After 1905, Einstein continued
working in the above areas. He made important contributions to the quantum theory,
but he wanted to extend the special theory of relativity to phenomena involving
acceleration. The key to an elaboration appeared in 1907 with the principle of
equivalence, a postulate that gravitational fields are equivalent to accelerations
of the frame of reference.
Einstein elevated this equivalence, which is embedded in the work of Isaac
Newton, to a guiding principle in his attempts to explain both electromagnetic
and gravitational acceleration according to one set of physical laws. He
proposed that if mass were equivalent to energy, then the principle of
equivalence required that gravitational mass would interact with the apparent
mass of electromagnetic radiation, which includes light.
For example people travelling in
a moving lift cannot, in principle, decide whether the force that acts on them
is caused by gravitation or by a constant acceleration of the lift.
On the basis of the general theory of
relativity, Einstein accounted for previously unexplained variations in the
orbital motion of the planets. By
1911, Einstein was able to make preliminary predictions about how a ray of
light from a distant star, passing near the Sun, would appear to be attracted,
or bent slightly, in the direction of the Sun's mass. At the same time, light
radiated from the Sun would interact with the Sun's mass, resulting in a slight
change toward the infrared end of the Sun's optical spectrum.
Einstein called his new work the general theory of relativity. After
several false starts the final form of the general theory was published in
1916. In it, the interactions of bodies, which heretofore had been ascribed to
gravitational forces, are explained as the influence of bodies on the geometry
of space-time (four-dimensional space, a mathematical abstraction, having the
three dimensions of Euclidean space and time as the fourth dimension).
This later phenomenon was confirmed during a solar eclipse in 1919. The confirmation became a media event and Einstein's fame spread worldwide.
Einstein's
Explanation
Needless to say, the General Theory of
Relativity is too difficult to explain. However Einstein had a very simple
explanation that says if you spend two hours speaking with a beautiful woman,
you feel, at the end of the two hours that you've spent only two minutes with
her; while if you spend two minutes sitting on hot tin, you feel you've spent
two days!
For the rest of his life Einstein devoted many efforts to generalize his theory even more. His last effort, a unified field theory was not completely successful. It was an attempt to understand all physical interactions in terms of the modification of the geometry of space-time between interacting entities.
