We learned about what’s inside. Inside the nucleus, there are protons and neutrons, two of the constituents. Don’t panic; they’re all right. They don’t hurt. We looked inside those, too. But in the course of this kind of study, we found, for example, something called “radioactivity.” You’ve heard about radioactivity. It turns out that radioactivity always involves a mysterious particle which escapes. When you see something radioactive in our research, it’s a kind of explosion. A particle explodes and gives rise to other particles. You study these particles that come off from the explosion, and you reproduce what the event was.
Now, there are certain guiding principles in all of this that help us. One of them is the conservation of energy. It says that the total amount of energy should stay the same in any process. Like, if you put 14 people in a room, they can interact with each other, they can yell at each other, scream at each other, but hopefully, at the end of the day, there are still 14 people. The number of people is conserved, in that sense. In the same way, energy — if you keep track of it — it should balance. And in these reactions, it didn’t balance. Something was missing.
For a while, physicists jumped out of second-story windows. They got very upset because they really loved conservation of energy, and it looked as if they were going to lose it as a principle. Until somebody said, “Maybe there’s a particle escaping. Let’s assume it is, and since it doesn’t show up in our apparatus, it must be electrically neutral, and because it doesn’t show up for other reasons, it must be very small. So they used the diminutive ending “ino,” which is Italian for “little, little guy.” So a neutral, little particle.
And because of other properties, It became a very mysterious particle head. It was like, “Little fly upon the wall, ain’t you got no…” Anyway, I’m not going to finish that. But the neutrino had no electric charge. it turned out that its mass was almost zero, if not zero. Even today, in 1992, we think the neutrino may have zero mass, or if it has any mass, it’s a teensy, weensy amount of mass, not much. Lots of particles are detected because they make collisions. A proton hitting into some piece of lead, three inches thick, will never get through the lead because it will hit something, some other nucleus, and be stopped. Neutrinos didn’t have this strong ability to collide, so it didn’t seem to have any forces. So here it is, almost not even a thing. It had no charge, no mass, no strong force, and yet, we knew it was robbing energy from reactions, and it was very important. As we got to know more and more about it, it became crucial.