Five major string theories
These "strings" vibrate in multiple dimensions, and depending on how
they vibrate, they might be seen in 3-dimensional space as matter,
light, or gravity. It´s the vibration of the string which determines
whether it appears to be matter or energy, and every form of matter or
energy is the result of the vibration of strings.
String theory then ran
into a problem. Another version of the equations was discovered. Then
another, and then another. Eventually, there were five major string
theories, all based on a 10-dimensional universe, and all of them
appeared to be correct.
That was a problem, because science can't have five seemingly
contradictory sets of equations to describe the same thing. In the mid
90s, a string theorist named Edward Witten of the Institute for Advanced
Study, and other important researchers, considered that the five
different versions of string theory might be describing the same thing
seen from different perspectives.
They proposed a unifying theory called
"M-Theory", in which the "M" is not specifically defined, but is
generally understood to stand for "membrane." M-Theory brought all of
the string theories together. It did this by asserting that strings are
really 1-dimensional slices of a 2-dimensional membrane vibrating in
In other words, string theories are theories that use one dimensional objects (strings) as the basic building block of the universe including matter. M-Theory
is a theory that unifies five different versions of the string theories
and super gravity together, all under one single framework.