Table of Contents
Quantum Gravity
Quantum gravity is the branch of theoretical physics concerned with developing a framework that reconciles general relativity-the classical theory of gravitation describing spacetime curvature at large scales-with quantum mechanics, which governs the behavior of matter and energy at subatomic scales. No experimentally confirmed theory of quantum gravity currently exists. The precise definition of the problem, the criteria a successful theory must meet, and whether unification is even the correct framing are themselves subjects of ongoing debate among physicists and philosophers of physics.
Current State of Knowledge
General relativity and quantum mechanics are each well-confirmed within their respective domains, yet the two frameworks rest on incompatible mathematical and conceptual foundations. General relativity treats spacetime as a smooth, continuous manifold whose geometry responds dynamically to matter and energy. Quantum mechanics, and its extension quantum field theory, describes discrete, probabilistic interactions in a fixed background spacetime. Attempts to quantize gravity using the perturbative methods that succeed for other fundamental forces produce non-renormalizable infinities that cannot be absorbed by standard techniques, suggesting that a more fundamental departure is required.
Several research programs have been developed in response to this incompatibility. String theory proposes that fundamental entities are one-dimensional extended objects (“strings”) whose vibrational modes correspond to observed particles, with gravity emerging from a specific mode; this framework requires extra spatial dimensions and yields a consistent perturbative quantum gravity in certain limits. Loop quantum gravity (LQG) takes a background-independent approach, quantizing spacetime geometry itself through networks of discrete quanta of area and volume. Other approaches include causal dynamical triangulations, causal set theory, asymptotic safety, and emergent gravity proposals. Each program has produced internal mathematical results, but none has yielded predictions tested at accessible energy scales.
The Planck scale-approximately 10⁻³⁵ meters and 10¹⁹ GeV-is where quantum gravitational effects are expected to become significant. Current particle accelerators reach energies roughly fifteen orders of magnitude below this threshold. Observational constraints have been sought through high-energy astrophysical phenomena, the cosmic microwave background, and black hole thermodynamics, but no conclusive empirical signal has been identified. The status of black hole information loss-whether information falling into a black hole is preserved or destroyed-remains an unresolved theoretical problem with direct implications for any candidate theory.
Consensus Status
There is broad agreement among physicists that a consistent quantum theory of gravity is necessary to describe regimes where both gravitational and quantum effects are significant, such as the interior of black holes and the very early universe. This agreement on the need for such a theory does not extend to which research program is most promising or what form a successful theory should take. See quantum-gravity-need-consensus-consensus.
Viewpoints
String theory / M-theory holds that a consistent, UV-complete theory of quantum gravity is provided within the string framework, with the AdS/CFT correspondence offering a precise non-perturbative definition in certain spacetime geometries. See Quantum Gravity - String Theory Viewpoint.
Loop quantum gravity holds that gravity should be quantized in a background-independent manner, treating spacetime geometry itself as the quantum variable, without requiring extra dimensions or supersymmetry. See Quantum Gravity - Loop Quantum Gravity Viewpoint.
Emergent gravity holds that spacetime and gravitational dynamics are not fundamental but arise from more basic degrees of freedom-thermodynamic, informational, or otherwise-so that quantizing the gravitational field directly is a category error. See Quantum Gravity - Emergent Gravity Viewpoint.
Conservative or pluralist positions hold that no current approach has sufficient evidential support to be regarded as the leading candidate, and that multiple research programs should be pursued in parallel without premature convergence. See quantum-gravity-pluralist-viewpoint.
Controversies
The black hole information paradox-whether unitary quantum evolution is compatible with Hawking radiation and black hole evaporation-has generated sustained dispute over the interpretation of both general relativity and quantum field theory in curved spacetime. See quantum-gravity-black-hole-information-controversy-controversy.
The sociological and methodological question of whether string theory has received disproportionate institutional resources and attention relative to competing programs has been a documented dispute within the physics community. See quantum-gravity-string-theory-dominance-controversy-controversy.
Related Pages
Footnotes
1. Carlo Rovelli, Quantum Gravity (Cambridge: Cambridge University Press, 2004). 2. Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (New York: W. W. Norton, 1999). 3. Steven Weinberg, “Ultraviolet Divergences in Quantum Theories of Gravitation,” in General Relativity: An Einstein Centenary Survey, ed. S. W. Hawking and W. Israel (Cambridge: Cambridge University Press, 1979), 790-831. 4. Juan Maldacena, “The Large N Limit of Superconformal Field Theories and Supergravity,” International Journal of Theoretical Physics 38, no. 4 (1999): 1113-1133. 5. S. W. Hawking, “Breakdown of Predictability in Gravitational Collapse,” Physical Review D 14, no. 10 (1976): 2460-2473. 6. Lee Smolin, The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next (Boston: Houghton Mifflin, 2006). 7. Abhay Ashtekar and Jerzy Lewandowski, “Background Independent Quantum Gravity: A Status Report,” Classical and Quantum Gravity 21, no. 15 (2004): R53-R152. 8. Erik Verlinde, “On the Origin of Gravity and the Laws of Newton,” Journal of High Energy Physics 2011, no. 4 (2011): 29.