Loop quantum gravity
← Theories of Everything · Glossary
Author: Abhay Ashtekar, Carlo Rovelli, Lee Smolin, from 1986
Published: Peer-reviewed physics literature
Reference: Loop quantum gravity
Spacetime itself is quantised — woven from discrete loops with a smallest possible area and volume. No extra dimensions required.
Standing: Active mainstream research; not experimentally confirmed.
The opposite bet from string theory
String theory quantises matter in a fixed background and hopes gravity follows. LQG quantises spacetime itself, and is background independent — there is no pre-existing stage on which things happen, which is arguably truer to what General Relativity actually says.
Its central result is that area and volume have discrete spectra: there is a smallest possible area, around the Planck scale. Space is not infinitely divisible. It needs no extra dimensions and no supersymmetry — a far leaner set of assumptions than its rival.
What it does not do
LQG is a theory of gravity, not of everything. It does not unify the other three forces or explain the particle spectrum, so calling it a theory of everything is a stretch its own proponents mostly avoid. It is included here because any TOE must contain a quantum theory of gravity, and this is the leading alternative route to one.
Like string theory it has no confirmed experimental prediction, though it makes one testable-in-principle claim string theory does not: at sufficient energies, discrete spacetime could produce energy-dependent variation in the speed of light. Gamma-ray burst observations have so far constrained this without confirming it.