How do elementary particles work?

The full narration of the video.

Let go of an apple, and it hits the floor. You know that without a physics class. But push three down quarks together, and what happens? You memorized their charges, and you still can't say. Someone online got stuck right here. He didn't want answers to memorize. He wanted rules to work them out.

Physicists do have those rules. Start with a game of Chinese chess. Nobody memorizes every game. But learn how the horse moves, and you can name every square it can reach. Particles work the same way. Skip the names. Learn three rules, and you can reason it out.

Rule one: the books must balance. Before and after any collision, the total charge stays the same, and so does the energy. A lone neutron falls apart in about fifteen minutes. It becomes a proton, an electron, and a tiny, nearly weightless particle. Zero becomes plus one, minus one, and zero. Still zero. The books balance.

Rule two: quarks never go out alone. Either three huddle together, or one pairs up with its opposite. Try pulling two quarks apart. A new pair pops out where it snaps, and each one has a partner again. Rule three is the best: anything the rules don't forbid happens sooner or later. The rules even tell you which outcomes are common and which are rare.

Can you trust them? In 1962, physicist Murray Gell-Mann used the rules to say one particle was missing. He even worked out its weight. Two years later, an experiment found it. Its weight was within one percent of his number. It gets more precise. The electron acts like a tiny magnet. Theory and measurement agree to better than one part in a trillion.

Two things to be clear about. First, simple rules can be hard to work through. Chess rules fit on a page, but winning takes deep calculation. How heavy is a proton? It took supercomputers, in 2008, to get that from the rules. Second, gravity isn't in this rulebook. About 85 percent of the matter in the universe is invisible, and it isn't in the rulebook either. Physicists are still looking.

Back to those three down quarks. Each carries minus one third, so together, minus one. Three in a huddle: rule two says yes. So it should exist, and experiments have seen it. But it's heavier than a neutron plus a small negative particle. Heavy things break into lighter ones whenever the books balance. It lasts so briefly that light only crosses a single proton in that time.

Now it's your turn. An up quark carries plus two thirds. Put three together. What's the charge?