The need for a theory that merges quantum mechanics and general relativity arises from the observation that these two frameworks, while individually successful in their respective domains, appear to be incompatible with each other under certain conditions.
Quantum mechanics provides a highly successful framework for describing the behavior of particles on very small scales, such as atoms and subatomic particles. It introduces probabilistic behavior, wave-particle duality, and the concept of superposition. Quantum mechanics has been extensively tested and confirmed through numerous experiments, and its predictions are in excellent agreement with observations.
On the other hand, general relativity is a theory of gravity that describes the behavior of spacetime in the presence of massive objects. It successfully explains the motion of planets, the bending of light, and the expansion of the universe. General relativity is based on the concept of spacetime as a curved geometric structure, and it is formulated in a framework that is consistent with Einstein's theory of relativity.
However, when attempts are made to combine quantum mechanics and general relativity, difficulties arise. One of the main challenges is that the mathematical frameworks of the two theories appear incompatible at a fundamental level. In particular, when attempting to describe the gravitational force within the framework of quantum mechanics, the calculations encounter infinities and inconsistencies.
Therefore, the search for a theory that merges quantum mechanics and general relativity is motivated by the desire to have a consistent and comprehensive framework that can describe the behavior of particles and the nature of spacetime in all regimes, including extremely high energies or small distances where both quantum effects and gravitational effects become important.
It is indeed possible to consider spacetime as a field where particles reside, but the challenge lies in reconciling the mathematical formalism and conceptual frameworks of quantum mechanics and general relativity. The theories have different mathematical structures and treat fundamental concepts, such as time and space, in different ways. This mismatch has led physicists to explore various approaches, such as string theory, loop quantum gravity, and other quantum gravity theories, in the hope of finding a consistent and unified framework.
While it is conceivable that a universal law combining quantum mechanics and general relativity may not exist as currently conceived, the quest for such a theory is driven by the desire for a deeper understanding of the fundamental nature of the universe and to address the questions that arise when both quantum and gravitational effects are important.