cam875 said:
than how did they discover that quarks exist
Edit: {
http://en.wikipedia.org/wiki/Deep_inelastic_scattering
Apparently they used Deep inelastic scattering to prove that they exist. But the mathematics had shown them to be useful. I guess you can ignore everything I originally wrote now...
}
Well, as far as my limited understanding goes (Someone PLEASE correct me if I'm wrong) there are certain quantities in physics which are conserved. Good examples are energy, linear momentum, etc. Some of these quantities are quantised, such as electrical charge (which always comes in units of the charge on the electron), baryon number, lepton number, spin etc. The fact that these are conserved is just experimental observation.
In a given process, these conserved quantities make calculations easier. For example, in beta decay, a neutron decays into a proton (or an up quark changes to a down) an electron, an (electron) antineutrino and photon(s)... So, a neutron has zero charge, so the sum of the charges of its product must also have zero charge. Now let's say that we didn't know that electrons exist. We observe the stuff we had to begin with (a neutron) and we observe what we know about, namely the proton, the antineutrino and the photon(s). We note that we've gained charge and lepton number from somewhere. This is a pain. Our theory predicts that charge is conserved. Either there's a lepton produced with negative charge that we don't know about (the electron) or the theory is wrong. In every other experiment, we've found that charge and lepton number are conserved, so the theory looks good. This leads people to search for the particle with the given properties. In a similar way, the theory predicts certain particles contained within baryons, which have certain qualities. These are the quarks. It's all a lot clearer with a diagram;
http://en.wikipedia.org/wiki/Image:Standard_Model_of_Elementary_Particles.svg
We've whittled the particles down to 16 fundamental or elementary particles of which everything else is made. Which is nice. And good. And simple. The quarks are predicted by theory essentially. And by counting numbers we know that they're there.