In theoretical physics, the question of how to reconcile general relativity with quantum mechanics - the project known as quantum gravity - is one of the deepest unsolved problems in the discipline. There is broad consensus among physicists that such a reconciliation is both necessary and not yet achieved. Beyond that foundational agreement, however, no consensus exists on which theoretical framework is correct, which predictions are empirically adequate, or which research program is most likely to succeed. This page documents the areas of genuine agreement and maps the terrain of persistent, unresolved disagreement between major expert communities.
There is near-universal agreement among theoretical physicists across institutions and research programs that general relativity and quantum field theory are mutually incompatible as currently formulated. General relativity treats spacetime as a smooth, continuous manifold governed by classical equations; quantum mechanics requires probabilistic, discrete descriptions of physical systems. At energy scales near the Planck scale (~10¹⁹ GeV), both theories are expected to break down, and neither alone can describe regimes such as the interior of black holes or the earliest moments of the Big Bang.1)2) This incompatibility is not a matter of interpretive disagreement; it follows from the mathematical structures of both theories and is accepted as a starting point by researchers across all major quantum-gravity programs.
There is likewise broad consensus that no quantum-gravity theory has yet been empirically confirmed. The Planck scale lies many orders of magnitude beyond the reach of current or near-future particle accelerators. Indirect observational strategies - searches for Lorentz invariance violation, signatures in the cosmic microwave background, gravitational wave phenomenology, and black hole thermodynamics - have constrained some theoretical parameter spaces but have not produced a confirmed positive detection of quantum-gravitational effects.3) The absence of empirical data capable of decisively testing competing frameworks is itself a point of consensus and is widely acknowledged as a defining challenge for the field.
There is substantial, cross-program agreement that any successful quantum-gravity theory must account for the thermodynamic properties of black holes - particularly the Bekenstein-Hawking entropy formula, which relates a black hole's entropy to its horizon area.4)5) This result, derived semi-classically and widely reproduced, functions as a benchmark. Both string theory and loop quantum gravity (LQG) communities have separately produced derivations of the Bekenstein-Hawking formula under specific conditions, though the methods and interpretations differ, and neither derivation is considered a full solution to the black hole information problem.6)7)
Within the string theory research community, there is broad agreement that the framework provides a mathematically consistent ultraviolet completion of gravity, that it predicts supersymmetry and extra dimensions, and that the AdS/CFT correspondence (anti-de Sitter space / conformal field theory duality) is a significant and productive result with applications beyond quantum gravity itself.8) String theorists broadly agree that the landscape of possible vacuum states is large and that this poses challenges for extracting unique predictions. These are within-community agreements; they are not shared across the full quantum-gravity research community. See Quantum Gravity - String Theory Viewpoint.
Within the LQG research community, there is broad agreement that space is discrete at the Planck scale, that area and volume operators have quantized spectra, and that the spin-foam formalism provides a covariant path-integral formulation of the theory.9) LQG researchers broadly agree that the framework does not require supersymmetry or extra dimensions. These again are within-community agreements not shared by the broader field. See Quantum Gravity - Loop Quantum Gravity Viewpoint.
The following questions remain genuinely open, with no cross-community consensus:
The following pages present viewpoints that challenge assumptions shared within one or more quantum-gravity research communities, or that dispute the methodological adequacy of leading frameworks:
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