namely, that there must be nothing, in observable phenomena, which
could be attributed to absolute motion of the observer.

In orthodox Newtonian dynamics the principle of relativity had a
simpler form, which did not require the substitution of local time
for general time. But it now appeared that Newtonian dynamics is only
valid when we confine ourselves to velocities much less than that
of light. The whole Galileo-Newton system thus sank to the level
of a first approximation, becoming progressively less exact as the
velocities concerned approached that of light.

Einstein's extension of his principle so as to account for gravitation
was made during the war, and for a considerable period our astronomers
were unable to become acquainted with it, owing to the difficulty
of obtaining German printed matter. However, copies of his work
ultimately reached the outside world and enabled people to learn more
about it. Gravitation, ever since Newton, had remained isolated from
other forces in nature; various attempts had been made to account
for it, but without success. The immense unification effected by
electro-magnetism apparently left gravitation out of its scope. It
seemed that nature had presented a challenge to the physicists which
none of them were able to meet.

At this point Einstein intervened with a hypothesis which, apart
altogether from subsequent verification, deserves to rank as one
of the great monuments of human genius. After correcting Newton,
it remained to correct Euclid, and it was in terms of non-Euclidean
geometry that he stated his new theory. Non-Euclidean geometry is
a study of which the primary motive was logical and philosophical;
few of its promoters ever dreamed that it would come to be applied