Question: what is string theory and loop quantum gravity?

James Verdon answered on 1 Jul 2012:

Hi pigsfly,

Quantum theories are not one of my strong points, but I’ll do my best. String theory and loop quantum gravity are two ideas that have been suggested by theoretical physicists to solve one of the largest remaining problems in physics.

Currently, the two dominant theories in physics are relativity (which includes gravity, and describes things that are very large – planets and suns – and travelling very quickly – close to the speed of light) and quantum mechanics (which describes very small things, the fundamental particles like electrones, quarks, neutrinos).

The problem is that they don’t quite match up – relativity doesn’t seem to work very well at the sub-atomic scale. A number of theories have been suggested to account for this problem, and to unify quantum and relativity together. Two of these are string theory and quantum loop gravity. Because neither have been experimentally confirmed beyond doubt, physicists are still arguing over which of these two, or a number of other theories, might be correct.

I’ll do my best to visualise them both: String theory essentially suggests that all the quantum particles (electrons, neutrinos, etc) are made up of multidimensional strings of energy. These strings are vibrating, and the frequency of vibration controls their properties. Quantum loop gravity describes a granular space, where each point in space is described by a multidimensional loop.

I don’t really think that’s a very good description, sorry. What they do is describe a theoretical basis where relativity can work at the quantum scale, though I’ve got no idea how they actually do that. I hope that helps a little though….