Musings

Sen gave a very beautiful talk on 1+1 dimensional noncritical string theory. He worked out the relation between the infinite number of conserved charges of the Matrix model and the infinite number of conserved charges of the continuum theory. This relation was known at $\mu=0$ a decade ago. Sen’s contribution was to generalize the formula to nonzero $\mu$.

He then went on to discuss the long-standing puzzle of the subject: where is the 2D blackhole in the Matrix model? Unfortunately (to my mind), he discussed the question at $\mu=0$ — which is to say, using the formulæ available a decade ago. The idea was simply to identify the state in the Matrix model with the same set of conserved charges as the blackhole ($Q_{1,0}=M$, $Q_{j,0}=0$ for $j\gt 0$). The “solution” is to have a very large number of fermions and holes, each with very low energy.

Much of the rest of the day was devoted to fluxes.

Kachru’s talk was a nice overview of developments in flux compactifications in F-theory.

In the afternoon, Graña gave a very nice talk about her work with Minasian et al $SU(3)$ structures and Hitchin’s Generalized Calabi-Yau manifolds.

The last talk of the day was by Ofer Aharony about the deconfinement transition in large-N gauge theories at finite volume. For small enough volume, the transition can (interestingly enough) be studied in perturbation theory.

For the free theory, the path integral reduces to a unitary matrix model (for the Wilson line around the thermal circle), and one finds a Hagedorn transition (for free N=4 SYM on $S^3$) at $T_H= -1/(\log(7-4\sqrt{3})R$. The interacting theory is still governed by a Matrix model, one whose effective action is more complicated, but which can be computed in perturbation theory. The phase structure depends on the signs of the coefficients in this effective action.

Some photos from the Registration at the École Normale Supérieur:

The hall at the Collège de France is quite beautiful, but too small for the assembled multitude. Sacha Polyakov gave the first talk on his recent work on new gauge theories from strings on certain homogeneous spaces for supergroups (like $Osp(2|4)$). His model is $kappa$-supersymmetric. There are two ways to attempt to deal with $\kappa$-supersymmetry. One is to quantize covariantly, which is generally very thorny, made all the thornier here by the nonpolynomial form of his action. His approach is to fix the $\kappa$-supersymmetry, in which case one needs to check covariance of the quantum theory. He said that $\kappa$-supersymmetry implies conformal invariance, which I don’t think is true, and which, in any case, does not seem to be the issue.

View from my hotel window, dusk.

He ended with a quote from Carl Jung, which seems only appropriate.

Mike Douglas’s talk also included a quote. This one from the poet Paul Valéry,

Tout ce qui est simple est faux, mais tout ce qui ne l’est pas est inutilisable.

a wise statement, but I still fear that Mike is studying the statistics of the empty set (or very nearly empty).

My favourite talk of the morning was Walcher’s on his work with Hori on D-branes in Landau-Ginsburg Theory.

I didn’t get very much out of Vasiliev’s talk in the afternoon, but Witten’s question afterwards was very clarifying. The conjecture seems to be that Vasiliev’s higher-spin gauge theory in the bulk AdS is equivalent to a free-field theory on the boundary. Nontrivial interactions of the bulk theory are obtained when we look at correlation functions of multi-linears in the boundary fields. Or something like that …

Rajesh Gopakumar gave a very interesting talk on rewriting free field gauge theory amplitudes in a stringy fashion (motivated by the AdS/CFT correspondence (I,II). The result looks intriguingly like an integral over the moduli space of genus-g surfaces with n punctures. The idea is to take the integrand and try to reconstruct the corresponding string theory.

I’ve blogged about the $a$-maximization conjecture before, but Ken Intriligator’s talk was a really nice summary, probably the highlight of the afternoon.

Thhhaaat’s all for today, folks…

If the thunderstorms in Dallas do not totally disrupt my plans (looks like I’ve already missed my original flight to Charles de Gaulle), I should be in Paris sometime tomorrow morning. [Update (from DFW): Yup, missed that flight. My new flight gets into CDG at 11am.]

My agenda for the day involves

1. Check into the hotel and take a well-deserved shower.
2. Around 3pm (8am CDT), avail myself of the free WiFi at the hotel and do a little iChat audio conference with S. and the kids. (So far, only one iSight camera in the family, so no video. )-:)} ) [Correction: There is no “free” WiFi at the hotel; there’s 10€/2 hours WiFi at the hotel, “thanks” to Orange. And the reception is crappy to boot. Gonna have to find something else. Hmmmm … there’s a café right down the street which advertises free WiFi. I wonder what delectables 10€ will buy me there?]
3. At 4 pm, register for Strings 2004 at the École Normale Supérieur.
4. At 5pm; there’s a concert of Islamic Music at the Louvre.

The rest of the day, I’m free. So if you’re in Paris tomorrow, send me an email and maybe we’ll meet up.

In the meantime, I’ve got some time to kill here at Austin-Bergstrom; here are some papers I’m looking at to while away the hour(s).

Katz and Sharpe have been looking at worldsheet instanton corrections to certain correlation function (which correspond to spacetime superpotential couplings) in (0,2) linear σ-models. One of the interesting things that they find is that different linear σ-model presentations of the same bundle lead to different extensions of the resulting sheaves over the compactification of the moduli space of instantons. This is related to an older puzzle, namely that the condition for the cancellation of worldsheet gauge anomalies is typically stronger than the corresponding spacetime condition that $c_2(\mathcal{E})=c_2(T)$. The presentation which solves the worldsheet gauge anomaly cancellation condition is the one to use.

Basu, Green and Sethi note that the holomorphic part of the $N=4$ super-Yang Mills action is in a supermultiplet with the superconformal currents. That implies Ward identities which determine the coupling constant dependence of the correlation functions of operators in short supersymmetry multiplets. They use this to prove nonrenormalization theorems for 2- and 3-point functions.

Karl Landsteiner does the Dijkgraaf-Vafa/Cachazo-Douglas-Seiberg-Witten analysis for $N=1$ supersymmetric theories without adjoint matter.

My colleagues, Aaron and Uday propose Loop-$E_8$ bundles as the topological setting for classifying fluxes in M-Theory/(massive) Type IIA. I’ve talked to them a lot about this, and have gone from thinking that it didn’t make any sense whatsoever to being quite intrigued.

As some reader may recall, a month ago I seized the opportunity and replaced my ailing G3 iBook with a new G4 model. When the new Apple Store opened, we decided to take the old machine in and see if it could be fixed. I’m happy to report that, not only did Apple fix it, they fixed it for free.

So now, my wife has a new computer, and I have three MacOSX machines to manage (Golem and the two iBooks).

One of the things you have to deal with when you have multiple machines is keeping your data in sync. That’s where rsync is your friend. Among that data are the many gigabytes of music in my iTunes Music Library. To simplify life, I have the command

rsync --progress -v -au -e ssh --delete "/Users/distler/Music/iTunes/iTunes Music/" golem.ph.utexas.edu:"/Users/distler/Music/iTunes/iTunes\ Music/"

aliased to put_music, which keeps things nicely up-to date. Less obvious, however, is how to fix things so that my wife can listen to the library of music that resides in my account on her iBook. The obvious Unixy things don’t seem to work. The only thing that does seem to work is to leave myself logged onto her machine, with iTunes running and “Share My Music” enabled. This just seems wrong.

Matt Mullenweg made it all the way to number 1 in a Google search for “Matt”, and made good on his promise to take down his web site, if only temporarily.

Curious, I decided to find out how I rank in a search for “Jacques.” Turns out that I’m number 17, following

1. Jacques Villeneuve
2. Brian Jacques
3. Jacques Cartier
4. Jacques Derrida
5. Jacques Torres Chocolates
6. Jean-Jacques Rousseau
7. The Jacques-Edouard Berger Foundation
8. Jacques Maritain
9. Jacques Brel
10. Jacques Cousteau
11. Jacques Whitford Engineering
12. Transat Jacques Vabre (a race from Havre to Bahia)
13. Jacques Vapillon
14. Jacques Delors
15. Jacques Attali
16. Jacques Tati

This is fairly remarkable, as I’ve heard of most of the other 16, whereas I’m pretty sure none of them have heard of me.

Unlike Matt, I promise not to take my website down in the unlikely event that I ever reach number 1.

I’m only about 6 months behind in writing this up.

Back when I first implemented Comment Validation, I relied on a very simple bit of social engineering to ensure that comments got run through the Validator before posting. Near the <textarea> where you enter/edit your comment, there’s a PREVIEW button, but no POST button. The POST button only appears on the Comment Preview form when you’ve previewed the comment, and it has successfully Validated.

If you then scroll down and edit the comment, you are supposed to click on PREVIEW again. But if you actually look at the code I suggested for the Comment Preview page, you quickly realize that nothing prevents you from scrolling back up and clicking on POST instead. I ran with this system for 7 months without a single invalid comment being posted. But just because nobody’s yet come around and rattled the door handle, there is no reason to leave the backdoor perpetually unlocked.

Besides, spambots and crapflooding scripts bypass the comment form entirely, and POST directly to the comment CGI script. They’re immune to social engineering. A better solution was called for.

So, 6 months ago, I created a plugin to solve the problem. Introducing MTHash.

Installation is easy:

1. The plugin’s only prerequisite is the Digest::SHA1 Perl module.
2. Put the plugin in your plugins directory.
3. Create a text file called salt.txt with some random gibberish in it, and place that, too, in your plugins directory.

The plugin offers two new MT Container Tags

<MTSHA1Hash>...</MTSHA1Hash>

Replaces its contents by an SHA1 hash of the contents.

<MTSHA1SaltHash>...</MTSHA1SaltHash>

Replaces its contents by an SHA1 hash of the contents salted with the aforementioned salt.txt file’s contents. Unlike the previous one, this can’t be pre-computed without access to the salt.txt file.

OK, so how do we use it?

1. In the Comment Preview form, we add a hidden form field

   <input type="hidden" name="validated" value="<MTSHA1SaltHash><MTCommentPreviewBody convert_breaks='0' sanitize='0'><$MTEntryID$><MTCommentPreviewIP><$MTCommentPreviewAuthor$><$MTCommentPreviewEmail$><$MTCommentPreviewURL$><$MTCommentPreviewSubject$><$MTCommentPreviewTextFilter$></MTSHA1SaltHash>" />

   (note that the MT tags in green, for the subject and text-filter of the comment, are special to my setup; you might need to omit them both here and below).
2. Then we modify lib/MT/App/Comments.pm

   --- lib/MT/App/Comments.pm.orig Thu Jul  1 15:29:58 2004
   +++ lib/MT/App/Comments.pm      Sun Aug  1 23:34:52 2004
   @@ -229,6 +277,24 @@
        if (!$q->param('text')) {
           return $app->handle_error($app->translate("Comment text is required."));
        }
   +    require Digest::SHA1;
   +    my $sha1 = Digest::SHA1->new;
   +
   +    $sha1->add($q->param('text') . $q->param('entry_id') . $app->remote_ip
   +               . $q->param('author') . $q->param('email') . $q->param('url') . $q->param('subject') . $q->param('convert_breaks') );
   +    my $salt_file = MT::ConfigMgr->instance->PluginPath .'/salt.txt';
   +    my $FH;
   +    open($FH, $salt_file) or die "cannot open file <$salt_file> ($!)";
   +    $sha1->addfile($FH);
   +    close $FH; 
   +               
   +    my $digest = $sha1->b64digest . "=";
   +
   +    if ($q->param('validated') ne $digest) {
   +    return $app->handle_error($app->translate(
   +            "Please preview your modified entry before posting it."));
   +    }
   +
        my ($comment, $commenter) = _make_comment($app, $entry);
        if (!$blog->allow_unreg_comments) {
           if (!$commenter) {

That’s it! If you modify your comment, now you must preview it again before posting. [Revised (8/1/2004) to be even more bulletproof. Revised (9/1/2004) for MT-3.1 compatibility.]

As a side benefit, spambots, which post directly to the Comment CGI script don’t work any more. Some of my friends who’ve been using this technique (without the Validation part) tell me it works wonders against spambots.

I wouldn’t know; I don’t get comment spam anymore (so far, only 15 spam comments in over 8 months).

[via Sean Carroll] Now, I know I’m not supposed to believe any physics papers published in Nature, but this one — if true — is pretty interesting. It seems that DØ has been reanalyzing their data on the top quark, and their new analysis pushes the central value for the mass up by 4 GeV. The previous world average value was $m_t = 174.3 \pm 5.14$ GeV. The new value is $178 \mp 4.3$ GeV.

This may not sound like much of a change, but if you recall, the Higgs mass in the MSSM depends rather sensitively on $m_t$. The one-loop contribution, up to logarithmic corrections, goes like $m_t^4$,

where $\lambda_t= m_t/\langle H\rangle$ is the top Yukawa coupling, $m_{\tilde{t}}$ is the stop mass, and $\tan(\beta)= \langle H\rangle/\langle\tilde{H}\rangle$.

To accommodate the current experimental lower bound, $m_h\gt 114$ GeV, one needs a very heavy stop, $m_{\tilde{t}} \gt 850$ GeV. And, even so, one has an upper limit of something like $m_h \lt 135$ GeV.

Shift the top mass up by $4$ GeV, and the upper limit on $m_h$ also shifts up to $m_h \lt 140$ GeV. But, more significantly, the central “best-fit” value goes up from the now-experimentally excluded $96$ GeV to $117$ GeV. Just out of range of what would have been seen at LEP, but about the first thing they’ll see at the LHC.

The mass of the lightest Higgs boson in the minimal supersymmetric standard model (MSSM). The predicted value is shown as a function of the parameter $\tan\beta$ (the other MSSM parameters are chosen such that they maximize the resulting value of the Higgs mass). The predicted Higgs mass is sensitive to the value of the top-quark mass used in the calculation. The solid line indicates the prediction using the new measurement of the top-quark mass from the DØ Collaboration; the white band indicates the uncertainty of the prediction that results from the error on the top-quark mass. The dashed line shows the situation before the new measurement (the previous experimental error of 5.1 GeV/c² is not shown). Based on the new value of the top-quark mass, an upper bound on the mass of the lightest MSSM Higgs boson of about 140 GeV/c² is established. [Nature 429 (10 June 2004) p. 614.]

It’s June and summer has arrived in Austin. This week, we saw a couple of 40 ℃ days. And there will be many more 38+ ℃ days before the weather gets nice again at the end of September.

Yesterday, when I embarked on my customary weekend 10.1 mile run (Mopac to the Longhorns Dam) on Town Lake¹, I had a new high-tech gadget, to accompany the heart monitor and the iPod: a Camelbak FlashFlo. The new gadget was purchased at S’s insistence, out of fear, I suppose, that I will keel over in the middle of one of my workouts². It may have been “only” 34 ℃, but I must say that sipping cold water, while listening to Eric Dolphy made the run much more pleasant.

Mopac (“A”) to Longhorns Dam (“F”) running trail on Town Lake.
(Mopac to 1^st St. (“C”) is also indicated.)

The trick with the Camelbak seems to be to fill with water, and stick the whole contraption in the freezer. Just remember to take it out far enough in advance, so that it’s half-defrosted by the time you hit the trail.

Summer in Austin may be tough, but now I’m ready.

Back when I was just starting this weblog, the Bogdanov brothers provided fodder for a number of highly amusing posts. It was a good way to get things rolling and, while this weblog has gone on to bigger (and hopefully better) things, I still maintain a soft spot for old Igor and Grichka B. and their antics. Many of you probably feel the same way.

Back in December, I received some correspondence which clearly indicated something was afoot on the Bogdanov front. And now, dear readers, your patience has been rewarded. The brothers have a new book out, Avant le Big Bang. It’s currently number 9 in sales on amazon.fr.

In it, they apparently claim that their erstwhile critics have retracted their criticisms. Fabien Besnard has been following up with the allegedly former critics.

I almost got to play along. Fortunately, my habitually prickly demeanor kept me out of trouble.

A minor update to my MovableType itexToMML plugin. With this update, the itex to MathML with parbreaks filter is much smarter about skipping over block-level tags. There’s no change to the itex2MML executable, which was last updated at the end of April.

If you’ve tried commenting on this blog, you’ll also have noticed that I have Textile with itex to MathML and Markdown with itex to MathML filters available. If you have Textile 1.1 and/or Markdown 1.0 installed on your system, you can get the same functionality on your blog by

1. Applying this patch to Textile and/or this patch to Markdown.
2. Installing the TextileMarkdownMML plugin.

(Anyone who wants to take a crack at getting this to work with Textile 2.x is welcome to try.)

Finally, a minor update to my WordPress patch, which enable my WordPress plugin to work nicely with the existing WordPress text filters (wptexturize, Textile1, Textile2 and Markdown). For those who’ve already applied my previous patch, the new bit is

--- wp-includes/functions-formatting.php.orig   Sun May 16 17:14:14 2004
+++ wp-includes/functions-formatting.php        Thu May 20 12:27:40 2004
@@ -103,7 +104,7 @@
        $content = preg_replace('/<category>(.+?)<\/category>/','',$content);

        // Converts lone & characters into &#38; (a.k.a. &amp;)
-       $content = preg_replace('/&([^#])(?![a-z]{1,8};)/i', '&#038;$1', $content);
+       $content = preg_replace('/&([^#])(?![a-z]+;)/i', '&#038;$1', $content);

        // Fix Word pasting
        $content = strtr($content, $wp_htmltranswinuni);

Update (8/25/2004): Updated for Markdown 1.0.

I’m always on the lookout for good review articles on various topic in string theory. For one thing, graduate students are always asking for them. For another, I find them useful for catching up on topics that I may not have been paying sufficiently close attention to.

It’s not often that I find one which is wholly satisfactory. Today, however, I get to sing the praises of Marcos Mariño’s Chern-Simons Theory and Topological Strings.

Marcos starts with a review of Chern-Simons Theory and topological string theory, and builds up to a review of Gopakumar and Vafa’s large-N duality, geometric transitions and the construction of the topological vertex.

All within a digestible 46 pages. Really a great read, about an imminently important subject. Highly recommended.