String Theory is a mathematical theory that was discovered by Gabriele Veneziano in the 1960s. He was a member of the European Organization of Particle Physics Research. It is now known as the Bosonic String Theory.
String theory became popular in the 1980s. It attempts to improve quantum physics by not relying on objects as points, but treating them as if they are string like particles. String theory uses one dimensional strings instead of particles. The strings use the Planck length (10^-35 meters), so they are very small. It has been predicted, however, that the strings could be as large as one millimeter in length.
String Theory formulas use more than the four dimensions we live in (length, width, depth, and time). Most require about ten dimensions. A dimension is “A property of space; extension in a given direction”. Evidently, this means that mathematically the dimensions are infinite. Time must be a line perpendicular to the first three dimensions that is measured using the usual seconds, hours, weeks, months, years, etc.
It is hopeful that string theory will result in a “theory of everything”. There are several variants of string theory, including the Bosonic String Theory, Super Symmetric String Theory, and M-Theory.
The first string theory was the Bosonic String Theory, which uses only bosons as the particles of nature. There are five types of Super Symmetric String Theory: Type 1, Type 11A, Type 11B, Heterotic Type O32 (HO) , and Heterotic Type E8xE8 (HE).
M-Theory tries to combine the five types of Super Symmetric String Theory as one theory.
Bosonic String Theory is unstable. The universe has stable matter made from fermions that satisfy the Pauli Exclusion Principle. Fermions and bosons are the two types of particles present in nature. This theory uses about 26 space time dimensions. String theory theorists found that using about ten space time dimensions and a super symmetric string theory with an equal number of bosons and fermions could be a stable theory.
There are three types of strings–open, closed, and Heterotic. Open strings are one dimensional structures in the shape of segments that have two endpoints, but can move flexibly. Closed strings are one dimensional structures that do not have endpoints, so must be circular in shape. Closed strings are the traditional type of strings. Heterotic strings combine bosons and super strings.
Super Symmetric String Theory uses about nine space dimensions and one time dimension. Type 1 uses both open and closed strings. In Type 11A Theory, the clockwise and counterclockwise vibrations of the strings require complex mathematics. Type 11B Theory has the clockwise and counterclockwise vibrations of the strings the same. The Heterotic String Theory of the Super Symmetric String Theory combines boson strings that use about 26 dimensions with Super Symmetric Theory, which only uses about 10 dimensions. This is accomplised by condensing the extra sixteen dimensions in a circular shape, which resulted in two shapes. The two shapes generated the two theories Heterotic HO and Heterotic HE.
In 1995 a new super string theory named M-Theory developed. It was introduced in a lecture at the University of Southern California by Professor Edward Witten. It combines the five theories of Super Symmetric Theory and adds supergravity. It uses about ten space dimensions and one time dimension. The extra dimension is used to combine all five string theories into one string theory. Some of the concepts that have evolved since the lecture include vibrating two dimensional membranes and three dimensional structures.
There are four known dimensions–length, width, depth, and time. One of the first to come up with the idea of an extra dimension was the Polish mathematician Theodor Kaluza in 1919. With the aid of the Swedish mathematician Oskar Klein in the 1920s, he claimed that there are more dimensions in tightly curled structures within the first three dimensions (length, width, and depth). He also said the new dimensions are very minute in size. The Calabi-Yau shape created by Eugenio Calabi and Shing-Tung Yau might envelop six of these dimensions. Adding the known four dimensions with the six dimensions in the Calabi-Yau structure could be enough for the Super Symmetric String Theory.