String physics: Encyclopedia II - String physics - Types of strings

String physics - Types of strings

String physics - 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 otherhand, 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 a lot.

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.

String physics - 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.

Adapted from the Wikipedia article "Types of strings", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki