Can quantum physics and gravity be united?

Atoms follow quantum rules while also falling in Earth’s gravitational field. If both descriptions already work, what remains to be united?

We can describe quantum matter in a gravitational field and build a low-energy quantum effective theory of gravity. The unfinished task is a complete, consistent description that also applies when geometry itself becomes quantum. As of the source check on 31 October 2026, no complete theory has been experimentally established. String theory and loop quantum gravity are research programmes, not confirmed final answers. Donoghue: the low-energy description, Plávala: the state of the question and conditional new tests, 2026 author version.

The matter producing gravity can be quantum

General relativity connects gravity with spacetime geometry: it determines distances, time intervals and possible freely falling trajectories. Geometry responds to matter’s energy and momentum. It is not simply a fixed stage on which actors move. An illustrated explanation of space and curvature.

Quantum theory permits superposition: a state with several coherent possibilities between which interference can occur. This differs from merely not knowing which of two boxes contains a classical ball. Imagine a body in a superposition of two positions. It has mass and gravitationally influences another body. What description is needed for that influence itself and its relationship to the quantum state?

We can start with a prescribed classical field and calculate quantum motion in it. But this does not yet describe the full coupled quantum dynamics of source, test body and geometry. This step distinguishes “a quantum particle under gravity” from the general problem of quantum gravity. Bose and colleagues: framing the question with two massive quantum bodies.

A shared working domain already exists

One bridge between the descriptions is quantum fields on a classical curved spacetime. This approximation is used, for example, to study Hawking radiation. The fields are quantum, while geometry is not quantized. The result therefore does not automatically answer what happens at the very end of black-hole evaporation.

Another bridge is effective field theory. It does not require knowing all physics at every energy: calculations are ordered in small parameters. At a fixed required accuracy, a finite set of contributions suffices. This allows low-energy quantum corrections to gravity to be obtained separately from unknown high-energy details. Donoghue’s original paper: introduction and effective description.

The difficulty arises if an ordinary perturbative expansion is required to work without any energy limit: ever more independent corrections must be introduced. This loses the predictive power of that unrestricted construction; it does not show that every low-energy calculation is meaningless. Addazi and colleagues: nonrenormalizability and effective theory, §1.

Four different levels of the question

Classical geometry

What is described: Motion of bodies and changes of spacetime in general relativity.
Limit of the inference: A successful classical prediction does not measure gravity’s quantum state.

Quantum fields on a prescribed background

What is described: Quantum matter or radiation in a classical geometry.
Limit of the inference: Geometry is not yet a complete quantum system.

Low-energy quantum gravity

What is described: Quantum corrections organized in an effective theory.
Limit of the inference: Predictivity within its domain does not mean a description at every energy.

A complete microscopic description

What is described: Coupled quantum dynamics of matter and gravity beyond an unreliable approximation.
Limit of the inference: An open task; no particular complete theory has been experimentally established.

A map of tasks and approximations, not a measured phase diagram or four sharply separated object sizes. Domains can overlap; the required description depends on state, energies, curvature and precision. This is not a ranking of candidate theories.

Where the approximation needs an extension

Extending classical equations to initial cosmological states or deep regions within a black hole can lead to singularities—limits at which that description ceases to be sufficient. This is not a direct measurement of physical infinity. We need to determine what new concepts and dynamics replace the unreliable extrapolation. Review of quantum-gravity tasks and approaches.

The division “relativity for large things, quantum physics for small things” is too crude here. Energy, curvature, quantum state and required precision matter. A quantum effect in a weak external field can be investigated in a laboratory without creating a black hole. And a large object size does not by itself cancel a quantum description.

Branch · scale and unitsOptional · you can skip this

What does the Planck length actually mean?

From the quantum constant $\hbar$ , gravitational constant $G$ and light speed $c$ , we can construct a length:

\ell_{\mathrm P}=\sqrt{\frac{\hbar G}{c^3}}.

Take rounded values $\hbar\approx1.055\times10^{-34}$ kg·m²/s, $G\approx6.674\times10^{-11}$ m³/(kg·s²), $c\approx3.00\times10^8$ m/s. The expression under the square root has units m² and is about $2.61\times10^{-70}$ m². Hence $\ell_{\mathrm P}\approx1.6\times10^{-35}$ m. CODATA 2022 constants, NIST table, formula and physical caveats.

This is a natural scale combining three constants, not an image of space’s smallest brick. The dimensional formula alone implies neither a spatial lattice nor a proven minimum measurable distance. A specific theory can introduce additional scales and consequences; weak indirect effects may also be tested over much larger distances.

Candidates offer different ways to extend the description

String theory seeks a quantum description including gravity within a broader construction of interactions. Connecting a particular version to our world requires additional choices, including the model’s state and geometry. The existence of a mathematical construction does not select its realization in nature.

Loop quantum gravity quantizes geometric variables without a fixed, unchanging background geometry. Moving from its fundamental construction to unambiguous observable predictions also requires work, often with additional approximations. A consequence of a simplified model cannot be presented as a compulsory prediction of the whole programme. Addazi and colleagues: assumptions of string and loop approaches, §1, §2.1.2 and §2.1.5.

Other approaches exist too. These two are not an exhaustive list and do not automatically have equal chances. Any candidate needs consistency, recovery of tested results in the appropriate limit and a connection to data capable of distinguishing it from alternatives.

Branch · a concrete piece of evidenceOptional · you can skip this

A quantum experiment with gravity is not yet a complete theory

The 2025 author version of an experiment by Dobkowski and colleagues describes an atomic interferometer: one part of a wave packet was held by a magnetic gradient while the other underwent free fall. After recombining the packets, quantum phase differences were measured and compared with calculations accounting for trajectory imperfections. Original experimental description, §II–IV.

The result tests quantum evolution of matter under a specified external field. This setup does not measure a quantum superposition of the source of Earth’s gravity. Agreement therefore does not mean that the experiment selected string theory, the loop programme or any complete microscopic theory. This is an inference about the scope of the tested setup, not a denial of the measurement’s value.

A test must distinguish physical explanations

One direction seeks entanglement, quantum correlations between two bodies created by their gravitational interaction. Proposed tests try to probe properties of the interaction through the state of matter. Gravity must be separated from electromagnetic forces, loss of coherence controlled and assumptions about mediation and dynamics made explicit. Bose and colleagues’ original proposal.

Even an inference of nonclassical interaction properties does not establish a unique complete theory. An indirect inference from a simpler experiment needs further assumptions. For example, Plávala’s 2026 paper explicitly states conditions under which tests of one delocalized system could constrain the behaviour of two; it is a theoretical argument with assumptions, not a report of an accomplished entanglement detection. Conditions, proofs and open questions in the original.

Quantum physics and gravity can thus already be combined in limited working descriptions. The major open question is what more complete description works beyond these approximations and how nature will let us test it. When reading news about “unification”, first separate three claims: a model was constructed, an effect was measured or data actually selected a particular theory.