In general relativity, spacetime is described as a dynamical entity that can be influenced by the presence of matter and energy. The curvature of spacetime is determined by the distribution of matter and energy, and it evolves in response to their presence and motion. This dynamical nature of spacetime is a key feature of general relativity.
On the other hand, in quantum field theory (QFT), which is the framework used to describe particle interactions and quantum phenomena, spacetime is treated classically as a fixed background. In QFT, particles and fields propagate on a fixed, pre-existing spacetime background without explicitly incorporating the dynamical aspects of general relativity.
The reason for this apparent discrepancy lies in the domain of applicability of each theory. General relativity describes the gravitational interactions at macroscopic scales, where the curvature of spacetime becomes significant. It provides a classical description of gravity that accounts for the dynamic nature of spacetime.
Quantum field theory, on the other hand, is primarily concerned with the behavior of quantum particles and fields at microscopic scales, where the effects of gravity are negligible compared to other fundamental forces. In this regime, spacetime can be treated as a fixed background since the quantum effects of gravity are typically negligible. QFT successfully describes the behavior of particles and fields within this framework, without explicitly incorporating the dynamical aspects of spacetime curvature.
It is important to note that reconciling general relativity and quantum field theory is an ongoing challenge in theoretical physics. The quest for a theory of quantum gravity aims to unify these two frameworks and provide a consistent description of both the quantum nature of particles and the dynamical nature of spacetime. Various approaches, such as string theory and loop quantum gravity, are being explored to address this fundamental issue and provide a coherent framework that encompasses both quantum mechanics and general relativity.