Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature

by Steven Weinberg

There was an obvious obstacle to any sort of unification along these lines. The apparent strengths of the forces in any field theory depend on two kinds of numerical parameter: the masses (if any) of the particles like W and Z particles that transmit the forces, and certain intrinsic strengths (also known as coupling constants) that characterize the likelihood for particles like photons or gluons or W and Z particles to be emitted and reabsorbed in particle reactions. The masses arise from spontaneous symmetry breaking, but the intrinsic strengths are numbers that appear in the underlying equation of the theory. Any symmetry that connects the strong with the weak and electromagnetic forces, even if spontaneously broken, would dictate that the intrinsic strengths of the electroweak and strong forces should (with suitable conventions for how they are defined) all be equal. The apparent differences between the strengths of the forces would have to be attributed to the spontaneous symmetry breaking that produces differences in the masses of the particles that transmit the forces, in much the same way that the differences between the electromagnetic and weak forces arise in the standard model from the fact that the electroweak symmetry breaking gives the W and Z particles very large masses, while the photon is left massless.