Theoritical Basic Properties of Superstrings According to Wikipedia:
Basic properties
String theory is formulated in terms of an
action principle, either the
Nambu-Goto action or the
Polyakov action, which describes how strings move through space and time. Like springs with no external force applied, the strings tend to shrink, thus minimizing their potential energy, but
conservation of energy prevents them from disappearing, and instead they oscillate. By applying the ideas of
quantum mechanics to strings it is possible to deduce the different vibrational modes of strings, and that each vibrational state appears to be a different particle. The mass of each particle, and the fashion with which it can interact, are determined by the way the string vibrates — the string can vibrate in many different modes, just like a guitar string can produce different notes. The different modes, each corresponding to a different kind of particle, make up the "
spectrum" of the theory.
Strings can split and combine, which would appear as particles emitting and absorbing other particles, presumably giving rise to the known interactions between particles.
String theory includes both
open strings, which have two distinct endpoints, and
closed strings, where the endpoints are joined to make a complete loop. The two
types of string behave in slightly different ways, yielding two different spectra. For example, in most string theories, one of the closed string modes is the
graviton, and one of the open string modes is the
photon. Because the two ends of an open string can always meet and connect, forming a closed string, there are no string theories without closed strings.
The earliest string model — the
bosonic string, which incorporated only
bosons, describes — in low enough energies — a quantum
gravity theory, which also includes (if open strings are incorporated as well)
gauge fields such as the photon (or, more generally, any
gauge theory). However, this model has problems. Most importantly, the theory has a fundamental instability, believed to result in the decay (at least partially) of space-time itself. Additionally, as the name implies, the spectrum of particles contains only bosons, particles which, like the photon, obey particular rules of behavior. Roughly speaking, bosons are the constituents of radiation, but not of matter, which is made of fermions. Investigating how a string theory may include
fermions in its spectrum led to the invention of
supersymmetry, a mathematical relation between bosons and fermions. String theories which include fermionic vibrations are now known as
superstring theories; several different kinds have been described, but all are now thought to be different limits of
M-theory.
While understanding the details of string and superstring theories requires considerable mathematical sophistication, some qualitative properties of quantum strings can be understood in a fairly intuitive fashion. For example, quantum strings have tension, much like regular strings made of
twine; this tension is considered a fundamental parameter of the theory. The tension of a quantum string is closely related to its size. Consider a closed loop of string, left to move through space without external forces. Its tension will tend to contract it into a smaller and smaller loop. Classical intuition suggests that it might shrink to a single point, but this would violate
Heisenberg's
uncertainty principle. The characteristic size of the string loop will be a balance between the tension force, acting to make it small, and the uncertainty effect, which keeps it "stretched". Consequently, the minimum size of a string is related to the string tension.
Linear Mode
