y the physical and chemical condition in which it moves.

It is comprehensible that a person could not have arrived at such a
far-reaching change of view by continuing to follow the old beaten
paths, but only by introducing some sort of new idea. Indeed,
Einstein arrived at his theory through a train of thought of great
originality. Let me try to restate it in concise terms.

THE EARTH AS A MOVING CAR

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 exten