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Introduction

“String theory” is a general term that has been manipulated through time and, in fact, represents numerous theories. The string theory introduced in the 1960s is certainly not as involved by the term today. Additions, such as supersymmetry and the research of noted physicists, have shaped string theory from its early beginnings.

Bosonic String Theory

The late 1960s witnessed the birth of string theory through the work of Gabriele Veneziano, a research fellow at CERN (European Organization of Particle Physics Research). This original version of string theory is now known as “bosonic string theory” and involved twenty-six spacetime dimensions. The naming of this theory is due to its use of only bosonic particles. This meant that the theory was lacking fermions and could not be a grand unification theory. More troubling than this, though, was the prediction of a “tachyon,” which was a particle whose mass was actually negative. This implication deemed the entire theory illogical.

Superstring Theory

Superstring theory or supersymmetric string theory added to the previous version of string theory by incorporating supersymmetry and realizing that bosonic patterns and fermionic patters came in pairs. That is, there was “symmetry” between the bosonic and fermionic patterns. Superstring theory caught on in 1980s; however, there was one significant problem. Superstring theory can actually be represented by five different theories. How could the one so-called theory of everything come in five different theories? The five theories - Type I theory, Type IIA theory, Type IIB theory, Heterotic type O(32), and Heterotic type E8 X E8 theory – are quite similar, though they differ in the minor intricacies of the theories. Each of these theories has ten dimensions (nine space dimensions and one time dimension)!

Type I
Type I string theory includes both open and closed strings. Additionally, the type I theory states that both orientations are equal. This theory resembles type IIB theory except for the lack of open loops in the type IIB theory.

Type IIA
In this theory, the clockwise and counterclockwise vibrations of the strings are opposite. Here, opposite refers to an idea requiring complex mathematics. Another way of describing this idea is stating that particles involved with this theory spin in different directions.

Type IIB
The opposite in nature of the type IIA theory, the type IIB string theory has clockwise and counterclockwise vibrations that are the same. Similarly, the spin of the particles is identical in this theory.

Heterotic String Theory
Heterotic string theory is very interesting because it combines bosonic strings, which need twenty-six dimensions, and supersymmetry, which involves ten dimensions. This combination results due to the vibrational patterns. Counterclockwise vibrational patters occupy twenty-six dimensions, while clockwise vibrational patterns occupy ten dimensions. This means that the additional sixteen dimensions are somehow condensed into a circular shape. Because there are two shapes that this circular structure can take, heterotic type O(32) [HO] and heterotic type E8 X E8 [HE] theory emerged to account for these two possibilities.

M-Theory

The “M” in “M-theory” is not known to stand for anything certainly, though many venture guesses.

M-Theory was developed as a result of the “second superstring revolution” in 1995. First introduced by Professor Edward Witten in a stunning lecture at the University of Southern California, M-theory unites the five different superstring theories and supergravity into one single theory. M-theory demands eleven dimensions (ten space dimensions and one time dimension) , and this added dimension that was previously overlooked allows for the combination of all of the five theories. In addition, M-theory involves a completely new host of concepts that captivate the mind. Features, such as vibrating two-dimensional membranes, “three-branes” (three-dimensional structures), and a myriad of other complex ideas, have surfaced after Professor Witten’s use of dualities with string theory. Dualities, which are theoretical models that can show the similarity between seemingly dissimilar concepts in physics, were used by Professor Witten to show the same underlying theory. M-theory is still relatively underdeveloped and requires more research. Part of the intrigue of M-theory comes from the name itself. The “M” in “M-theory” is not known to stand for anything certainly, though many venture guesses (“mystery,” “membrane,” etc…).

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