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{{Short description|Open problem of reconciling quantum theory with gravity}}
{{Quantum book backlink|Advanced and frontier topics}}

The '''quantum gravity problem''' is the open problem of reconciling [[Physics:Quantum mechanics|quantum mechanics]] with the gravitational description of spacetime. Quantum theory describes matter and fields through states, operators, probabilities, and quantum fields, while [[Physics:General relativity|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.<ref name="SEPQuantumGravity">{{Cite web |last=Weinstein |first=Steven |title=Quantum Gravity |url=https://plato.stanford.edu/entries/quantum-gravity/ |website=Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=7 May 2026}}</ref>

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.<ref name="SEPQuantumGravity" />

<div style="float:right; border:1px solid #e0d890; background:#fff8cc; padding:6px; margin:0 0 1em 1em; width:360px;">
[[File:Quantum_gravity_problem_yellow.png|340px]]
<div style="font-size:90%;">The quantum gravity problem asks how quantum theory and dynamical spacetime fit into one consistent framework.</div>
</div>

== 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.<ref name="PolchinskiString">{{Cite journal |last=Polchinski |first=Joseph |title=String theory to the rescue |year=2015 |arxiv=1512.02477 |archivePrefix=arXiv}}</ref>

== 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.<ref name="RovelliLQG">{{Cite journal |last=Rovelli |first=Carlo |title=Loop Quantum Gravity |journal=Living Reviews in Relativity |year=1998 |volume=1 |article-number=1 |doi=10.12942/lrr-1998-1}}</ref>

== 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 ==

{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}

=References=
{{reflist|3}}

== Further reading ==

* {{Cite book |last=Rovelli |first=Carlo |title=Quantum Gravity |publisher=Cambridge University Press |year=2004 |isbn=978-0-521-83733-0}}
* {{Cite book |last1=Kiefer |first1=Claus |title=Quantum Gravity |publisher=Oxford University Press |year=2012 |isbn=978-0-19-958520-5}}
* {{Cite book |last1=Thiemann |first1=Thomas |title=Modern Canonical Quantum General Relativity |publisher=Cambridge University Press |year=2007 |isbn=978-0-521-84263-1}}
* {{Cite book |last1=Polchinski |first1=Joseph |title=String Theory, Volume 1: An Introduction to the Bosonic String |publisher=Cambridge University Press |year=1998 |isbn=978-0-521-63303-1}}
* {{Cite book |last1=Polchinski |first1=Joseph |title=String Theory, Volume 2: Superstring Theory and Beyond |publisher=Cambridge University Press |year=1998 |isbn=978-0-521-63304-8}}

{{Author|Harold Foppele}}

{{Sourceattribution|Quantum gravity problem|1}}

Physics:Quantum gravity problem - Revision history

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