The standard model was designed within a framework known as Quantum Field Theory (QFT), which gives us the tools to build theories consistent both with quantum mechanics and the special theory of relativity. With these tools, theories were built which describe with great success three of the four known interactions in Nature: Electromagnetism, and the Strong and Weak nuclear forces. Furthermore, a very successful unification between Electromagnetism and the Weak force was achieved (Electroweak Theory), and promising ideas put forward to try to include the Strong force. But unfortunately the fourth interaction, gravity, beautifully described by Einstein's General Relativity (GR), does not seem to fit into this scheme. Whenever one tries to apply the rules of QFT to GR one gets results which make no sense. For instance, the force between two gravitons (the particles that mediate gravitational interactions), becomes infinite and we do not know how to get rid of these infinities to get physically sensible results.

One mode of vibration, or `note', makes the string appear as an
electron, another as a photon. There is even a mode describing the
graviton, the particle carrying the force of gravity, which is an
important reason why String Theory has received so much attention. The
point is that we can make sense of the interaction of two gravitons in
String theory in a way we could not in QFT. There are no infinities!
And gravity is not something we put in by hand. It *has* to be
there in a theory of strings. So, the first great achievement of
String Theory was to give a consistent theory of quantum gravity,
which resembles GR at macroscopic distances. Moreover String Theory
also possesses the necessary degrees of freedom to describe the other
interactions! At this point a great hope was created that String
Theory would be able to unify all the known forces and particles
together into a single `Theory of Everything'.

By introducing *Supersymmetry* to Bosonic String Theory,
we can obtain a new theory that describes both the forces and the
matter which make up the Universe. This is the theory of
*superstrings*. There are three different superstring theories
which make sense, i.e. display no mathematical inconsistencies. In two
of them the fundamental object is a closed string, while in the third,
open strings are the building blocks. Furthermore, mixing the best
features of the bosonic string and the superstring, we can create two
other consistent theories of strings, Heterotic String Theories.

However, this abundance of theories of strings was a puzzle: If we are searching for the theory of everything, to have five of them is an embarrassment of riches! Fortunately, M-theory came to save us.

For simplicity, it is usually assumed that the extra dimensions are wrapped up on six circles. For realistic results they are treated as being wrapped up on mathematical elaborations known as Calabi-Yau Manifolds and Orbifolds.

There is still a third possibility for the M in M-theory. One of the islands that was found on the M-theory planet corresponds to a theory that lives not in 10 but in 11 dimensions. This seems to be telling us that M-theory should be viewed as an 11 dimensional theory that looks 10 dimensional at some points in its space of parameters. Such a theory could have as a fundamental object a Membrane, as opposed to a string. Like a drinking straw seen at a distance, the membranes would look like strings when we curl the 11th dimension into a small circle.

Many other problems are still open, but the application of string theory to the study of Black Holes promises to be one of the most interesting topics for the next few years.

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