Musings

Austin is truly Wifi Heaven.

I’m sitting in JP’s Java (a stone’s throw from my office, but much nicer), enjoying the cappuccino and classical music. I’m surrounded by three Dell’s, two Sony Viao’s, two iBooks (counting my own), and a powerbook G4.

UT has Wireless Access in the classroom, libraries, and even on the West Mall. But what’s astounding is the number of free WAPs which have sprung up all over town. Everyone from Schlotzky’s Deli to the Alamo DraftHouse have gotten into the act, as have my favourite Cafés (Triumph and Trianon) near the house.

Of course, Starbucks still wants to charge $50/month for WiFi access, but we all know that they’re evil, don’t we?

Sendmail 8.12.7 is out. If you are, as I am, running sendmail, it’s time to update.

Be sure to verify the PGP signature:

gpg -v sendmail.8.12.7.tar.gz.sig
tar xzf sendmail.8.12.7.tar.gz

before proceeding with building and installing it.

Whoops! Mine’s finished compiling and installing itself. Gotta run…

I should have said that Gottfried Curio also has a derivation of the complex Chern-Simons superpotential I discussed previously. I would have said so earlier, but I’m afraid I find his paper nearly impenetrable.

There’s definitely good stuff there, though. Looks like hyperbolic 3-manifolds are rearing their ugly heads in the context of M-theory.

Just when you thought it was safe to go outside again …

Perhaps it’s a bit too early (he hasn’t yet posted any “real” entries to his blog), but here’s a hearty welcome to the latest physics blogger, Demian Cho.

Hey Demian and Aaron! Does Blogger afford you the ability to produce an RSS feed of your blog? If they do, you should flip the relevant switch.

Update: Answered my own question. Guys, just go to voidstar.com or logicerror.com and follow the instructions there.

I added a few more RSS feeds to this blog (and modified the existing ones a bit). You can now choose from a “plain-Jane” RSS 0.9.1 feed — a basic format, understood by all News Aggregators — or one of the “second generation” RSS feeds (RSS 1.0 or RSS 2.0), which are extensible using XML namespaces.

There’s really not much difference between “1.0” and “2.0”, except for the egos of the developers involved. They look different — open them in your web browser and see — but they are functionally equivalent.

If your News Aggregator supports the additional data supplied by these feeds, you can use whichever one is convenient. Some clients, like NetNewsWire can actually make use of the full-content feed, which includes the full text content of the entries in this blog. You can subscribe to the “Full Content” version and read this blog in NetNewsWire, without ever opening your web browser.

I don’t actually recommend that, as the HTML rendering (especially of math formulae) is pretty lousy. And you can’t view or post comments. Generally, it’s a lot less useful.

There’s also the issue of bandwidth. The 0.91 feed is 5K. The 1.0 and 2.0 feeds are 12K and 10K, respectively. The RSS 2.0 Full Content feed is 36K. So, you see, one pays for the extra information.

I’d be interested in hearing back from subscribers to these feeds, about which format(s) their client software supports, and which features they would like added to the feed(s).

There have been several papers recently on compactifications of M-Theory on “manifolds”, X, of G₂ holonomy. I put ‘manifolds’ in quotes because to get nonabelian gauge groups with chiral matter, we need certain types of singularities.

Friedmann and Witten have written about threshold corrections in such theories. At least in the limit that they consider, the threshold corrections are given by the Ray-Singer torsion of the associative 3-manifold, Q, where X can be thought of as a K3 fibration over a base isomorphic to Q, and where the generic K3 fiber has a A₄ singularity. In layman’s terms, the threshold corrections are entirely due to massive adjoints of SU(5); there is no contributions (in this limit) from massive fundamentals or other representations.

I’ve been meaning to post some comments on this paper; I guess I’ll get around to that eventually.

More recently, Bobby Acharya has written about the superpotential for models with G-flux. He finds that, in addition to the usual contribution linear in G, there’s also a term in the superpotential proportional to a complex Chern-Simons term on Q.

This ends up fixing the moduli of the compactification, in a manner reminiscent of what happens in some (doubtless related) Type IIA orientifold models. Of course, he ends up with a theory with unbroken N=1 supersymmetry in (3+1)-dimensional anti-de Sitter space. Perhaps (with considerable handwaving) after supersymmetry breaking we might end up with a vanishing or small positive cosmological constant. But understanding the resolution of that question seems as far away as ever.

I “upgraded” this blog from XHTML 1.0-Transitional to XHTML 1.1. The idea is that if I ever decide to take another stab at dabbling in MathML, it’ll be an easy transition.

XHTML is modular, and the W3C has already published an XHTML 1.1+ MathML+SVG DTD. You can also load other standard modules into your DTD, e.g. for frames or for the “target” attribute. I can finally say truthfully that the whole weblog (including the MathML stuff) validates.

All in all, it wasn’t too bad a transition: a few modifications of the templates to eliminate some deprecated constructions, and a bit of fixing of some old entries. XHTML 1.1 is stricter than XHTML 1.0 about what can go inside a <blockquote> and insists that

<div>
<form>
...
</form>
</div>

be rearranged to

<form>
<div>
...
</div>
</form>

Holler back if something broke for you, but it seems to me that anyone whose browser can handle XHTML 1.0-Transitional can handle XHTML 1.1 just fine.

While we’ve all been distracted by the travails of Trent “I am not a racist” Lott, the Administration is pressing on with some astounding revelations about the focus of their economic policy. Fortunately, Slate seems to be on the ball.

First came Tim Noah’s article on the increasing prevalence in Republican circles of the notion that the poor pay too little in taxes.

“The Post reports that economists at the Treasury Department are drafting new ways to calculate the distribution of tax burdens among different income classes,” i.e., looking for ways to rig the numbers, “to highlight what administration officials see as a rising tax burden on the rich and a declining burden on the poor.” The Council of Economic Advisers will offer up some sort of tax-the-poor rationale in its forthcoming “Economic Report to the President,” according to the Post. At the aforementioned Dec. 10 AEI conference, the CEA’s chairman, R. Glenn Hubbard, complained, “The increasing reliance on taxing higher-income households and targeted social preferences at lower incomes stands in the way of moving to a simpler, flatter tax system.”

What’s wonderful about this is that they have — rhetorically speaking — barely begun to warm up. Clearly, the poor receive an “unfairly large” proportion (relative to their income) of government services. And, the logic goes, this largess should be deducted from their “effective” tax burden.

Even simple things, like say the money the government spends educating a poor child, while much less in absolute terms than that spent per child in an affluent school district, constitutes a much larger proportion of the parent’s income — and hence an unfair tax advantage conferred on the poor.

“Time to put a stop to this damn freeloading!” says a chorus both inside and outside the Administration.

And now comes How Reaganomics Became Rubinomics from Michael Kinsley on the Administration’s astonishing embrace of deficit spending.

At least the Republicans are no longer pretending that deficits, if they happen to occur, are detritus left behind by the previous administration like all those McDonald’s wrappers behind the dresser in the Lincoln Bedroom. Instead, Republicans embrace the coming deficits as their own and pooh-pooh any desire for a balanced budget as some kind of liberal Democratic folly. This is breathtakingly dishonest on three levels…

At long last, the silly facade of “compassionate conservatism” has been abandoned, and the Republicans have firmly embraced their role as the Party of Fiscal Irresponsibility.

In case no one noticed (and how could you not?), I finally made explicit the licencing terms of this blog and its contents. It’s made available to you, dear reader, under the terms of the Attribution-NonCommercial Licence from the Creative Commons project. I would have made my intentions explicit from the start, but the CC project only “went live” with their web site on December 16.

Thanks to James Boyle, Michael Carroll and Lawrence Lessig for providing the legalese for the licence and the inspiration for the project.

MacOSX 10.2.3 is out.

Though it’s not mentioned in the Release Notes, this update fixes a security vulnerability in CUPS, the printing subsystem used in Jaguar.

Time to fire up Software Update …

Tom Banks and Willy Fischler have been pushing a peculiar view of what quantum gravity in de Sitter space should look like. Taking off from the observation that de Sitter has a finite entropy, they argue that the Hilbert space of quantum gravity in de Sitter ought to be finite-dimensional.

Remember that a single harmonic oscillator has an infinite-dimensional Hilbert Space, and a single free scalar field corresponds to an infinite number of harmonic oscillators, and you see just how weird that statement is. Moreover, their recent paper highlights (in yet another way) the familiar fact that systems with finite-dimensional Hilbert spaces do not have classical limits.

One thing that has always bothered me about their proposal is the question of de Sitter invariance. Recall that in asymptotically flat space, we mod out only by those diffeomorphisms (“gauge transformations”) which go to the identity at infinity. Call this subgroup of diffeomorphisms Diff_c. The quotient Diff/Diff_c is a finite-dimensional group isomorphic to the Poincaré group. The elements of this group act as global symmetries of our theory, and we can decompose our Hilbert space into representations of Poincaré. And, indeed, the gauge-invariant observables of quantum gravity in asymptotically flat space are S-matrix elements, representing the scattering of quanta that make it off to infinity.

Similarly in asymptotically anti-de Sitter space. Here “infinity” is off in a spacelike direction (it was off in a null direction in the asymptotically flat case). Again, if we take as our gauge group Diff_c, the diffeomorphisms that go to the identity on the boundary, Diff/Diff_c is isomorphic to the anti-de Sitter group, which is also the conformal group of the conformally-rescaled boundary. The observables of quantum gravity in this case turn out to be the correlation functions of a quantum field theory living on this conformal boundary.

De Sitter, too, has a notion of “infinity”, but this time it lies in the time-like direction. If we do the same construction as before, we end up with the de Sitter group, SO(d,1), acting as global symmetries in our quantum theory. If this were right, it would fly in the face of the claim that the Hilbert space is finite-dimensional, as SO(d,1) has no finite-dimensional unitary representations.

OK, but Banks and Fischler would object that there is no super-observer who has access to the hypersurface at timelike infinity. It’s not so clear what behaviour one should demand of diffeomorphisms as one approaches timelike infinity. The global symmetry in their formulation should be SO(d-1)×R. Susskind et al concur . However, the reasoning, in both cases — that this is the subgroup of SO(d,1) which preserves the horizon does not make sense in a theory of quantum gravity where the metric (and hence the horizon) fluctuates.

But how do we get SO(d-1)×R? If we mod out by all diffeomorphisms, we get no global symmetries. If we mod out by those which preserve timelike infinity, we get SO(d,1). How to get something in between?

One obvious answer is the following. Pick a timelike worldline, γ. Recall that we are in asymptotically de Sitter space, that is, we consider only metrics which approach the de Sitter metric at timelike-infinity. So we will demand that γ approach a geodesic
(of the de Sitter metric) at timelike-infinity. Let Diff_γ be those diffeomorphisms which carry this “asymptotic geodesic” into itself (i.e. φ: γ → γ', with γ' asymptoting to γ as we approach infinity). As before, our gauge group is Diff_c, the diffeomorphisms which go to the identity at infinity. The quotient Diff_γ/Diff_c = SO(d-1)×R.

Note that this depends on a choice of worldline γ. This makes sense from the point of view of Banks et al. In de Sitter space, local observers fall out of causal contact if you wait long enough, so the formulation of the quantum theory ought to involve a choice of a particular local observer. If he waits long enough, he will not be able to compare notes with anyone else.

What about the other diffeomorphisms, which do not preserve the asymptotic form of our chosen worldline, γ? In this conjectured way of formulating quantum gravity in asymptotically de Sitter space, they simply don’t act as operators on the Hilbert space, as the Hilbert space is somehow based on the physics as seen by a local observer following the worldline γ.

It is still mysterious, however, how this setup is supposed to become isomorphic to the usual flat-space S-matrix formulation as Λ→0. In particular, the seemingly extraneous choice of γ and the restriction that only diffeomorphisms in Diff_γ and not all of Diff act on the Hilbert space look very odd in that context.

OK, OK … I just couldn’t resist this brilliant Gilbert & Sullivan parody.

“Our customers are all crooks” seems to be the motto of the members of the RIAA and the MPAA. “Without DRM, and unless we can crush online filesharing, no one will buy our stuff.”

Tim O'Reilly sees things differently. He’s a successful publisher, who’s embraced online distribution of his books, without worrying about DRM. In his experience, shoplifting is a bigger crimp on his revenues than online “piracy”.

The vast majority of writers and artist, says O'Reilly, stand to benefit more than they stand to lose from online sharing of their work (a point others have argued too). But unlike many, he sees a future for publishers, too, in the world of online distributions. Word-of-mouth doesn’t scale, so someone has to serve as the middleman to bring together millions of customers with thousands of writers and artists. And, as any cable TV subscriber will tell you, people will embrace a feature-rich pay service in preference to a feature-poor, but free alternative.

But O'Reilly doesn’t just talk the talk. He walks the walk, and makes money at it too. Read the whole article here.

I haven’t been blogging much recently. There are several things that have been keeping me otherwise occupied, but one reason is that my iBook (from which I do most of my blogging) has been consistently crashing these past few weeks.

I keep my iBook running all the time, putting it to sleep when I need to transport it, or during periods of inactivity. Usually, it only gets rebooted for System Updates. [I don’t understand why Apple insists on a reboot when a simple kextunload/kextload or restarting a daemon would do, but that’s another story …].

Or that was the case until a few weeks ago. Then, the iBook started crashing every time I woke it up from sleep. The symptom was always the same. It would awake from sleep OK, but then, a few seconds to a minute later, the disk would start churning … cha, cha, cha, chung, … cha, cha, cha, chung, …cha, cha, cha, chung …, the machine would become unresponsive, and I would eventually be forced to reboot it.

Grrrr… Must be something to do with one of the recent Software Updates, but what? Then I noticed an — at first apparently unrelated — oddity. When I ran perl scripts from the commandline, I got a warning message I had never seen before:

% somescript.pl
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LC_ALL = (unset),
        LANG = "en_US"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
...

Hmmm… The LANG environment variable didn’t used to be set. And it isn’t set on Golem, which is nominally running the same system software (MacOX 10.2.2 and Perl 5.8.0). It was easy to fix this problem, though.

% setenv LC_ALL C

overrides the LANG setting, and Perl scripts executed from the commandline now performed normally.

Then I noticed that ProcessWizard wasn’t displaying its list of running processes, as it was supposed to. Looked inside the App-wrapper and, aha! ProcessWizard relies on a bunch of perl scripts to gather and parse the information about running processes. Clearly, it was getting the same warning message, which was confusing it.

So how to set the environment variable LC_ALL so that an Application like ProcessWizard will see it? Much unsuccessful mucking about until I found this explanation. Okey Dokey! So I created a ~/.MacOSX/environment.plist file to set the environment variable LC_ALL. After logging out and back in, ProcessWizard worked again and commandline perl scripts executed correctly.

But now for the kicker! I had cured my “Wake-From-Sleep” problem too! My iBook hasn’t crashed since.

My best surmise is that the wakeup routine executes some perl script (or other locale-dependent code) which was getting screwed up by the same bug that was screwing up ProcessWizard. Setting the environment variable fixed the problem.

Bottom line: My iBook is back in service! So maybe I’ll start blogging again.

Amazon.com’s software is intended to produce that sought-after personalized shopping experience. Buyers of, say, Pat Robertson’s latest book are gently steered towards other books of a similarly “uplifting” nature.

Those ever-sharp observers at The Register recently caught this wonderful software … erm … in the act.