The discussion in the What Are Sub-Atomic paricles composed Of? thread reminded me of a question I’ve been pondering. deltasigma gave a link to a recent article with the quote
What if the quarks were massless? Say only the Up and Down, or even maybe only the Up quark was massless. Would they still combine to form protons and neutrons, or would their being massless prevent them from doing so? Would they simply have a slightly lower mass, or would they have very different properties? If they’d be very different, would that difference only occur if they were truly massless, or would there be big differences if they had a tiny mass, maybe similar to neutrinos?
If they were massless they’d have to move at the speed of light in all reference frames. If just the ups were massless and the downs weren’t, I don’t see how they could make a meaningful combination since they’d have to be separating at the speed of light.
They wouldn’t have to move at the speed of light in a straight line. They could move in circles, for example. Gluons are massless, and stay confined in the proton and neutron. But maybe they need the Up and Down as anchors.
Frank Wilczek considers the case of a ‘QCD Lite’ with three massless quarks (u, d, s), and three quarks of infinite mass (c, t, b) in his article on *The Origins of Mass *. It’s not too different from our everyday world, IIRC (and there are massive protons and neutrons).