“A successful unification of quantum theory and relativity would necessarily be a theory of the universe as a whole. It would tell us, as Aristotle and Newton did before, what things are made of, and what kind of laws those things obey. Such a theory will bring about a radical shift—a revolution—in our understanding of what nature is. It must also have wide repercussions, and will likely bring about, or contribute to, a shift in our understanding of ourselves and our relationship to the rest of the universe.” (Smolin )
Question 13: What can be considered the big problem facing physicists today?
We use general relativity for the physics of the very massive (planets, stars) and we use quantum mechanics for the physics of the very small (electrons, protons), so in most situations, they do not overlap. However, there are at least two situations that would be small and massive. The first is in black hole theory (black holes are very dense), and the second is in analyzing theory for the whole universe at the moment of the big bang.
In trying to combine the theories of general relativity and quantum mechanics, physicists currently get nonsense answers like infinity for calculated probabilities. The two theories at present cannot co-exist. This is the big problem for physicists today: The reconciliation of quantum mechanics and general relativity into a unified theory. It does not sit well with physicists when they have to stick two theories together that do not fit properly, like a piece-wise defined function.