Albert Einstein works on new Theory of Gravity
By 1912 Einstein was hot on the heels of a new theory of gravity that would incorporate his strong equivalence principle.
By calling on this principle, he realized, he could avoid dealing with gravity as a force altogether. Move in the right way, by free-falling, and you don't feel gravity: in an inertial frame, you're weightless and gravity drops out of the picture. But Einstein also realized that the Lorentz transformation of special relativity wouldn't carry over to a more general setting because the way you have to move to cancel out gravity is different in different locations. What he needed was some mathematical way to stitch together local inertial frames in different places so that gravity canceled out everywhere. Although he wasn't yet sure what form his new theory of gravity would take, he did know this: If all accelerated systems are equivalent [with respect to the laws of physics], then Euclidean geometry cannot hold in all of them.
We also know that around 1913 Einstein and his friend Michele Besso worked out the perihelion advance implied by the so-called Entwurf theory of gravity that Einstein had developed with the help of Marcel Grossmann. They were disappointed to find that the theory actually predicted a negative value for the precession, making the anomaly even worse. They decided not to publish the derivation.
Despite the failure to account for Mercury’s precession, Einstein was initially enthusiastic about the Entwurf theory, but gradually he began to lose confidence in it, and resumed the search for a satisfactory theory.
1913-He is appointed Professor of theoretical physics at the Federal Institute of Technology, Zurich and works on his new Theory of Gravity