Section 11.5 Universal Symmetry
One of the goals of grand unified theories is to show that the strong, weak, and electromagnetic interactions are really different aspects of a single type of universal interaction. As we've just seen, by studying how the strengths of these interactions vary with energy and distance, one learns that at high enough energies, the interaction strengths would become equal (see Figure 11.7). At this energy, one could observe directly what is known as universal symmetry. All the interactions would be on the same footing, look the same, and have the same strength.
At this grand unification energy, all interactions would be equally possible, including ones mediated by the \(X\)-boson, mentioned above in proton decay. In order to really see the \(X\)-boson in action, we would need to have energies available comparable to the \(X\)-boson's rest energy \(\sim 10^{17}\Xunits{MeV}\text{.}\) Recall that the world's biggest accelerator, the Large Hadron Collider in Europe, runs at a paltry \(10^7\Xunits{MeV}\text{.}\) A modern linear accelerator of \(10^{17}\Xunits{MeV}\) would have to extend beyond the outer reaches of Pluto's orbit around the sun. So how can we hope to study such high energies where universal symmetry would be apparent?
The answer is during the early universe! Shortly after the Big Bang, the average thermal energies of particles were well in excess of the grand unification scale. In those early moments, reactions occurred that determined the character of our present universe. Since we believe that the same laws of physics governed then as now, we can use the early universe as the testing ground for modern particle physics. The details of the physics of the early universe are the subject of the next chapter.