How many types of elementary particles are there?
We have a thing called the standard model, which is based on about 60 particles, but there may be many more. These are just the ones that have a low energy, so we can detect them. The s and s could be considered a heyday of particle physics, when many subatomic particles—and not just elementary ones, it turns out—were being discovered.
Could you talk a little bit about the events leading up to your discovery of the quark? That was very dramatic for me. I had been working for years on the properties of particles that participated in the strong interaction. This is the interaction responsible for holding the nucleus of the atom together. The family of strongly interacting particles includes the neutrons and protons; those are the most familiar ones. But now tens, dozens, hundreds of other particles were being discovered in experiments in which protons collided with each other in particle accelerators.
There were lots and lots of energy states in which we saw relatives—cousins—of the neutrons and protons. They are produced in a particle collision in an accelerator, and they decay after a short time. After a tiny fraction of a second, they fall apart into other things. One particle that I predicted, the omega-minus , can decay into a neutral pion and xi-minus, and then the pion decays into photons, and the xi-minus decays into a negative pion and a lambda.
And then the lambda decays into a negative pion and a proton. The interior of the sun has a very high temperature, but even that very high temperature is not enough to make all of these things. Do all these exotic particles exist anywhere outside of physics experiments? They existed right after the Big Bang, when temperatures were incredibly high. And they occur in cosmic-ray events. Looking at the table of known particles and the experimental data, it was clear that the neutron and proton could be made up of three particles with fractional charges, which I called quarks.
The neutron and proton were no longer to be considered elementary. It was not a difficult thing to deduce. What was difficult was believing it, because nobody had ever heard of making the neutron and proton composite. Nobody had ever heard of these fractional charges.
Nobody had ever heard of particles being confined permanently inside observable things and not directly attainable.
As time goes on, physicists seem to find more and more particles. Could there be an infinite number of them? All of us theorists believe in simplicity. Simplicity has always been a reliable guide to theory in fundamental physics. But the simplicity may not lie in the number of named particles. It may be that the theory, expressed simply, gives rise to huge numbers of particle types. The particles might go on forever, but you detect only the ones that are light enough to play a role in your experiments.
Now researchers are pinning a lot of hope on finding yet another set of predicted particles in experiments at the Large Hadron Collider. Do you think this will bring some clarity?
With Murray at Caltech
Well, there is another possibility, that they find some phenomenon that is utterly unexpected. He had had a very substantial library, a huge library. And when the bad times struck—the Depression—he had to get rid of them when we moved to a tiny apartment. He had to have the furniture taken away. He paid somebody five dollars to take away his library. But he had a few books left, 50 books or something like that. One of them was a book that gave etymologies of English words borrowed from Greek and Latin.
So I learned all these Greek and Latin roots and how they went to make up English words. It was exciting. That started me on etymology, and I have loved etymology ever since. I was always OK in math. Actually I loved math, loved studying it, loved using it. I loved history.
Concepts of physics 1
I was particularly in love with archaeology and linguistics. And I could discuss anything with my brother—archaeology, etymology, anything at all. He never did anything with it, but he was very, very intelligent and very knowledgeable about all sorts of things. He was passionate about birds and other living things.
Not so much the scientific principles of ornithology, but just seeing the birds and identifying them and knowing where they were, and what kind of nest they had, and what songs they sang. Going with him on a bird trip was the best thing—the best thing—I did in those years. My brother taught me to read from a cracker box when I was 3. When you were going into college, you were interested in studying archaeology, natural history, or linguistics, but your father wanted you to make money as an engineer. If I designed something it would fall down.
Then how did you settle on physics? General relativity, quantum mechanics, you will love it. I never took his advice on anything else. He told me how beautiful physics would be if I stuck with it, and that notion of beauty impressed me. My father studied those things. He was a great admirer of Einstein. Inside the protons and neutrons, we find the quarks, but these appear to be indivisible, just like the electrons.
Quarks and electrons are some of the elementary particles we study at CERN and in other laboratories. But physicists have found more of these elementary particles in various experiments, so many in fact that researchers needed to organize them, just like Mendeleev did with his periodic table. This is summarized in a concise theoretical model called the Standard Model. Today, we have a very good idea of what matter is made of, how it all holds together and how these particles interact with each other.
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