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Attempts were made, starting in 1971 by Pierre Ramond, than later by John H. Schwarz and Andrei Neveu, to incorporate fermionic patterns of vibration into string theory. Surprisingly, what was found is that these fermionic and bosonic patterns of string vibration actually came in pairs. I mentioned that there were problems with the original version of string theory: called bosonic string theory.
Pierre Ramond
Andrei Neveu and John H. Schwarz (2002)
The idea that these three theorists proposed was one that would include particles with spin. These Neveu-Schwarz-Ramond strings eventually became the superstring theory.
Nevertheless, the work of Raymond, Schwarz and Neveu will be put into its proper light in 1977, by Ferdinand Gliozzi, Joel Scherk and David Olive.
Ferdinand Gliozzi
David Olive
Their insight was that string theory, included supersymmetry, taking on the name: superstring theory. This superstring theory avoided the problem of the tachyon, and could account for both patterns of string vibration: bosonic and fermionic.
The idea behind string theory is that all of the point particles of the Standard Model, can be modeled as different vibrational states of a 1-dimensional string. The way the string vibrates will determine which subatomic particle it will be observed to be on larger distance scales. One mode of the string's vibration, corresponds to the graviton (no mass and a spin of 2). For this reason, string theory, is a proposed theory of quantum gravity, the field of theoretical physics that seeks to reconcile general relativity with quantum mechanics. The hope is that string theory can resolve this incompatibility. Plus, the scale of the string, will be the scale of quantum gravity: the Planck length or 10^-35 meters. At this scale, the force of gravity can be unified with the other fundamental interactions: electromagnetism and the strong and weak nuclear forces.
The length scale of the string is the Planck length. The Planck length is 10^-35 meters. This is where the quantum effects of gravitation become manifest. This scale could be the quantization of space that makes up the fabric of the universe. This is also believed the smallest meaningful length. Below the Planck length, the notions of space and time cease to exist. In fact, high energy experiments, seeking to probe scales smaller than this, would produce a black hole.
John Wheeler, in 1955, coined the term "quantum foam" to describe this scale.
The problem was that supersymmetry could be incorporated into string theory in five distinct ways. These were the five ten-dimensional versions of superstring theory that existed in the 1980s: type I, type IIA, type IIB, heterotic E 8 x E 8 and heterotic SO(32).
Superstring theory came about in the 1980s, to attempt to solve the issue of why string theory only has bosons and not fermions. Supersymmetry, was incorporated into string theory in the 1980s, to include these fermionic vibrations. Supersymmetry is a principle that a theory might have. It is a transformation, or, a relationship between these two families of particles. Each boson will have a superpartner fermion and vice-versa. Superstring theory also removed the presence of the tachyon. The theory came about in the 1980s and is 10 dimensional and there were 5 different versions:
Type I
Has both open and closed strings. This is the only version of superstring theory that has open strings. It is also the only theory where strings are unorientated or both orientations are equivalent. Type I string theory involves both open and closed strings.
Type IIA
Has only oriented closed strings. Type IIA superstring theory is non-chiral or parity conserving. This means that massless fermions spin both ways. Type IIA string theory involves closed strings with left-right symmetric vibrational patterns.
Type IIB
Has only oriented closed strings. Type IIB superstring theory is chiral or is not parity conserving. This means that massless fermions can only spin one way. Type IIB string theory involves closed strings with left-right asymmetric vibrational patterns.
Heterotic SO (32) and Heterotic E 8 x E 8
Has only closed strings. Heterotic strings are a kind of hybrid of a bosonic string and a type I superstring, where the right moving and left moving strings differ. The two kinds depend on their two different gauge groups: SO(32) and E 8 x E 8. Heterotic SO(32) string theory and Heterotic E 8 x E 8 string theory involve closed strings. These strings’ right moving vibrations resemble the pattern of the type II string. The left moving vibrations of the heterotic strings resemble the pattern of the bosonic string. The two flavors of heterotic string theory only differ in subtle ways.
These are the 5 versions of superstring theory that emerged in the 1980s, that are formulated in 10 dimensions.
Type I
Type IIA
Type IIB
Heterotic SO (32)
Heterotic E 8 x E 8
The difference between the theories are not all that important. The difference is in some minor mathematical details, such as if open strings are allowed, or only closed strings. At any rate, it was an embarrassment for string theorists that there were 5 different theories, as only one, if any, could be a correct description of nature. We live in one universe, not five. Thus, we need one theory, not five theories. At any rate, these theories share some basic features. The vibrational state of the strings determines the mass and charges of these particles. The theories must also be formulated in ten spacetime dimensions. Their extra dimensions must also take on the geometry of the Calabi-yau shape. A Calabi-yau shape is a geometry into which the extra dimensions required by string theory can be curled up, while maintaining consistency with the equations of string theory.
String theory says that all of the particles (quarks, electrons, ect.) are all different harmonics of a 1-dimensional vibrating filament of energy: a string.
String theory, also, since it is a theory of everything, can in one consistent theoretical framework, describe gravity, electromagnetism, the strong and weak nuclear forces.
However, the price for this unification of general relativity, quantum mechanics, and the laws of nature, is that there are extra dimensions, that may be curled up to the size of the Planck length (10^-35 meters) on the geometry of the Calabi-yau manifold, or, our universe could just a 4-dimensional subset, confined on an object called a brane, emdedded in some higher dimensional bulk, or hyperspace.
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