### String theory and extra dimensions

#### Loose ends

Will the string tie the Standard package? Hot on the heels of the Standard Model, some physicists are working to support an 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 predictions 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.

#### Secret dimensions

In everyday life, we inhabit a space of three dimensions – a vast ‘cupboard’ with height, width and depth, well known for centuries. Less obviously, we can consider time as an additional, fourth dimension, as Einstein famously revealed. But just as we are becoming more used to the idea of four dimensions, some theorists have made predictions wilder than even Einstein had imagined.

String theory intriguingly suggests that six more dimensions exist, but are somehow hidden from our senses. They could be all around us, but curled up to be so tiny that we have never realized their existence.

#### What is a dimension?

Dimensions are really just the number of co-ordinates we need to describe things. We can compare this to a tightrope walker travelling along a rope. For the acrobat there is only one dimension – forwards or backwards, and we can state her or his position with just one number. But if we look on a smaller level, for an ant crawling about on the rope there would be two dimensions of travel: we’d need to know how far *around* the rope it is, as well as how far *along*. If we zoom in even further, for atoms inside the rope, the world would be in three dimensions, the x, y and z of everyday coordinates. Who is to say that as we go smaller and smaller the number of directions to travel in, the number of dimensions, does not increase even further?

#### Beyond the third dimension

Some string theorists have taken this idea further to explain a mystery of gravity that has perplexed physicists for some time – why is gravity so much weaker than the other fundamental forces? Why do we need objects the size of planets in order to feel its force when we can experience the electromagnetic force with just a small magnet? And if gravity’s quantum carrier, the graviton, exists, how can we find it? The idea is that we do not feel gravity’s full effect in the everyday world. Gravity may appear weak only because its force is being shared with other spatial dimensions.

To find out whether these ideas are just products of wild imaginations or an incredible leap in understanding will require experimental evidence. But how?

High-energy experiments could prise open the inconspicuous dimensions just enough to allow particles to move between the normal 3D world and other dimensions, manifesting itself in the sudden appearance or disappearance of a particle. Or we might detect some of the new phenomena that a world with extra dimensions predicts. Who knows where such a discovery could lead!

Read more about Extra dimensions in CMS via the link below