Albert Einstein's theory of general relativity, which he formulated in 1915, describes the force of gravity as a curvature of spacetime caused by mass and energy. It has been highly successful in explaining a wide range of physical phenomena, such as the bending of light around massive objects and the behavior of celestial bodies.
However, it is important to note that Einstein spent much of his later career searching for a unified theory that would go beyond general relativity and encompass other fundamental forces, such as electromagnetism, in a single framework. He was dissatisfied with the fact that general relativity did not account for the behavior of subatomic particles and quantum phenomena.
Einstein's quest for a theory of everything led him to explore the field of unified field theory, aiming to find a single set of equations that would unify gravity and electromagnetism. He made several attempts to develop such a theory, but ultimately, he was not successful in his lifetime.
In his later years, Einstein expressed frustration and acknowledged the difficulties he faced in finding a complete unified theory. However, it is important to note that he never completely abandoned his pursuit and continued to explore and discuss ideas related to a unified theory until his death in 1955.
It's worth mentioning that the search for a theory of everything remains an ongoing endeavor in modern theoretical physics, and several approaches, such as string theory and loop quantum gravity, have been proposed. However, a definitive theory that unifies all fundamental forces and explains all aspects of the physical world is yet to be discovered.