Warped Passages - Lisa Randall [117]
Because the proton can decay in a Grand Unified Theory that links quarks and leptons, all familiar matter would ultimately be unstable. However, the decay rate of the proton is very slow—the lifetime would far exceed the age of the universe. That means that even as dramatic a signal as a proton decaying would not stand much chance of being detected: it would happen much too rarely.
To find evidence of proton decay, physicists had to build extremely large and long-lasting experiments that studied a huge number of protons. That way, even if any single proton is unlikely to decay, a large number of protons would greatly increase the odds that the experiment could detect the decay of one of them. Even though your likelihood of winning the lottery is small, it would be much greater if you bought millions of tickets.
Physicists did build such large, multi-proton experiments, including the Irvine/Michigan/Brookhaven (IMB) experiment located in the Homestake Mine in South Dakota, and the Kamiokande experiment, a vat of water and detectors buried a kilometer deep underground in Kamioka, Japan. Although proton decay is an extremely rare process, these experiments would already have found evidence of it if the Georgi-Glashow GUT were correct. Unfortunately for grand ambitions, no one has yet discovered such decay.
This doesn’t necessarily rule out unification. In fact, thanks to more precise measurement of the forces, we now know that the original model proposed by Georgi and Glashow is almost certainly incorrect, and only an extended version of the Standard Model can unify forces. As it turns out, in such models the predictions for the proton lifetime are longer, and proton decay shouldn’t have been detected yet.
Today, we don’t actually know whether unification of forces is a true feature of nature or, if it is, what it signifies. Calculations show that unification could happen in several models I’ll discuss later, including supersymmetric models, the Hořava-Witten extra-dimensional models, and the warped extra-dimensional models that Raman Sundrum and I developed. The extra-dimensional models are particularly intriguing because they could bring gravity into the unification fold and truly unify all four known forces. These models are also important because in the original unification models it was assumed there were no new particles to be found above the weak scale other than those with GUT scale masses.* These other models demonstrate that unification might happen even if there are many new particles that could be produced only at energies above the weak scale.
However, fascinating as unification of forces can be, physicists are currently divided about its theoretical merits according to whether they favor a top-down or a bottom-up approach to physics. The idea of a Grand Unified Theory embodies a top-down approach. Georgi and Glashow made a bold assumption about the absence of particles with mass between one thousand and one thousand trillion GeV and hypothesized a theory based on this assumption. Grand Unification was the first step in the particle physics debate that continues today with string theory. Both theories extrapolate physical laws from measured energies to energies at least ten trillion times higher. Georgi and Glashow later became skeptical about the top-down approach that string theory and the search for Grand Unification represent. They have since reversed their tracks and now concentrate on lower-energy physics.
Although unified theories have some appealing features, I’m not really sure whether studying them will lead to correct insights into nature. The gap in energy between what we know and what we extrapolate to is huge, and one can