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The theory of relativity: Albert Einstein
by Dr. Edwin E. Slosson
Reprint of an article by Dr. Slosson in Science Service, in 1949.
A friend of mine--I don't know him personally, but any man who buys a book of mine is a friend of mine--writes to me: "If you will put Einstein's theory of relativity in words of one syllable perhaps I can understand it."
Now, that is a foolish notion--even though he is a friend of mine. Short words may be easier to pronounce, but not easier to understand.
But anything to oblige a friend. So here goes:
If you were on a train and saw a train on the side track slip by your pane of glass you could not tell which trained moved if yours did not jolt. You might think that your train was at rest, and that one moved back, or that both moved, but not at the same rate or the same way. It would be all the same which way you looked at it.
If now you were in a tight box or chest as big as a room that rests on the ground you would feel a down pull, which we call your weight. It is said to be due to a "force." But if the box is off in space where there is no force from the earth to act on it, and the box is pulled up by a rope at the same rate as a mass falls to the earth, you would feel the floor press up on your feet just the same as when you stood on the ground.
You know how it feels when you are in a lift that goes up with a jerk. If, while you were in this box off in space, you should throw a ball up in the air, it would go up a ways then fall down to the floor. So it looks to you, though to a man not in the box it seems that the floor moves so fast that it must catch up with the slow ball.
If you should fire a shot straight from the right side of the box to the left, its path would seemed curved down at the end as it would on the earth. So, then, a ray of light, which too, we say, moved straight, would seem to you curved when it passed through the box as though it, like the shot, had been pulled down by some force. But there is no down force in this case, for the box is not near the earth. It is due to the fact that the box moves up with more and more speed in the same way as a mass falls to the earth.
Then we must think that a ray of light near a large mass would not move in a straight line but in a curve. It would act just as if there were a force to pull it in. This has been found to be so. As the light from a star goes past the sun its track is bent to the sun as though the sun pulled the ray, as it does the earth, in a curved path. So when the sun is made dark by the moon the stars round about it seem pushed out of place. They do not stand so close as they do on the star map when the sun is not in their midst.
Then, too, the sphere that moves around the sun and is most near to it does not quite close up the ring of its path at the end of a year as it should by the old law. The new law shows why this is so.
A third test of the new law is still to be passed. The light and dark lines that are seen in a beam of light when it is bent out of its course by a wedge of glass should be pushed to the red end of the band if the light comes from large stars like the sun. A long light wave like the red should show more shift than the short waves. This point has not yet been proven for sure. Such a shift has been seen, but does not seem to be of the right size.
Some strange things must be true if the new law holds good. First, we must say that mass and weight are not fixed, but change when the thing moves, though the change is slight save at high speeds. But near the speed of light the change is great. A thing must weigh more when it moves fast. If a rod goes at great speed in the line of its length it will not seem so long as if it were at rest. No mass can be made to move as swift as light.
A clock in a state of rest does not show the same time as a clock that moves at high speed. As it moves fast through space it seems to slow up. A man would not seem to grow old if he could move with the speed of light.
It is a matter of choice if we say that the earth goes round the sun or that the sun goes round the earth.
If a ring is seen to be one foot through when a rule is laid on it, it will be Pi (3.14159 and so on) times that length round the rim. But if there is a great weight put in the mid point of the ring, then the line will be less that Pi times the line that cuts through the ring at its mid point. If a thin steel disk whirls round fast, its rim will seem to shrink like a hot tire on the wheel of a cart.
It seems then that the scheme of points and lines that we got from the Greeks, and that is taught in our schools yet, is not quite true when we come to deal with time and space as a whole. Space would be naught if there were no time. Time would be naught if there were no space. The two must join to form a sort of fixed frame or mesh in which all things are set.
At each point, say the point where you stand, four lines cross and lead out straight in the four ways. One line runs up and down, the next runs right and left, the third runs back and forth, and the fourth runs from time past to time to come. To fix a thing we must know its point on the time line as well as its points on the three space lines. To place an act we must know when as well as where it came to pass.
Mass will wrap this mesh of space and time. A mass as it moves forms a sort of crease or ridge. A mass that is at rest in space, of course, moves on the time line. A mass, as it moves from this point to that must take the track that is most long through the mesh of space and time.
Space as a whole may be closed up in the form of a sphere or roll, and in that sense may be said to have no end, though it may not be so large as we used to think. A ray of light that starts out from the sun may not go on straight for all time, but may round the sphere of space and come back at the end of a long time to the place it set out from.
NOTE: Much has happened since this article was written in 1949. One of which was the new law passing its third test. Read articles on Einstein and relativity.
Sources:
1. Slosson, Dr. Edwin E. Science Service: "The theory of relativity." Science Service: NY 1949
2. Editors. Science Milestones. Windsor Press: NY 1954
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