Loop quantum gravity

Loop quantum gravity (LQG) is a proposed quantum theory of spacetime which seems to explain the seemingly incompatible theories of quantum mechanics and general relativity. The fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity. Previous theories of quantum mechanics, including string theory depended on exactly such a non-existent fixed spacetime background. This inconsistency drove the effort to find a theory explaining quantum mechanics in a relativistic spacetime.

In LQG, the fabric of spacetime is a foamy network of interacting loops mathematically described by spin networks. These loops are about 10^-35 meters in size, called the Planck scale. The loops knot together forming edges, surfaces, and vertices, much as do soap bubbles joined together. In other words, spacetime itself is quantized. Any attempt to divide a loop would, if successful, cause it to divide into two loops each with the original size. In LQG, spin networks represent the quantum states[?] of the geometry of relative spacetime. Looked at another way, Einstein's theory of general relativity is (as Einstein predicted) a classical approximation of a quantized geometry.

An important principle in LQG is that there are no observers outside the universe. All observers must be a part of the universe they are observing. However, because light cones[?] limit the information that is available to any observer, the Platonic idea of absolute truths does not exist in a LQG universe. Instead, there exists a consistency of truths in that every observer will report consistent (not necessarily the same) results if truthful.

Unlike string theory and M-theory, LQG makes experimentally testable hypotheses. The path taken by a photon through a discrete spacetime geometry would be different from the path taken by the same photon through continuous spacetime. Normally, such differences should be insignificant, but Giovanni Amelino-Camelia[?] points out that photons which have travelled from distant galaxies may reveal the structure of spacetime. LQG predicts that more energetic photons should travel ever so slightly faster than less energetic photons. This effect would be too small to observe within our galaxy. However, light reaching us from gamma ray bursts in other galaxies should manifest a varying spectral shift over time. In other words, distant gamma ray bursts should appear to start off more bluish and end more reddish.

The recent result that gravity propagates at the speed of light is consistent with LQG. However, the result significantly constrains string theory and probably M-theory because large numbers of dimensions would allow gravity to propagate along extra dimensions. This result does not by itself rule out all forms of string theory.

Loop quantum gravity theorists:

Books on LQG include The End of Time[?] by Julian Barbour[?] and Three Roads to Quantum Gravity[?] by Lee Smolin.

Information about LQG may be found at http://www.qgravity.org/.