String theory is a mathematical theory that tries to explain certain phenomena which is not currently explainable under the standard model of quantum physics.

### The Basics of String Theory

At its core, string theory uses a model of one-dimensional strings in place of the particles of quantum physics. These strings, the size of the *Planck length* (10^{-35} m), vibrate at specific resonant frequencies. Some recent versions of string theory have predicted that the strings could have a longer length, up to nearly a millimeter in size, which would mean they're in the realm that experiments could detect them. The formulas that result from string theory predict more than four dimensions (10 or 11 in the most common variants, though one version requires 26 dimensions), but the extra dimensions are "curled up" within the Planck length.

In addition to the strings, string theory contains another type of fundamental object called a brane, which can have many more dimensions. In some "braneworld scenarios," our universe is actually "stuck" inside of a 3-dimensional brane (called a 3-brane).

String theory was initially developed in the 1970s in an attempt to explain some inconsistencies with the energy behavior of hadrons and other fundamental particles of physics.

As with much of quantum physics, the mathematics that apply to string theory cannot be uniquely solved. Physicists must apply perturbation theory to obtain a series of approximated solutions. Such solutions, of course, include assumptions which may or may not be true.

The driving hope behind this work is that it will result in a "theory of everything," including a solution to the problem of quantum gravity, and to reconcile quantum physics with general relativity, thus reconciling the fundamental forces of physics.

### Variants of String Theory

The original string theory focused only on boson particles.

Superstring theory (short for "supersymmetric string theory") incorporates bosons with another particle, fermions, as well as supersymmetry to model gravity. There are five independent superstring theories:

- Type 1
- Type IIA
- Type IIB
- Type HO
- Type HE

**M-Theory**: A superstring theory, proposed in 1995, which attempts to consolidate the Type I, Type IIA, Type IIB, Type HO, and Type HE models as variants of the same fundamental physical model.

One consequence of the research in string theory is the realization that there is an immense number of possible theories that could be constructed, leading some to question whether this approach will ever actually develop the "theory of everything" that many researchers originally hoped. Instead, many researchers have adopted a view that they are describing a vast string theory landscape of possible theoretical structures, many of which do not actually describe our universe.

### Research in String Theory

At present, string theory has not successfully made any prediction which is not also explained through an alternative theory. It is neither specifically proven nor falsified, though it has mathematical features which give it great appeal to many physicists.

A number of proposed experiments might have the possibility of displaying "string effects." The energy required for many such experiments is not currently obtainable, although some are in the realm of possibility in the near future, such as possible observations from black holes.

Only time will tell if string theory will be able to take a dominant place in science, beyond inspiring the hearts and minds of many physicists.