The need for a quantum theory of gravity arises from the incompatibility between quantum mechanics and general relativity, which are the two fundamental theories of physics that describe the behavior of particles and the nature of gravity, respectively.
Quantum mechanics provides a highly successful framework for understanding the behavior of particles at the microscopic level. It describes the behavior of fundamental particles, such as electrons and photons, through wave functions and probabilistic interpretations. It has been extensively tested and validated in various experiments.
On the other hand, general relativity provides a powerful theory of gravity that describes the behavior of gravity in terms of the curvature of spacetime caused by massive objects. It successfully explains the motion of planets, the bending of light by gravity, and the expansion of the universe. However, general relativity is a classical theory and does not incorporate the principles of quantum mechanics.
The problem arises when attempting to apply both theories simultaneously, particularly in extreme scenarios like the early universe or inside a black hole. In these regimes, the effects of gravity become significant at the microscopic level, requiring a quantum mechanical description. However, the equations of general relativity break down when applied to such small scales.
To have a consistent and complete understanding of the fundamental nature of the universe, it is essential to develop a theory that unifies quantum mechanics and general relativity, known as a theory of quantum gravity. This theory would provide a framework that can explain the behavior of particles and the nature of gravity on all scales, from the subatomic to the cosmological.
Moreover, a quantum theory of gravity is necessary for addressing fundamental questions in theoretical physics, such as the nature of singularities, the behavior of spacetime at the Planck scale, and the origin of the universe itself. It is also expected to provide insights into the fundamental structure of spacetime and potentially reconcile apparent inconsistencies between quantum mechanics and general relativity.
While there are various approaches and candidate theories for a quantum theory of gravity, such as string theory, loop quantum gravity, and causal dynamical triangulation, the field is still an active area of research, and a definitive theory remains elusive. Nonetheless, the pursuit of a quantum theory of gravity is driven by the goal of achieving a more complete and unified understanding of the fundamental laws of physics.