"The central fact which has been proved--and which is of great interest
and importance--is that the natural phenomena involving gravitation
and inertia (such as the motions of the planets) and the phenomena
involving electricity and magnetism (including the motion of light)
are not independent of one another, but are intimately related, so
that both sets of phenomena should be regarded as parts of one vast
system, embracing all Nature. The relation of the two is, however, of
such a character that it is perceptible only in a very few instances,
and then only to refined observations."

Already before the war, Einstein had immense fame among physicists,
and among all who are interested in the philosophy of science,
because of his principle of relativity.

Clerk Maxwell had shown that light is electro-magnetic, and had reduced
the whole theory of electro-magnetism to a small number of equations,
which are fundamental in all subsequent work. But these equations
were entangled with the hypothesis of the ether, and with the notion
of motion relative to the ether. Since the ether was supposed to be
at rest, such motion was indistinguishable from absolute motion. The
motion of the earth relatively to the ether should have been different
at different points of its orbit, and measurable phenomena should
have resulted from this difference. But none did, and all attempts to
detect effects of motions relative to the ether failed. The theory of
relativity succeeded in accounting for this fact. But it was necessary
incidentally to throw over the one universal time, and substitute
local times attached to moving bodies and varying according to their
motion. The equations on which the theory of relativity is based are
due to Lorentz, but Einstein connected them with his general principle,