In the 20th century, we were proposed with two revolutionary theories in physics: quantum mechanics and relativity. Quantum mechanics deals with small scale (molecular/atomic and smaller) behaviour. Max Planck, a German physicist, initiated this field of physics. Some well-known statements of quantum mechanics are that matter can be viewed as a wave which describes the chances of finding it at a given position (proposed by Schrodinger), and that it is impossible to measure a particle’s position and momentum with a low enough uncertainty (proposed by Heisenberg).
In 1915, Einstein published his renowned theory of general relativity. Relativity transformed our understanding of space, time, and gravity, showing that the former two are in fact one continuum, known as spacetime, and gravity is a result of a bend in the geometry of this continuum.
A great thing that Einstein’s relativity predicted was the existence of black holes. Theoretically speaking, any object, when shrunk to a small enough radius (Schwarzchild radius) will collapse into a singularity known as a black hole. Every black hole has a boundary known as the event horizon, past which nothing can escape. If there’s any place in the universe that is truly unreachable, it is past the event horizon of a black hole. There are theories on what happens when one gets past the event horizon, but there doesn’t seem to be a way to prove them.
At the singularity, spacetime and thus all known laws of physics breaks down. Moreover, a black hole’s singularity is large enough that it cannot be ignored from general relativity, but is also small enough that one cannot ignore the effects of quantum mechanics. As it turns out, quantum mechanics and general relativity do not make a great fit together. One main reason for this conflict is the perception of time by either theory. In general theory, time is viewed as dynamic and stretchable, but quantum mechanics views it as fixed and merely there for quantum states to evolve. Hence, physicists are on a quest to unify the two theories by reformulating gravity in terms of quantum mechanical principles. There are quite a few theories that have been described so far, with the most successful being loop quantum gravity theory and string theory. Other theories include spin foam models, causal set theory, and shape dynamics. However, unifying theory has not been discovered yet. It is interesting that two theories that are highly influential and greatly supported by experimental evidence turn out to be incompatible with each other. Hopefully a reformulation of either or both theories will allow us to solve this puzzle in physics very soon.