## Loose ends

Will the string tie the Standard package? Hot on the heels of the Standard Model, some physicists are working to support a new idea called string theory. This attempts to tie up the loose ends in the Standard Model by explaining all the fundamental particles and forces (including gravity) in a unified framework.

Underlying string theory is the radical idea that fundamental particles are not really like points or dots, but rather small loops of vibrating strings. All the different particles and forces are just different oscillation modes of a unique type of string. Bizarrely, the theory also implies that besides the familiar three–dimensional world and the fourth dimension of time, there are six additional spatial dimensions! These extra dimensions are apparently 'curled up' so small that we do not see them.

### Which string?

String theory is conceptually complex, with a fascinating but very difficult mathematical structure. This has so far prevented researchers from deriving concrete hypotheses from the theory for comparison with experimental results. Not only does string theory involve the complex study of the geometry of extra dimensions, but the way the structure of the dimensions are chosen appears arbitrary and can lead to different outcomes.

For instance, there seem to be many possible ways to curl up the extra dimensions, by choosing different shapes and sizes. This leads to many alternative versions of the theory. In certain cases, the sizes of the extra dimensions are very small and it will be difficult to obtain direct evidence for them. In others, the sizes are far larger and could be observed at new accelerators such as CERN’s Large Hadron Collider.