The physics textbooks tell me that protons and neutrons are both made up of three quarks. These little things amuse me, because no-one has actually ‘seen’ one. (By ‘seen’, I don’t literally mean seen visibly, but rather I mean some method by which a quark can be isolated and leave a tell-tale quark-signature in some way). But the real issue is that the quark theories tell us that we can’t see individual quarks – they can only come in groups of three (such as the proton and neutron) or groups of two (the mesons). Trying to isolate one just doesn’t work.
So we have a theory that seems inherently untestable. That’s not much of a scientific theory. Think about the following:
Fred: Why does a helium balloon go upwards?
Bill: It’s the magic pixies that are carrying it
Fred: But I haven’t seen a magic pixie
Bill: Of course not, you can’t see them, they are invisible.
Fred: But I can’t hear them either
Bill: They’re silent. Plus, before you ask, tasteless, smell-less…
Now, I’m not impying for a moment that the quark model is wrong, but rather, conveniently untestable (at least, directly). But that might change. The ALICE experiment at the LHC is designed to create a temperature so high that even the protons and neutrons can melt, leaving behind a soupy thing containing free quarks. If that works, quarks will be ‘seen’ for the first time.
Of course, the natural question to ask then is ‘What are quarks made of?’. Magic pixies, most probably.