This article traces the historical development of attempts to reconcile quantum gravity - the unification of general relativity and quantum mechanics - from the early twentieth century to the present. For the current state of competing research programs, see Quantum Gravity - Consensus. For the debate over loop quantum gravity specifically, see Quantum Gravity - Loop Quantum Gravity - Viewpoint Debate.
The problem of quantum gravity emerged as a consequence of two independent revolutions in theoretical physics. Max Planck introduced the quantum of action in 1900, and Albert Einstein extended this framework with the special theory of relativity (1905) and the general theory of relativity (1915). General relativity described gravitation as the curvature of spacetime geometry, while quantum mechanics - developed through the contributions of Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and Paul Dirac across the 1920s - described matter and energy in terms of probabilistic wave functions and discrete quanta. No framework existed to describe gravitation in quantum mechanical terms.
Einstein himself noted the tension between his field equations and quantum theory as early as 1916, observing that quantum effects should in principle disturb the spacetime metric.(1) The incompatibility was recognized but not systematically pursued during the 1920s, as the primary task of physicists was the elaboration of quantum mechanics itself.
Léon Rosenfeld made the first formal attempt to quantize the gravitational field in 1930, applying methods drawn from quantum electrodynamics to linearized general relativity.(2) He found that the procedure generated divergences analogous to those encountered in other quantum field theories, a difficulty that would persist throughout subsequent decades.
Fierz and Pauli analyzed the properties of a hypothetical spin-2 graviton in 1939, establishing the theoretical particle associated with the gravitational field within a flat-spacetime approximation.(3) Their work demonstrated that a consistent linear theory of a massless spin-2 particle reproduced the predictions of general relativity to first order.
During the 1940s, Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga developed renormalization techniques that successfully handled the infinities of quantum electrodynamics. This success encouraged the expectation that similar methods might eventually be applied to gravity, though the structural differences between electromagnetism and general relativity were already apparent to many workers in the field.
Two broad research programs took shape in the 1950s. The covariant approach, pursued by Feynman, Bryce DeWitt, and others, treated the graviton as a perturbative excitation of flat spacetime and attempted to apply Feynman diagram techniques. Feynman presented early calculations along these lines at the 1957 Chapel Hill Conference on the Role of Gravitation in Physics, a landmark meeting that established quantum gravity as a recognized research area.(4)
The canonical approach, developed primarily by Peter Bergmann and Paul Dirac, sought instead to apply the Hamiltonian quantization procedure directly to general relativity as a constrained dynamical system. Dirac systematically analyzed the constraint structure of general relativity between 1958 and 1959, identifying the primary and secondary constraints that would later anchor canonical quantization programs.(5)
Bryce DeWitt synthesized both strands in a series of papers between 1967 and 1968. His canonical quantization of general relativity produced the Wheeler-DeWitt equation - a functional differential equation governing the quantum state of the universe - derived independently and discussed in parallel by John Archibald Wheeler.(6) The Wheeler-DeWitt equation raised deep interpretive questions about the role of time in quantum cosmology, since the equation contains no explicit time variable.
Through the 1970s, systematic perturbative calculations confirmed that quantum general relativity is non-renormalizable: divergences at two loops and beyond cannot be absorbed into a finite number of counterterms.(7) Gerard 't Hooft and Veltman established one-loop results in 1974; Marc Goroff and Augusto Sagnotti's two-loop calculations in 1986 definitively demonstrated the problem at higher order.(8) This result effectively closed off the straightforward application of perturbative quantum field theory to gravity.
Supergravity, introduced by Freedman, van Nieuwenhuizen, and Ferrara in 1976, proposed that supersymmetry - a symmetry relating bosons and fermions - might cancel the divergences that plagued pure quantum gravity.(9) Extended supergravity theories, particularly N=8 supergravity, attracted significant attention through the late 1970s and early 1980s, though the cancellation of all divergences was not established.
In 1984, Michael Green and John Schwarz demonstrated that certain anomalies in ten-dimensional superstring theory cancel precisely, triggering what became known as the first superstring revolution.(10) String theory, which had been investigated since the early 1970s in the context of the strong nuclear force, was recognized as a candidate for a unified theory incorporating gravity. In string theory, the graviton appears as a massless spin-2 excitation of a closed string, and the theory is finite to all perturbative orders.
Edward Witten, along with Gross, Harvey, Martinec, and Rohm, rapidly developed the heterotic string theory in 1985.(11) By the late 1980s, string theory had attracted a large fraction of the theoretical physics community working on quantum gravity. Compactification schemes - mechanisms by which the extra dimensions required by string theory are made unobservably small - were extensively studied.
An alternative research program developed in parallel. In 1986, Abhay Ashtekar introduced a reformulation of general relativity using new variables - now called Ashtekar variables - that cast Einstein's equations in a form resembling Yang-Mills gauge theory.(12) This reformulation made canonical quantization more tractable.
Carlo Rovelli and Lee Smolin extended Ashtekar's work, introducing loop variables in 1988 and developing what became loop quantum gravity (LQG).(13) By the early 1990s, Rovelli and Smolin had identified a basis of quantum states of the gravitational field - spin network states - and had derived discrete spectra for geometric operators such as area and volume.(14) These results suggested that spacetime geometry itself might be quantized at the Planck scale.
The LQG program differed from string theory in its methodological commitments: it sought to quantize general relativity directly without introducing additional dimensions or new matter content, and it maintained background independence - the property that the theory does not presuppose a fixed spacetime background. For competing assessments of this program, see Quantum Gravity - Loop Quantum Gravity - Viewpoint Debate.
In 1995, Witten proposed that the five distinct ten-dimensional superstring theories and eleven-dimensional supergravity were all limiting cases of a single eleven-dimensional framework he called M-theory.(15) This proposal, which inaugurated the second superstring revolution, unified previously distinct string theories through a web of duality relations.
Juan Maldacena's 1997 paper proposing the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence became one of the most cited papers in theoretical physics.(16) The correspondence - also known as the holographic principle in this context - conjectured that a string theory in a bulk spacetime with negative cosmological constant is exactly equivalent to a conformal quantum field theory on its boundary. AdS/CFT provided a non-perturbative definition of string theory in certain backgrounds and offered tools for studying strongly coupled quantum systems.
The 2000s saw both the maturation of existing programs and the emergence of new ones. Within loop quantum gravity, the spin foam formalism - a covariant, path-integral version of LQG - was developed by Reisenberger, Rovelli, Barrett, Crane, and others from the late 1990s onward, providing a four-dimensional spacetime picture complementing the canonical approach.(17)
Causal set theory, originating with Bombelli, Lee, Meyer, and Sorkin in 1987 and further developed through the 2000s, proposed that the fundamental structure of spacetime is a discrete partial order of events.(18) Causal dynamical triangulations (CDT), advanced by Ambjørn, Jurkiewicz, and Loll, provided numerical evidence that a four-dimensional spacetime emerges dynamically from simplicial building blocks under certain causal conditions.(19)
The experimental accessibility of quantum gravity remained a central problem throughout this period. The relevant energy scale - the Planck energy, approximately 10¹⁹ GeV - lies far beyond the reach of any conceivable particle accelerator, driving interest in indirect observational windows such as the cosmic microwave background, gravitational wave astronomy, and Lorentz invariance tests.
The first direct detection of gravitational waves by the LIGO collaboration in 2015 opened a new observational channel for probing strong-field gravity, though at energy scales far below the Planck regime.(20) The detection confirmed a central prediction of general relativity and provided new tests of the theory in the strong-field, high-velocity limit.
The firewall paradox, introduced by Almheiri, Marolf, Polchinski, and Sully in 2012, sharpened debates about black hole information loss that had persisted since Hawking's 1974-1975 papers on black hole radiation.(21) Proposed resolutions, including the ER=EPR conjecture of Maldacena and Susskind (2013), connected quantum gravity to questions about quantum entanglement and spacetime connectivity.
Progress in LQG included the development of spinfoam vertex amplitudes and ongoing work on the low-energy limit of the theory. String theory research continued to develop AdS/CFT applications across condensed matter physics and quantum information, while questions about the string theory landscape - the vast number of possible vacuum states - intersected with debates about the multiverse and the limits of physical explanation.
Whether string theory constitutes a scientific theory in the empirically testable sense has been disputed by physicists and philosophers of science since at least the 1980s; see Quantum Gravity - Debate.(22)
Whether loop quantum gravity successfully recovers classical general relativity in the appropriate limit remains contested within the research community; see Quantum Gravity - Loop Quantum Gravity - Viewpoint Debate.(23)
Whether the proliferation of vacuum states in the string landscape constitutes a failure of predictivity or an expected feature of a correct fundamental theory is a matter of ongoing dispute; see Quantum Gravity - Debate.(24)
Some historians and physicists have disputed whether sociological factors, including the concentration of funding and positions around string theory in the 1990s and 2000s, distorted the development of quantum gravity research; see Quantum Gravity - Debate.(25)
1. Albert Einstein, “Näherungsweise Integration der Feldgleichungen der Gravitation,” Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften (1916), 688-696.
2. Léon Rosenfeld, “Zur Quantelung der Wellenfelder,” Annalen der Physik 397 (1930): 113-152.
3. Markus Fierz and Wolfgang Pauli, “On Relativistic Wave Equations for Particles of Arbitrary Spin in an Electromagnetic Field,” Proceedings of the Royal Society A 173 (1939): 211-232.
4. Cécile DeWitt-Morette, ed., The Role of Gravitation in Physics: Report from the 1957 Chapel Hill Conference (Berlin: Max Planck Research Library, 2011).
5. Paul A. M. Dirac, “The Theory of Gravitation in Hamiltonian Form,” Proceedings of the Royal Society A 246 (1958): 333-343.
6. Bryce S. DeWitt, “Quantum Theory of Gravity. I. The Canonical Theory,” Physical Review 160 (1967): 1113-1148; John A. Wheeler, “Superspace and the Nature of Quantum Geometrodynamics,” in Battelle Rencontres, ed. C. DeWitt and J. Wheeler (New York: Benjamin, 1968), 242-307.
7. Stephen 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.
8. Marc H. Goroff and Augusto Sagnotti, “The Ultraviolet Behavior of Einstein Gravity,” Nuclear Physics B 266 (1986): 709-736.
9. Daniel Z. Freedman, Peter van Nieuwenhuizen, and Sergio Ferrara, “Progress Toward a Theory of Supergravity,” Physical Review D 13 (1976): 3214-3218.
10. Michael B. Green and John H. Schwarz, “Anomaly Cancellations in Supersymmetric D=10 Gauge Theory and Superstring Theory,” Physics Letters B 149 (1984): 117-122.
11. David J. Gross, Jeffrey A. Harvey, Emil J. Martinec, and Ryan Rohm, “Heterotic String Theory (I): The Free Heterotic String,” Nuclear Physics B 256 (1985): 253-284.
12. Abhay Ashtekar, “New Variables for Classical and Quantum Gravity,” Physical Review Letters 57 (1986): 2244-2247.
13. Carlo Rovelli and Lee Smolin, “Loop Space Representation of Quantum General Relativity,” Nuclear Physics B 331 (1990): 80-152.
14. Carlo Rovelli and Lee Smolin, “Discreteness of Area and Volume in Quantum Gravity,” Nuclear Physics B 442 (1995): 593-619.
15. Edward Witten, “String Theory Dynamics in Various Dimensions,” Nuclear Physics B 443 (1995): 85-126.
16. Juan M. Maldacena, “The Large N Limit of Superconformal Field Theories and Supergravity,” Advances in Theoretical and Mathematical Physics 2 (1998): 231-252.
17. Michael P. Reisenberger and Carlo Rovelli, “'Sum over Surfaces' Form of Loop Quantum Gravity,” Physical Review D 56 (1997): 3490-3508.
18. Luca Bombelli, Joohan Lee, David Meyer, and Rafael D. Sorkin, “Space-Time as a Causal Set,” Physical Review Letters 59 (1987): 521-524.
19. Jan Ambjørn, Jerzy Jurkiewicz, and Renate Loll, “Emergence of a 4D World from Causal Quantum Gravity,” Physical Review Letters 93 (2004): 131301.
20. B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Observation of Gravitational Waves from a Binary Black Hole Merger,” Physical Review Letters 116 (2016): 061102.
21. Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully, “Black Holes: Complementarity or Firewalls?” Journal of High Energy Physics 2013, no. 2 (2013): 62.
22. Peter Woit, Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law (New York: Basic Books, 2006); Lee Smolin, The Trouble with Physics (Boston: Houghton Mifflin, 2006).
23. Thomas Thiemann, Modern Canonical Quantum General Relativity (Cambridge: Cambridge University Press, 2007), chaps. 10-11.
24. Leonard Susskind, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design (New York: Little, Brown, 2005); David Gross, “The Landscape of Theory,” remarks at Strings 2005.
25. Smolin, The Trouble with Physics, chaps. 15-16; responses in American Scientist 95 (2007).