String theory  
Superstring theory


Superstring theory is an attempt to explain all of the particles and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetric strings. Superstring theory is a shorthand for supersymmetric string theory because unlike bosonic string theory, it is the version of string theory that incorporates fermions and supersymmetry. Here, the word "super" has the meaning of supersymmetry.
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The deepest problem in theoretical physics is harmonizing the theory of general relativity, which describes gravitation and applies to largescale structures (stars, galaxies, super clusters), with quantum mechanics, which describes the other three fundamental forces acting on the atomic scale.
The development of a quantum field theory of a force invariably results in infinite (and therefore useless) probabilities. Physicists have developed mathematical techniques (renormalization) to eliminate these infinities which work for three of the four fundamental forces – electromagnetic, strong nuclear and weak nuclear forces  but not for gravity. The development of a quantum theory of gravity must therefore come about by different means than those used for the other forces.
The basic idea is that the fundamental constituents of reality are strings of the Planck length (about 10^{−33} cm) which vibrate at resonant frequencies. Every string in theory has a unique resonance, or harmonic. Different harmonics determine different fundamental forces. The tension in a string is on the order of the Planck force (10^{44} newtons). The graviton (the proposed messenger particle of the gravitational force), for example, is predicted by the theory to be a string with wave amplitude zero. Another key insight provided by the theory is that no measurable differences can be detected between strings that wrap around dimensions smaller than themselves and those that move along larger dimensions (i.e., effects in a dimension of size R equal those whose size is 1/R). Singularities are avoided because the observed consequences of "Big Crunches" never reach zero size. In fact, should the universe begin a "big crunch" sort of process, string theory dictates that the universe could never be smaller than the size of a string, at which point it would actually begin expanding.
Our physical space is observed to have only three large dimensions and—taken together with duration as the fourth dimension—a physical theory must take this into account. However, nothing prevents a theory from including more than 4 dimensions, per se. In the case of string theory, consistency requires spacetime to have 10 (3+1+6) dimensions. The conflict between observation and theory is resolved by making the unobserved dimensions compactified.
Our minds have difficulty visualizing higher dimensions because we can only move in three spatial dimensions. One way of dealing with this limitation is not to try to visualize higher dimensions at all, but just to think of them as extra numbers in the equations that describe the way the world works. This opens the question of whether these 'extra numbers' can be investigated directly in any experiment (which must show different results in 1, 2, or 2+1 dimensions to a human scientist). This, in turn, raises the question of whether models that rely on such abstract modelling (and potentially impossibly huge experimental apparatus) can be considered scientific. Sixdimensional CalabiYau shapes can account for the additional dimensions required by superstring theory. The theory states that every point in space (or whatever we had previously considered a point) is in fact a very small manifold where each extra dimension has a size on the order of the Planck length.
Superstring theory is not the first theory to propose extra spatial dimensions; the KaluzaKlein theory had done so previously. Modern string theory relies on the mathematics of folds, knots, and topology, which were largely developed after Kaluza and Klein, and has made physical theories relying on extra dimensions much more credible.
Theoretical physicists were troubled by the existence of five separate string theories. A possible solution for this dilemma was suggested at the beginning of what is called the second superstring revolution in the 1990s, which suggests that the five string theories might be different limits of a single underlying theory, called Mtheory. However, unfortunately, to this date this remains a conjecture.
String Theories  

Type  Spacetime dimensions 
Details 
Bosonic  26  Only bosons, no fermions means only forces, no matter, with both open and closed strings; major flaw: a particle with imaginary mass, called the tachyon 
I  10  Supersymmetry between forces and matter, with both open and closed strings, no tachyon, group symmetry is SO(32) 
IIA  10  Supersymmetry between forces and matter, with closed strings only, no tachyon, massless fermions spin both ways (nonchiral) 
IIB  10  Supersymmetry between forces and matter, with closed strings only, no tachyon, massless fermions only spin one way (chiral) 
HO  10  Supersymmetry between forces and matter, with closed strings only, no tachyon, heterotic, meaning right moving and left moving strings differ, group symmetry is SO(32) 
HE  10  Supersymmetry between forces and matter, with closed strings only, no tachyon, heterotic, meaning right moving and left moving strings differ, group symmetry is E_{8}×E_{8} 
The five consistent superstring theories are:
Chiral gauge theories can be inconsistent due to anomalies. This happens when certain oneloop Feynman diagrams cause a quantum mechanical breakdown of the gauge symmetry. The anomalies were canceled out via the GreenSchwarz mechanism.
Please note that the number of superstring theories given above is only a highlevel classification; the actual number of mathematically distinct theories which are compatible with observation and would therefore have to be examined to find the one that correctly describes nature is currently believed to be at least 10^{500} (a one with five hundred zeroes). This has given rise to the concern that superstring theories, despite the alluring simplicity of their basic principles, are, in fact, not simple at all, and according to the principle of Occam's razor perhaps alternative physical theories going beyond the Standard Model should be explored. This is aggravated by the fact that it is exceedingly hard to make predictions from any superstring theory which can be falsified by experiment, and in fact no current superstring theory makes any falsifiable prediction.
General relativity typically deals with situations involving large mass objects in fairly large regions of spacetime whereas quantum mechanics is generally reserved for scenarios at the atomic scale (small spacetime regions). The two are very rarely used together, and the most common case in which they are combined is in the study of black holes. Having "peak density", or the maximum amount of matter possible in a space, and very small area, the two must be used in synchrony in order to predict conditions in such places; yet, when used together, the equations fall apart, spitting out impossible answers, such as imaginary distances and less than one dimension.
The major problem with their congruence is that, at Planck scale (a fundamental small unit of length) lengths, general relativity predicts a smooth, flowing surface, while quantum mechanics predicts a random, warped surface, neither of which are anywhere near compatible. Superstring theory resolves this issue, replacing the classical idea of point particles with loops. These loops have an average diameter of the Planck length, with extremely small variances, which completely ignores the quantum mechanical predictions of Planckscale length dimensional warping.
There are five ways open and closed strings can interact. An interaction in superstring theory is a topology changing event. Since superstring theory has to be a local theory to obey causality the topology change must only occur at a single point. If C represents a closed string and O an open string, then the five interactions are OOO, CCC, OOOO, CO and COO.
All open superstring theories also contain closed superstrings since closed superstrings can be seen from the fifth interaction, and they are unavoidable. Although all these interactions are possible, in practice the most used superstring model is the closed heterotic E8xE8 superstring which only has closed strings and so only the second interaction (CCC) is needed.
The single most important equation in (first quantisized bosonic) string theory is the Npoint scattering amplitude. This treats the incoming and outgoing strings as points, which in string theory are tachyons, with momentum k_{i} which connect to a string world surface at the surface points z_{i}. It is given by the following functional integral which integrates (sums) over all possible embeddings of this 2D surface in 26 dimensions.
The functional integral can be done because it is a Gaussian to become:
This is integrated over the various points z_{i}. Special care must be taken because two parts of this complex region may represent the same point on the 2D surface and you don't want to integrate over them twice. Also you need to make sure you are not integrating multiple times over different parameterizations of the surface. When this is taken into account it can be used to calculate the 4point scattering amplitude (the 3point amplitude is simply a delta function):
Which is a beta function. It was this beta function which was apparently found before full string theory was developed. With superstrings the equations contain not only the 10D spacetime coordinates X but also the Grassman coordinates θ. Since there are various ways this can be done this leads to different string theories.
When integrating over surfaces such as the torus, we end up with equations in terms of theta functions and elliptic functions such as the Dedekind eta function. This is smooth everywhere, which it has to be to make physical sense, only when raised to the 24th power. This is the origin of needing 26 dimensions of spacetime for bosonic string theory. The extra two dimensions arise as degrees of freedom of the string surface.
DBranes are membranelike objects in 10D string theory. They can be thought of as occurring as a result of a KaluzaKlein compactification of 11D MTheory which contains membranes. Because compactification of a geometric theory produces extra vector fields the Dbranes can be included in the action by adding an extra U(1) vector field to the string action.
In type I open string theory, the ends of open strings are always attached to Dbrane surfaces. A string theory with more gauge fields such as SU(2) gauge fields would then correspond to the compactification of some higher dimensional theory above 11 dimensions which is not thought to be possible to date.
For a 10 dimensional supersymmetric theory we are allowed a 497component Majorana spinor. This can be decomposed into a pair of 16component MajoranaWeyl (chiral) spinors. There are then various ways to construct an invariant depending on whether these two spinors have the same or opposite chiralities:
Superstring Model  Invariant 

Heterotic  
IIA  
IIB 
The heterotic superstrings come in two types SO(32) and E8xE8 as indicated above and the type I superstrings include open strings.
It is commonly believed that the 5 superstring theories are approximated to a theory in higher dimensions possibly involving membranes. Unfortunately because the action for this involves quartic terms and higher so is not Gaussian the functional integrals are very difficult to solve and so this has confounded the top theoretical physicists. Edward Witten has popularised the concept of a theory in 11 dimensions MTheory involving membranes interpolating from the known symmetries of superstring theory. It may turn out that there exist membrane models or other nonmembrane models in higher dimensions which may become acceptable when new unknown symmetries of nature are found, such as noncommutative geometry for example. It is thought, however, that 16 is probably the maximum since O(16) is a maximal subgroup of E8 the largest exceptional lie group and also is more than large enough to contain the Standard Model. Quartic integrals of the nonfunctional kind are easier to solve so there is hope for the future. This is the series solution which is always convergent when a is nonzero and negative:
In the case of membranes the series would correspond to sums of various membrane interactions that are not seen in string theory.
Investigating theories of higher dimensions often involves looking at the 10 dimensional superstring theory and interpreting some of the more obscure results in terms of compactified dimensions. For example Dbranes are seen as compactified membranes from 11D MTheory. Theories of higher dimensions such as 12D Ftheory and beyond will produce other effects such as gauge terms higher than U(1). The components of the extra vector fields (A) in the Dbrane actions can be thought of as extra coordinates (X) in disguise. However, the known symmetries including supersymmetry currently restrict the spinors to have 32components which limits the number of dimensions to 11 (or 12 if you include two time dimensions.) Some commentators (e.g. John Baez et al.) have speculated that the exceptional lie groups E_{6}, E_{7} and E_{8} having maximum orthogonal subgroups O(10), O(12) and O(16) may be related to theories in 10, 12 and 16 dimensions; 10 dimensions corresponding to string theory and the 12 and 16 dimensional theories being yet undiscovered but would be theories based on 3branes and 7branes respectively. However this is a minority view within the string community. Since E_{7} is some sense F_{4} quaternified and E_{8} is F_{4} octonified, then the 12 and 16 dimensional theories, if they did exist, may involve the noncommutative geometry based on the quaternions and octonions respectively. From the above discussion it can be seen that physicists have many ideas for to extend superstring theory beyond the current 10 dimensional theory but so far none have been successful.
Since strings can have an infinite number of modes, the symmetry used to describe string theory is based on infinite dimensional Lie algebras. Some KacMoody algebras that have been considered as symmetries for MTheory have been E_{10} and E_{11} and their supersymmetric extensions.
