Symmetry is a property of our universe. The fact that physical laws are the same at every point in space and every moment in time is a symmetry. The statement in relativity that all observers are treated equally - symmetrically - is also a symmetry. Another kind of symmetry, like rotational symmetry, exists in the spin of particles. It is not completely the same as what we think of as spin, there is a quantum mechanical twist to it, but you can think of it like this. A particle with spin of zero looks the same from all directions, like a dot or a circle. Particles of spin-1 look the same every full turn (360°), and spin-2 particles look the same every 180°. Spin-½ is difficult to visualize in the real world; particles of spin-½ look the same every two full revolutions, or 720°. Spin-½ particles are matter particles (fermions), and all other particles are virtual force particles (bosons). When spin was taken into account, it was discovered in 1971 that there is one more kind of symmetry that is mathematically possible - supersymmetry.

In the early 1970s, it was determined that if supersymmetry is a property of our universe, then particles must come in pairs. These pairs are known as superpartners, and their spins differ by ½. In other words, every matter particle has a corresponding virtual particle and vice versa. Then, in the mid-1970s, it was proven that none of the known particles can be superpartners of one another. So, for every particle, there must be a superpartner that has not been discovered yet. For example, the electron must have a superpartner called a selectron (short for supersymmetric-electron). The superpartners of quarks and neutrinos would be called squarks and sneutrinos. Virtual particles would also have superpartners. For photons there would be photinos, for gluons gluinos, and for W and Z bosons there would be winos and zinos.

One reason for us to believe in the existence of supersymmetry has to do with grand unification. The idea behind grand unification is that at a certain temperature, or equivalently at a certain small distance, the strengths of the three non-gravitational forces are equal. For example, Sheldon Glashow, Abdus Salam, and Steven Weinberg showed that at a high enough temperature, like the temperature at a fraction of a second after the big bang, the electromagnetic and weak force dissolve into each other, creating the electroweak fields. When the temperature drops (like it has done since the big bang), the two forces crystallize out into different forms through a process called symmetry breaking. It was supposed that the strong force was related to the electroweak force in the same way. In 1991, it was discovered that this is not the case. The graph shows that the force strengths almost, but not quite, meet. However, the slight changes in the graph created with the addition of supersymmetry are just enough to make the force strengths equal at a certain temperature.

The full name for string theory is actually superstring theory, because it incorporates supersymmetry. It is called string theory for short. If supersymmetry is proven to be an attribute of our universe by the discovery of superpartners, this would greatly strengthen the case for superstring theory.