String (physics)

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String theory v • d • e
bosonic string theory superstring theory type I string type II string heterotic string M-theory (simplified) string field theory strings branes
Related topics
quantum field theory gauge theory conformal field theory topological field theory supersymmetry supergravity general relativity quantum gravity
See also
string theory topics

A string is one of the main objects of study in a branch of theoretical physics called string theory. There are different string theories, the most recent version being M-theory. A string is an object with a one-dimensional spatial extent, unlike an elementary particle which is zero-dimensional.

The characteristic length scale of strings may be estimated by the Planck length of quantum gravity:

$\ell_P =\sqrt{\frac{\hbar G}{c^3}} \cong 1.616 24 (12) \times 10^{-35}$ m

On much larger length scales, such as the scales visible in physics laboratories, such objects would look very similar to zero-dimensional point particles. According to string theory, the various elementary particles in the standard model of quantum field theory correspond to different vibrational modes of tiny fundamental strings.

Propagating in spacetime, strings sweep out a two-dimensional surface, called a worldsheet, analogous to the one-dimensional worldline traced out by a point particle.

1 Types of strings
- 1.1 Closed and open strings
- 1.2 Orientation
2 See also
3 References

[edit] Types of strings

[edit] Closed and open strings

Strings can be either open or closed. A closed string is a string that has no end-points, and therefore is topologically equivalent to a circle. An open string, on the other hand, has two end-points and is topologically equivalent to a line interval.

Not all string theories contain both open and closed strings. However, any theory which contains open strings must also contain closed strings as interactions between open strings can always result in closed strings.

The oldest superstring theory containing open strings was type I string theory. However, the developments in string theory in the 1990s have shown that the open strings should always be thought of as ending on a new type of objects called the D-branes, and the spectrum of possibilities for open strings has increased greatly.

Open and closed strings both have their own characteristic vibrational patterns. One of the vibration modes of a closed string can be identified as the graviton. In certain string theories the lowest-energy vibration of an open string is a tachyon and can undergo tachyon condensation. One of the other vibration modes of an open string exhibits properties similar to a photon, and so on.

[edit] Orientation

Strings can also possess an orientation, which can be thought of as an internal "arrow" which distinguishes the string from one with the opposite orientation. By contrast, an unoriented string is one with no such arrow on it.