Tuesday, March 27, 2007

Frank Wilczek talk, part two

Frank Wilczek gave his second talk at Rice, "The lightness of being", about the origins of mass and the "feebleness" of gravity. He demonstrated the relative weakness of gravity very effectively by jumping up and down, showing that by using a tiny amount of chemical energy, he could overcome (temporarily) the gravitational attraction of the entire planet. I'll admit that I was a bit disappointed in this talk, in the sense that there was more overlap with yesterday's public lecture than I was expecting. I did come away having learned a new way to think about the origin of the mass of the nucleons, though. Wilczek's most famous contribution to physics is asymptotic freedom of quarks, which can be summarized as this: unlike the other forces that weaken with interparticle distance, the gluon-mediated color charge interaction between quarks grows as the separation between quarks is increased. One result of this is that there are no free quarks - if you try and separate a lone quark, the energies involved in the strong interaction become large enough to favor creation of quark-antiquark pairs. So try to build a nucleon out of three quarks. The quarks have to be pretty localized relative to each other, so that from far away there is no unscreened color charge. Localizing quantum mechanical objects leads to a particle-in-a-box type kinetic energy, though. You can think of this as coming from the uncertainty principle. It's this internal kinetic energy that is the source of 95% of the mass of the proton, via m = E/c^2. Voila - mass comes about due to quantum confinement. "Nano" concepts at work on the "femto" scale.

Another interesting point that Wilczek made: the near-perfect conservation of mass law identified by Lavoisier in chemical reactions is a great example of an emergent law. Strictly speaking, mass isn't conserved - energy is. That's very clear at particle accelerators, where a colliding e-e+ pair can produce particles massing 30000x that of the two electrons. The reason that chemistry doesn't see this effect is very much in the spirit of condensed matter. The excitation spectrum of the bound quarks is very strongly gapped. There are no available excited states of the coupled quark system at the few-eV energies relevant to chemical reactions. This basic idea, that processes can be suppressed because of a lack of available states, is also prevalent in much nanoscale physics.

I asked him about the proton "spin problem", as discussed recently here. At issue is where does the intrinsic angular momentum of the proton come from. Wilczek pointed out that there actually isn't any discrepancy with theory; lattice QCD does give spin-1/2 as the final total. What rubs people the wrong way is that the calculations run counter to most intuition. Rather than that angular momentum coming from the spins of the quarks, it appears that much of it comes from the gluon field. There you have it.

Finally, in the Q&A period, someone asked Wilczek about the possibility of extra dimensions - from context, I assume "large" ones. Wilczek really doesn't like this idea; he favors supersymmetry-driven Planck-scale grand unification. He said that it's hard enough accomplishing that and not running into problems like proton decay, and that pushing unification to lower energies (as would happen in the large extra dimension case) would cause all kinds of difficulties like that. I hadn't heard this said before, and would be curious to know more about it. Presumably the proponents of these extra dimension ideas have thought about this.


wolfgang said...

> lattice QCD does give spin-1/2 as the final total

this is correct, but as far as I know the distribution (valence quarks, gluons, ..) does not come out correct yet, which may be simply due to various technical issues.

Doug Natelson said...

Wolfgang - this is exactly Wilczek's point. He says that the lattice QCD distribution is correct. It is what it is, even if that doesn't jibe with people's intuitive expectations of what it should be.

Peter Woit said...

Wilczek is very fond of the fact that in the minimal extension of the standard model, running the couplings up in energy, they unify much more accurately than in the standard model himself. Besides the reasons lots of people are impressed by this, one reason he is so fond of it is that he was one of the people who did this calculation.

Large extra dimensions ruin this calculation pretty much completely, so if they exist, lots of people will be very excited, but one of his possible claims for a second Nobel prize will be shot.