Instring theory, aheterotic string is a closed string (or loop) which is a hybrid ('heterotic') of asuperstring and abosonic string. There are two kinds of heterotic superstring theories, the heterotic SO(32) and the heterotic E8 × E8, abbreviated toHO andHE. Apart from that there exist seven more heterotic string theories which are notsupersymmetric and hence are only of secondary importance in most applications.[1] Heterotic string theory was first developed in 1985 byDavid Gross,Jeffrey Harvey,Emil Martinec, andRyan Rohm[2] (the so-called "Princeton string quartet"[3]), in one of the key papers that fueled thefirst superstring revolution.
Instring theory, the left-moving and the right-moving excitations of strings are completely decoupled for a closed string,[4] and it is possible to construct a string theory whose left-moving (counter-clockwise) excitations are treated as a bosonic string propagating inD = 26 dimensions, while the right-moving (clockwise) excitations are treated as a superstring inD = 10 dimensions.
The mismatched 16 dimensions must be compactified on an even,self-dual lattice (adiscrete subgroup of a linear space). There are two possible even self-dual lattices in 16 dimensions, and it leads to two types of the heterotic string. They differ by thegauge group in 10 dimensions. One gauge group isSO(32) (the HO string) while the other isE8 × E8 (the HE string).[5]
These two gauge groups also turned out to be the only twoanomaly-free gauge groups that can be coupled to theN = 1 supergravity in 10 dimensions. (Although not realized for quite some time, U(1)496 and E8 × U(1)248 are anomalous.[6])
Every heterotic string must be aclosed string, not anopen string; it is not possible to define anyboundary conditions that would relate the left-moving and the right-moving excitations because they have a different character.
String duality is a class of symmetries in physics that link different string theories. In the 1990s, it was realized that the strong coupling limit of the HO theory istype I string theory — a theory that also containsopen strings; this relation is calledS-duality. The HO and HE theories are also related byT-duality.
Because the various superstring theories were shown to be related by dualities, it was proposed that each type of string was a different limit of a single underlying theory calledM-theory.