THE EINSTEIN THEORY.*
" ALL fine things are difficult," said Spinoza ; and if the diffi-
culty is a measure of the fineness, to understand the Einstein theory—still more to explain it olearly—must assuredly be one of the finest things in the world. It can only be adequately expressed, indeed, in mathematical language of excessive complexity. But a general comprehension of its meaning and its effect is now within the reach of the average reader, thanks to the six recent books which are named at the foot of this column, all of which profess to appeal to the non-mathe- matical student. The first r is the authorized translation— excellently done by Dr. R. W. Lawson, of the University of Sheffield—of Professor Albert Einstein's own " popular exposi- tion," intended " to give an exact insight into the theory of relativity to those readers who, from a general scientific and philosophical point of view, are interested in the theory, but who are not conversant with the mathematical apparatus of theoretical physics." It need hardly be said that this is an admirable and classioal work, but we cannot honestly describe it as easy reading, and the author is decidedly optimistic in saying that its comprehension merely presumes " a standard of education oorresponding to that of a University matriculation examination." We can hardly believe that this was true even in pre-war Berlin.
• A more intelligible book for the general reader is the fascinat- ing, though still not very easy, volumes in which Professor A. S. Eddington devotes his well-known powers of lucid exposi- tion to a mainly non-mathematical presentation of the Einstein theory. " For the first nine chapters the task is one of inter- preting a clear-cut theory, accepted in all its essentials by a large and growing school of physicists—although perhaps not every one would accept the author's views of its meaning. Chapters X. and XI. deal with very recent advances, with regard to which opinion is more fluid. As for the last chapter, containing the author's speculations on the meaning of nature, since it touches on the rudiments of a philosophical system, it is perhaps too sanguine to hope that it can ever be other than controversial."
The reader who goes on from Professor Eddington's masterly explanations to Professor Wildon Carr's illuminative essay on " The General Principle of Relativity in its Philosophical and Historical Aspect " may justly flatter himself that he knows as much as it is possible for anyone but a very eminent mathematical physicist to know of the Einstein theory. Pro- fessor Wildon Carr writes not as a mathematician but as a philosopher, and thus provides a work which is a useful comple- ment to that of Professor Eddington. " I have tried," he says, " to expound the reformed concepts of space and time and movement which fire the justification and the foundation of the new working formulae. I have not attempted to indicate or explain, even in non-mathematical terms, the formulae themselves. I have not, for example, tried to show how Einstein worked out the formula of the precession of the peri- helion of Mercury, the displacement of light from stars observed in the eclipse observation, or the shift of the spectral lines. What I have tried to show is the exact meaning in philosophy of the new concept of the framework of nature." What Professor Wildon Carr deliberately omits forms the subject-matter of Professor Eddington's book.
A very brief statement of the facts is also given by Sir Joseph
Larmor in the appendix which he contributes to a new edition of Clerk Maxwell's classic treatise on Matter and Motion.' For the reader who wants to know something of Professor Einstein's work without having to " intend his mind " to it we. may recommend Mr. Slosson'a entertaining and really very clever popularization s of the Einstein theory for the average American in a hurry. To write such a book is much more difficult than the reader may think, and Mr. Slosson deserves great credit for his success in making the Einstein theory bright, breezy, and entertaining. In an appendix he reprints Professor Einstein's own account of his work from the Times of December, 1918.
Lastly, we may call attention to Professor Whitehead's
• (I) Relativity. By Albert Einstein. London : Methuen. (5s. net.1- (2) Spam Time. and Gravitation. By A. 8. Eddington, F.E.8. Cambridge: at the University Press. 115a. net.]—(3) 2'he General Principle of Relativity. By H. Wildon Carr. London : Macmillan. [7s. 6d. net.]---(4) Matter and Motion. By J. Clerk Maxwell. F.H.B. With Notes and Appendices by Sir Joseph Larmor. F.H.B. London : [5s. net.)—(5) Easy Lessons in Einstein. By Edwin E. Simeon. London : Houtiedge. (be. oast.]---(6) The Concept of Nature. By A. N. Whitehead, F.B.B. Cambridge : at the University Press. 114a. net.] incisive criticism of some of Einstein's own methods of inter- preting his results, as set forth in the first series of the Tamer Lectures, given at Cambridge in the autumn of 1919.6 " There is a general agreement," says Professor Whitehead, " that Einstein's investigations have one fundamental merit irrespective of any criticisms which we may feel inclined to pass .upon them. They have made us think. But when we have admitted so far, we are most of us faced with a distressing perplexity. What is it that we ought to think about ? The purport of my lecture this afternoon will be to meet this difficulty and, so far as I am able, to set in a clear light the changes in the background of our scientific thought which are necessitated by any acceptance, however qualified, of Einstein's main positions." The lecture in question, which was delivered before the Chemical Society at the Imperial College of Science last spring, is here appended to the Tamer Lectures as a convenient summary of their doctrine, and may be read with advantage by those who wish to understand the meaning of " the four-dimensional space-time manifold which the theory of relativity presupposes."
One of the most striking features of the Einstein theory is the adoption of Minkowski'a principle that the universe must be regarded (we quote from Professor Wildon Carr) " not, as hitherto, as a three-dimensional continuum enduring in a one- dimensional time, to which it is indifferent, but as a four- dimensional continuum in which the three dimensions of space and the dimension of time are the axes of co-ordination. . . . The beauty of this theory is that its apparent strangeness when first propounded tends to give way to familiarity and obvious- ness ; for when we come to think of it we recognize that the world of our living experience is four-dimensional." Every one who has ever made an appointment to meet a friend knows that— sometimes by painful experience. It is not enough to arrange to meet under the clock at Charing Cross Station—i.e., by specifying the three dimensions of space. One must also " make a date," as the • Americans say—must introduce the fourth dimension of time, and add the magio words " at seven p.m. on April 1st." Or, as Professor Whitehead puts it, the statement that " Cleopatra's Needle is on the Thames Embankment " is incomplete or even misleading until the fourth dimension— the time dimension—is introduced. "If an angel had made the remark some hundreds of millions of years ago, the earth was not in existence, twenty millions of years ago there was no Thames, eighty years ago there was no Thames Embankment, and when I was a small boy Cleopatra's Needle was not there." This, of course, is only one element in the Einstein theory, and by no means the most important, though in some ways it is the most curious and interesting. One of the most fascinating fairy-tales of science might be written about the special principle of relativity, and the remarkable fashion in which it has thus far been verified in its bearing on the theory of gravitation by its explanation of the discrepancies hi Mercury's orbit and the bending of light-rays which pass close to the sun. But our space is already exceeded, and for this we must refer the reader in particular to Professor Eddington's book.
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