physics question #2050

James Hope, a 17 year old male from Gloucester asks on April 16, 2004,


I recently read a book on the big bang, and it said that protons and neutrons are made up of 3 quarks. It also said that up quarks have a + charge and down quarks have a - charge. If I am correct then a proton is made up of 2 ups and a down quark, this leaves a + charge. And a neutron is made up of 2 downs and a up quark, surely this would leave a - charge. Why are neutrons neutral if this is the case?

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the answer

Jess Brewer answered on April 18, 2004, A:

Well, either the book left out an important aspect of quarks or you read too fast. The weirdest things about quarks is that their charges are not "unit" charge + or - (like a proton or an electron) but +2/3 and -1/3 respectively for up and ddown quarks. (I think I have that right.) Anyway, this explains how two up plus 1 down can have charge +1 while two down plus 1 up can have zero charge. (The proton & neutron respectively.) You might well ask, wouldn't particles with such weird charges be easy to detect? Probably they would, if you could ever "see" a single bare quark. But you can't, because the force between quarks gets BIGGER as they get further apart (almost like a spring -- but not quite) so by the time one quark starts to get off by itself, the vacuum between it and its compatriots is torn apart, releasing quark-antiquark pairs that even things up again. You might ask, then, are quarks really real? Good question. For decades people considered them to be just a cartoon version of a mathematical symmetry obeyed by the strong interaction; but it turned out to be such a GOOD cartoon (all aspects of the theory seem to be embodied in this picture) that the argument boils down to a quibble over what you mean by "real". Crazy stuff, eh?

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