THE EINSTEIN THEORY

Dominion, Volume 14, Issue 257, 25 July 1921, Page 9

THE EINSTEIN THEORY

HOW IT STANDS TO-DAY THE NEW CONCEPTION OF SPACE In visw of Professor Einstein’s visit to England during the coming week, an attempt to estimate the present position of the theory of relativity may not be Inopportune, wrote J. 11. Jeans, F.R.S. (lato Stokes Lecturer in Applied Mathematics at Cambridge, in the London "Observer" of May 29. As is well known, the theory falls naturally into two parts—the earlier or "restricted" theory and the later or "generalised" theory, the latter being, as its name implies, a development of the former. The earlier theory arose out of the apparent inability of experimental physics to determine the earth’s absolute velocity in space. If is easy to determine Tlio earth’s velo/ity relative to the sun or to any star or stars we please, but we cannot in this way ever arrive nt a velocity which has any right to be regarded as absolute, for no astronomical body has any special claim to be regarded as fixed in space. Throughout the nineteenth century,, however, it was believed that wo were surrounded, and indeed permeated, by an ocean of ether. The parts of this ocean were not supposed to move relatively to one another, so that the ether might be regarded as providing an absolute standard of rest. Physicist after physicist attempted to measure our velocity through the ether by carefully-devised optical and electrical experiments, but one and all failed; no velocity of the earth through the ether could ever bo detected. The restricted theory of relativity was based upon the conjecture that these repeated failures were rendered inevitable by a general principle in Nature. It was assumed that a, general law of Nature, dominating all other laws, made it inherently impossible over to determine the absqjwle velocity of the earth, or, of course, of any other body, by any experimental means whatever. This principle was called the principle of relativity. It was soon found to have consequences which reached far outside the sciences of optica and electro-mag-netism in which it had its birth. In particular, it was seen to shatter our previous conceptions of the nature and meaning of space, time, and motion. The present position of the restricted theory of relativity can be expressed, •imply but completely; in two words: It stands. The only* other possibility would have been total and irrevocable downfall. Had a single well-authenticat-ed law of Nature been found to be in opposition to tlie supposed dominating principle, there'would have been an immediate end to the principle, and with it to the theory of relativity. But no such opposing law has appeared, or is now likely to appear. When a supposed law has seemed to violate the principle of relativity, further experiment has invariably shown that the law had to be discarded and not the principle.

It was early noticed that a violation of the principle was involved in the famous Newtonian law of gravitation, that every particle of matter attracts every other with a force proportional to the inverse square of their distance apart. Clearly, either Newton’s law or the principle of relativity had to go. Einstein, pinning his faith to the former alternative, set to work to examine whether it was not possible to account for gravitational phenomena without discarding the principle of relativity. The result was the generalised theory of relativity. Imagine a ship rounding the Lizard and bound for the mouth of the St. Lawrence River. Both points lie on the same parallel of latitude, so that an unsophisticated passenger might reasonably expect the ship to sail always due west along this parallel of latitude. In actual fact the course set at first will be considerably north of west, while later it will be considerably south of west, and tho ship’s track, as marked on its Mercator chatt, will appear excessively curved and roundabout. It will nevertheless bo the shortest course between the two points; tho apparent curvature .arises from the curvature of the earth’s surface and not from the vagaries of the navigating officers. So it is an essential part of Einstein’s generalised theory that the apparent curvature of the track of a cricket ball or a comet arises from the curvature of the space in which the motion occurs. Tho earth does not emit i. gravitational force which pulls the cricket ball down; it imposes a curvature on the surrounding space bo that the path of the cricket ball appears curved, although it pursues the shortest course open to it. Tho space on which this curvature is impressed is not, however, the ordinary three-dimensional space in which, until recently, we believed all events to occur. It is a new four-dimen-sional space in which this old space is inextricably blended with time. The restricted theory of relativity had already shown that this four-dimensional space is objectively real, and that the old three-dimensional space is a mere subjective 'illusion—different, for each of us, like a rainbow or a nightmare. A planet such as Mercury pursuing its shortest course through tho curved space round tho sun would not follow quite the same course ns under Newton’s law. On tho relativity explanation of. gravitation. Einstein showed that Mercury’s perihilion ought, in opposition to Newton's law, to advance about forty-three seconds of arc per century. This was precisely tho amount of the outstanding discrepancy in tho motion of Mercury which had been known since the time of

I<everricr. The other outstanding discrepancies in the planetary motions hav also been shown to be in agreement with the theory of relativity. According to this theory, a ray of light also would pursue the shortest course in the curved four-dimensional space around the sun, and Einstein was able to predict that a ray of light which just grazed the sun's surface ought to show an apparent bending equal to 1.75 seconds of are., a prediction which was subsequently confirmed at the eclipse of 1919. Since the eclipse results were made known an attempt .has been made to explain tho observed bending as' a consequence fl of refraction by a solar atmosphere, but it appeared that the requisite atmosphere would bo of a highly artificial kind, and I du not think this alternative explanation has received any support. Recently the actual eclipse measurements have been rediscussed by Professor 11. N. Eu-sscll. He finds evidence of an instrumental distortion, unsuspected by the observers, and if the measures aro corrected to allow for this, they aro found to agree very closely with Einstein’s prediction. A third prediction of Einstein has given rise to much discussion and experiment. He believed his theory to require that the frequency of tho light emitted by an atom of, say, calcium in tho sun should be less than from a terrestrial atom of. tile same kind, so that the Fraunhofer lines in tho solar spectrum Ought to show a shift towards tho red. The difficulties of testing this prediction are very great, nnd Ihe attempts so far made have shown discordant results. The best-equipped observer. Professor St. John, of Mount Wilson, finds no appreciable shift nt all: other observers claim to detect a shift of the kind predicted, but its amount is generally too

small. On tho whole the balance of evidence seems to turn against Einstein's prediction. At present this is the only cloud in an otherwise clear sky, but it ought to be added that some mathematicians are loss convinced than Einstein that tho predicted shift forms an essential part of his theory. In conclusion, reference may bo made to an interesting extension of tho generalised theory, which promises to be of far-reaching significance. Maxwell and

Faraday taught that electro-magnetic phenomena were tho perceptible manifestations of actions taking place in the other—for instance, electric force resulted from a state of tension in tho ether, an electric spark showed the release of this tension, and so forth. Einstein’s teaching has abolished at least this kind of ether, and has shown force, at any rate in so far as gravitational force is concerned, to be an illusion. How, then, are wo to explain electric force? Quito recently one of the most brilliant of German mathematicians, Professor Weyl, of Gottingen, has shown that space can bo distorted in more ways than were ever imagined by Einstein. The comparatively simple distortions which Einstein inflicted on space may result in apparent gravitational "forces,” but the most genoral distortions which are possible will add to these whole sets of now "forces." And those new .forces are found to have precisely and identically the properties possessed by the familiar "forces" of electricity and magnetism. In the new interpretation of the universe which the theory of relativity ie thrusting upon us, this solution of the riddle of electricity and magnetism appears likely to play a predominant par —