I was just wondering what is wrong the quantum theory? What are its drawbacks?
The only drawback that I know of is that it is a background dependent theory - spacetime is where physics happens, but nothing happens to spacetime in the process - spacetime is above and behind the physics. In contrast in GR, matter curves spacetime and spacetime makes matter go in curved paths, so they interact, and there is nothing above and behind the physics.
The job of making the quantum theories rigorous is still a work in progress, but I don't consider that a flaw. Newton's physics and Maxwell's EM had foundational problems too.
The most commonly recognized shortcoming of quantum theory is that, much like GR, it produces nonsensical answers when used to describe a singularity. Under normal circumstances, across the very small distances generally involved in QT, the curvature of space-time is negligible, so it is ignored. In areas of extreme gravitation, where the curvature of space-time cannot be ignored, all attempts to use the equations that model GR with those of QT have produced answers that cannot be true. Location probabilities of 100%, infinite mass densities, etc.
This is why you will frequently hear people say that the TOE will have to incorporate elements of both GR and QT. Each is well supported by evidence that shows it to be excellent at predicting and describing the real universe. So long as we don't try to use both at once.
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