The Secrets of the Universe The Nature of the Physical
World. By A. S. Eddington. (Cambridge University Press. 12s. 6d.) nR. EririnvOrow reprehends Dr. Johnson for thinking that to kick a large stone refutes the theory that there can be no real existence outside the mind of the individual thinker. It would be less illogical to find a refutation of Berkeley's " ingenious sophistry " in the appearance of such a profound and fascinating book as Dr. Eddington has now given us. It is easier to believe that it comes from an external source than that it is a by-product of the reader's own mind. Very few readers, indeed, will claim the ability even to criticize this masterly exposition of the ideas of modern physics, as revo- lutionized by Einstein and Minkowsky. We are content to he grateful for the union of lucid statement with lambent humour and shrewd common sense which makes Dr. Eddington's recast of his 1927 Gifford Lectures the most intelligible account yet issued of recent ideas concerning the nature of the physical world.
It must not be supposed, however, that Dr. Eddington's book is easy reading. As the older Spectator observed, " It is hard for a Reader, who has not rolled this Thought in his own Mind, to follow in such an abstracted Speculation." To expound the work of Einstein without mathematics is more difficult than to explain colour to a blind man. Illustration and analogy, which Dr. Eddington handles so aptly and grace- fully, go a long way, but not always all the way. It-needs much concentrated attention to accompany the author through the four-dimensional world which we are now sup- posed to inhabit—the three familiar dimensions of - Space being combined with, Time as a fourth dimension—and to comprehend human existence as that of a " four-dimensional worm ". to which Past and Future have the same kind of spatial relation as East and West. But the mental effort is well- rewarded, even though the brain may sometimes reel in the attempt to realize Frames of Space and to discriminate between Primary and Secondary Laws.
The modern physicist obeys the adjuration to live in Metro-land." His science is based on (more or less) exact measurements, or " pointer-readings." With the aid of a scale and a clock, Einstein has fathomed the secrets of the universe. It is true, indeed, that all measures are relative— the metre rod itself changes length according to its speed and orientation. It shrinks to zero at the speed of light, though fortunately the change is only two or three inches in the diameter of the earth at the trifling speed of our planet. Yet even so, a certain accuracy can be attained. The Einstein theory of gravitation has now supplanted that of Newton, which held the field for two centuries. Not only has the new theory solved certain practical problems—relating to such matters as the bending of light in passing close to the sun, the motion of the perihelion of Mercury, and the shift-of stellar spectral lines towards the red—which beat the older one, but it has explained the cause of gravitation, which Newton never attempted to do. The four-dimensional Time- space in which we live is finite, though unbounded—somewhat as the earth's surface, though by no means infinite, has no bounds. And this Time-space has a measurable curvature, so that rays of light emitted from the sun, instead of wandering away for ever straight, pursue a curved path and may return ultimately to their starting-point. Gravitation turns out to be a necessary consequence of the particular curvature which Einstein attributes to our Time-space : there is no longer any need to postulate an external force. It is enough to be assured that the ten principal co-efficients of curvature are zero in empty space—and the apple has no option but to fall.
Though the universe is finite, it is almost inconceivably vast. Dr. Eddington reckons that light, travelling 186,000 miles per second, would take 200 million years to cross our world, which another calculation makes a thousand times greater. Yet the atom is so tiny that, " if we eliminated all the unfilled space in a man's body and collected his protons and electrons into one mass, the man would be reduced to a speck just visible with a magnifying glass." It is not long since the atom was first envisaged as a kind of solar system, in which the central nucleus was encircled by one or more planetary electrons. One scheme after another has since variously described the grouping and orbits of the protons and electrons. The advance has been rapid, and finality is not yet reached. Dr. Eddington still admits that he visualizes the electron as a tiny hard red ball ; but in one of the latest theories the electron has wholly lost its individuality. Schro- (linger reduces it to a Grand Perhaps, an undulating proba- bility associated with a stormy area in the aether. As man is assuredly made of electrons, we now understand why
Montaigne found him divers et ondoyant. - The finite nature of Time-space forbids us to consider. Dr. Eddington's subtle criticism of the new physical doctrines,
though this is perhaps the most important part of his book.
His discussion of the philosophical consequences of the rela- tivity doctrine is elaborate and convincing. It starts from the sole fixed point of the Here-Now, and ends, after searching through all the dimensions of Time-space, on a note in accord with that sounded by a- spiritual progenitor of Einstein :-
"Behold, I go forward, but he is not there ; and backward, but I cannot perceive him : on the left hand, Nhere he doth work, but I cannot behold him : he hideth himself on the right hand, that I cannot see him."
After all invocations of Hamiltonian differentiation and the Tensor Calculus, reason still has to take refuge in an 0 Altitudo ! Even so, the Second Law of Thermodynamics remains Unshaken ; and, as Dr. Eddington concludes
" Amid all our faulty attempts at expression, the kernel of scientific truth steadily grows ; and of this truth it may be said— the more it changes, the more it remains the same thing,"
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