String theory

From Wikinfo

A string theory is a physical model whose fundamental building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that were the basis of most earlier physics. For this reason, string theories are able to avoid problems associated with the presence of pointlike particles in a physical theory. Detailed study of string theories has revealed that they contain not just strings but other objects, variously including points, membranes, and higher-dimensional objects. As discussed below, it is important to realize that no string theory has yet made firm predictions that would allow it to be experimentally tested.

The term 'string theory' properly refers to both the 26-dimensional bosonic string theories and to the 10-dimensional superstring theories discovered by adding supersymmetry. Nowadays, 'string theory' usually refers to the supersymmetric variant while the earlier is given its full name 'bosonic string theory'. In the 1990s, Edward Witten and others found strong evidence that the different superstring theories were different limits of an unknown 11-dimensional theory called M-theory. These discoveries sparked the Second Superstring Revolution.

Interest in string theory is driven largely by the hope that it will prove to be a theory of everything. It is one viable solution for quantum gravity, and in addition to gravity it can naturally describe interactions similar to electromagnetism and the other forces of nature. Superstring theories also include fermions, the building blocks of matter. It is not yet known whether string theory is able to describe a universe with the precise collection of forces and matter that we observe, nor how much freedom to choose those details the theory will allow.

The observed 4 dimensions of the universe would seem to be in contradiction with the 10 or 11 dimensions one finds in string/M-theory. This is usually solved in one of two different ways. The first is to compactify the extra dimensions. In other words, this means that the 6 or 7 extra dimensions are so small as to not be detectable in our experience. In the 6-dimensional case, this is done with Calabi-Yau spaces. In 7 dimensions, they are termed G₂ manifolds. Essentially these extra dimensions are "compactified" by causing them to loop back upon themselves. A standard analogy for this is to consider multidimensional space as a garden hose. If we view the hose from sufficiently far away, it appears to have only one dimension, its length. This is akin to the 4 macroscopic dimensions we are accustomed to dealing with every day. If, however, one approaches the hose, one discovers that it contains a second dimension, its circumference. This "extra dimension" is only visible within a relatively close range to the hose, just as the extra dimensions of the Calabi-Yau space are only visible at extremely small distances, and thus are not easily detected.

Another possibility is that we are stuck to a 3+1 dimensional subspace of the full universe. This is known as a braneworld theory. An interesting byproduct is that these would allow (but not necessitate) observations of quantum gravity effects even at the soon to open Large Hadron Collider at CERN in Geneva. While intriguing, this possibility is not widely believed.

On a more concrete level, string theory has led to advances in the mathematics of knots, Calabi-Yau spaces and many other fields. Much exciting new mathematics in recent years has its origin in string theory. String theory has also led to much insight into supersymmetric gauge theories, a subject which includes possible extensions of the standard model.

Problems with string theory

String theory suffers from two major problems. The first problem is that, as with any current theory of quantum gravity, it does not yet make any firm predictions that are currently subject to experimental verification. There do exist certain models, such as the braneworlds mentioned above, that could lead to observation of stringy behavior in the next decade, but this is not required by string theory, only allowed. Other possibilities include cosmological observations that may reflect stringy physics. Finally, one cannot discount that other possibilities may arise in the future. Nonetheless, while these possibilities for confirmation, however remote, do exist, as things now stand string theory can not be disproven by experiment, which is a serious problem for any theory of physics.

The second problem is that much of theory is still only formulated perturbatively (as a series of approximations rather than as an exact solution). While much progress has been made in nonperturbative techniques including conjectured complete definitions in space-times satisfying certain asymptotics, a full nonperturbative definition of the theory is still lacking.