Science › Black holes & quantum gravity
Loop quantum gravity
1986· Theory
SpeculativeA serious idea with no experimental proof yet.
A speculative theory in which space itself is made of discrete, indivisible chunks woven into a network.
See it on the Science map →How it works
Starting from Abhay Ashtekar's 1986 rewriting of general relativity, Carlo Rovelli, Lee Smolin and others built a quantum version of space itself. Its states are "spin networks": nodes carry tiny chunks of volume and links carry chunks of area. Area and volume come in fixed minimum sizes near the Planck scale. Unlike string theory, it quantizes gravity alone, without extra dimensions or new particles.
Why it matters
It offers a concrete picture of grainy space and suggests the Big Bang may have been a "Big Bounce" from an earlier contracting universe.
Think of it like
Like a parcel map where land can only be sold in whole lots, never in fractions.
How we know
No experimental confirmation. Gamma-ray burst timing (Fermi telescope, 2009) found high- and low-energy light arriving together, ruling out some simple grainy-space versions.
Still a mystery
It has not yet shown that smooth, everyday spacetime emerges from its networks, or how to include the other forces.
Connections (3)
▸Quantum gravity: the missing link has a candidate in Loop quantum gravitySpeculative
The relationship
Loop quantum gravity takes a different route: instead of adding new ingredients, it applies quantum rules directly to the geometry of space in general relativity. The result is a space built from discrete chunks of area and volume. It aims to remain faithful to relativity's principle that space is not a fixed stage but dynamic.
How the link was found
Abhay Ashtekar's 1986 reformulation of relativity; Carlo Rovelli and Lee Smolin developed loop methods from 1988 onward.
Is it proven?
No experimental confirmation. It gives discrete area and volume and a finite entropy for black holes, but has not shown that ordinary smooth space emerges at large scales.
Open questions and mysteries
Can it reproduce general relativity at everyday scales? Can it include the other forces of nature?
▸Loop quantum gravity says space is grainy at The Planck scale: the smallest meaningful sizeSpeculative
The relationship
In loop quantum gravity, measuring the area of a surface or the volume of a region can only yield certain allowed values, like energy levels in an atom. The smallest nonzero area is about the Planck length squared. Space is therefore grainy at the Planck scale: no smooth continuum below it, just a network of quantum chunks.
How the link was found
Carlo Rovelli and Lee Smolin derived discrete area and volume in 1995; Ashtekar and Lewandowski refined it in 1997.
Is it proven?
A mathematical result within the theory, not observed. Gamma-ray burst timing by the Fermi telescope (2009) found no light-speed variation, which rules out some simple grainy-space models.
Open questions and mysteries
Whether the graininess is physically real and could ever leave a measurable trace.
▸Loop quantum gravity predicts, in simplified models, The Big BounceSpeculative
The relationship
Loop quantum gravity says space comes in smallest chunks, so it cannot be squeezed indefinitely. Applied to a uniform universe, this adds a correction to the equations of expansion that matters only near the Planck density. There gravity effectively turns repulsive: contraction slows, stops at a maximum density and reverses into expansion, with no infinity.
How the link was found
Martin Bojowald showed singularity avoidance in 2001. Abhay Ashtekar, Tomasz Pawłowski and Parampreet Singh computed a clean bounce in 2006 by following a quantum universe's evolution numerically.
Is it proven?
Mathematically solid in the simplified models, which assume a perfectly uniform universe. Whether the full theory gives the same result is not established, and no observation tests it.
Open questions and mysteries
Do the simplifications hide effects that would change the result? Could the bounce leave a measurable imprint on the microwave background?