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February 27, 2005

Cole Cr#%

Juan Cole is an indispensable source of information and insight into the situation in Iraq. He really knows the country, and collates an impressive amount of information from both the Western and Arabic Press.

His blog is compelling reading, but it would be that much more compelling, if it were not laced with tendentious crap like

AP reports that the one-day total for war-related violence in Iraq, including the police station bombing in Tikrit reported here yesterday morning, came to 30. That is about 11,000 persons a year if the rate were constant and extrapolated out.

Do you really want to take a one-day death count (and an anomalously high one, at that) and extrapolate that forward for a year? Do you think the resulting number has any meaning?

With a couple of minutes more work, you could take a 5-day running average death count (as reported on his blog and cross-referenced with iraqbodycount) and extrapolate that number forward. Isn’t 4745 deaths/year bad enough?

Given the inadequacies of the reportage, it’s likely to be an underestimate. But, at least, it won’t be instantly dismissed — by any reader with an ounce of sense — as a statistical fluctuation.

Posted by distler at 9:45 AM | Permalink | Followups (2)

Conversations with Greg

Greg Moore was in town for a few days, and we had — as always — some very interesting discussions. Among the topics was his recent paper with Dabholkar, Denef and Pioline.

I’ve talked before about the Ooguri-Strominger-Vafa proposal relating the entropy of a charged N=2N=2 black (which appears as a nontrivial solution of type-IIA strings compactified on some Calabi-Yau, MM) to the topological string partition function for the same Calabi-Yau. Specifically,


Ω(p,q)=dϕ|e iπ2F(p+iϕ,256)| 2e πqϕ \Omega(p,q) = \int d\phi \left|e^{i\frac{\pi}{2}F(p+i\phi,256)}\right|^2 e^{\pi q\cdot\phi} where (p I,q I)(p^I,q_I), I=0,,h 1,1(M)I=0,\dots,h^{1,1}(M) are the electric and magnetic charges, F(X I,λ 2)F(X^I,\lambda^2) is the holomorphic topological string free energy, and Ω\Omega is a microcanonical partition function — the number, or perhaps some index, of the number of states of charge (p,q)(p,q).

There are three questions about this formula

  1. What contour of integration should be chosen (if one exists) so that the integral is well-defined?
  2. Exactly what is Ω(p,q)\Omega(p,q) counting?
  3. Is the formula right?
Posted by distler at 1:59 AM | Permalink | Followups (1)

February 23, 2005


Back in second grade, I was dissatisfied with the algorithm we were being taught for doing subtraction. So I “invented” my own

– 185

5 is bigger than 3, so we subtract them in the opposite order (5–3=2) and take the tens-complement (8) of the result. As usual, we borrow from the 6 (which becomes a 5) and we repeat: 8–5=3 and take the tens-complement (7). Finally 2–1=1, so the answer is 178.

While only slightly different from the conventional algorithm, I felt this one to be an improvement because I never had to know how to subtract from numbers larger than 10 (e.g. who cares what 13–5 is?).

I haven’t thought much about this little juvenile act of rebellion until a couple of months ago, when I was going over my daughter’s 3rd grade math homework with her. She was doing similar subtraction problems. But, in keeping with the times, she was charged with explaining her methods for arriving at the answer.

Imagine my surprise when she explained her method to me. It was exactly the same “unconventional” algorithm that I had used when I was her age. It was not what the teacher had taught; she had figured it out on her own.

[Her method was the same, but her accuracy was not the greatest. So I taught her the other trick that I learned in that era: check your work by doing arithmetic modulo 9: 178=1+7+8= 7 mod 9, 185=1+8+5=5 mod 9. So 178+185=5+7=12=1+2 = 3 mod 9, which agrees with 363=3+6+3=3 mod 9.]

Now, I don’t know what this has to do with Larry Summers’ remarks on the dearth of women in the Hard Sciences (at least in this country). My personal experience echoes that of the AIP Study. An alarmingly large majority of the women who arrived at Harvard the year I did, intending to major in Physics, had decided by sophomore year to do something else. As a consequence, it was unsurprising that, by the time I started graduate school, there was only one woman in an entering class of 28. Sean Carroll takes on the thankless task of confronting Summers hypotheses with the data. I’m afraid I can’t muster the energy.

I’m much too busy trying to nurture that spark of creativity in my daughter, hoping that, a decade from now, she doesn’t face the stark choice that my classmates at Harvard/Radcliffe faced a generation ago.

Posted by distler at 4:28 PM | Permalink | Followups (6)

February 17, 2005

Internationalization and Trackbacks

The last straw was when I received a Korean trackback, encoded in euc-kr.

The Trackback Specification makes no mention of character encodings, and MovableType’s original implementation was blissfully ignorant of any such notion. The sender of a Trackback ping sent a string of bytes (which represented a string of characters in charset of his blog) and the recipient dutifully published that string of bytes on his blog. If the recipient’s charset happened not to be the same as that of the sender, well, then, the result was gibberish.

The most recent versions of MovableType convey the sender’s charset in the HTTP headers of the Trackback. But the recipient doesn’t actually do anything with the information.

As a result, I had a slowly increasing number of gibberish Trackbacks on my blog, with no end in sight.

If you want something done right …

Posted by distler at 1:04 PM | Permalink | Followups (18)

February 16, 2005

Coming Soon to a Hard Drive Near You

Once upon a time, a megabyte was a lot of data. In 1989, when Joanne Cohn first started emailing preprints to a couple of hundred colleagues, people quickly found themselves exceeding their mail quotas. And not everyone was interested in every paper. So why waste bandwidth and precious disk space on all that junk?

The idea of centralizing the storage, and sending people only the papers they requested, prompted Paul Ginsparg to start the hep-th archive in 1991.

Flash forward 14 years.

250 GB hard drives are cheap, and a laptop with less than 60 GB seems positively claustrophobic. Hep-th has grown tremendously. But even with over two hundred submissions a month, it’s still a puny amount of data by today’s standards. The entire archive from 1991 to the present (8GB of PDF files) fits easily on an iPod.

So, in reversal of history, Joanna Karczmarek is gearing up to distribute the whole shebang via bittorent. She’s currently offering 2004 (pdf papers and a plain-text list of abstracts) as a modest 850 MB torrent.

A year’s worth of physics for your iPod Shuffle.

Posted by distler at 3:27 PM | Permalink | Followups (6)

February 13, 2005

Words to Live By

“Don’t fire a gun while you’re driving a car.”
— my 9 year old daughter, admonishing her 4 year old brother

Posted by distler at 12:42 AM | Permalink | Post a Comment

February 11, 2005

Berkovits Update

I’ve spent the past few days in a lengthy email correspondence with Nathan Berkovits about some of the questions raised in my previous post about his “pure spinor” approach to the superstring.

The main progress has been in clarifying his construction of the bilocal operators, b^(y,z)\hat{b}(y,z), which play the role of anti-ghosts in his theory. Recall that I was more than a little worried about the apparent zz-dependence of the amplitudes.

Posted by distler at 11:43 PM | Permalink | Post a Comment

February 8, 2005

MathML News

“Not another one!” I hear you groan.

Posted by distler at 10:34 PM | Permalink | Followups (6)

February 5, 2005

Who Do You Trust?

The debate over Google’s new rel="nofollow" attribute for “untrusted” links continues to simmer. I explained our (Musings and the String Coffee Table’s) policy a while back. Trackbacks and Comment-Author Links are innoculated with rel="nofollow".

But then I got to thinking. There is, surely, one class of Comment-Author Link that I do trust: authors who have gone to the trouble to PGP-sign their comments. Previously, PGP-signing your comments gave you that warm feeling of knowing that you cannot be impersonated, nor the text of your comments tampered-with, without that being evident to anyone who clicks on the verification link. But now, PGP-signing your comments buys you that extra little ε of Google PageRank as well.

Comment-Author Links of PGP-signed comments are exempt from the rel="nofollow" policy.
So … go generate yourself a PGP key, put your public key on your website (make sure it’s served right), start signing your comments here, and watch your PageRank soar.

Well, OK, maybe not the last one. But this is a wee bit more incentive to do what you should be doing anyway.

Posted by distler at 12:41 AM | Permalink | Followups (11)

February 4, 2005

Multiloop Amplitudes

Surprising to say, at this late date, but there’s been considerable recent progress in multiloop string perturbation theory.

D’Hoker and Phong have a pair of new papers, looking at genus-2 scattering amplitudes. I’ve written about their previous work in some detail. The current papers extend their story to N-point functions at genus-2.

Meanwhile, prodded by Luboš who, in his weblog post and privately, has been championing Nathan Berkovits’s pure-spinor approach to the covariant Green-Schwarz superstring, I decided to take a closer look.

The action, in Nathan’s theory looks deceptively simple:

(1)S=d 2z[12x m¯x mp α¯θ αp˜ αθ˜ α+w α¯λ α+w˜ αλ˜ α] S = \int d^2z[-\textstyle{\frac{1}{2}} \partial x^m\overline{\partial} x_m - p_\alpha\overline{\partial}\theta^\alpha - \tilde{p}_\alpha\partial\tilde{\theta}^\alpha + w_\alpha\overline{\partial}\lambda^\alpha + \tilde{w}_\alpha\partial\tilde{\lambda}^\alpha ]

The only wrinkle is that the commuting ghost fields λ α\lambda^\alpha, λ˜ α\tilde{\lambda}^\alpha obey a pure-spinor condition

(2)λ tγ μλ=0,λ˜ tγ μλ˜=0 \lambda^t \gamma^\mu \lambda = 0,\qquad \tilde{\lambda}^t \gamma^\mu \tilde{\lambda} = 0

so, despite appearances, this is not a free field theory. For compatibility with the pure-spinor constraint, the antighosts, w αw_\alpha, have a gauge-invariance

(3)w αw α+Λ m(γ mλ) α w_\alpha \to w_\alpha + \Lambda^m(\gamma_m\lambda)_\alpha

The “BRST operator” is

(4)Q =λ αd α d α =p α12γ αβ mθ βx m18γ αβ mγ mγδθ βθ γθ δ \array{\arrayopts{\colalign{right left}} Q & =\oint \lambda^\alpha d_\alpha\\ d_\alpha & = p_\alpha -\frac{1}{2}\gamma^m_{\alpha\beta} \theta^\beta \partial x_m - \frac{1}{8}\gamma^m_{\alpha\beta}\gamma_{m\gamma\delta} \theta^\beta\theta^\gamma\partial\theta^\delta }

Because of the gauge-invariance, however, w αw_\alpha can only appear in gauge-invariant combinations like

(5)N mn=12w α(γ mn) β αλ β,J=w αλ α N_{m n} = \frac{1}{2} w_\alpha (\gamma_{mn})^\alpha_\beta \lambda^\beta,\qquad J= w_\alpha \lambda^\alpha

and correlation functions involving these objects (and the λ\lambdas), says Nathan, can be computed using free fields. Unfortunately, there’s no candidate (composite) local operator, bb, which satisfies {Q,b}=T\{Q,b\}= T. Instead, Nathan has a rather strange prescription to contruct a bilocal operator, of ghost number zero, which satisfies

(6){Q,b^(y,z)}=T(y)Z B(z) \{Q,\hat{b}(y,z)\} = T(y)Z_B(z)


(7)Z B=12B mnλγ mndδ(BN) Z_B = \frac{1}{2} B_{mn} \lambda\gamma^{mn}d \delta (B N)

for some constant antisymmetric tensor BB. Aside from the strange bilocality, we by construction break the Lorentz-invariance in our definition of the b^\hat{b}s.

Similarly, the dimension-(1,1) (integrated) vertex operators are not built from the dimension-0 BRST cohomology, VV, by acting with bb. Instead, they’re constructed in an ad-hoc way as ghost-number zero fields satisfying [Q,U]=V[Q,U]= \partial V. And, in order to define the amplitudes, one needs a plethora of further insertions of non-Lorentz-invariant “Picture-changing Operators” (of which Z BZ_B above was an example).

All of these various sources of non-Lorentz-invariance, says Nathan, only change the integrand by surface terms. And, if you use a certain prescription for integrating over the zero modes of the λ\lambdas (remember, it’s a nonlinear space), all will be OK.

As you can tell, I have many, many questions about this — very interesting — proposal. But I’ll close with four:

  1. Is it true that

    (8) zb^(y,z)=[Q,] \partial_z \hat{b}(y,z) = [Q,\cdot]

    (as surely is required for a sensible amplitude)? The expression for b^(y,z)\hat{b}(y,z) is deucedly complicated, and I can’t see why this is true.

  2. The usual relation that b 1b¯ 1b_{-1}\overline{b}_{-1} acting on the dimension-0 (fixed-location) vertex operator gives you the dimension-(1,1) (integrated) vertex operator is crucial to the proof of unitarity of multiloop amplitudes (so crucial, that we rarely think about it). What replaces that here?
  3. The unphysical poles that one encounters in multiloop NSR amplitudes when one naïvely uses the picture-changing formalism is a consequence of the index theorem applied to the bosonic ghosts (not, as implied in footnote 12, of bosonizing those ghosts). One might worry that similar poles arise here.
  4. Is it really true that the only legacy of the nonlinear nature of the pure-spinor constraint is in the zero-mode integration?
Posted by distler at 12:30 PM | Permalink | Followups (5)

February 1, 2005

Trackback Spammers

The group of spammers I blogged about previously, the ones using crapflooding techniques (multiple POSTs from behind anonymous proxies) for comment spam, finally returned, this time as Trackback spammers. Lotta people seem to have been hit hard.

Since it took the Crapflooders only a week or so to figure out that Trackback flooding was easier and more fun than Comment flooding, I was wondering when these spammers would come to the same realization. Last night, they finally did. Golem received several hundred trackback attempts, in two concerted waves.

When the crapflooders were at it, a throttle on the number of trackbacks in a given time-period was my main defence. That throttle is now built-into MT 3.1x.

Since then, I’ve wised up a bit, and block submissions (of Trackbacks and Comments) from open HTTP Proxies. Thanks to Brad Choate’s plugin, modified to use the DNSBL list of open Proxies (instead of the irrelevant list of open SMTP servers), all of the hundreds of would-be spam Trackbacks were blocked.

The modifications to Brad’s plugin are easy,

--- plugins/   Thu Nov 11 11:06:29 2004
+++ plugins/        Thu Nov 11 11:08:58 2004
@@ -12,9 +12,9 @@
     my ($eh, $app, $comment) = @_;
     my $remote_ip = $app->remote_ip;
     my ($a, $b, $c, $d) = split /\./, $remote_ip;
-    if (checkdnsrr("$d.$c.$b.$")) {
+    if (checkdnsrr("$d.$c.$b.$")) {
         $app->log("Blocked comment post from known open proxy: $remote_ip");
-        my $url = "$remote_ip";
+        my $url = "$remote_ip";
         # we're forcing out the header here and exiting since I can't find
         # a cleaner way to force a redirection to the site...

The only surprising thing was how well it performed.

Update (2/2/2005):

Zack is, alas, correct. My “internal working version” is a little more heavily hacked than I let on (or even remembered). So, pending Brad releasing a new version of his plugin, here’s a (slightly neatened-up) canned replacement which filters both Comments and Trackbacks.

Update (2/4/2005):

Ever the scientist, I decided to check whether the success of the open proxy list in covering the particular proxies used by these spammers in their recent Trackback Spam runs was due to dumb luck or to genuine comprehensiveness. So I decided to look up a much larger sample of IP addresses, used in recent weeks by these lowlifes for referrer spam and their (feeble attempts at) comment spam. The result is that lists only about half of those IP addresses1. In other words, I got lucky2.

So I’ve begun to deploy some other countermeasures against them, which I will surely write about anon. In the meantime, there’s a wee buglet in the plugin I posted the other day. If you downloaded it, please download it again.

1 Looking back at a weeks-old list of proxies may not be a reliable measure. Many of these may once have been open, but are now closed and delisted. So this surely understates the effectiveness of the Blitzed list. By how much is hard to tell.

2 In case you’re wondering about the Central Limit Theorem, a spam run seems to use about a dozen different proxies, but these are likely not uncorrelated. If the spammers use trojanned PCs for their spam run, none of them will show up on the Blitzed list. Conversely, if they happen to use open proxies, which have previously been used to connect to certain IRC channels (monitored by the BOPM), they all will appear.

Posted by distler at 5:57 PM | Permalink | Followups (16)