Quantum gravity

What makes this fascinating

• Two theories that contradict each other — General relativity treats spacetime as smooth; quantum mechanics treats everything as probabilistic. Combine them at tiny scales and you get nonsense — infinities.

• The Planck scale — The conflict only bites at about 10⁻³⁵ metres, far beyond any experiment — so theorists work almost without data.

• Rival maps of reality — String theory and loop quantum gravity are the leading candidates, but neither has yet made a prediction we can test and confirm.

Frequently asked questions

Has quantum gravity been solved?

No. Merging general relativity with quantum mechanics into one consistent theory is unsolved. Candidate frameworks such as string theory and loop quantum gravity exist, but none is experimentally confirmed.

Why are general relativity and quantum mechanics incompatible?

General relativity treats spacetime as smooth and deterministic; quantum mechanics is probabilistic and assumes a fixed background. Naively quantizing gravity produces infinities that cannot be removed, and the conflict only bites at the tiny Planck scale, far beyond any experiment.

Is string theory the theory of quantum gravity?

It is the most developed candidate, but unconfirmed. String theory and loop quantum gravity are competing approaches, and with no data at the Planck scale neither has been shown to be the way nature actually works.

More summits in Physics

• The nature of dark matter

  85% of the universe's matter is invisible and unidentified. What is it?

• The nature of dark energy

  Something is pushing the universe apart, faster and faster. We have no idea what.

• The quantum measurement problem

  Why does a particle's haze of possibilities snap into one outcome when we look?

• High-temperature superconductivity

  Some materials carry current with zero loss — and we still can't explain why.