Ask Yan - What is string theory?

Physicists don't have a theory that explains everything – the theory of the very small, quantum mechanics, is inconsistent with the theory of the very large, Einstein's general theory of relativity. String theory is an attempt to join the two.

In quantum mechanics, the basic building blocks of matter are treated as point-like particles (no length or breadth or height), but in string theory the building blocks are treated as strings (they have length, but still no breadth or height). The length of a string is near the Planck length, which is thought to be a natural limit on how short an object can be. This is tiny, even compared to a proton. London is to a proton what a proton is to a string.

Just like on a violin, the string can vibrate, and the different notes represent different particles. Unlike on a violin, the strings aren’t fixed. They are free to move around and vibrate in space-time, or even join their ends together and form a little loop.

Physicists are excited because strings can represent particles called gravitons, which could explain the gravitational force. However, gravitons have not yet been observed. It also builds upon another theory, called supersymmetry, which also hasn't been seen yet. The Large Hadron Collider at CERN will search for supersymmetry and indirect evidence of gravitons – either would improve string theory's credibility.

It's hard to know if string theory will result in life-improving breakthroughs. Quantum mechanics gave us many unforeseen discoveries, including microwave ovens, semiconductors, computers, mp3 players, nuclear power, and medical diagnostics and therapies. Perhaps new technologies will result from deeper insights.

- Answered by Stephen Serjeant and Robert Flack, 13 August 2009