**M-Theory**, developed by Edward Witten of the Institute for Advanced Study in Princeton, N.J., unites five different superstring theories, by placing them in the same arena, like 5 islands in a large sea. It is one of the leading candidates for what has been colloquially called the theory of everything; however it has yet to produce testable predictions. Dr. Witten is quoted as saying, "String theory is 21st century physics which fell into the 20th century by accident." The theory makes heavy use of the principle of "duality" (detailed below). Two theories are "dual" to each other if they can be shown to be equivalent under a certain interchange. The five superstring theories enclosed by M-Theory are:

**type I**, **type IIA**, **type IIB**, **E8 X E8 heterotic** ( or **HE**t), and **SO(32) heterotic** (or **HO**t).

- The
**type II**theories have two supersymmetries in the ten-dimensional sense, the rest just one. - The
**type I**theory is special in that it is based on unoriented open and closed strings. - The other four are based on oriented closed strings.
- The
**IIA**theory is special because it is non-chiral (parity conserving). - The other four are chiral (parity violating).

In each of these cases there is an 11th dimension that becomes large at* strong coupling*. In the** IIA** case the 11th dimension is a circle. In the **HE** case it is a line interval , which makes eleven-dimensional space-time display two ten-dimensional boundaries. The strong coupling limit of either theory produces an 11-dimensional space-time. This eleven-dimensional description of the underlying theory is called** "M- theory"**. A string's space-time history can be viewed mathematically by functions like

**X**^{μ}(σ,τ)

that describe how the string's two-dimensional sheet coordinates **(σ,τ)** map into space-time **X ^{μ}**

There are other functions on the two-dimensional sheet that describe other degrees of freedom, for instance those associated with supersymmetry? and gauge symmetries.
Classical string theory dynamics are denoted by an invariance that conforms with 2D quantum field theory. This *conformal invariance* is symmetry under a change of length scale. One-dimensional strings differ from higher dimensional analogs due to the fact that the 2D theory is renormalizable (contains no glitches of short-distance infinities).

Objects with **p** dimensions, i.e, "**p-branes**," have a **(p+1)-dimensional world volume theory**.
For **p > 1**, these theories are non-renormalizable. This feature gives strings a special status, even though higher dimensional p-branes do occur in superstring theory.

Insight into* non-perturbative* properties of superstring theory apparently stems from the study of a special class of p-branes called Dirichlet p-branes(** D-branes**). This name results from the boundary conditions assigned to the ends of open strings. Normal open strings of the** type I** theory satisfy the *Neumann boundary condition* which states " no momentum flows on or of the end of a string." On the other hand, **T duality** infers the existence of dual open strings with specified positions known as Dirichlet boundary conditions in the dimensions that are T-transformed. Generally, in **type II** theories, we can imagine open strings with specific positions for the end-points in some of the dimensions. This lends an inference that they must end on a preferred surface. Superficially, this notion seems to break the *relativistic invariance* of the theory, possibly paradoxical. The resolution here of this paradox is that strings end on a **p-dimensional** dynamic object, the D-brane. D-branes have been studied for a number of years, their significance explained by Polchinski just recently. Why are we mentioning them ?

The importance of D-branes stems from the fact that they make it possible to study the excitations of the brane using the renormalizable 2D quantum field theory of the open string instead of the non-renormalizable world-volume theory of the D-brane itself. In this way it becomes possible to compute non-perturbative phenomena using perturbative methods. Many of the previously identified p-branes are D-branes ! Others are related to D-branes by duality symmetries, so that they can also be brought under mathematical control. D-branes have found many useful applications, the most remarkable being the study of black holes. *Strominger and Vafa have shown that D-brane techniques can be used to count the quantum microstates associated to classical black hole configurations. *The simplest case first explored was **static extremal charged black holes** in five dimensions. Strominger and Vafa proved for large values of the charges the entropy ** S = log N,** where N is equal to the number of quantum states that system can be in, agrees with the [[**Bekenstein-Hawking**]] prediction (1/4 the area of the event horizon).
This result has been generalized to black holes in 4D as well as to ones that are near extremal (and radiate correctly) or rotating, a remarkable advance. It has not yet been proven that there is any problematic breakdown of quantum mechanics due to black holes.

M-theory Simplified for a non-technical easier explanation.

- Michael J. Duff,
*The Theory Formerly Known as Strings*, Scientific American, February 1998, online at http://www.sciam.com/1998/0298issue/0298duff.html