Strings

Strings are unlike the zero-dimensional point like constituents of particle physics. They are actually 1-dimensional extended objects. This has caused string theory to be an area of active interest, as sometimes, nature prefers to have these kinds of constituents. Most importantly, a theory of strings, following the rules of quantum mechanics, could describe quantum gravity. 

Strings may be open or closed. If they are open, they form a line segment with two end points. If they are closed, they form a loop shape. These strings may have other special properties. String theories of particle physics predict that these strings are very small, much smaller than can be observed today by high even the most high energy of experiments. This length scale or scale of string is considered to be the Planck length or 10^-35 meters. This is the scale where the effects of quantum gravity are believed to become significant. Thus, on larger length scales, the strings would be observed to be their corresponding subatomic particle. The vibrational state of the string would determine which particle this is. The strings appear as zero-dimensional points in physics laboratories. Strings vibrate as harmonic oscillators. As I mentioned previously, different harmonics would correspond to different subatomic particles. These various string vibrations can constitute the members of the Standard Model of particle physics. 

Strings, as they propagate through spacetime, do not sweep out worldlines, as point particles do. However, the higher dimensional analogue to the worldline is the worldsheet. The world sheet is actually a 2-dimensional surface. Thus, string physics can be described by a 2-dimensional conformal field theory or CFT, as they are also known as. This 2-dimensional conformal field theory is associated with the worldsheet. 

Not all string theories contain open strings. All string theories do, however, contain closed strings. This is because interactions between open strings can always result in closed strings. 

The type I theory was the oldest superstring theory to contain open strings. However, developments in the 1990s have shown that the end points of open strings are actually required to lay on constructions known as D-branes. 

Open and closed strings are often identified with certain vibrational modes as corresponding to particular particles. For example, one of the vibrational states of the closed string corresponds to the graviton. In some string theories, the lowest energy vibration of an open string corresponds to a tachyon. This tachyon can undergo tachyon condensation. The photon and gluon are also examples of open string vibrations.

Strings can also possess an orientation. This is an internal arrow. It distinguishes a string from the opposite orientation. An unoriented string has no such arrow in it.