In the early part of the 20th century, Albert Einstein revolutionized physics. Einstein is probably best known for formulating the Special Theory of Relativity which explains the relationship between Mass and Energy, formulating the General Theory of Relativity which explains Gravity, and formulating the Photo-Electric Principle which explains the interactions between Photons and Electrons. He won the Nobel Prize for for formulating the Photo-Electric Principle, not Relativity.
Einstein never really understood the Theory of Quantum Mechanics which was derived from the math of his General Theory of Relativity; because, Einstein was a Classical Physicist. In Quantum Mechanics, you can only know probabilities; but, in Classical Physics, exact values must be deriveable. So, Einstein concluded that the Theory of Quantum Mechanics was incomplete.