The challenge of reconciling the theory of relativity (specifically, general relativity) with quantum physics is one of the biggest open problems in theoretical physics. The two theories, despite their remarkable successes in their respective domains, have different conceptual frameworks and mathematical formulations, which makes their unification difficult. This mismatch arises due to several fundamental differences between the two theories:
Scale: General relativity describes the behavior of gravity and the dynamics of spacetime on macroscopic scales, such as planets, stars, and galaxies. Quantum physics, on the other hand, deals with the behavior of particles and fields on microscopic scales, such as atoms, subatomic particles, and their interactions. The scales at which these theories operate are vastly different, making it challenging to directly combine them.
Determinism vs. Probabilistic Nature: General relativity is a deterministic theory, meaning that it predicts the exact future evolution of a system given its initial conditions. Quantum physics, however, introduces inherent uncertainty and probabilistic behavior into the description of microscopic particles. This probabilistic nature is encapsulated in quantities like the wave function, which represents the probabilities of different outcomes. The deterministic nature of general relativity and the probabilistic nature of quantum physics create a tension between the two frameworks.
Spacetime Structure: General relativity describes spacetime as a dynamic, curved, and continuous four-dimensional manifold influenced by matter and energy. Quantum physics, on the other hand, treats spacetime as a fixed, non-quantized background within which quantum fields and particles evolve. The attempts to quantize gravity and treat spacetime as a quantized entity have encountered significant theoretical challenges and have not yet been fully realized.
Renormalization: Quantum field theories, which are the foundation of particle physics and quantum physics, employ a mathematical technique called renormalization to deal with infinities that arise in certain calculations. However, when applied to gravity in the context of general relativity, this technique encounters difficulties, leading to non-renormalizable infinities. This poses a significant obstacle to achieving a consistent quantum theory of gravity.
These differences and challenges make it difficult to directly merge the theory of relativity with quantum physics. Various approaches, such as string theory, loop quantum gravity, and other quantum gravity frameworks, have been proposed to tackle this problem, but a fully satisfactory and experimentally confirmed theory of quantum gravity that unifies these two pillars of modern physics is yet to be established.