M-THEORY - 1
As we have already mentioned, this system refers to a still mysterious theory, and hence the various interpretations on the meaning of M, in which the various String Theories would not be in contrast with each other, but they would simply cover a specific aspect of it. These theories are:
Bosonic String Theory: the initial version of the String Theory developed in the 1960s and then supplanted by supersymmetry and superstrings. It just predicts the existence of bosons and not fermions, therefore only of forces and not of matter, and the presence of hypothetical particles, the tachyons, which would travel faster than light, something impossible according to current physics. It also has other inconsistencies that are resolved with a spacetime in 26 dimensions, 25 spatial. However, the 22 extra dimensions can be rolled up into a toroidal structure leaving only those perceived by us
Type I String Theory: Type I is the most peculiar theory among those existing, and therefore the most difficult one to link with the others. It was the first theory on superstrings and sustains that photons are included in the excitations of open strings, while gravitons in those of closed strings. The model, being in ten dimensions, can exclude the tachyons
Type II String Theory: this superstring theory is divided into two:
A: there are only closed strings, no tachyons, fermions are massless and spin in both directions, parity is conserved, and are non-chiral (a chiral object is an object that cannot be superimposed on its mirror image)
B: there are always only closed strings, no tachyons, fermions are massless but have spin only in one direction and therefore parity is violated (they are chiral)
Heterotic String Theory (HO): as we have already mentioned, the heterotic string is a hybrid between a bosonic string and a superstring. They are closed strings: vibrations moving counter-clockwise can be treated as bosonic ones, while those going in a clockwise direction as ten-dimensional superstrings. There are no tachyons
Heterotic Strings Theory (HE): it maintains the same characteristics of the HO but has a different kind of symmetry
Overview of theories that are part of M-Theory
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