General relativity vs quantum theory
Pro Research Analysisby 
Searched over 200M research papers
General Relativity vs Quantum Theory: A Synthesis
Introduction to General Relativity and Quantum Theory
General relativity and quantum theory are two pillars of modern physics, each describing different aspects of the universe. General relativity, formulated by Einstein, is a deterministic theory that describes the gravitational force and the structure of spacetime. It operates on a macroscopic scale and is characterized by a non-fixed causal structure . On the other hand, quantum theory is a probabilistic framework that governs the behavior of particles at the microscopic level, with a fixed causal structure .
Challenges in Unifying General Relativity and Quantum Theory
Conceptual and Technical Difficulties
The unification of general relativity and quantum theory has been a longstanding challenge in theoretical physics. Several approaches to quantizing general relativity have been proposed, including canonical, sum-over-histories, and source theory methods. However, these approaches often face issues of arbitrariness, ambiguity, and technical difficulties, making them less than satisfactory . The fundamental problem lies in the differing formulations: quantum mechanics is based on systems extended in time, while general relativity is based on events limited in both space and time .
Probabilistic Theories with Non-Fixed Causal Structure
One promising approach to bridging the gap between these theories involves creating a framework for probabilistic theories with non-fixed causal structures. This method combines elements of both general relativity and quantum theory, using a concept called physical compression to relate quantities in a spacetime region. This framework introduces a new mathematical object called the "causaloid," which can calculate probabilistic correlations without imposing a fixed causal structure .
Progress in Quantum Gravity
Loop Quantum Gravity and String Theory
Recent progress in string theory and loop quantum gravity has provided new insights, although a complete solution remains elusive. Loop quantum gravity, for instance, attempts to define a non-perturbative, background-independent theory of quantum gravity. This approach suggests a fundamental discreteness of spacetime, providing substantial evidence for a theory where the gravitational field acts as a natural UV cut-off . String theory, on the other hand, posits that fundamental particles are one-dimensional "strings" rather than point particles, offering another route to unification .
Quantum Field Theory and General Relativity
Another approach involves recasting quantum field theory within the framework of general relativity. This method has led to the development of new conceptual schemes and insights into the foundations of physics. For example, the quantum field theoretic description of general relativity describes gravitational force as being carried by spin-2 gravitons. In the classical limit, this theory aligns with Einstein's field equations .
Reconciling General Relativity with Quantum Theory
Quantization and Gravitons
Most versions of quantization predict the existence of gravitons, hypothetical particles that mediate the gravitational force. However, it is also possible to formulate a quantum theory with classical gravity, where the expectation values of the stress-energy tensor constitute the sources of the classical metric field . This dual approach allows for the exploration of both quantum and classical aspects of gravity.
Information Theory and General Relativity
The intersection of quantum information theory and general relativity has also provided new perspectives. For instance, the Einstein-Podolsky-Rosen paradox has been reexamined in light of Shannon's information theory, revealing that horizons in general relativity act as one-way membranes for the propagation of quantum information, particularly in the context of black holes .
Conclusion
The quest to unify general relativity and quantum theory continues to be one of the most profound challenges in physics. While significant progress has been made through various approaches such as loop quantum gravity, string theory, and quantum field theory, a complete and satisfactory theory remains out of reach. The ongoing efforts to reconcile these two foundational theories promise to deepen our understanding of the universe and the fundamental nature of reality.
Sources and full results
Most relevant research papers on this topic