Now that’s a hard job because, if you try to calculate, using the best information we had, how much material — let’s say steel. How thick a steel wall do you need if a given neutrino coming into the steel wall should have a very good chance of never getting out? How long does that steel wall have to be? Ten feet, a hundred feet, a mile, ten miles? Turned out the answer is a hundred million miles. So we went to the authorities, and we said, “We need a hundred million miles of steel. We want to catch neutrinos.” No, of course, we didn’t do that! We thought more clearly, and it turns out, if you have two neutrinos, you only need half that thickness. And if you have a billion neutrinos — or a billion, billion neutrinos — then you might need a kind of detector that you could think of building. It would still have to be very massive and detailed. It turned out that we hit on a way of doing this with a detector, which, for that time, the 1960s, was very massive. It was ten tons of material, and it’s not just ten stupid tons of steel sitting there. You had to look inside to see the collision. So it had to be, in some sense, semi-transparent.
Anyway, the experiment was wildly successful. We discovered, in fact, that they weren’t one neutrino, but there were two kinds of neutrinos, and that’s what was giving us all the confusion. The number of neutrinos was doubled. And that, these two types of neutrinos, really set us on a road towards what we now call the “Standard Model,” a compact summary of all of this data that I’ve been telling you about — the data — lots of data that came out in the laboratories all over the world since 1960. So it became known as the “two-neutrino experiment.” Of course, when you tell somebody who’s not a scientist about two neutrinos, they say it sounds like an Italian dance team. “Ladies and gentlemen, we now have the Two Neutrinos!”