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When constructing his general theory of relativity, Einstein made the following observation: a freely falling object in a gravitational field will not be able to detect the existence of the field by making local measurements ("a falling man feels no gravity").
The correct theory of gravitation is now believed to be Einstein's General Theory of Relativity.
His fame grew immensely in 1960 when Robert Pound and Glen Rebka used this effect to prove the red shift of gamma radiation in the gravitational field of the Earth; this Pound–Rebka experiment was one of the first experimental precision tests of Albert Einstein's general theory of relativity.
His thesis work was on alternative theories of gravity to Einstein's General Theory of Relativity; he is co-author of the Lightman-Lee and Lee-Lightman-Ni theories of gravity (see Alternatives to General Relativity).