Quantum mechanics... developed through some rather messy, complicated processes stimulated by experiment. While it's a very rich and wonderful theory, it doesn't quite have the conceptual foundation of general relativity. Our problem in physics is that everything is based on these two different theories and when we put them together we get nonsense.
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About Edward Witten
Edward Witten was American theoretical physicist. Edward Witten is an American theoretical physicist known for his contributions to string theory, topological quantum field theory, and various areas of mathematics. He is a professor emeritus in the school of natural sciences at the Institute for Advanced Study in Princeton. Read more on Wikipedia →