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May 29, 2004
Movin’ On Up
MT 3.0
Yes, if you look down at the sidebar, you see that Musings and the String Coffee Table have been upgraded to MT 3.0. That’s 29 plugins1 and 657 lines2 of (unified diff) patches to the MT source code. Actually, 657 lines is a good bit shorter than my accumulated patches for MT 2.661. 6A actually fixed several of the bugs on my list in 3.0, while introducing, so far, only two new ones.
Remarkably, the whole thing seems to work.
(Well, OK, the comment-posting code was busted for a while, and people could preview, but not post their comments. Thanks to Srijith for catching this.)
iBook
I type these words on my new 14" G4 iBook. My aging G3 iBook was suffering from the dreaded backlight problems, and intermittent trackpad wonkiness. Keeping an external monitor and a USB mouse plugged in kinda defeats the concept of “laptop.” So, when I had a chance to purchase a brand new 1 GHz 14" iBook for under $1000 (ya gotta know the right people ;-), I decided to go for it. It arrived yesterday. I installed the RAM upgrade (to 640 MB), booted the machine in FireWire Disk Mode, and proceeded to clone my old machine’s hard drive onto the new one3. Upon rebooting, iTunes demanded that I authorize the new machine, Mathematica demanded that I re-enter my License Code (it demands that, whenever you so much as sneeze), but everything else worked flawlessly.
My only miscalculation was the naive presumption that I could re-use the Airport card from the old machine. Nope! “Airport Extreme-Ready” means incompatible with the old cards. Guess I’ll be adding another $100 to the cost of this baby …
1 The 30th plugin, MT-Blacklist, awaits a 3.0-compatible update, and another, was rendered superfluous by 3.0. Six of the 29 were plugins of mine.
2 More precisely, that was: 14 files patched, 289 lines added and 29 lines removed from the MT codebase. This doesn’t count patches to other people’s plugins. I just copied the plugins over from my MT 2.x plugins directory.
3 N.b. I had MacOSX 10.3.4 (build 7H63) installed, a later build than the one included with the machine. This is important.
May 27, 2004
High Energy Supersymmetry
I’ve really gotta stop posting about the Landscape. Posts on the subject rot your teeth and attract flies.
Still, it helps to sort out one’s thinking about anthropic ideas, which definitely clash with the sort of “explanation” we have become used-to in field theory. In any scientific framework, one needs to understand what’s just given — input data, if you will — what needs to be “explained” and (most importantly) what counts as an explanation. There’s a temptation to mix and match: to envoke the anthropic principle to explain some things, and “technical naturalness” to explain others. But that is simply inconsistent; a statistical distribution in the space of couplings does not favour technically-natural ones over others.
Consider the question: why is the QCD scale so much lower than the Planck scale (or the GUT scale)?
We are accustomed to saying that this large hierarchy is natural because it arises from renormalization-group running. The QCD coupling starts out moderately small at the GUT scale (), and increases only logarithmically as we go down in energy.
But, in the Landscape, there’s a probability distribution for values of , which might just as easily be , or . What sounded like a virtue now sounds like a vice. The ratio depends exponentially on , and so is an exquisitely sensitive function of the moduli — exactly the sort of thing about which it is hard to make statistical predictions.
Instead, there’s an anthropic explanation for the value of . Namely, the proton mass (which is essentially determined by the QCD scale) is tightly constrained. Vary by a factor of a few, and stars cease to exist. Hence must be pretty close to , otherwise, we aren’t here.
Similarly, point out Arkani-Hamed and Dimopoulos, the electroweak scale cannot be vastly different from . For the ratio enters into the neutron-proton mass difference. If the neutron were lighter than the proton, there would be no atoms at all. If it were much heavier, all heavy elements would be unstable to beta decay, and there would be only hydrogen. Either way, we would not exist.
If the electroweak scale is anthropically-determined, is there any reason to expect any beyond-the-Standard-Model particles below the GUT scale? We don’t need low-energy supersymmetry to make natural. Arkani-Hamed and Dimopoulos posit a scenario where supersymmetry is broken at a high scale, with squarks and sleptons having masses in the GeV range (more on that below), whereas the “'inos” (the higgsino, the gluino, the wino, zino and photino) survive down to low energies.
Light fermions are, of course, technically natural. But there’s no reason to expect the theory to have approximate chiral symmetries. So technical naturalness is not, in this context an explanation for the light fermions. Instead, Arkani-Hamed and Dimopoulos argue that low-energy supersymmetry does have one great virtue — it ensured the unification of couplings around GeV. The “'inos” contribute to the -function at 1-loop, so the 1-loop running in this model is exactly as in the MSSM. The squarks and sleptons contribute at 2-loops (as they come in complete multiplets, their 1-loop contribution does not affect the unification of couplings), and removing them from low energies actually improves the fit somewhat.
Arguing for coupling constant unification sounds equally bogus until you turn the argument on its head (thanks to Aaron Bergman for helping me see the light). Assume that at short distances one has grand unification. Then one needs light “'inos” so that the 3-2-1 couplings flow to their anthropically-allowed values at long distances.
Once we’ve abandoned low-energy, SUSY breaking, why not let the SUSY breaking scale be all the way up at the GUT scale? The reason is, again, anthropic. The gluino is a light colour-octet fermion, and hence very long-lived (it decays only via gravitino exchange). If you push the SUSY breaking scale up too high, the long-lived gluino creates problems for cosmology. Arkani-Hamed and Dimopoulos favour a SUSY-breaking scale, GeV.
This gives a big improvement over low-energy SUSY in the context of the landscape. Flavour-changing neutral currents are no longer a problem. And it ameliorates, but does not really solve the problem of proton decay.
Proton decay via squark exchange, with two R-parity violating vertices.
Recall that there