Within the past few years Professor Einstein has made his theory of
Relativity famous, but it is probable that the theory itself is not
clearly understood by the general public, and the following brief
exposition may be of help in this connection.
If you give a boy a piece of apple pie, he may consider it a large
piece until you give another boy a bigger slice, when he will at once
consider his own small. That is to say it is small _relatively_ to the
other piece.
Professor Einstein first brought home to us the fact that all motion
in our physical universe is not “absolute” but “relative” motion--that
is to say, its speed cannot be measured from any stationary point, for
there is no stationary point on our world from which to measure it.
The earth on which we live is itself moving on its axis, and that axis
“wobbles,” and the earth is also revolving around the sun at a speed of
approximately eighteen miles a second, and our whole solar system is
rushing through space at an unknown speed!
If one man is on a moving ship, walking up and down the deck, and he
passes another man, walking up and down the deck of another boat,
moving in another direction, there are all sorts of complicated
motions which must be taken into account, and this gives you an idea
of the difficulty of obtaining a stationary point, from which to make
measurements of other motions (that is to say, matter _in_ motion).
The first point, therefore, is that all motion is merely relative
motion; and Einstein also proved that _time_ on bodies moving at
different rates of speed was not the same length of time--that is to
say, that an “_hour_” on a body moving at very great velocity was not
the same length of time as an hour on one moving at a slower rate of
speed. Time is, therefore, only relative, just as motion and space are
relative. Both depend upon the velocity of the moving body, and both
vary as the speed of that body varies.
These are two of the simplest conclusions of the theory of relativity.
There are, of course, many more complicated theories involved within
it, into which we have not time to enter now.
The theory itself explains several astronomical phenomena which had
been puzzles until the theory was propounded, such as certain minor
motions of the planet Mercury. The _proof_ of the theory rests largely
upon a certain historical experiment, conducted some years ago, in
which it was proved that a beam of light, coming from one of the
distant stars, was bent or diverted by the pull of gravity exercised
upon it by the sun. Very briefly, the experiment was as follows:
A photographic chart of the heavens having been made, certain stars
were found to occupy certain fixed positions when the sun was not
present in the heavens. When the sun _was_ present it was, of course,
impossible to photograph these stars, because the extreme brightness
of the sun prevented the stars from being seen by the eye of the
camera. When, however, the sun was eclipsed, these stars could be seen,
and during the eclipse of the sun photographs were taken of these same
stars and the plates were compared.
It was then found that the position of these stars did not coincide
exactly. The stars themselves had not varied relatively to the
surrounding stars, as subsequent photographs proved. What, then, had
happened? Light rays coming from these stars had been slightly bent or
pulled inward by the gravitational pull of the sun! This deviation was
exactly that required on the Einstein theory of relativity.