THE ECLIPSE.

New Zealand Herald, Volume LIX, Issue 18192, 11 September 1922, Page 9

THE ECLIPSE.

EINSTEIN UNDER TEST. THE WALLAL EXPEDITION. STAB-BEAMS .AND RELATIVITY. No. 11. BY H. HARDCASTLE. Perhaps if the Kinstein theory had never been published, the solar eclipse of September 21 would still have gathered many scientists together, even at such a forlorn and little-known place as Wallal. a spot named on few maps, and located on the nearly deserted coast of Western Australia, but whose name may yet echo through history. The eclipse is attractive because it will be an unusually long one, with more than five minutes of totality at Wallal, whose dry, clear atmosphere promises good observation. But it is certainly the fact that the Einstein theory is to stand its trial there that has made the eclipse so "popular" an event, though few people will actually see it. The eclipse will be used to test the most remarkable structure of mathematical logic ever placed before the world. In the earliest days of philosophical speculation, men thought, excusably enough, that the earth was the centre of the universe, and that the sun and the stars revolved round it—an idea in full conformity with the pleasing notion that everything was created solely for the admiration and delight of man. When it had been argued and proved that most of the heavenly motion that is observed is due to the. motion of the earth, it was, again not unjustifiably, assumed that all the visible heavenly bodies except the moon, the planets, and the comets, were "fixed stars," and indeed so they are still very commonly called. But one cannot ponder for any time about the structure of the universe and the laws that govern it without realising that the stars cannot bo fixed in their positions. The law of gravitation inevitably destroys any such idea. As a. natural consequence of realising this fact, there arose a desire to detect and, if possible, to measure the velocity of the earth through space. It is now 41 years since the most famous experiment for this purpose, now universally known as the Michelson-Morley experiment, was tried. The earth was, of course, known to be in motion round the sun, and its velocity in its orbit was known; but, until recently, there was no certainW as to whether the sun had any definite motion through space; indeed it is not known even now, except by circumstantial evidence. What is known, as the result of research with the spectroscope, is that the sun is in motion with respect to the general mass of the stellar system, and that, taking the average of the apparent velocities of a great number of stars relative to the solar system, the sun has a proper motion through the stellar universe in an apparently straight line at the rate of about 12J miles a second. But, for all wo know, those stars, which form the system of reference for measuring the sun's velocity, may themselves have a velocity which the 'sun shares with them, and which lias hitherto utterly defied detection. There is no known fixed milepost to judge by.

The Velocity oi Light. But there is a moving mile-poit. Experiment suggests at) d theory asserts beyond possibility of doubt that light, from no matter what source. travels through free space always at the same velocity, and it has long been known that, within a small margin of error, that velocity is approximately 186,000 jniles per second. For the purposes of deciding the present problem, it u not the exact velocity, but the constancy of the velocity, that is important., because the problem is merely one of comparison. If two rays of Herht pass an observer in opposite directions, and be has means for comparing the velocities with which they pass him, he can ascertain his own velocity by a simple sum "in subtraction and division. If the difference in the velocities is 100 miles per second, his own velocity is 50 miles per second in the direction that the apparently slower ray is travelling. The difficulty is not in the calculation, but in the observation, especially as it seems to bo impossible to compare the velocities of rays travelling in opposite j directions at right angles; and even then the rays have to bo passed both ways along each path, because of the peculiar ontical difficulties of the problem. The effect off] this is that a large proportion of the difference of the velocities cancels out, and what remains, when expressed as a length in the apparatus, is too small for any microscope to detect it. But there are means for detecting even smaller differences, depending on the phe- , nomenon known as the interference of litrbt, and these were applied in the Michelson-Morlev experiment. Though very simple in theory, the experiment was one of extraordinay difficulty in execution. Nevertheless it was carried out several times, and the experimenters were certain that if there was a difference to be measured due. to the velocity of the earth, they could have measured it with ease. A Strange Answer. The result was rather appalling. No velocity whatever of the earth through the ether, not even that of the journey round the sun, was indicated. It seemed* as if the ether surrounding the earth shared its motion; yet both theory and independent experiment give a positive denial of that suggestion. It has been necessary to refer to the Mi'chelson-Morley experiment, because its negative result is the soil in which the theorv of relativity has grown. Various scientists, grappling with the new problem that had arisen, made the apparently extraordinary suggestion that when a body is in motion it is contracted in the direction of motion, but not across that direction ; and that the shortening is just such as will defeat any sucli experiment as that which was attempted. The effect of this shortening is that an egg-shaped body moved endwise £ends to become a sphere, or a sphere to become flattened. But, no matter what the velocity of the earth might be, and what the degree of flattening, it could never bo detected, because every means adopted to measuro the deformation would be deformed in the same degree. Difficult though this explanation is to believe at first, it is quite simple, and,, given ample thought, it is acceptable as a basis for argument; and what Einstein did was to take it up and argue it to its logical conclusions by means of the higher mathematics. His calculations fully confirmed the contraction hypothesis; and from that he developed a series oJ what appear to be inevitable consequences, which radically affect all the laws of physics. In physical .science there are only three fundamentals—length, mass, and time. Tn terms of these it is possible to give mathematical expressions for any physical phenomenon whatever; and it is clear that if one of these is altered by velocity then the others are also; and Einstein uhows by his mathematics that mass and ■time are as non-absolute ns length when velocity is involved. Still further developing his mathematical structure, he produced what has. of all his theory, paused the most talk-the gravitational theory. The sensation which 'his theory caused was due to the fact .that, starting quite independently of gravitational elements it crave a new law which Is, only slightly different from the very simple law exnounded by Nfcwton, but which seems to provide for certain difficulties in astronomical problems that were beyond the power of Newton's law *o explain. The Fitzgerald-Lorentz contraction, to give it its usual name, defies detection because of its generality. The difference between the Einstein and the Newtonian laws of gravitation defies detection b«caugD ei its minuteness. But tfiero are two tests for it; and up to the present the Einstein theory has been remarkably supported by theM ietta.

One of these is that the Einstein liw explains with close accuracy the astronomical anomaly known as the advance of the perihelion of Mercury. The other test is* the verification or otherwise of a forecast, made by Einstein of the effect of gravitation upon light. He stated that if a ray of light passes through the gravitational field of any body, it will be diverted from its normal straight path as jf it had weight and was gravitationally attracted by the body. He suggested that, to test his assertion, the light of stars passing close to the sun should be examined, and he calculated the amount —very small —of the deviation that would he observed. It is- often said that Einstein argued that light has weight and is attracted; but that is not a correct statement. What he claims is that light behaves as if it has weight. This behaviour is attributed, not to any weightiness in light, but to the argument in the theory of relativity that gravitation is mathematically and physically equal in its effects to an actual constant change in the velocity of the mass which supplies the gravitational field, and this argument holds in spite of the fact that the mass mav in fact not be in motion at all.

Although this seems to be "utter theory," with no possibility of likelihood, to anyone not fully in sympathy with mathematical processes, photographic evidence in favour of it was obtained during the last solar eclipse, in May, 1919. Photographs were taken by night of the stars in the region where the nun was to be oclipsed ; and other photographs v.ere taken during the eclipse, showing a number of stars close to the sun. "When • these were compared, the star images were seen to have been displaced, in some of the photographs, in the direction foretold by Einstein (outward from the sun) and, as nearly as could be measured, to about the extent he had anticipated. The photographs were not, however, unanimous; in any case the "shifts " were _ very minute; and relativists and anti-relativists were left equally eager for an opportunity to make a further test. That opportunity is approaching; and at Christmas Island, at Wallal, and at various other points in Australia, astronomers have set up telescopes and cameras for the purpose of securing the most accurate records possible, by which the famous theory will be more or less confirmed or refuted.