Einstein's Real Motivation Behind GR

[CASPIDE]

I wonder what really motivated Einstein to formulate his general theory of relativity:

a) would be due to the principle of equivalence?
b) it would be because after the relativistic formulation of Maxwell's equations, there was only the classical gravitation with the factor 1 / r ^ 2?
c) would be to find that there was no reason for there to be a privileged reference frame (ie the inertial frame of reference presents the special theory of relativity)?
d) would be because the Newtonian gravitation had presupposed an action to the instantaneous distance, violating the principle of the constancy and the speed of light limit?

In fact, the physics textbooks (in the area of gravity or relativity) induce the readers to believe that Einstein postulated his GR mainly due to the principle of equivalence. But that would be too simplistic!

 

[stevendaryl]

In fact, the physics textbooks (in the area of gravity or relativity) induce the readers to believe that Einstein postulated his GR mainly due to the principle of equivalence. But that would be too simplistic!

I think that the motivation for looking for a theory of gravity was pretty straight-forward: SR (Special Relativity) successfully reconciled mechanics (the dynamics of massive particles) with electrodynamics, but the theory of gravity, with its instantaneous action-at-a-distance, was incompatible with SR. So the most immediate motivation was to have a theory of gravity that was compatible with his theory of relativity. There was a second motivation, which was not obviously connected with the first, which was to extend his principle of relativity from the equivalence of all inertial reference frames to the equivalence all frames, inertial or not.

The "equivalence principle", which was the idea that locally there was no difference between being at rest in a gravitational field and being at rest in an accelerated reference frame, turned out to connect those two motivations. It was the clue as to how to formulate a relativistic theory of gravity that treated all motion as "relative".

 

[pervect]

I concur. The main motivation for GR was to have a theory of gravity that was compatible with special relativity, and experiment. Some early attempts at this yielded a theory of gravity that was compatible with SR, but incompatible with observations - Nordstrom's theory, which described gravity with a scalar field. When the scalar field approach failed, and vector field approaches failed, Einstein went to a tensor field approach. He made remarks to the effect that the path was more difficult an abstract than he expected.

A secondary motivation for GR was the equivalence principle. Basically there wasn't any reason we should have a "gravitational mass" and an "inertial mass" that just happened to be equal to the gravitational mass. So Einstein started thinking about theories where the equality wasn't just a coincidence, but had a theoretical basis.