Easy Lessons in Einstein

Part 1

DR. ALBERT EINSTEIN IN HIS STUDY AT BERLIN.]

EASY LESSONS IN EINSTEIN

A DISCUSSION OF THE MORE INTELLIGIBLE FEATURES OF THE THEORY OF RELATIVITY

EDWIN E. SLOSSON, M.S., Ph.D.

Literary Editor of The Independent, Associate in the Columbia School of Journalism. Author of “Great American Universities,” “Major Prophets of To-day,” “Six Major Prophets,” “Creative Chemistry,” etc.

With an Article by Albert Einstein and a Bibliography

ILLUSTRATED

NEW YORK HARCOURT, BRACE AND HOWE 1920

COPYRIGHT, 1920, BY HARCOURT, BRACE AND HOWE, INC.

THE QUINN & BODEN COMPANY RAHWAY, N. J.

Deepest of all illusory Appearances, for hiding Wonder, as for many other ends, are your two grand fundamental world-enveloping Appearances, Space and Time.--CARLYLE.

Henceforth Space in itself and Time in itself sink into mere shadows and only a kind of union of the two can be maintained as self-existent.--MINKOWSKI.

A PREFATORIAL DIALOGUE

(The Purpose of which is to Prevent the Prospective Reader from buying the Book under False Pretenses)

SCENE: A street car in uniform movement of translation in any direction.

TIME: The present.

The Reader: (looking over the top of a morning paper): Here’s something queer--a whole page taken with a new discovery in physics--“Eclipse Observations Confirm Einstein’s Theory of Relativity.” Anything about it in your paper?

The Author: Yes. Here’s a cartoon on it by McCutcheon.

The Reader: Must be something to it then. McCutcheon always knows what’s news. (Reads on with audible fragments) “Most sensational discovery in the history of science”--“Greatest achievement of the human intellect”--“Upsets Galileo, Newton, and Euclid”--“Revolution in philosophy and theology.” It looks as though I ought to know something about this, doesn’t it?

The Author: I think you will have to sometime. And you might as well do it now and get it over with.

The Reader: (running down the column and hitting the high spots): “Parallel lines meet”--“a man moving with the speed of light never grows old”--“gravitation due to a warp in space”--“length of a measuring stick depends upon direction of its motion”--“mass is latent energy”--“time as a fourth dimension”--why, the man is crazy, isn’t he?

The Author: Well, definitions of insanity are so uncertain that it is not safe to say who is crazy. But it seems there’s method in his madness--otherwise how could he have hit upon the exact extent of the sun’s attraction on light?

The Reader: (Picks up his paper and reads aloud with concentrated attention) “Postulate I. Every law of nature which holds good with respect to a coördinate system K must also hold good for any other system K′, provided that K and K′ are in uniform movement of translation.” Say, do you know anything about this business?

The Author: Well, yes, a little. I have followed the controversy--at a safe distance--for a number of years.

The Reader: Can you tell me in plain language what it is all about?

The Author: Yes. Just that. I can tell you what it is about, though I can’t tell you what it is. Einstein says that there are only twelve men in the world capable of understanding his latest paper.

The Reader: Are you one of the twelve?

The Author: No, nor the thirteenth. But without plunging into the mathematics of it, we might talk over some of the interesting aspects of the theory of relativity and in the end I could put you on track of the twelve so you could read up on the subject if you liked.

Part 2

The Reader: All right. That’s fair. This is a slow car anyhow. Go ahead.

The Author: (See following pages)--

EASY LESSONS IN EINSTEIN

“A warp in nature has been found, No line is straight, no circle round; For Isaac Newton had unsound Ideas of gravitation.”

Why is it that our newspapers are sending out their reporters to interview astronomers as well as actresses and devoting pages to speculations on the nature of space and time as well as on the state of the market? It is--to get at the bottom of it--merely because a few photographs taken during the eclipse of the sun on May 29, 1919, by two telescopes, one at Sobral in northern Brazil and the other on the island of Principe off the west coast of Africa, showed an abnormal shift of less than one-324,000th of a right angle in the position of the stars. When these photograph films were laid over films taken before the eclipse it was found that the star-images about the darkened disk of the sun did not exactly coincide with the images when the sun was not in their midst. Measured with a micrometer the displacement of the stars from their ordinary positions was found to be 1.60 seconds of arc on the African plates and 1.98 seconds on the Brazilian plates. Average these two observations and you get 1.79. This is extremely close to the 1.73 predicted by Professor Einstein of Berlin and twice as large as the deflection calculated according to Newton’s law of gravitation which would be .87 of a second.

When the announcement of this result was made at the meeting of the Royal Society of London on November 6 all eyes were turned toward Sir Oliver Lodge, for last February he had been rash enough to express the hope, if not the prediction, that the results of the eclipse expedition would support Newton rather than Einstein. But instead of taking part in the discussion Sir Oliver got up and walked out. It was suspected that he had “gone off mad,” as we Americans would put it, because the starlight would not follow his preferred path. But he put a stop to any such rumors by a letter to The Times in which he explains that his departure was not due to any dissatisfaction with the universe but to the necessity of catching the 6 o’clock train. He frankly acknowledges that “the eclipse result is a great victory for Einstein; the quantitative agreement is too close to allow much room for doubt” but he adds “a caution against a strengthening of great and complicated generalizations concerning space and time on the strength of this splendid result: I trust that it may be accounted for, with reasonable simplicity in terms of the ether of space.”

This caution is wise, but we cannot hold our breath till 1922, when the next eclipse comes, to see if these observations are verified and we may in the meantime consider some of the implications of Einstein’s theory of relativity.

Sir Joseph Thomson, President of the Royal Society, in making the momentous announcement in the session of the Society, said:

If his theory is right, it makes us take an entirely new view of gravitation. If it is sustained that Einstein’s reasoning holds good--and it has sustained two very severe tests in connection with the perihelion of Mercury and the present eclipse--then it is the result of one of the highest achievements of human thought. The weak point in the theory is the great difficulty in expressing it. It would seem that no one can understand the new law of gravitation without a thorough knowledge of the theory of invariants and of the calculus of variations.

What is this theory of relativity and why is it so important? The mathematics of it are too much for most of us, but we can get some notion of it by a familiar illustration.

Suppose you wake up some morning in a Pullman berth and look out of the window to see where you are. You find your view blocked by a passing train on the next track. Now if you do not feel any jar of your car and cannot catch sight of the landscape beyond the other train you cannot tell whether (1) your train is moving forward and the other train is standing still, or (2) your train is standing still and the other train is moving backward, or (3) whether both trains are moving in opposite directions, or (4) whether both trains are moving in the same direction, but your train faster. It is obvious that the trains are getting past one another. You can measure their speed of parting as accurately as you please. But all you can perceive is the relative motion of the two trains. You begin to wonder whether there is any such thing as absolute motion; whether there is any real difference between rest and motion. Is there any possible way of telling whether your train is in motion or not if all you can see out of the window is some object that itself be moving? Suppose the windows were all curtained, how could you find out whether you were moving forward or backward or standing still?

You discuss this curious question with your fellow passengers at the breakfast table and one of them makes the brilliant suggestion that it might be possible to determine the absolute motion of the car by reference to the air. If the car is moving forward the air would stream from front to rear and the reverse if it were moving backward. “Suppose,” says the ingenious experimentalist, “that you stand at one end of the car and I at the other. We will shout at each other alternately and time the passage of the sound with our stop watches. Since sound is carried by air waves it will take longer for the shout to go against the air current than with it, and from that measurement it might be possible for us not only to determine which way the car is moving but how to calculate how fast it travels, assuming, of course, that there is no wind blowing.” That strikes you as a crucial experiment, but you point out one possible difficulty, that the doors at the ends of the car may be closed and the air inside is being carried along with the car, so no difference would be observable in the speed of the sound even though the car were moving. “All right,” replies your scientific friend, “we will make a preliminary test to see if the enclosed air is carried along with the car, and if we find that it is not then we will try the second experiment with the sound signals to see which way the air current is moving. These two experiments must settle it, for either the air is moving with the car or it is moving through the car. Can you conceive of any other possibility than these two?” No, you cannot, so you proceed to try the two experiments. First you visit both ends of the car and find both doors open; the air then is not being carried along with the car. You turn then with confidence to the second experiment and you find, of course, that there is a difference in the speed of sound whether it moves with the air drift or against it.

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