General theory of relativity (GTR) describes gravity as a geometric property of space-time fabric. It combines specific relativity and Newton’s law of gravitation. GTR which accounts for gravity and all of the things it dominates: orbiting planets, colliding galaxies, colliding black holes, formation of planets etcetera. Easiest definition of what gravity is that, it is an attractive force between massive bodies.
Quantum mechanics on the other hand deals with sub-atomic particles like electrons. It tells the minimum size of any object, including the universe. There are four fundamental forces in nature namely, gravitational force, weak and strong nuclear force, and electromagnetic force. All forces except gravitational force are quantized. Now, what does it mean when a force is quantized? It means that the force is restricted to attain certain discrete magnitudes. For example light is quantized into packets of energy. Quantum mechanics handles all three forces at subatomic level but gravity is exceptional. It is meaningless to ask about how gravity works at infinitesimally smaller distances such as in case of subatomic particles.
Both theories describe reality differently. Both perceptions are genuinely incompatible. On one hand GTR asks the universe to be continuous and deterministic. While, in quantum mechanics events produced by the interaction of subatomic particles happen in jumps with probabilistic rather than definite outcomes. All quantum particles show dual behavior (both, particle and wave) and their positions are determined by Heinsberg’s uncertainty principle. Subatomic particles show superposition that is; they can exist in all possible states at a time. But, when we talk about macroscopic particles, due to gravity they cannot show superposition. Gravity which is the weakest form of background noise, forces them to attain one state from all possible states, making the universe deterministic.
Is there a solution? Maybe
String theory provides the grand unification of Quantum mechanics and theory of relativity. String theory is a theoretical composition where point-like particles of particle physics are replaced by one-dimensional strings of length 10^-33 cm (Planck’s length). String theory explains interactions and vibrations of these strings, which in turn explains fundamentals forces and forms of matter. String theory is the theory of quantum gravity, where it tries to unify quantum gravity with other three fundamental forces. In string theory, one of the many vibrational states of the string corresponds to the graviton, a possible quantum mechanical particle that carries gravitational force.
However, string theory is often accused of taking lots of assumptions and predicting a lot in its attempt to unify physical laws of universe. It discards the possibility of multiverse. There is no “outside” to our universe.