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.

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 E_{8} x E_{8}

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!