Physics:Quantum gravity problem

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Short description: Open problem of reconciling quantum theory with gravity

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The quantum gravity problem is the open problem of reconciling quantum mechanics with the gravitational description of spacetime. Quantum theory describes matter and fields through states, operators, probabilities, and quantum fields, while general relativity describes gravity as the curvature of spacetime. A theory of quantum gravity would explain regimes where both quantum effects and gravitational effects are important.

The problem is especially important near black holes, in the very early universe, and at distances close to the Planck scale. In these regimes, treating spacetime as a fixed classical background is expected to fail. A complete theory should explain how spacetime, geometry, causality, and gravitational dynamics are related to quantum principles.[1]

No experimentally confirmed, complete theory of quantum gravity is currently known. Several major approaches exist, including string theory, loop quantum gravity, spin foam models, causal sets, asymptotic safety, and effective field theory methods. These approaches differ in how they treat spacetime, background independence, unification, and the relation between geometry and quantum states.[1]

The quantum gravity problem asks how quantum theory and dynamical spacetime fit into one consistent framework.

Basic issue

Quantum mechanics and general relativity are both highly successful, but they are built on different conceptual foundations. In ordinary quantum theory, physical systems are described by quantum states evolving with respect to time. In general relativity, time and space are not fixed external structures; they are part of a dynamical spacetime geometry.

This creates a basic tension. Quantum field theory is usually formulated on a fixed spacetime background. General relativity, by contrast, makes the geometry of spacetime itself a physical field. A theory of quantum gravity must therefore explain whether spacetime is fundamental, emergent, quantized, or only an approximate large-scale description.

Planck scale

Quantum-gravitational effects are expected to become important near the Planck length and Planck energy. The Planck length is extremely small, which makes direct experimental tests difficult. This is one reason why quantum gravity remains an open problem.

At ordinary laboratory energies, gravitational effects between elementary particles are extremely weak. At very high energies or very small distances, however, quantum fluctuations of geometry may become important. A complete theory should describe what replaces the classical concept of smooth spacetime at such scales.

Black holes

Black holes are central to the quantum gravity problem because they combine gravity, thermodynamics, and quantum field theory. Semiclassical calculations suggest that black holes emit Hawking radiation. This leads to deep questions about entropy, information, and the microscopic degrees of freedom of spacetime.

The black hole information paradox is one of the most important clues that general relativity and quantum mechanics cannot simply be combined without modification. It asks whether information that falls into a black hole is preserved, lost, or encoded in some more subtle way.

Early universe

Quantum gravity is also expected to be important in the early universe, where densities and curvatures may have been extremely large. A complete theory may be needed to understand the initial singularity of classical cosmology, possible pre-Big-Bang phases, or the quantum origin of cosmic structure.

The early universe is therefore a natural testing ground for ideas about quantum spacetime, although direct evidence remains difficult to obtain.

String theory

String theory is an approach in which the fundamental objects are not point particles but extended strings. Different vibrational modes of a string can appear as different particles. A key reason string theory is important for quantum gravity is that its spectrum naturally includes a spin-2 excitation that can be interpreted as the graviton.

String theory also led to ideas such as duality, D-branes, holography, and the AdS/CFT correspondence. These tools have provided deep insights into black holes, quantum field theory, and the relation between gravity and quantum information. However, a direct experimentally verified version of string theory is not yet known.[2]

Loop quantum gravity

Loop quantum gravity attempts to quantize general relativity while preserving background independence. In this approach, geometry itself is described by quantum states. Areas and volumes may have discrete spectra, so the smooth geometry of general relativity appears as an approximation at large scales.

Loop quantum gravity is one of the main non-string approaches to quantum gravity. It focuses directly on the quantum structure of spacetime rather than on unifying all forces in one framework.[3]

Spin foam and causal approaches

Spin foam models provide a path-integral-like formulation related to loop quantum gravity. They attempt to describe quantum spacetime histories rather than only quantum states of space. In these approaches, spacetime geometry is represented through combinatorial and algebraic structures rather than through a fixed smooth manifold.

Causal set theory is another approach. It proposes that spacetime may be fundamentally discrete and ordered by causal relations. In such a view, the continuum spacetime of general relativity would emerge only at large scales.

Effective field theory

Even without a complete theory of quantum gravity, gravity can be treated as an effective field theory at energies far below the Planck scale. This allows physicists to calculate small quantum corrections to gravitational processes while accepting that the theory will break down at sufficiently high energies.

The effective-field-theory viewpoint is useful because it separates low-energy predictions from unknown Planck-scale physics. However, it does not by itself provide a complete ultraviolet theory of quantum gravity.

Why it remains unsolved

The quantum gravity problem remains unsolved for several reasons. First, direct experiments at the Planck scale are far beyond current technology. Second, the conceptual foundations of quantum mechanics and general relativity are deeply different. Third, many candidate theories are mathematically complex and can be difficult to connect with observable predictions.

The problem is therefore not simply to quantize another force. It is to understand the quantum nature of spacetime itself.

Relation to the hierarchy problem

The hierarchy problem is a different problem. It concerns the large separation between the electroweak scale and the Planck scale, especially the question of why the Higgs boson mass is so small compared with quantum corrections involving very high energy scales.

The hierarchy problem is related to gravity through the Planck scale, but it is not the same as the quantum gravity problem. Quantum gravity asks how gravity and spacetime become quantum; the hierarchy problem asks why certain particle-physics scales are so widely separated.

Status

There is no single accepted theory of quantum gravity. String theory, loop quantum gravity, spin foam models, causal sets, asymptotic safety, and other approaches each address part of the problem, but none has yet provided a complete, experimentally confirmed theory.

For this reason, the quantum gravity problem remains one of the central open problems in modern theoretical physics.

See also

Table of contents (184 articles)

Index

Full contents

1. Foundations (11)↑
  1. Physics:Quantum basics
  2. Physics:Quantum Postulates
  3. Physics:Quantum Hilbert space
  4. Physics:Quantum Observables and operators
  5. Physics:Quantum mechanics
  6. Physics:Quantum mechanics measurements
  7. Physics:Quantum state
  8. Physics:Quantum system
  9. Physics:Quantum superposition
  10. Physics:Quantum probability
  11. Physics:Quantum Mathematical Foundations of Quantum Theory
2. Conceptual and interpretations (13)↑
  1. Physics:Quantum Interpretations of quantum mechanics
  2. Physics:Quantum Wave–particle duality
  3. Physics:Quantum Complementarity principle
  4. Physics:Quantum Uncertainty principle
  5. Physics:Quantum Measurement problem
  6. Physics:Quantum Bell's theorem
  7. Physics:Quantum Hidden variable theory
  8. Physics:Quantum nonlocality
  9. Physics:Quantum contextuality
  10. Physics:Quantum Darwinism
  11. Physics:Quantum A Spooky Action at a Distance
  12. Physics:Quantum A Walk Through the Universe
  13. Physics:Quantum The Secret of Cohesion and How Waves Hold Matter Together
  14. Physics:Quantum measurement problem
3. Mathematical structure and systems (13)↑
  1. Physics:Quantum Density matrix
  2. Physics:Quantum Exactly solvable quantum systems
  3. Physics:Quantum Formulas Collection
  4. Physics:Quantum A Matter Of Size
  5. Physics:Quantum Symmetry in quantum mechanics
  6. Physics:Quantum Angular momentum operator
  7. Physics:Quantum Runge–Lenz vector
  8. Physics:Quantum Approximation Methods
  9. Physics:Quantum Matter Elements and Particles
  10. Physics:Quantum Dirac equation
  11. Physics:Quantum Klein–Gordon equation
  12. Physics:Quantum pendulum
  13. Physics:Quantum configuration space
4. Atomic and spectroscopy (14)↑
    Quantum atomic structure and spectroscopy: orbitals, energy levels, and emission and absorption spectra.
  1. Physics:Quantum Atomic structure and spectroscopy
  2. Physics:Quantum Hydrogen atom
  3. Physics:Quantum number
  4. Physics:Quantum Multi-electron atoms
  5. Physics:Quantum Fine structure
  6. Physics:Quantum Hyperfine structure
  7. Physics:Quantum Isotopic shift
  8. Physics:Quantum defect
  9. Physics:Quantum Zeeman effect
  10. Physics:Quantum Stark effect
  11. Physics:Quantum Spectral lines and series
  12. Physics:Quantum Selection rules
  13. Physics:Quantum Fermi's golden rule
  14. Physics:Quantum beats
5. Wavefunctions and modes (9)↑
    A quantum wavefunction showing probability amplitude in space; the square of its magnitude gives the probability density.
  1. Physics:Quantum Wavefunction
  2. Physics:Quantum Superposition principle
  3. Physics:Quantum Eigenstates and eigenvalues
  4. Physics:Quantum Boundary conditions and quantization
  5. Physics:Quantum Standing waves and modes
  6. Physics:Quantum Normal modes and field quantization
  7. Physics:Number of independent spatial modes in a spherical volume
  8. Physics:Quantum Density of states
  9. Physics:Quantum carpet
6. Quantum dynamics and evolution (17)↑
  1. Physics:Quantum Time evolution
  2. Physics:Quantum Schrödinger equation
  3. Physics:Quantum Time-dependent Schrödinger equation
  4. Physics:Quantum Stationary states
  5. Physics:Quantum Perturbation theory
  6. Physics:Quantum Time-dependent perturbation theory
  7. Physics:Quantum Adiabatic theorem
  8. Physics:Quantum Scattering theory
  9. Physics:Quantum S-matrix
  10. Physics:Quantum tunnelling
  11. Physics:Quantum speed limit
  12. Physics:Quantum revival
  13. Physics:Quantum reflection
  14. Physics:Quantum oscillations
  15. Physics:Quantum jump
  16. Physics:Quantum boomerang effect
  17. Physics:Quantum chaos
7. Measurement and information (9)↑
  1. Physics:Quantum Measurement theory
  2. Physics:Quantum Measurement operators
  3. Physics:Quantum Projective measurement
  4. Physics:Quantum POVM
  5. Physics:Quantum Weak measurement
  6. Physics:Quantum Measurement collapse
  7. Physics:Quantum entanglement
  8. Physics:Quantum Zeno effect
  9. Physics:Quantum limit
8. Quantum information and computing (10)↑
  1. Physics:Quantum information theory
  2. Physics:Quantum Qubit
  3. Physics:Quantum Entanglement
  4. Physics:Quantum Gates and circuits
  5. Physics:Quantum Computing Algorithms in the NISQ Era
  6. Physics:Quantum Noisy Qubits
  7. Physics:Quantum random access code
  8. Physics:Quantum pseudo-telepathy
  9. Physics:Quantum network
  10. Physics:Quantum money
9. Quantum optics and experiments (5)↑
    Experimental quantum physics: qubits, dilution refrigerators, quantum communication, and laboratory systems.
  1. Physics:Quantum Nonlinear King plot anomaly in calcium isotope spectroscopy
  2. Physics:Quantum optics beam splitter experiments
  3. Physics:Quantum Ultra fast lasers
  4. Physics:Quantum Experimental quantum physics
  5. Physics:Quantum optics
    6. Template:Quantum optics operators
10. Open quantum systems (9)↑
  1. Physics:Quantum Open systems
  2. Physics:Quantum Master equation
  3. Physics:Quantum Lindblad equation
  4. Physics:Quantum Decoherence
  5. Physics:Quantum dissipation
  6. Physics:Quantum Markov semigroup
  7. Physics:Quantum Markovian dynamics
  8. Physics:Quantum Non-Markovian dynamics
  9. Physics:Quantum Trajectories
11. Quantum field theory (20)↑
    Structural dependency map of quantum field theory.
  1. Physics:Quantum field theory (QFT) basics
  2. Physics:Quantum field theory (QFT) core
  3. Physics:Quantum Fields and Particles
  4. Physics:Quantum Second quantization
  5. Physics:Quantum Harmonic Oscillator field modes
  6. Physics:Quantum Creation and annihilation operators
  7. Physics:Quantum vacuum fluctuations
  8. Physics:Quantum Propagators in quantum field theory
  9. Physics:Quantum Feynman diagrams
  10. Physics:Quantum Path integral formulation
  11. Physics:Quantum Renormalization in field theory
  12. Physics:Quantum Renormalization group
  13. Physics:Quantum Field Theory Gauge symmetry
  14. Physics:Quantum Non-Abelian gauge theory
  15. Physics:Quantum Electrodynamics (QED)
  16. Physics:Quantum chromodynamics (QCD)
  17. Physics:Quantum Electroweak theory
  18. Physics:Quantum Standard Model
  19. Physics:Quantum triviality
  20. Physics:Quantum confinement problem
12. Statistical mechanics and kinetic theory (9)↑
  1. Physics:Quantum Statistical mechanics
  2. Physics:Quantum Partition function
  3. Physics:Quantum Distribution functions
  4. Physics:Quantum Liouville equation
  5. Physics:Quantum Kinetic theory
  6. Physics:Quantum Boltzmann equation
  7. Physics:Quantum BBGKY hierarchy
  8. Physics:Quantum Relaxation and thermalization
  9. Physics:Quantum Thermodynamics
13. Condensed matter and solid-state physics (13)↑
  1. Physics:Quantum Band structure
  2. Physics:Quantum Fermi surfaces
  3. Physics:Quantum Semiconductor physics
  4. Physics:Quantum Phonons
  5. Physics:Quantum Electron-phonon interaction
  6. Physics:Quantum Superconductivity
  7. Physics:Quantum Topological phases of matter
  8. Physics:Quantum well
  9. Physics:Quantum spin liquid
  10. Physics:Quantum spin Hall effect
  11. Physics:Quantum phase transition
  12. Physics:Quantum critical point
  13. Physics:Quantum dot
14. Plasma and fusion physics (8)↑
    Conceptual illustration of plasma physics in a fusion context, showing magnetically confined ionized gas in a tokamak and the collective behavior governed by electromagnetic fields and transport processes.
  1. Physics:Quantum Fusion reactions and Lawson criterion
  2. Physics:Quantum Plasma (fusion context)
  3. Physics:Quantum Magnetic confinement fusion
  4. Physics:Quantum Inertial confinement fusion
  5. Physics:Quantum Plasma instabilities and turbulence
  6. Physics:Quantum Tokamak core plasma
  7. Physics:Quantum Tokamak edge physics and recycling asymmetries
  8. Physics:Quantum Stellarator
15. Timeline (8)↑
  1. Physics:Quantum mechanics/Timeline
  2. Physics:Quantum mechanics/Timeline/Pre-quantum era
  3. Physics:Quantum mechanics/Timeline/Old quantum theory
  4. Physics:Quantum mechanics/Timeline/Modern quantum mechanics
  5. Physics:Quantum mechanics/Timeline/Quantum field theory era
  6. Physics:Quantum mechanics/Timeline/Quantum information era
  7. Physics:Quantum mechanics/Timeline/Quantum technology era
  8. Physics:Quantum mechanics/Timeline/Quiz
16. Advanced and frontier topics (16)↑
  1. Physics:Quantum topology
  2. Physics:Quantum battery
  3. Physics:Quantum Supersymmetry
  4. Physics:Quantum Black hole thermodynamics
  5. Physics:Quantum Holographic principle
  6. Physics:Quantum gravity
  7. Physics:Quantum De Sitter invariant special relativity
  8. Physics:Quantum Doubly special relativity
  9. Physics:Quantum arithmetic geometry
  10. Physics:Quantum unsolved problems
  11. Physics:Quantum Yang-Mills mass gap
  12. Physics:Quantum gravity problem
  13. Physics:Quantum black hole information paradox
  14. Physics:Quantum dark matter problem
  15. Physics:Quantum neutrino mass problem
  16. Physics:Quantum matter-antimatter asymmetry problem

References

  1. ↑ 1.0 1.1 Weinstein, Steven. "Quantum Gravity". Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/entries/quantum-gravity/. 
  2. ↑ Polchinski, Joseph (2015). String theory to the rescue. 
  3. ↑ Rovelli, Carlo (1998). "Loop Quantum Gravity". Living Reviews in Relativity 1. doi:10.12942/lrr-1998-1. 

Further reading

  • Rovelli, Carlo (2004). Quantum Gravity. Cambridge University Press. ISBN 978-0-521-83733-0. 
  • Kiefer, Claus (2012). Quantum Gravity. Oxford University Press. ISBN 978-0-19-958520-5. 
  • Thiemann, Thomas (2007). Modern Canonical Quantum General Relativity. Cambridge University Press. ISBN 978-0-521-84263-1. 
  • Polchinski, Joseph (1998). String Theory, Volume 1: An Introduction to the Bosonic String. Cambridge University Press. ISBN 978-0-521-63303-1. 
  • Polchinski, Joseph (1998). String Theory, Volume 2: Superstring Theory and Beyond. Cambridge University Press. ISBN 978-0-521-63304-8. 


Author: Harold Foppele





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