Warped Passages - Lisa Randall [116]
A single interaction strength at high energy is a prerequisite for a Grand Unified Theory. That means that the three lines representing interaction strength as a function of energy must all intersect at a single energy. But we already know how the strengths of the three nongravitational forces change with energy. And because quantum mechanics tells us that large distance is interchangeable with low energy and that short distance is interchangeable with high energy,* the results of the previous section can be interpreted equally well in terms of energy. At low energies the electromagnetic and weak forces are less powerful than the strong force, but they strengthen at higher energies, whereas the strong force weakens.
In other words, the strengths of the three nongravitational forces are becoming more comparable at high energies. They might even be converging to a single strength. This would mean that the three lines representing interaction strength as a function of energy intersect at high energies.
Two lines meeting at a single point is not such an exciting result—it is bound to happen when the lines approach each other. But three lines meeting at a point is either a strong coincidence or evidence of something more meaningful. If the forces do converge, their single interaction strength could be an indication that there is only a single type of force at high energy—in which case we would have a unified theory.
Although unification to this day remains a conjecture, the unification of forces, if true, would be a major leap towards a simpler description of nature. Because unifying principles are so intriguing, physicists studied the strength of the three forces at high energies to see whether or not they converge. Back in 1974, nobody had measured the interaction strengths of the three nongravitational forces with very great accuracy. Howard Georgi, Steven Weinberg, and Helen Quinn (who was then an unpaid Harvard postdoctoral fellow, now a physicist at the Stanford Linear Accelerator Center and president of the American Physical Society) used the imperfect measurements that were then available and did a renormalization group calculation to extrapolate the strength of the forces to high energies. They discovered that the three lines representing the strength of nongravitational forces did indeed appear to converge to a single point.
The famous 1974 Georgi-Glashow paper on their Grand Unified Theory begins with these words: “We present a series of hypotheses and speculations leading inescapably to the conclusion…that all elementary particle forces (strong, weak, and electromagnetic) are different manifestations of the same fundamental interaction involving a single coupling strength. Our hypotheses may be wrong and our speculations idle, but the uniqueness and simplicity of our scheme are reasons enough that it be taken seriously.”* Perhaps those were not the most modest of words. However, Georgi and Glashow did not really think that that uniqueness and simplicity were sufficient evidence that their theory was the correct description of nature. They also wanted experimental confirmation.
Although an enormous leap of faith was required to extrapolate the Standard Model to ten trillion times the energy anyone had directly explored, they realized that their extrapolation had a testable consequence. In their paper, Georgi and Glashow explained that their GUT “predicts that the proton decays,” and that experimenters should try to test this prediction.
Georgi and Glashow’s unified theory predicted that protons wouldn’t last for ever. After a very long time, they would decay. Such a thing would never happen in the Standard Model. Quarks and leptons are ordinarily distinguished by the forces they experience. But in a Grand Unified Theory, the forces