  How arestring theories related?  A new picture of string
theory      At one time, string theorists believed there were
five distinct superstring theories: type I, types IIA and IIB,
and heterotic SO(32) and E8XE8 string theories. The thinking was
that out of these five candidate theories, only one was the
actual correct Theory of Everything, and that theory was the
theory whose low energy limit, with ten dimensions spacetime
compactified down to four, matched the physics observed in our
world today. The other theories would be nothing more than
rejected string theories, mathematical constructs not blessed by
Nature with existence.     But now it is known that this naive
picture was wrong, and that the the five superstring theories
are connected to one another as if they are each a special case
of some more fundamental theory, of which there is only one. In
the mid-nineties it was learned that superstring theories are
related by duality transformations known as T duality and S
duality. These dualities link t