String theory

String theory:

In string theory, it is hypothesized that the most fundamental or elementary i.e. smallest particles do not exist as individual points but instead are tiny one-dimensional strings (each of about 10-33 cm).

The particles that are observed in particle accelerators would then be the result of excitations or vibrations of these strings, rather like how musical notes result from the vibrations or excitations of strings on a musical instrument such as a guitar.

The theory arose from papers of Leonard Susskind, Yoichiro Nambu and Holger Nielsen

Not all string theories incorporate fermions but those that do require supersymmetry (in which every boson has a corresponding supersymmetric fermion). 

Closed-type string.
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Open-type string.
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String theories are of two main types: those that incorporate strings that remain closed (as loops that cannot be opened) and those that are loops that can be opened (see

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