How do we reconcile gravity with quantum mechanics?
General relativity describes smooth spacetime geometry; quantum mechanics describes probabilistic fields on a fixed background. They collide at the Planck scale — inside black holes and at the Big Bang — and no experiment has yet tested any theory of quantum gravity.
Why it is hard
The Planck energy is ~10^16 times beyond collider reach. Contenders (string theory, loop quantum gravity, asymptotic safety, causal sets) differ on the fundamental nature of spacetime, and cosmological signatures — primordial gravitational waves, black-hole evaporation — are extraordinarily faint.
Current evidence
Indirect constraints exist: black-hole thermodynamics (Bekenstein–Hawking entropy), the observed light-element abundances consistent with hot Big Bang + inflation, and no observed Lorentz violation in gamma-ray burst photon arrival times. No direct quantum-gravity signature has been detected.
Possible approaches
Primordial B-mode polarization searches, tabletop entanglement tests of gravity's quantum nature, black-hole ringdown spectroscopy, Lorentz-invariance tests with high-energy photons and neutrinos, and mathematical development of candidate theories toward falsifiable predictions.
Related topics
Sources
- Loop Quantum Cosmology: A Status Report — Living Reviews in Relativity
- Dark Charge Could Make Exploding Black Holes More Common — APS Physics
- Quantum Theory of Gravity. I. The Canonical Theory — American Physical Society
- Gravitational-Wave Science — LIGO Scientific Collaboration