Everyone knows that a person may be sitting in any kind of a vehicle
without noticing its progress, so long as the movement does not vary
in direction or speed; in a car of a fast express train objects fall
in just the same way as in a coach that is standing still. Only when
we look at objects outside the train, or when the air can enter the
car, do we notice indications of the motion. We may compare the earth
with such a moving vehicle, which in its course around the sun has
a remarkable speed, of which the direction and velocity during a
considerable period of time may be regarded as constant. In place
of the air now comes, so it was reasoned formerly, the ether which
fills the spaces of the universe and is the carrier of light and of
electro-magnetic phenomena; there were good reasons to assume that the
earth was entirely permeable for the ether and could travel through it
without setting it in motion. So here was a case comparable with that
of a railroad coach open on all sides. There certainly should have
been a powerful "ether wind" blowing through the earth and all our
instruments, and it was to have been expected that some signs of it
would be noticed in connection with some experiment or other. Every
attempt along that line, however, has remained fruitless; all the
phenomena examined were evidently independent of the motion of the
earth. That this is the way they do function was brought to the front
by Einstein in his first or "special" theory of relativity. For him
the ether does not function and in the sketch that he draws of natural
phenomena there is no mention of that intermediate matter.
If the spaces of the universe are filled with an ether, let us suppose
with a substance, in which, aside from eventual vibrations and other
slight movements, there is never any crowding or flowing of one part
alongside of another, then we can imagine fixed points existing in it;
for example, points in a straight line, located one meter apart, points
in a level plain, like the angles or squares on a chess board extending
out into infinity, and finally, points in space as they are obtained
by repeatedly shifting that level spot a distance of a meter in the
direction perpendicular to it. If, consequently, one of the points
is chosen as an "original point" we can, proceeding from that point,
reach any other point through three steps in the common perpendicular
directions in which the points are arranged. The figures showing how
many meters are comprized in each of the steps may serve to indicate
the place reached and to distinguish it from any other; these are, as
is said, the "co-ordinates" of these places, comparable, for example,
with the numbers on a map giving the longitude and latitude. Let
us imagine that each point has noted upon it the three numbers that
give its position, then we have something comparable with a measure
with numbered subdivisions; only we now have to do, one might say,
with a good many imaginary measures in three common perpendicular
directions. In this "system of co-ordinates" the numbers that fix
the position of one or the other of the bodies may now be read off
at any moment.
This is the means which the astronomers and their mathematical
assistants have always used in dealing with the movement of the
heavenly bodies. At a determined moment the position of each body
is fixed by its three co-ordinates. If these are given, then one
knows also the common distances, as well as the angles formed by the
connecting lines, and the movement of a planet is to be known as soon
as one knows how its co-ordinates are changing from one moment to
the other. Thus the picture that one forms of the phenomena stands
there as if it were sketched on the canvas of the motionless ether.