For the last few decades of his life, Einstein tried to find a Grand Unified Theory where Electromagnetism, Gravity, and the Weak and Strong Nuclear forces could be explained with a simple single theory.
General Relativity explains gravity, and quantum explains the nuclear forces. The two theories, which are experimentally extremely efficient at explaining phenomenon, are incompatible.
Relativity breaks down when trying to explain the extremely small, and quantum is pretty much unusable when explaining the macro world.
The GUT is still the holy grail of physics and string theory is a possible way of getting there. GUT is what Einstein was devoted to but string theories had yet to be developed before Einstein’s death.
TweetyBird:
Einsteins general theory does explain gravity as warping of the fabric of spacetime by the effect of a large mass.
This explains how light, which has no mass, could be effected by gravity. Light always travels in a straight line but "straight" is relative to the space in which it defined.
If the spacetime is curved then 'straight' turns out to be curved as it follows the warped fabric. So the path light travels appears to be bent due to the effect of gravity.
