 |
| 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 E8 x E8 |
If we ask how to get from ten space-time dimensions to four space-time dimensions, then the number of string theories grows, because there are so many possible ways to make six dimensions much smaller than the other four in string theory. However, the way that these other dimensions shrink is not just arbitrary. This process of compactification of unwanted space-time dimensions yields interesting physics on its own. It leads to important properties of the particles, such as mass, electric charge, and "color" charge (which will be seen later).
But the number of string theories has also been shrinking in recent years, because string theorists are discovering that what they thought were completely different theories were in fact different ways of looking at the same theory! This period in string history has been given the name the second string revolution.
And now the big rush in string theory research is to collapse the table above into one theory, which some people want to call M theory, for it is the Mother of all theories.
For more on-line information about string theories, you may enjoy visiting Professor John Schwarz's site at Cal Tech! Or, if you are audio inclined, here is a public lecture on the history and origins of string theory by Professor Ed Witten of Princeton.
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