The purpose of unifying quantum theory with Einstein's theory of general relativity is to develop a comprehensive framework that can describe the behavior of matter and energy on both the microscopic and macroscopic scales, as well as in extreme conditions such as those found in black holes or the early universe. This pursuit is often referred to as the quest for a theory of quantum gravity.
Quantum mechanics and general relativity are two foundational theories of modern physics, each successful in its respective domain but with different mathematical formulations and conceptual frameworks. Quantum mechanics provides a description of the behavior of matter and energy at the microscopic level, while general relativity describes the nature of gravity and the curvature of spacetime on a macroscopic and cosmological scale.
At present, these theories are not fully compatible and have different mathematical structures and assumptions. When attempting to apply both theories simultaneously, mathematical inconsistencies arise, particularly in extreme situations where gravity and quantum effects are both significant, such as inside black holes or during the early stages of the universe.
The unification of these theories is sought after for several reasons:
Understanding the early universe: A theory of quantum gravity would provide insights into the behavior of matter and energy during the extremely high-energy conditions of the early universe, allowing us to study phenomena like the Big Bang.
Black holes and singularities: Black holes are regions of spacetime where gravity is extremely strong. Understanding the quantum nature of black holes could shed light on the nature of singularities and the behavior of matter under extreme gravitational conditions.
Fundamental physics: Unifying quantum theory with general relativity is a fundamental pursuit in physics, seeking a coherent and comprehensive description of the fundamental building blocks of the universe.
Resolving theoretical issues: The unification of quantum mechanics and general relativity is expected to resolve certain theoretical issues, such as the nature of spacetime at small scales, the quantization of gravity, and the reconciliation of deterministic and probabilistic principles.
Several approaches and theoretical frameworks have been proposed in the quest for a theory of quantum gravity, such as string theory, loop quantum gravity, and causal sets. However, a definitive and experimentally confirmed theory of quantum gravity remains an ongoing challenge for physicists, and the unification of quantum mechanics and general relativity continues to be an active area of research.