| From before 1920 until his death in 1955, Einstein struggled
to find laws of physics far more general than any known before.
In his theory of relativity, the force of gravity had become
an expression of the geometry of space and time. The other forces
in nature, above all the force of electromagnetism, had not
been described in such terms. But it seemed likely to Einstein
that electromagnetism and gravity could both be explained as
aspects of some broader mathematical structure. The quest for
such an explanation -- for a "unified field" theory that would
unite electromagnetism and gravity, space and time, all together
-- occupied more of Einstein's years than any other activity.
With Peter Bergmann and Leopold Infeld.