I’ve reached a point in the book-in-progress where I find myself needing to talk a little about particle physics. As this is very much not my field, this quickly led to a situation where the dog asked a question I can’t answer. But, hey, that’s why I have a blog with lots of smart readers…
The question is this:
What are all these extra particles for?
Or, to put it in slightly more physics-y terms: The Standard Model contains twelve material particles: six leptons (the electron, muon, and tau, plus associated neutrinos) and six quarks (up-down, strange-charm, top-bottom). The observable universe only uses four, though: every material object we see is made up of electrons and up and down quarks, and electron neutrinos are generated in nuclear reactions that move between different arrangements of electrons and up and down quarks. The other eight turn up only in high-energy physics experiments (whether in man-made accelerators, or natural cosmic ray collisions), and don’t stick around for very long before they decay into the four common types. So, to the casual observer, there doesn’t seem to be an obvious purpose to them. So why are they there?
Presumably, there is some good reason why the universe as we know it has to have twelve particles rather than just four. Something like “Without the second and third generations of quarks and leptons, it’s impossible to generate enough CP violation to explain the matter-antimatter asymmetry we observe.” Only probably not that exact thing, because as far as I know, there isn’t any way to explain the matter-antimatter asymmetry we observe within the Standard Model. But something along those lines– some fundamental feature of our universe that requires the existence of muons and strange quarks and all the rest, and would prevent a universe with only electrons and up and down quarks.
If there is such an explanation of the “extra” generations, though, I don’t think I’ve run across it. Or else I’ve completely forgotten it. So if you know of a good explanation of why the universe we live in couldn’t be made with only up and down quarks, electrons, and electron neutrinos, leave it (or a pointer to it) in the comments. The dog and I will thank you, both on-line and in print.