Quantum gravity

Quantum gravity is a field of theoretical physics that aims to formulate a consistent description of gravity that accords with the principles of quantum mechanics. It addresses physical regimes in which both strong gravitational effects and quantum phenomena are simultaneously significant, such as the immediate aftermath of the Big Bang and the extreme environments around black holes and other compact astrophysical objects. Whereas electromagnetism, the strong interaction and the weak interaction are well described within quantum mechanics and quantum field theory, gravity at present remains formulated through the classical framework of general relativity. Reconciling these two highly successful yet conceptually distinct theories is one of the foremost open problems in modern physics.

Motivation and Conceptual Challenges

General relativity portrays gravity as the curvature of spacetime caused by the presence and motion of matter and energy. Its geometric formulation successfully explains a wide range of gravitational phenomena, from planetary motion to gravitational waves. However, the theory exhibits several conceptual and empirical limitations. The prediction of spacetime singularities within black holes and at the origin of the universe signals an incompleteness at small length scales. Furthermore, the introduction of dark matter and dark energy to account for astronomical observations, together with the cosmological constant problem—where the vacuum energy predicted by quantum field theory exceeds observed values by many orders of magnitude—indicates a mismatch between gravitational theory and quantum physics.
At length scales comparable with the Planck length, around 10−3510^{-35}10−35 metres, quantum fluctuations of spacetime itself are expected to become significant. A satisfactory theory of quantum gravity must therefore describe situations in which spacetime geometry is influenced by quantum effects. Similarly, quantum properties of matter should, in turn, influence the geometry in a fully dynamical and mutually consistent way.

Approaches to Quantum Gravity

Multiple theoretical frameworks have been developed to address the problem of quantising gravity. Although differing in methodology and scope, they share the goal of identifying the quantum behaviour of the gravitational field.

• String theory and M-theory attempt to unify all known forces within a single framework in which the fundamental constituents are one-dimensional strings rather than point particles. In these models, gravity emerges naturally from the vibrational modes of the string, and quantum consistency imposes additional symmetries that potentially eliminate problematic divergences.
• Loop quantum gravity focuses specifically on the quantisation of spacetime geometry, treating the gravitational field as a network of discrete quantum states. This approach aims to quantise gravity without incorporating the other fundamental interactions.
• Causal dynamical triangulations and non-commutative geometry offer alternative formulations, describing spacetime in terms of discrete building blocks or modified algebraic structures. Twistor theory provides yet another mathematical framework for analysing spacetime geometry and quantum fields.

These approaches differ in their ambitions: some pursue a theory of everything, whereas others restrict attention to gravity alone.

Observational and Phenomenological Considerations

One of the most significant obstacles in quantum gravity research is the absence of direct experimental data. Quantum gravitational effects are thought to become prominent only near the Planck scale, requiring energies far beyond those attainable with present laboratory technology. Nevertheless, indirect approaches are actively explored. Quantum gravitational signatures might be encoded in the structure of the early universe, accessible through cosmological observations. In addition, new experiments in laboratory-scale quantum systems aim to test aspects of gravitational–quantum interplay, and so-called phenomenological quantum gravity develops theoretical models that could be constrained by forthcoming experiments.
Thought experiments have also been proposed as tools to investigate foundational questions, such as the role of spin in sourcing gravity or the limits of spacetime localisation. Advances in quantum sensing, interferometry and tabletop experiments suggest that experimental probes of quantum gravitational effects may become feasible in the coming decades.

Quantum Mechanics and General Relativity

Quantum field theory in flat spacetime describes matter fields in the presence of a fixed geometric background. Extending this framework to curved spacetime introduces mathematical subtleties, while allowing the gravitational metric to become a quantum variable introduces deeper difficulties. In general relativity, the causal structure of spacetime—whether two locations are timelike, spacelike or lightlike separated—depends on the metric. In a quantum theory, the metric itself may be in superposition, leading to superposed causal structures. Reconciling such scenarios with quantum mechanics requires novel conceptual and mathematical tools.

The Graviton

The conceptual analogy between gravity and the other fundamental interactions motivates the hypothesis of a graviton, a hypothetical massless spin-2 particle that would mediate the gravitational interaction in a quantum field theory. Under broad assumptions, the linearised limit of general relativity possesses the degrees of freedom of such a particle. Gravitons are therefore central to many unifying theories developed since the 1970s. However, because they interact extremely weakly, they are generally believed to be undetectable with foreseeable technology.

Nonrenormalisability of Gravity

Treating gravity as a quantum field in the same way as electromagnetism leads to a perturbatively nonrenormalisable theory. In perturbative quantum field theory, renormalisability requires that only a finite set of parameters need to be fixed by experiment. In quantised gravity, an infinite tower of independent counterterms appears, meaning that infinitely many parameters would need to be determined. At low energies the renormalisation group ensures that quantum corrections reduce to classical general relativity, but at high energies the proliferation of parameters renders the theory unpredictive.
Several strategies have been proposed to overcome this issue. One is the asymptotic safety programme, which posits the existence of a non-perturbative ultraviolet fixed point. Another is the introduction of new symmetries, as in string theory, that may remove or constrain the problematic divergences.

Quantum Gravity as an Effective Field Theory

Despite being perturbatively nonrenormalisable, gravity can be formulated successfully as a low-energy effective field theory, in which only the first few terms of the infinite parameter set contribute measurably. Higher-order terms are suppressed by powers of the Planck scale, ensuring that classical general relativity remains a good approximation at energies accessible to present-day experiments. This viewpoint permits systematic computations of low-energy quantum corrections to gravitational phenomena and aligns with the broader philosophy of effective theories in particle physics.

Outlook

Quantum gravity remains one of the most challenging and compelling problems in theoretical physics. Bridging the conceptual and mathematical divide between general relativity and quantum mechanics is expected to yield insights into the fundamental nature of space, time and matter. Whether through the development of a unifying theory such as string theory, the quantisation of geometry in loop quantum gravity or emerging phenomenological approaches, progress in this field promises to deepen our understanding of the universe at its most fundamental level.