Quantum Gravity
How do gravity (Einstein) and the quantum world fit together? There are candidate theories but no measurement that decides between them.
Open in the interactive tree →Relativity describes the large, quantum mechanics the small; inside black holes and at the Big Bang both are needed, and the calculations break down. Candidates include string theory (roughly 10^500 possible solutions), loop quantum gravity, asymptotic safety and others. Quantum-gravity effects are expected only at the Planck length (10^-35 m), far beyond any accelerator.
As of October 2026
October 2026: no breakthrough, and no experiment separates the approaches. Hope rests on indirect tests: experiments checking whether two small bodies become entangled through gravity alone (nanodiamond interferometers and others, not yet realized), precision data on the cosmic microwave background and gamma-ray bursts. Even a successful entanglement test would not unambiguously show that gravity is quantum, because open questions of interpretation remain.
What is missing
- A measurable quantity that makes Planck-scale effects or quantum behavior of gravity visible
- A mathematically consistent, complete theory (string theory: the landscape problem; loop gravity: connection to smooth spacetime)
- An understanding of the black-hole information paradox and of the initial singularity
- Quantum control of massive objects (microgram scale) to test gravity-induced entanglement
Becomes possible once solved
- A unified theory of all forces
- The physics of the first instant of the Big Bang and of black-hole interiors
- An answer to whether space and time are fundamental or emergent
Open steps
- Gravity-induced entanglement test Low AI leveragePut two micro-scale masses into quantum superposition, shield them from decoherence, and test whether gravity alone entangles them.
- Mapping the string landscape High AI leverageCharacterize the huge set of string compactifications (about 10^500 solutions) to find which, if any, reproduce our particle content and constants.
- Black-hole information, singularity Low AI leverageUnderstand how information escapes evaporating black holes and how the initial singularity is resolved, with calculations that can be checked in toy models.
- Loop gravity and safety numerics Medium AI leverageShow that loop quantum gravity or asymptotic safety reproduces smooth spacetime at large scales, using large numerical calculations (spin-foam sums, renormalization-group flows).
- Planck-scale signatures in cosmic data Medium AI leverageFind tiny deviations from standard physics in cosmic microwave background and gamma-ray-burst data that could constrain quantum-gravity models.
Where AI could help
Low AI leverage. The gap is a measurable prediction and a consistent theory; AI speeds side calculations but cannot supply missing Planck-scale data.
- Compute string-compactification geometry (Calabi-Yau metrics) and scan the landscape faster
- Search for closed-form amplitude formulas and check them symbolically
- Optimize designs and noise models for tabletop gravity-entanglement experiments
- Analyze CMB and gamma-ray-burst data for tiny deviations from standard physics
Shown so far
- In February 2026 OpenAI reported that GPT-5.2 simplified low-point results and proposed a closed form for single-minus gluon tree amplitudes, which physicists then proved and checked (a related field-theory result, not gravity itself). source
- In October 2019 Ashmore, He and Ovrut showed that machine learning computes numerical Ricci-flat Calabi-Yau metrics (needed for string compactifications) one to two orders of magnitude faster than the classical algorithm. source
Prerequisites
- Theory of Relativity1905
- Quantum Mechanics1925
- Standard Model of Particles1973
- Black Hole Thermodynamics1974Black hole thermodynamics is the best clue to quantum gravity