On March 29, 1919, there was an eclipse of the Sun. It was a particularly important eclipse. For years astronomers had eagerly awaited it, since it would enable them to check a revolutionary new theory in physics, proposed four years earlier by a scientist named Albert Einstein.
On the day of the eclipse one group of astronomers was stationed in northern Brazil, another on an island of the western coast of Africa. Delicate cameras were set up and waiting. Pictures would be taken during the eclipse — not of the eclipsed Sun, but of the stars that appear in the suddenly darkened sky around the Sun.
Einstein had said that the position of the stars would be somewhat changed, since the rays of starlight passing near the Sun would be bent by the Sun's mass. To many scientists this sounded impossible. How could light, which was immaterial, they argued, be affected by gravity? If Einstein were correct, the picture of the Universe built up by the great Newton more than two hundred years earlier would have to be considerably revised.
The eclipse came. The pictures were taken and developed. The distances of the stars from the Sun and from one another were carefully measured. There could be no doubt about the results. Einstein was right. The light rays had been bent by the attraction of the Sun. One of the key points of Einstein's theory had been experimentally confirmed.
It was said that only twelve persons in the world really understood exactly at that time what Einstein meant in his theory of relativity. Yet throughout the civilized world everyone who read the newspapers knew that Einstein was a genius, that he had overthrown the foundations on which physics, chemistry, and astronomy had rested for two hundred years, and upset all earlier concepts of the Universe. Later they learned that this revolution had made possible the development of the photoelectric cell, television, a whole series of electronic inventions, and, finally, the harnessing of atomic energy.
More than fifty years have now passed since Einstein put forward the principle of relativity. His theory, which seemed a mere flight of the imagination to many at first, is now the cornerstone of modern physics. Indeed, modern physics cannot exist without the theory of relativity, just as it cannot exist without the concept of atoms and molecules. A vast number of physical phenomena could never be explained without the theory of relativity. Particle accelerators and calculations of nuclear reactions are based on it.
It is not too difficult to grasp the simpler aspects of the theory of relativity. We have little difficulty in understanding that a fly walking along the top of a moving train moves at one speed relative to the train and at another relative to the ground. It would appear to be moving at still another speed from the point of view of an observer in space who could consider the motion of the Earth' in addition to the motion of the train and the fly. From these facts it is not hard to go on to Einstein's contention that there is no absolute motion and no absolute rest: that all motion in the Universe is relative to some other motion. Indeed, the very notions of space and time are relative. The interval between two events, like the distance between them in space, has to refer to some frame if it is to have meaning. The simple words "at one and the same time" are just as meaningless as the words "in one and the same place".
From here on, however, the way becomes more difficult. Our story must stop before it reaches the point where sentences turn into equations. If some of you, however, feel determined to learn more about this remarkable theory, there is a very good book about it, written in simple and understandable language. It is a short book of only 60 pages, with many pictures, and is called "What Is the Theory of Relativity?". The authors are the late Academician Lev Landay and Y. Rumer. An English edition of the book was published by the Foreign Languages Publishing House in Moscow.
© 2003 - 2006 Елькин В.И.
Electric Light Bulb
Cameras in the Beginning
Tape Recorder (and Walkman)