Warped Passages - Lisa Randall [115]
Though surprising, the growth of strong interactions with distance is sufficient to explain all the distinctive properties of the strong force. It explains why the strong force is powerful enough to keep quarks bound up into protons and neutrons, and quarks trapped inside jets—the strong force grows at long distance to the point where a particle that experiences the strong force cannot be separated overly far from other strongly interacting particles. Fundamental strongly interacting particles such as quarks are never found in isolation.
A well-separated quark and an antiquark would store an enormous amount of energy, so much so that it would be more energy-efficient to create additional physical quarks and antiquarks in between than to let them remain isolated. If you were to try to pull the quark and antiquark further apart, new quarks and antiquarks would be created from the vacuum. Just as in Boston traffic, where you can never be more than a car-length behind the car in front of you without a car coming in from the next lane, those new quarks and antiquarks would hover near the original ones so that no single quark or antiquark would become any more isolated than when it started—some other quark or antiquark is always nearby.
Because the strong force at large distances is so strong that it doesn’t allow strongly interacting particles to be isolated from one another, particles that are charged under the strong force are always surrounded by other charged particles in strong-force-neutral combinations. The consequence is that we never see isolated quarks. We only see strongly bound hadrons and jets.
Grand Unification
The results of the previous section tell us about the distance dependence of the strong, weak, and electromagnetic forces.21 In 1974, Georgi and Glashow made the bold suggestion that these three forces change with distance and energy in such a way that they unify into a single force at high energy. They called their theory a GUT, short for Grand Unified Theory. Whereas the strong force symmetry interchanges three colors of quarks (as discussed in Chapter 7) and the weak force symmetry interchanges different particle pairs, the GUT force symmetry acts on and interchanges all types of Standard Model particles, quarks and leptons.22
According to Georgi and Glashow’s Grand Unified Theory, early in the evolution of the universe, when the temperature and energy were extremely high—the temperature was higher than one hundred trillion trillion degrees kelvin, and the energy was higher than one thousand trillion GeV—the strength of each of the three forces was the same as that of the others and the three nongravitational forces fused into a single one, “The Force.”
As the universe evolved, the temperature dropped and the unified force split into three distinct forces, each with its own distinct energy dependence, through which they evolved into the three nongravitational forces we know today. Although the forces began as a single force, they ended up with very different interaction strengths at low energies because of the different influences that virtual particles had on each of them.
The three forces would be like identical triplets who developed from a single fertilized egg, but matured into three rather different individuals. One triplet might now be a punk rocker with dyed, spiked hair, one a marine with a crewcut, and one an artist with a long ponytail. They would nonetheless share the same DNA, and when they were babies would have been pretty much indistinguishable.
In the early universe, the three forces would also have