The need for a consistent description of gravity that combines Einstein's theory of general relativity (GR) and quantum field theory (QFT) arises from the desire to have a unified framework that can account for both the large-scale behavior of gravity and the quantum behavior of elementary particles.
Einstein's general relativity is a classical theory that successfully describes gravity as the curvature of spacetime caused by the presence of mass and energy. It provides a geometric understanding of gravity, where massive objects cause spacetime to curve, and the motion of particles is influenced by this curvature. General relativity has been extensively tested and confirmed in various astrophysical and cosmological observations.
On the other hand, quantum field theory is a theoretical framework that combines quantum mechanics with special relativity to describe the behavior of elementary particles and their interactions. It has been tremendously successful in explaining the behavior of fundamental particles and electromagnetic, weak, and strong nuclear forces through the framework of quantum fields.
However, when it comes to gravity, the efforts to develop a consistent quantum theory of gravity have encountered significant challenges. The main difficulty lies in reconciling the fundamentally different nature of the two theories—general relativity being a classical theory and quantum field theory being a quantum theory.
At extremely small scales, such as those near the Planck scale (10^(-35) meters), where both quantum effects and gravitational effects are expected to be important, the classical description of gravity breaks down. In these regimes, a quantum theory of gravity is needed to understand the underlying dynamics.
Various approaches have been proposed to reconcile general relativity with quantum field theory, such as string theory, loop quantum gravity, and others. These attempts aim to construct a consistent framework where both gravity and quantum mechanics are incorporated. However, a fully satisfactory and experimentally verified theory of quantum gravity remains an open question in theoretical physics.
The unification of general relativity and quantum field theory is a subject of ongoing research and is considered one of the major challenges in theoretical physics. It is hoped that such a theory, often referred to as a theory of quantum gravity, will provide a deeper understanding of the fundamental nature of the universe, particularly in extreme conditions where both quantum effects and gravitational effects are significant.