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We went to the moon using just Newton's laws of motion and gravity. Newtonian dynamics we call it. So then we find out, "Well, this works because there's certain regimes we've never tested it in." Had we done so, we would show that it didn't work: For example, at very high speeds, very high gravity, Newton's laws fail. They just fail. You need Einstein's laws of motion and gravity. Those would be his special theory of relativity and general theory of relativity. Now you invoke those and it works.
