The vast majority of modern scientists are agnostics in that they reject the claim of the metaphysical realist who presumes to have discovered substance and true being in the outside world. They will claim that substance and the thing in itself are unknowable, or at least that these elude rational investigation, and that the objective world of science is nothing but a mental construct imagined for the purpose of co-ordinating our sense impressions. But, once this point is admitted, they will recognise that this mentally constructed objective universe must to all intents and purposes be treated as a reality pre-existing to the observer who discovers it bit by bit. This last expression of opinion is not the result of some philosophical system. It is imposed upon scientists as an inevitable conclusion; for had it been proved impossible to imagine a common objective universe, the same for all men, science could never have existed, since it would have reduced to individual points of view which could never have been co-ordinated. In other words, knowledge would have lacked generality; and without generality there could have been no such thing as science.
Had the theory of relativity, for instance, failed to furnish us with a common objective world, it could never have gone very far. All we can correctly state is that the objective world of absolute duration and distance has faded away, that space and time as the fundamental constituents of the objective world have sunk to mere shadows. Expressed in other words, the so-called primary qualities of shape, size, duration and mass erstwhile considered absolutes must now be regarded as relatives having no definite value until the observer is specified. But this work of destruction has been followed by one of reconstruction, as a result of which a new objective world, that of space-time with its invariant intervals, has taken the place of the discarded world of separate space and time.
Idealistic metaphysicians have a habit of passing rapidly over space-time as if it were of no particular importance. They lose sight thereby of the entire significance of Einstein’s theory. As Einstein tells us himself: “Without space-time the general theory of relativity would perhaps have got no farther than its long clothes.” Even though we were to limit our investigations to the bare facts of the theory, without studying it more deeply, the idealistic interpretation would be open to serious objections.
Consider, for example, the phenomenon of gravitation. It is obviously impossible to assert that the apparent bending of a ray of starlight as it passes near the sun is due entirely to the idiosyncrasies of the observer; for we know that were the sun to be removed suddenly during an eclipse the ray of starlight would automatically cease to bend, in spite of the fact that the purely subjective space and time idiosyncrasies of the astronomer at his telescope would have had no occasion to vary. Again, in the special theory, the space and time computations vary from one observer to another only when relative motion exists between the observers. The norms of Peter do not differ from those of Paul because Peter is Peter and Paul is Paul, but only because Peter and Paul are in relative motion. Were their relative motion to cease, their norms would cease to differ.
But this is not all. According to the idealistic views, the norms of the various observers are so adjusted, owing to some miraculous pre-established harmony, that the velocity of light appears the same to all. As a matter of fact, this statement is incorrect, for the velocity of light is invariant only for Galilean observers; it is variable from place to place when accelerated observers are considered, and it is also variable in the neighbourhood of matter. Again, if we revert to Michelson’s experiment, we see that its essential feature is to have demonstrated that no matter what the velocity of a sphere as a whole may be, rays of light which leave the centre at the same instant of time all return to the centre at the same instant, after having suffered a reflection against the sphere’s inner surface. In this experiment, we are therefore dealing with a coincidence, with waves of light which pass the same point at the same time. The space and time elongations produced by the idiosyncrasies of the observer could never create or disrupt a coincidence of this sort. They might cause a duration or a distance to appear longer or shorter; but they could never cause two rays of light to intersect on the surface of some object if they did not intersect there in the opinion of all other observers. Thus an intersection or a coincidence is an absolute, for the simple reason that no amount of magnification will ever cause a point to appear as two separate points. A point has no dimensions which can be torn apart by magnification. It follows that in an experiment such as Michelson’s it is impossible to attribute the observed coincidence to the idiosyncrasies of the observer; there is obviously a mysterious something in nature which will have to be conceived of as existing independently of the observer and as situated in the objective world of science. This something is found to be represented by the intersections of the world-lines in absolute space-time, i.e., in the objective world of science.
As a matter of fact, in the general theory the concepts of space and time become completely indeterminate, and it is only in the very simplest type of problems that it is feasible to reason in terms of space and time. As Bertrand Russell points out, the idealistic interpretation appears to have arisen from the continual reference to “the observer” in Einstein’s theory. It should be realised, however, that “the observer” is a very loose term. A photographic camera and clock, or any other mechanical registering device, would be just as appropriate for purposes of observation as would a living human being. In short, the principal weakness of the exponents of the idealistic interpretation seems to be their desire to cling to space and time in spite of everything. They fail to see that the objective world of relativity cannot be built up of space and of time in the same simple way as that of classical science. Summing up, we may say that whether space-time be considered a metaphysical reality, or whether it be regarded as a mental construct devised for the purpose of co-ordinating our sensory experience, in either case it plays the part of an objective reality (in the scientific sense). We must conceive of it, therefore, as an entity pre-existing to the observer who explores it and locates events in its substance.
Now the criticism has often been raised that there is nothing in Einstein’s space-time model which enables us to explain the reason for that most fundamental fact of consciousness, our awareness of the flowing of time. It is difficult to see why a criticism of this sort should be directed with any greater justification against space-time than against the classical theory. The fact that in classical science space and time were separate did not allow us to explain the phenomenon of the flowing of time any better than does the space-time theory. To state with Newton that time flows teaches us nothing about the cause of this flow. Hence, we must conclude that whatever merit there may be in the critic’s objections, they would apply with equal force to classical science. But when we analyse the reason for these objections we find that the critic is starting from the assumption that the passage of time must be conceived of as pre-existing in the objective universe, and is thus denying its purely subjective nature. Speculations of this kind can never lead us very far in the present state of our knowledge; but, if need be, there is nothing to prevent us from assuming that space-time presents dynamic properties which urge natural processes on from past to future along the bundle of time directions. Those, however, who suggest views of this sort have not accomplished much thereby; they have merely removed the mystery of the passage of time from human consciousness and placed it elsewhere in the objective universe. It seems safer to concede that our awareness of the passage of time is in all probability of a subjective nature, connected with our consciousness of being alive. In this case we should have to assume that it is our consciousness which rises along a world-line, discovering, as it proceeds on its course, events situated in the cone of its absolute past. This has always been the view defended by the most competent authorities, in particular by Minkowski, Einstein, Weyl and Eddington. As Weyl tells us:
“However deep the chasm may be that separates the intuitive nature of space from that of time in our experience, nothing of this qualitative difference enters into the objective world which physics endeavours to crystallise out of direct experience. It is a four-dimensional continuum, which is neither ‘time’ nor ‘space.’ Only the consciousness that passes on in one portion of this world experiences the detached piece which comes to meet it and passes behind it, as history, that is, as a process that is going forward in time and takes place in space.... It is remarkable that the three-dimensional geometry of the statical world that was put into a complete axiomatic system by Euclid has such a translucent character, whereas we have been able to assume command over the four-dimensional geometry only after a prolonged struggle and by referring to an extensive set of physical phenomena and empirical data. Only now the theory of relativity has succeeded in enabling our knowledge of physical nature to get a full grasp of the fact of motion, of change in the world.”
As can be seen from the preceding quotation, our awareness of the passage of time is ascribed to the passage of our consciousness through the space-time continuum. According to the views of Minkowski, a material body is then represented by a world-line, or continuous chain of events, the entire length of which is subjected to a modification of structure which we come to interpret as connoting the presence of matter. So long as we reason in terms of space-time, this is all we can say. But when in our habitual perception of things, we, as sentient observers, split the world up into an appropriate space and time, what we perceive of the world-line is its momentary intersection with our cone of the passive past. If our world-line and that of the body which is being observed are parallel, the body will be said to be at rest. But if the two world-lines are not parallel, then, when interpreting things in terms of space and time, the body will be said to be in relative motion.
We may illustrate these same views by considering the following example: We shall assume for reasons of convenience that space-time is reduced to three dimensions: two of space and one of time.