The boundary at which general relativity (GR) stops working and quantum mechanics (QM) takes over is a topic of ongoing research and debate in theoretical physics. GR describes the behavior of gravity on large scales, such as the motion of planets and galaxies, while QM describes the behavior of matter and forces on small scales, such as the behavior of atoms and subatomic particles.
In most everyday situations, the effects of gravity are well described by GR, and QM effects are negligible. However, as we delve into extremely high-energy regimes, such as those found in the early universe or near black holes, both GR and QM become relevant, and their interplay becomes important.
One of the key challenges in theoretical physics is reconciling GR and QM into a unified theory of quantum gravity. This theory would describe gravity in the framework of QM and provide a consistent description of the universe at all scales. Various approaches, such as string theory, loop quantum gravity, and others, have been proposed as potential candidates for a theory of quantum gravity. However, currently, there is no widely accepted and experimentally confirmed theory that unifies GR and QM completely.
Therefore, it is difficult to pinpoint an exact energy scale or scenario at which GR definitively breaks down and QM takes over. The search for a theory of quantum gravity is an active area of research, and progress in this field may eventually shed light on the boundary between GR and QM.