Where did Einstein prove the theory of relativity?

Answer 1

Einstein did not "prove" his postulates in the sense that he conducted tests to determine whether or not they fit experimental evidence. Instead, he wrote these postulates in his special theory of relativity while working as a patents clerk in Geneva in 1905 followed by the general theory in Germany in 1912.

Both theories have been tested extensively over the decades, and no variance from either theory has ever been found. Indeed, GPS systems would not work if relativistic corrections were not added to their calculations. Thus, relativity is as established a part of our understanding of our Universe as is gravity.