The main difference between a quantum field theory (QFT) and a quantum gravity theory lies in the specific phenomena they aim to describe and the fundamental principles they incorporate.
Quantum Field Theory (QFT): QFT is a framework that combines quantum mechanics with special relativity to describe the behavior of elementary particles and their interactions. It provides a way to understand and calculate the properties and interactions of particles by treating them as excitations or disturbances in their respective quantum fields. Each type of particle is associated with a specific quantum field, and the interactions between particles are mediated by the exchange of other particles (mediator particles). QFT successfully describes three of the four fundamental forces in nature: electromagnetic, weak nuclear, and strong nuclear forces, through quantum electrodynamics (QED), electroweak theory, and quantum chromodynamics (QCD), respectively. However, QFT does not incorporate the effects of gravity.
Quantum Gravity Theory: Quantum gravity theory, on the other hand, aims to provide a framework that unifies quantum mechanics and general relativity to describe the gravitational force on a quantum level. General relativity describes gravity as the curvature of spacetime caused by massive objects, while quantum mechanics deals with the behavior of particles and fields on a small scale. In the realm of quantum gravity, scientists seek to understand the behavior of spacetime itself, including its quantized nature and the quantum properties of gravitons (hypothetical particles mediating gravity). The ultimate goal of quantum gravity is to provide a consistent and mathematically coherent theory that can describe the behavior of gravity at extremely high energies and in the presence of quantum phenomena.
Developing a consistent quantum theory of gravity is a significant challenge, and various approaches have been proposed, including string theory, loop quantum gravity, and others. These theories go beyond QFT and attempt to incorporate the gravitational force into the framework of quantum mechanics, accounting for the quantization of spacetime itself.
In summary, QFT focuses on describing the behavior of particles and their interactions within the framework of quantum mechanics, while quantum gravity theories aim to unify quantum mechanics and general relativity to describe the gravitational force at the quantum level.