Scientists have come up with theories in the shower, on barren
mountains, while driving to work and even in their sleep. But
what all theories have in common is that their predictions are
eventually tested in experiments, where nature determines which
inspirations are right and which are wrong. Then there is string
theory, the ambitious, profoundly mathematical attempt to knit
together all of physics -- from gravity to quantum mechanics to
subatomic forces -- into a single sublime formalism. Though
string theorists first suspected they might be onto a ''theory
of everything'' in the mid-1980's, and the field is the hottest
area in theoretical physics, string theorists have yet to devise
a make-or-break laboratory test for their ideas. In part, that
is because the theorized strings cannot be observed directly;
they are thought to be vibrating entities smaller than a
trillionth of a trillionth the size of an atom. Different string
vibrations somehow correspond to different particles in nature,
but scientists ha