Theoretical “discoveries” have to be tested in the real world to see if the world actually behaves the way a researcher thought it would.

Einstein did not do experiments; he looked at the work of past scientists, found some things that didn’t look right, and thought up a new theory to explain them. His breakthrough in working on his theory of relativity was that the speed of light had to be constant; and the mind-boggling result of that insight was that time had to pass at different speeds depending on the position and speed of the observer.

The first “proof” of his theory of relativity did not come until the late nineteen-teens; other experiments supported the theory after that. “Proof” is in quotes because we know that there are some problems with Einstein’s theory — it isn’t perfect. But it has held up very well.