General relativity and quantum field theory (QFT) are two fundamental theories in physics that describe different aspects of the universe. While general relativity successfully describes gravity as the curvature of spacetime, it is not considered a true quantum field theory for several reasons:
Non-renormalizability: In a quantum field theory, certain calculations involve infinities that need to be removed through a process called renormalization. However, when attempts are made to quantize general relativity using standard techniques, it leads to infinite results that cannot be renormalized in a consistent manner. This indicates that general relativity is non-renormalizable, which means it cannot be treated in the same way as other successful quantum field theories, such as quantum electrodynamics (QED).
Incompatibility with quantum principles: General relativity and QFT are based on different theoretical frameworks. General relativity is a classical theory that describes gravity in terms of smooth and continuous spacetime geometry. On the other hand, QFT is a quantum theory that incorporates discrete particles and wave-particle duality. Combining these two frameworks consistently has proven to be a significant challenge, as they have different mathematical structures and principles.
Lack of a consistent theory of quantum gravity: While there have been attempts to develop a theory of quantum gravity that unifies general relativity and quantum mechanics, such as string theory and loop quantum gravity, a complete and experimentally validated theory is not yet available. These theories are still under active research, and their predictions and implications are subjects of ongoing investigation.
Energy and curvature: In general relativity, the curvature of spacetime is determined by the distribution of matter and energy. However, in QFT, the energy associated with quantum fields can fluctuate wildly due to the uncertainty principle. These quantum fluctuations can introduce significant changes to the curvature of spacetime, leading to difficulties in reconciling the two theories consistently.
While general relativity and quantum mechanics have been extremely successful in their respective domains, reconciling them into a single consistent framework remains a major challenge in theoretical physics. The quest for a theory of quantum gravity that unifies these theories is an active area of research and one of the key open questions in fundamental physics.