The evolution of scientific thought from Newton to Einstein

by A. D'Abro

One of the chief difficulties attendant on an atomistic theory such as Lorentz’s was due precisely to this interpretation of observable effects in terms of invisible ones about which practically nothing could be known and where hypotheses ad hoc had to be invoked at each stage. As for the mathematical difficulties, they, of course, grew in proportion to the complexity of the hidden mechanisms which we had to postulate. Einstein’s theory is thus a return to universal principles induced from experiment, and in this respect is analogous to the physics of the general principles which appeared in the course of the nineteenth century. There again such comprehensive universal principles as those of entropy, of least action, of the conservation of energy, mass and momentum, took the place of the hidden mechanisms whereby the atomists had endeavoured to account for observed phenomena.

These periodic swings in the scientific viewpoint have always been necessitated by the continuous advance of knowledge. Both attitudes are fruitful and have yielded important results. In the present case Lorentz had progressed about as far as it was possible to go in his speculations on hidden mechanisms, and a theory such as Einstein’s was the necessary antidote to the increasing difficulties which were hampering further developments.

We have now to consider certain important consequences of Einstein’s theory which affect our traditional concepts of space and time. The FitzGerald contraction is no longer a real physical contraction, as it was assumed to be in Lorentz’s theory. It no longer arises as a result of a perfectly concrete motion of a body through a stagnant ether; the stagnant ether with respect to which velocity acquired its significance having been banished by Einstein. The FitzGerald contraction is now solely due to the relative motion existing between the observer and the body observed.

If the observer remains attached to the body, there is no contraction; if the observer is moving with respect to the body, or the body moving with respect to him, the FitzGerald contraction appears. If the observer once more changes his velocity relative to the body, the FitzGerald contraction of the body will likewise change in magnitude. Nothing has happened to the body and yet its length has altered. Obviously, physical length is not what we once thought it to be; it can in no wise be immanent in the body, since a body has no determinate length until the relative motion of the observer has been specified. A length is therefore but the expression of a relationship between the observer and the observed, and the two partners of the relationship must be specified before the length can have any meaning.

In the same way the colour of an opal has no meaning. It is red from here, green from there, blue from elsewhere, and yet the opal has not changed. It is our position with respect to the opal which has changed, and the colour of the opal is indeterminate until such time as we have specified our relative position. In other words, length and the colour of the opal both express relations and not immanent characteristics.

Similar considerations apply to the slowing down of time. Duration, and time are mere relatives, mere expressions of relationships, and have no absolute significance per se. This does not mean that the duration we sense is a myth; for as our consciousness always accompanies our human body wherever we go, we always experience the same flow of time. All that is implied is that this rate of time-flow cannot be credited with any unique significance in nature, and a comparison of time-flows characteristic of various Galilean frames will reveal differences in the rate of flow.

We have mentioned elsewhere a further illustration of relativity, when discussing electric and magnetic forces. We showed how it was that an electric field, as such, was indeterminate; how according to the observer it would present itself as a purely electric or as an electromagnetic field. As all these conditions of observation are on the same footing, there is no sense in distinguishing between apparent fields and real fields, or apparent lengths and real lengths, or apparent durations and real durations. All these concepts, of themselves, are mere phantoms, to which substance can be given only when the conditions of observation are specified.

A certain number of lay philosophers have been confused by this continual reference to the observer in Einstein’s theory, and have assumed that all things occurred in the observer’s mind, the outside common objective world of science playing no part.[49] But this extreme idealistic interpretation cannot be defended.

The word “observer” is a very loose term and does not necessarily mean a living human being. We might replace the observer’s eyes by a photographic camera, his computation of time-flow by a clock—in fact, all his senses and measurements by recording instruments of a suitable nature, whose readings any man situated in any frame could check later. The results would still be the same.

Now it might be feared that, with this vanishing of the absoluteness of such fundamental forms of perception as duration and distance, the entire objective world of science would sink to a shadow; and without a common objective world, science would be impossible. However, we need have no fear of any such catastrophe; for, as will be explained in later pages, a new common objective world of space-time will take the place of the classical one of space and time. But even at the present stage, without appealing to space-time, we can see that objectivity is not denied us for the following reasons: