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アインシュタインは統一理論も考えていた

http://en.wikipedia.org/wiki/Unified_field_theory
Later, Albert Einstein used the constancy of the speed of light in Maxwell's theory to unify our notions of space and time into an entity we now call spacetime.
Einstein then expanded the theory of special relativity to a description of gravity, General Relativity, using a field to describe the curving geometry of four dimensional spacetime.

In 1921 Theodor Kaluza extended General Relativity to five dimensions and in 1926 Oscar Klein proposed that the fourth spatial dimension be curled up into a small, unobserved circle.
In Kaluza-Klein theory, the gravitational curvature of the extra spatial direction behaves as an additional force similar to electromagnetism.
These and other models of electromagnetism and gravity were pursued by Albert Einstein in his attempts at a classical unified field theory.
By 1930 Einstein had already considered the Einstein-Maxwell-Dirac System [Dongen]. This system is (heuristically) the super-classical [Varadarajan] limit of (the not mathematically well-defined) Quantum Electrodynamics.
One can easily extend this system to include the weak and strong nuclear forces to get the Einstein-Yang-Mills-Dirac System.