Sometimes the biggest puzzle in physics seems like the worst relationship in the universe. Quantum mechanics and general relativity are the two best theories in physics, but they have never been able to get along.
Quantum mechanics successfully describes the world of the very small, where nothing is predictable and objects don’t have precise positions until they are observed.
General relativity does well with describing massive objects. It says that the world behaves in a precise, predictable way, whether or not it’s observed.
Neither one has ever failed an experimental test. But so far no experiment has been able to show which — if either — of the two theories will hold up in the places where the two converge, such as the beginning of the universe and the center of a black hole.