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How many string theories are there?
. There are several ways theorists can build string theories. Start with the elementary ingredient: a wiggling tiny string. Next decide: should it be an open string or a closed string? Then ask: will I settle for only bosons ( particles that transmit forces) or will I ask for fermions, too (particles that make up matter)? (Remember that in string theory, a particle is like a note played on the string.)
. If the answer to the last question is "Bosons only, please!" then one gets bosonic string theory. If the answer is "No, I demand that matter exist!" then we wind up needing supersymmetry, which means an equal matching between bosons (particles that transmit forces) and fermions (particles that make up matter). A supersymmetric string theory is called a superstring theory. There are five kinds of superstring theories, shown in the table below.
. The final question for making a string theory should be: can I do quantum mechanics sensibly? For bosonic strings, this question is only answered in the affirmative if the spacetime dimensions number 26. For superstrings we can whittle it down to 10. How we get down to the four spacetime dimensions we observe in our world is another story.

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A Brief Table of 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 E8 x E8
. If we ask how to get from ten spacetime dimensions to four spacetime dimensions, then the number of string theories grows, because there are so many possible ways to make six dimensions much much smaller than the other four in string theory. This process of compactification of unwanted spacetime dimensions yields interesting physics on its own.
. 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 biggest rush in string 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.
. Stay tuned to this web site, we may some day soon be changing the name to The Official M Theory Web Site!


What is theoretical physics? // Particles and relativity // Why strings? // What is string theory? // How many are there?
How are string theories related? // Is there a more fundamental theory?


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