in it? If we are to continue to believe in it, it must be on purely philosophical grounds. It may be that the purely philosophical arguments in its favor are convincing, but they can no longer invoke the prestige of science in their support.
Our scientific knowledge of the ultimate constituents of matter is partial knowledge. In tracing the position of an electron, for instance, we can say what the probabilities are that it is in one place rather than in some other place, and our knowledge of its motion can be represented by waves of probability. Sometimes the probability may be, as it were, diffused, so that the electron might be anywhere in a comparatively large volume. At other times the probability may be condensed, so that we can say definitely where the electron is. But in all cases our knowledge is probable knowledge. Whatever the objective reality may be, our knowledge of it is partial. In this sense, matter will always be mysterious.