In general, most people have an idea what symmetry is. However, physics has different standards of symmetry than the ordinary sense of the word. Brian Greene's The Elegant Universe defines symmetry as "A property of a physical system that does not change when the system is transformed in some manner. For example, a sphere is rotationally symmetrical since its appearance does not change if it is rotated." Nature is considered by physicists to have a number of symmetries; for example, the equivalence principle of general relativity, which states that physical laws are the same for all frames of reference, even those in accelerated motion. Nature also possesses rotational symmetry, which states that the laws of physics are the same for all orientations. Furthermore, nature observes gauge symmetries, or symmetries stating that the laws of physics remain the same despite changes in the states of particles that determine their responsiveness to the fundamental forces. However, the gauge symmetries have no direct connection to spacetime. Within this restriction, Sidney Coleman and Jeffrey Mandula proved in 1967 that no more basic symmetries existed. However, they overlooked a particle property called spin.
Based on curious experiments involving light absorbed and emitted by atoms, it was determined that electrons' magnetic properties arise, like all magnetism, from the motion of electric charge. This implied that electrons rotate. However, their rotation is (surprise) a little different from the rotation we are used to - it is a constant intrinsic property of electrons, just as other rotational states are intrinsic properties of other particles. This property is called spin and it plays an important part in the symmetry missed by Coleman and Mandula.
More specifically, physicists found that all matter particles and their antimatter partners have spin-1/2 (defined as meaning the angular momentum of the particle is /2). Messenger particles, those that transmit forces, generally have spin 1 - this applies to photons, gluons, and both types of weak-gauge bosons. However, the graviton is a special case - it has spin-2. In should be noted that, as with all particle properties under string theory, spin arises from a string's vibrational patterns.
By 1971, after decades of research involving far too many dedicated physicists to be named here, an answer emerged. There was one symmetry beyond those stated by Coleman and Mandula that was mathematically possible: supersymmetry.
Physicists calculated that, if the universe obeys supersymmetry, each particle should come with a partner called a superpartner whose spin differs by half a unit. At first, physicists thought this meant that the mass and force particles had been connected - but this thought was incorrect. In reality, if the universe obeys supersymmetry, each particle has another undiscovered superpartner whose spin is 1/2 unit less than its own. An interesting nomenclature has arisen from this; the theorized superpartner of the electron is the selectron (supersymmetric electron), the superpartner of the neutrino is the sneutrino and that of the quark, the squark. Since these are all mass particles, their superpartners should all have spin-0. For the force particles, the superpartners should have spin-1/2. The photon's superpartner is the photino; the gluon, the gluino; and for W and Z bosons, the wino and zino. This plethora of particles led many to dismiss supersymmetry as being highly unlikely, since none of these particles had been discovered.
Even before string theory, there were many convincing arguments in support of supersymmetry, despite its apparent excesses. First, there was the aesthetic viewpoint: is it possible that nature would observe all but one of the mathematically possible symmetries available? Second, there was the evidence provided by the standard model. Some problematic details are swiftly dealt with if the theory is formulated supersymmetrically. (Click here for details.)
The third and perhaps the strongest pre-string argument for supersymmetry was grand unification, or the merging of the nongravitational forces into one at high enough temperatures. The electromagnetic and weak forces were shown to unify into the electroweak force at 1015 Kelvin, a temperature reached in the first moments of the Big Bang. The string force was then shown to merge with the electroweak force at around 1028 Kelvin. (This is the equivalent of about 4 orders of magnitude less than the Planck mass.)
These mergers are explained in quantum terms by the fact that forces are affected, like everything else, by quantum fluctuations. From far away, the electromagnetic field is "shrouded" by the haze of particle/antiparticle interactions that constitute quantum fluctuations. As one approaches the source of an electromagnetic field - say an electron - more of its strength is revealed through the shroud. (This effect is distinguished from ordinary increase of force strength with decreasing distance by the term "increasing intrinsic strength.") Conversely, the strong and weak forces are actually amplified by the quantum shroud; their strengths decrease with decreasing distance. Thus the force strengths are shown to actually become equal at sufficiently short distances (10-29 cm). The high energies required to probe such small distances explain the merger based on temperature.
Later refinements of experimental data, however, showed that without supersymmetry the strengths of the nongravitational forces almost, but do not quite, meet. Reformulated with supersymmetry, the three forces' strengths unify. Although these reasons are not "set in stone," they provided a powerful stimulus to study the implications of supersymmetry. String theorists did just that - integrated supersymmetry into string theory.
The original string theory of the 1960s was a bosonic theory, meaning that all the vibrational patterns it yielded were those of bosons, or particles with whole-number spins (force particles). Of course, any theory that is trying to describe all particles must incorporate fermions. But there was another problematic feature of this formulation: it yielded a vibrational pattern whose mass (or precisely, mass squared) came out negative. This theoretical particle, called a tachyon, had been previously studied by physicists, but they had found it impossible to formulate a tachyon-containing theory that had no logical inconsistencies.
In 1971, Pierre Ramond of the University of Florida began the work of formulating a string theory that included fermions. Moreover, in the new version of the theory, bosons and fermions appeared to come in pairs, a hallmark of supersymmetry. This was the birth of supersymmetric string theory, shortened to superstring theory (but now universally called simply string theory). However, the introduction of supersymmetry into the point-particle quantum field theories yielded great success, eventually resulting in the supersymmetrical standard model. Strings, never truly embraced by the mainstream, fell by the wayside until the first superstring revolution in 1984.
By 1985, physicists realized that supersymmetry and string theory could be fused in not just one but five different ways, each of which yields boson/fermion pairing, but differs in other fundamental details. These five versions are called Type I theory, Type IIA theory, Type IIB theory, Heterotic type O(32) theory (read "oh-thirty-two"), and Heterotic type E8 × E8 theory (read "e-eight times e-eight"). These names are not important now, but they will be discussed later in this series. The five versions of what has been advertised as a theory of everything is a problem - we only exist in one universe, so only one can possibly describe it. As Edward Witten put it, "If one of the five theories describes our universe, who lives in the other four worlds?" Many physicists believe that one of the hallmarks of a true theory of everything is inevitability, or the conclusion that a theory is true because no other theory can be. In other words, a theory of everything must be "the only game in town." In actuality, as will be shown later in this series, these five formulations are all different ways of stating the same theory, tentatively termed M-Theory. This, if true, means that string theory does have the stamp of uniqueness.