The search for a theory of quantum gravity arises from the desire to unify two foundational theories of physics: quantum mechanics and general relativity.
Quantum mechanics is the theory that describes the behavior of matter and energy at the microscopic scale. It successfully explains phenomena such as the behavior of particles and the interactions between subatomic particles. However, quantum mechanics is incompatible with general relativity, which is the theory of gravity at the macroscopic scale.
General relativity, formulated by Albert Einstein, describes gravity as the curvature of spacetime caused by mass and energy. It provides a comprehensive framework for understanding the behavior of gravity on large scales, such as the motion of planets and galaxies. However, at extremely small distances and high energies, such as those found near black holes or during the early stages of the universe, the effects of quantum mechanics become significant.
The problem is that when attempts are made to directly apply the principles of quantum mechanics to general relativity, mathematical inconsistencies arise. This mismatch between the two theories has led physicists to seek a theory of quantum gravity that can successfully reconcile them.
Quantum gravity is expected to provide a more complete understanding of gravity on all scales, including those where quantum effects cannot be ignored. It aims to describe the behavior of spacetime and gravity at the microscopic level, where quantum effects dominate. Additionally, a theory of quantum gravity may help explain phenomena such as the behavior of black holes, the nature of the early universe, and the fundamental structure of spacetime itself.
While general relativity is a highly successful theory in describing gravity, it does not incorporate the principles of quantum mechanics. Thus, the pursuit of a theory of quantum gravity is driven by the desire to achieve a more comprehensive and unified understanding of the fundamental forces and particles that govern our universe.