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. 

To put it more simply:

Strings, essentially, are entities postulated to exist by string theory. Strings are different from elementary particles. Elementary particles are 0-dimensional, and strings by definition, are 1-dimensional extended objects. This is actually a property of the final or fundamental theory may have according to some theorists, that fundamental entities are extended strings rather than point particles. The most important example of this is the graviton, the hypothetical massless spin-2 force mediating tensor boson for Gravitation, which arises from a vibrational mode of a closed string. 

Strings may be open or closed in string theory. If they are open, they have two endpoints. If they are closed, they form a closed loop shape. Strings are very small in string theory and particle physics, far too small to be observed in today's particle accelerators. The length scale of the string is the order of the Planck length or 10^-35 meters. This is the scale of quantum gravity, where the effects of both general relativity and quantum mechanics become significant. This is why on larger length scales, such as observed in physics laboratories, a string appears as a 0-dimensional point particle. 

Strings, which vibrate as harmonic oscillators, and different harmonics or vibrational states will determine which particle it is deemed to be at observable distance scales. These different harmonics constitute the different particles of the Standard Model of particle physics. 

As a string propagates in and through spacetime, it sweeps out a 2-dimensional volume or a surface called a 'worldsheet.' This is a concept taken from General relativity, a world line is a 1-dimensional path traced out by a particle in Einstein's theory. Thus, string physics is described by a 2D conformal field theory, describing the physics of the string worldsheet.

I mentioned that strings may be open or closed loops in shape. A closed string, topologically speaking, is equivalent to a closed loop or circle. An open string has two end points, topologically analogous to a line interval. Only Type I superstring theory has open strings, the other four superstring theories, Type IIA, IIB, heterotic SO(32) and E 8 x E 8 all have only closed strings. Type I is the exception, allowing for both open and closed string descriptions.

Certain particles are characterized by not only their vibrational state, but also on whether the string is open or closed. For example, the graviton has been identified as a vibrational state of a closed string. While with open strings, in some theories, the lowest energy vibrational mode is the tachyon. Open strings can also exhibit properties of photons and gluons. 

Strings may also possess an orientation. This may be thought of as an internal arrow, distinguished from its opposite direction.