PERHAPS to-day is the first time in the history of science when the universe has really become mysterious. In the days before Galileo the universe, although not explained on scientific lines, was nevertheless regarded as a perfectly understandable affair. Everything that existed was supposed to have its bearing on the destiny of man, and in this bearing was found the reason for its existence. Thus the precise way in which things happened was not a matter of much interest; the real point was why they happened. And, as everything was supposed to be satisfactorily explained from this point of view, there was nothing mysterious about the universe.
The science of Galileo and his contemporaries sprang from a different kind of curiosity. They were interested in the actual behavior of things. How, precisely, do stones fall? This was one of Galileo’s problems, and he said that he did not think this question had occurred to any philosopher before him, although many of them had written a great deal about motion. As we know, he made experiments to find out, and his results contradicted the Aristotelian teaching of the time. This secured for them a good deal of attention, not because the precise behavior of falling stones was regarded as a matter of any importance, but because it was thought shocking that anyone should contradict Aristotle. As another result of the attempt to find out how things behave came the discovery that the earth moves round the sun. This discovery also was objected to because the idea of a moving earth violated certain religious prejudices.
It was, perhaps, a fortunate thing for science that its early discoveries were regarded as shocking, for this fact secured for them a great deal more attention than they would otherwise have received. The mere behavior of things was not a subject which, in itself, would arouse any general interest. When Kepler went on to show that Copernicus was wrong in supposing that the earth moved round the sun in a circle, and that the actual path, as more precise examination showed, was an ellipse, the result, although of the greatest scientific moment, raised no popular interest. The damage had already been done. That the earth moved at all was the significant point; the precise way in which it moved interested none but scientific men.
These early discoveries, although they upset certain prepossessions, did not make the universe mysterious. The laws of behavior might have been unexpected, but they were not at all bewildering. And the subsequent generalization of these results by Newton introduced no element of mystery into the universe. Rather, it abolished mystery.
Nature and Nature’s laws lay hid in night: God said, ‘Let Newton be!’ and all was light.
This couplet expressed the general feeling of his time. It was thought that the key to the universe had been found — that all natural phenomena would prove to be explainable by the Newtonian technique.
During the nineteenth century this conviction grew, for the triumphs achieved by the Newtonian set of ideas were dazzling. The universe was so far from being mysterious that a nineteenth-century engineer, given the time and materials, could have made it. There were still certain details where our knowledge was not quite complete, but in all its main lines the universe was known. The only task of future science, it was confidently believed, would be to carry the measurements to another place of decimals.
The understanding was most complete, of course, for the material sciences. The nature of matter was known, and so were the laws of its behavior. The understanding of living things was not, it is true, so complete. The origin of life was an unsolved puzzle, for experiment had shown that life always came from preexisting life. But, leaving that puzzle on one side, the theory of evolution had shown how all the existing forms of life could have developed, by purely natural processes, from some primitive living germ. And it was confidently anticipated that, when science was further pursued along these lines, all mental phenomena would receive an equally natural explanation.
Thus the Victorian scientific universe, large and grand as it was, was a universe from which all mystery had been abolished. It seemed, for a period, that science had reached finality. The first indication that the scientific scheme was not yet complete came when Maxwell published his electromagnetic theory of light. This theory, brilliantly confirmed by experiment, could not be fitted into the general scientific framework. It raised a doubt as to whether the conception of the universe as a huge machine was really quite satisfactory. Nevertheless, the old outlook, in all essentials, persisted up to the close of the nineteenth century. It was admitted that there were problems not yet resolved, but it was supposed that they would, in time, yield to the methods that had hitherto proved so successful. These problems were difficulties, not mysteries.
It is during the present century that the outlook which dominated science from its inception has completely collapsed, and with its collapse has come the realization that the universe is mysterious as it has never before appeared to be in the history of science. The matter may be put briefly by saying that science has been brought to realize, to a far greater extent than ever before, the gulf between appearance and reality. There has always been such a gulf, of course. There has always been a gap between the teachings of science and the plain testimony of the senses. Almost any great scientific doctrine is an example of this. The theory that a gas consists of an immense number of little solid particles flying about in all directions, that heat is ‘really’ the energy of motion of such particles, that light consists of minute waves in the ether, that sound consists of waves in air, are all doctrines which give us a different picture from that provided by our direct sense impressions. But these doctrines all repose on conceptions that are familiar to us. The difference is chiefly one of scale. The notion of an atom is merely an extension of our experience of a minute particle — such as a grain of sand. Light waves are smaller than any waves we have seen — but we have seen waves. Nature, as science revealed it, is evidently a much more fine-spun structure than it appears, but the lines on which it is built, as it were, are familiar. The reality behind appearance, as it had been revealed by science, although strange and unexpected, was not unimaginably different from anything we had experienced.
The universe of modern science has gone beyond the limits of the pictorial imagination. In fact, it is not too much to say that the pictorial imagination is now more of a hindrance than a help. The very basis of what we have called understanding — the reduction of the new things to familiar images — has been taken from us. Further, fundamental ways and habits of thought which we had always regarded as necessary have been shown to be misleading. Our minds have been developed to deal with the world of appearance, and, now that we have begun to penetrate beneath the world of appearance, the reality we find is so strange that we may well doubt whether man will ever be able to understand it.
Consider, for instance, those fundamental elements of our experience, space and time. There is nothing more basic and pervasive in our consciousness — except perhaps our awareness of our own existence. Space and time form the conditioning framework for every form of experience. And in our immediate and unquestioning apprehension of them they appear as distinct. Among all the objects of our awareness there are no two things more distinct than space and time. Modern science requires us to believe that this distinction is illusory. The reality that physical science investigates knows nothing, it appears, of this distinction.
According to relativity theory, space and time are not independent realities; they are merely aspects of something which includes them both. The fact that we separate them is, so far as the physicist is concerned, merely a psychological peculiarity of ours. He has found, also, that this peculiarity depends to some extent on our state of motion. The inhabitants of a planet moving very much faster than the earth would be living in a very different space and time from ours. Events which are simultaneous in our time system would be successive in theirs. They would see objects as near together that we see as far apart. But their scientific men would discover the same reality behind these appearances that we have discovered. Their space and time would prove to be as illusory as our own.
Now this scientific vision of reality, if we take it seriously, upsets our most natural convictions. In fact it is doubtful whether, with the best will in the world, we can possibly take it seriously. In the four-dimensional reality of science there is no essential difference between time and space. There is no creative advance of nature. Nothing comes into existence from the future, and nothing vanishes out of existence into the past. Everything exists now. As Weyl has said, events do not happen; we come across them.
This notion, that time is unreal, is perhaps the most baffling of all the ideas that modern science has put forward. All our speculations about the past and the future assume the reality of time. Even if events do not happen, yet our coming across them would itself seem to be an event in time. It seems impossible to carry through consistently this idea of a timeless universe. For this reason many people think that the philosophical significance of relativity theory has been exaggerated, or at least distorted, by some of its expositors. The ‘time’ of the mathematicians is probably too thin and inadequate an abstraction to be a satisfactory rendering of the time of our intuitions. And there are indications that, considered merely as a mathematical device, the merging of time and space has only a limited application. For phenomena on the grand scale, when the whole material cosmos is considered, time and space, according to some authorities, once more separate out.
Yet the notion that time is unreal is not at all a new idea. It has been held by many philosophers and mystics. It doubtless has some kind of intuitive basis. It is not merely a product of the ingenuity of the mathematician. But it seems impossible, in the present stage of thought, to formulate clearly our intuition of time. All the formulations hitherto given lead to unresolvable paradoxes. So that it is not a fatal objection to the time of relativity theory that it is still somewhat incoherent. And the theory of the four-dimensional continuum has achieved so much that, in spite of its puzzles and inadequacies, we may be confident that it will be an essential factor in any further synthesis.
This theory of space and time is the most ‘ philosophical ’ of all the new ideas that modern science has introduced. But this is the less surprising since space and time have always proved baffling to anybody who thought about them. They may have acquired new difficulties, but they have always been difficult. The old notion of infinite space, although it seemed to be a ‘necessity of thought,’ brought with it grave difficulties. For are we to suppose the stars to be scattered throughout this infinite space, or are we to suppose that only a finite region of it is occupied by matter and that around this region, in all directions, stretch infinite expanses of empty space? Either view presents difficulties. On the first view it can be shown that, owing to gravitational attraction, there should be stars having very much greater velocities than they are observed to have. On the other hand, the idea that all the matter in the universe is collected into an island region surrounded everywhere by infinite emptiness seems somehow unreasonable. Einstein, as we know, solved this difficulty by his theory of a finite but unbounded space. This is a perfectly reasonable conception, provided we attribute a non-Euclidean geometry to space, and this is precisely what Einstein does. But, here again, attempts to picture this kind of space are not likely to be successful.
Recently the mathematicians have put forward the idea that this finite space is expanding. It is expanding faster and faster, and there are no limits to the size it may reach. Again we are presented with an idea that seems to be incomprehensible. For how can space expand? What is it expanding into? It would seem that the idea must be self-contradictory. It is not likely, however, that the mathematicians should have embraced a theory which can be so simply demolished. We have here another instance of the fact that in modern science reason has outstripped imagination. As long as the theory is logically consistent, the fact that we cannot picture it is irrelevant. Nevertheless it is certainly odd to reflect that every body in the universe is flying away from us with an ever-increasing velocity, so that our particular system will finally be left marooned, as it were, in space. It seems to add to the extraordinary pointlessness of this mysterious scientific universe.
The three fundamental entities of physical science used to be space, time, and matter. Space and time, as we have said, always presented difficulties when they were thought about hard enough. But it is only of recent times that matter has become equally incomprehensible. The further the scientific analysis of matter has been carried, the more unfamiliar has matter become, until, in the present stage of science, it is strictly unimaginable.
The first stage in this analysis, the reduction of matter to atoms, was straightforward enough. For these atoms were only small particles. They retained all the familiar properties of matter — in particular the quality of substantiality. For most purposes it was good enough to regard them as little hard spheres, although it was apparent that there must be several kinds of them, differing from one another in their chemical properties and their weights. Certain scientific men, however, surmised that atoms were not simple homogeneous bodies, but that they probably possessed structure of some kind, different atoms having different structures. There was nothing in any of this which made any great strain on the imagination.
During the nineties a group of brilliant researches carried the analysis of matter a stage further, and some entirely new conceptions were imported into science. The surmise that atoms possess structure was found to be fully confirmed, and the first representation of this structure was given. It was found that an atom consists of a number of little electric particles. The element of queerness in this idea came from the fact that these electric particles are wholly electrical. They are not subatomic particles of matter carrying electric charges. They are constituted of nothing but electricity. As the writers of the time described them, they are ‘disembodied charges of electricity.’ In an atom, according to this theory, there is no ‘ordinary’ matter at all. All atoms, and hence the whole of matter, are wholly electrical. This idea was found so baffling chiefly because the primary quality of matter, its substantiality, seemed to be taken from it. Electricity, like light and heat, had been classed as one of the ‘imponderables’ of nature.
But what is this quality of substantiality that we attribute to matter? It seems to be a mixed idea deriving from the facts that matter has weight and that it requires force to move it. It is this latter fact which is the more important. At a sufficient distance from the earth a piece of matter would weigh nothing, but it would still be regarded as substantial. It would still possess inertia. When it was shown, therefore, that electricity also possesses inertia, so that, for example, an electrically charged body has more inertia than an exactly similar uncharged body, the electrical constitution of matter became somewhat more comprehensible. It was granted that electricity might possess enough of the quality of substantiality to play the part of matter. But there was still a great deal that was puzzling about the theory. The laws obeyed by these little particles made it clear that natural phenomena take place in discontinuous jerks and not uniformly, as had always been supposed. And, a much graver difficulty, it appears that the behavior of these ultimate constituents of matter is not strictly determined. So far as we can make out, they will sometimes do one thing and sometimes another, although the surrounding circumstances remain exactly the same. The best we can do is to say what the chances are that they will do one thing rather than another. We replace exact prediction by statistics, as a life insurance actuary does. Is this because we do not know enough, or is it because a real element of chance, or sort of free will, enters into natural processes?
Contemporary opinion is very much divided on this point, and it is probably the most important issue that is at present before the scientific world. Some of our leading scientific authorities hold that the ultimate processes of nature really are indeterminate, that the strict sequence of cause and effect hitherto assumed by science is merely an idea based on insufficient experience. Other authorities not only disagree with this, but find the assertion nonsensical. We are here in the presence of differences of opinion which are not dependent solely on scientific evidence; philosophical predilections here play an active part.
This is characteristic of many of the modern scientific theories. That purely ‘objective’ judgment on which science used to pride itself is seen to be an illusion. Scientific theories always assumed a metaphysical background, of course, but in most cases the assumption was made unconsciously. The scientific man, like most other men, described his metaphysical assumptions as ‘necessities of thought’ when they were merely very deepseated prejudices. But science has now become more self-conscious, although there is not yet a perfectly frank recognition of the fact that science has reached a point where it is inextricably intertwined with philosophy.
The mystery of matter has, of late years, become even more profound. The endeavor to represent the ultimate constituents of matter as little electrical particles has broken down. The ‘particle’ conception has been found inadequate. At the same time there is no other familiar conception with which it can be replaced. There are experiments in which these ultimate constituents manifest particle characteristics quite unambiguously. In other experiments they just as indubitably manifest themselves as waves. These two characteristics are not of a nature that can be combined. We can make a blend of the words, — Eddington has suggested the composite word ‘wavicle,’ — but we cannot blend the conceptions. We know of no synthetic conception, no ‘higher unity,’ of which wave and particle are aspects. The two characteristics are combined in nature, but we can form no picture of the resulting entity. The reason for this inability must lie very deep. Any picture of a physical entity that we can form presupposes a space-andtime setting. It has been suggested that this fact is expressive merely of a mental limitation, and that, by insisting that these physical entities shall occupy a space-and-time framework, we so distort them, as it were, that we make them incomprehensible. The entities themselves, it has been suggested, actually transcend space and time. The difficulties they present come from the way we think about them.
If this be true, then the next step forward must come from such an effort of the scientific imagination as led to the creation of the non-Euclidean geometries. In that great effort what had been taken for two thousand years to be necessities of thought were shown to have alternatives. This example alone should be sufficient to warn us that we cannot be confident of any insuperable barriers to man’s imagination.
Thus this mysterious universe may not always remain mysterious. The fact that we get, in many cases, contradictory answers to our questions may merely indicate that our questions are inappropriate. The stock of concepts with which we have approached the task of investigating nature happens to be inadequate. They have originated and developed in the course of the evolutionary experience of man, and have been adapted to the needs of his form of life on this planet. This, it appears, is not sufficient, and indeed there is no a ’priori reason why it should be. As the founders of science realized, science is an adventure which may or may not be successful. There is no guarantee in the nature of things that it must be successful. But the success so far achieved, and the fact that the mind of man is a developing thing, give us a reasonable hope that the universe will not remain permanently mysterious. This is not to say that it may not always contain unexplored depths and present fresh problems. But progress may be continuous although it never reaches its goal.
At the present time, however, it must be admitted that the scientific imagination has not proved adequate to its new material. It seems that radically new concepts are required, but it has not yet been found possible to formulate them. In the meantime, scientific men have to do the best they can with the ideas already in stock. These ideas have been greatly subtilized and refined in the attempt to adapt them to the new scientific universe, but they were originally created to serve an entirely different outlook. It is for this reason that their present application seems to lead to results which contradict one another, such results as that a light-quantum is, at one and the same time, large enough to fill the lens of the 100-inch telescope at Mount Wilson and also small enough to enter an atom. This contradiction probably indicates that the notion of spatial extension should not be attributed to a light-quantum. But we do not yet know how to think about it in other terms.
The other fundamental entities of physics have similarly paradoxical qualities. Eddington has gone so far as to suggest that these paradoxes may prove irresolvable, that the universe may turn out to be irrational. Such a conclusion may be theoretically possible, but it is difficult to believe that scientific men would ever accept it. They will continue to believe that a logically self-consistent scheme of science will one day be constructed. In the absence of such a scheme, science can be no more than a collection of recipes of greater or less practical utility, and the chief motive for the prosecution of science would be gone. Without the belief that nature is rational and harmonious, science, to the true scientific man, would not be worth pursuing.
The present state of physics may be summarized by saying that the attempt to present the material universe as a rigidly determined scheme fixed in space and time has broken down. At the present time two directions are being explored. It has been shown that the law of strict causality cannot be maintained if we insist on fixing phenomena in space and time. If we give up the space-and-time representation, however, it may be that the law of causality can be preserved. In either case we have to get rid of some of our most deep-rooted habits of thought. And, even if we do this, there is no certainty that all our difficulties will disappear. That the universe is now so mysterious is due, it appears, to the way we think about things. But it may well be that we shall never hit on a way of thinking about things that will abolish that mystery.