What is the theory of relativity, and did Einstein ever express doubts about its correctness?

The theory of relativity is a set of two interrelated physical theories proposed by Albert Einstein. The first one, called special relativity, was published in 1905 and deals with the laws of physics in the absence of gravity. The second theory, called general relativity, was published in 1915 and includes the effects of gravity.

The theory of relativity revolutionized the way physicists understand space, time, and gravity. It shows that space and time are not absolute but are relative to the observer's frame of reference. The theory also predicted some remarkable phenomena, such as time dilation, length contraction, and black holes.

As for Einstein's doubts about the theory's correctness, it is true that he expressed some skepticism about quantum mechanics, a branch of physics that deals with the behavior of particles on a very small scale. Einstein believed that quantum mechanics was incomplete and that there should be a more fundamental theory that could explain everything. However, he never doubted the correctness of the theory of relativity, and it remains one of the most well-established and thoroughly tested theories in physics.