© David Kaplan.

One of the things that distinguishes the different forces of nature is their strength, which is directly related to a parameter in the theory called "the coupling." If the three forces are all part of the same force, there must be a sense in which their couplings are the same. It turns out that if you probe the forces at shorter and shorter distances, the coupling changes, as discussed in Sections 4 and 5. Here, we show two plots that show how the couplings for the strong, weak, and electromagnetic forces change with energy in the Standard Model (left) and supersymmetry (right). We see that in the Standard Model, the couplings which would be measured at higher and higher energies come closer and closer together. In extensions of the standard model theory, that extrapolation can be different. The supersymmetric theory, which includes the Standard Model in the simplest way, actually shows the three couplings meeting at the same energy. This suggests that at that particular energy, the three forces of QED, QCD and the weak force, are all part of the same force, and above that energy a single line would represent the coupling of all three. (Unit: 2)