338 | Ryan Patterson on the Physics of Neutrinos

Hello, everyone. Welcome to the Mindscape podcast. I'm your host, Sean Carroll. These days in particle physics, in quantum field theory, in the attempt to understand the universe at its deepest levels, as you know, if you've been listening to this podcast, we have this great theory, the standard model of particle physics. But we also have plenty of reasons to believe that the standard model is not the final answer. It doesn't play well with gravity. It doesn't explain dark matter. It gives us a bunch of sort of tempting clues that somehow there should be grander unification of some sort. So there's all sorts of reasons to go beyond the standard model to invent new particles and new properties. And indeed, there's lots of papers out there in the scientific literature where people propose new particles. I've proposed new particles. I don't know many of my friends in the theoretical physics world who haven't proposed a new particle or two. But back in the day, long ago, the standards were different. You know, when physics a hundred years ago was going on, there was so much data that we had that hadn't been explained by theory yet. The people weren't just proposing new particles just for the heck of it. They would wait until they really were forced to do that by some experimental phenomenon. A great example of this, of course, is the neutrino. The idea of the neutrino was proposed around 1930 by Wolfgang Pauli because there was this known phenomenon namely beta decay. This is the decay of the neutron. Neutron is a little bit heavier than a proton, so it can decay into a proton, which is a positively charged particle, and an electron, which is a negatively charged particle. But there's a couple of things that weren't quite working out about that idea of a neutron going to a proton and an electron. After all, the neutron is spin one half, the proton is spin one half, and the electron is spin one half. Like, where did the extra angular momentum come from? And more careful experiments showed that even energy wasn't really being conserved in these interactions. So there was what by modern standards was a perfectly obvious thing to do, propose a new invisible particle that carried away a little bit of the spin and a little bit of the energy. But in 1930, that was just not done, so Wolfgang Pauli eventually did it. He proposed there was a little particle, but he was embarrassed to do so. He was like, sorry. You know? Maybe it's an idea. Don't take it too seriously. Of course, he was embarrassed because he didn't think that the particle could be detected. In order to fit the data, in order to explain what he needed to explain, it had to be a very, very weakly interacting particle. And indeed, the neutrino as we now know it is a pretty weakly interacting particle, but we're …