Question: what is quantam loop theory?

Nathan Langford answered on 2 Jul 2012:

Loop quantum gravity is another theory which tries to come up with a quantum mechanical description of gravitational force. Unlike String Theory, it doesn’t aim to be a Grand Unified Theory of everything – instead it just applies the concepts of quantum physics directly to the gravitational field.

One of the most interesting and perhaps also confusing features of loop quantum gravity goes back to Einstein’s original theory of general relativity. In general relativity, Einstein suggested that the concept of “space-time” as an independent thing is unnecessary and that, in his theory, space-time itself is just determined by the underlying gravitational field, which can itself evolve and change. In the theory of loop quantum gravity, this idea is extended to the quantum regime: there is no space-time, the gravitational field doesn’t exist within space-time, instead the field itself defines space-time.

In a quantum theory, the gravitational field must also be a quantum field, and must therefore itself become “lumpy” or “granular” (“quantised” is the technical term). These elementary lumps or “excitations” of the quantum gravitational field are called “loops”, hence the name of “loop quantum gravity”. At low energies, these loops are elementary particles, called “gravitons”, which carry gravitational force, like gluons carry nuclear force. In the theory of loop quantum gravity, the structure of space-time itself is therefore defined by the intersections and connections in the network of these loops.