## March 26, 2008

### Bagger-Lambert

The low-energy theory on a (stack of) D2-brane(s) is a maximally-supersymmetric gauge theory in 2+1 dimensions. The Yang-Mill multiplet has a gauge field and 7 real scalars in the adjoint representation. At least if you are out on the Coulomb branch, where the gauge symmetry is Higgsed down to the Cartan, you can dualize the gauge fields to another scalar, which is circle-valued.

The M2-brane is obtained as the strong-coupling limit of this theory. The radius of the circles (one for each element of the Cartan subalgebra) go to infinity, and the $SO(7)$ R-symmetry is promoted to $SO(8)$. This strong-coupling limit is superconformal, but the above description is effective only for the free theory, where the M2-branes are separated (away from the origin, the moduli space looks like $\mathbb{R}^{8n}/S_n$). The theory of coincident M2-branes is an interacting SCFT which, so far, does not have a Lagrangian description. But there’s no theorem that rules out a Lagrangian description, so there may just be one.

Bagger and Lambert recently proposed a very interesting maximally supersymmetric interacting 2+1D Lagrangian field theory which, at least classically, seems to be superconformal. It does not arise as the dimensional reduction of some higher dimensional theory, and so it was missed in previous attempts at tackling this problem.

I never got around to blogging about Bagger and Lambert’s paper, but Bandres, Lipstein and Schwarz wrote a nice followup, which gives me an excuse to return to the subject.

#### Update:

Whoops! Even as was typing this, Mark van Raamsdonk came out with a paper making some of the points below. I’d better hurry up and post this, before there are yet-more followup papers to discuss.
Posted by distler at 11:37 AM | Permalink | Followups (9)

## March 21, 2008

Are you a (graduate or undergraduate) student with some free time this summer? Wanna earn \$4500 doing a great public service? Google Summer of Code will pay you a stipend to work on WebKit. Among the project ideas listed, the one most relevant to readers of this blog is the project to bring MathML support to WebKit.

If you’ve tried reading this blog in Safari, or on an iPhone, you know how great that would be.

Just don’t let someone cajole you into settling for a stylesheet.

Posted by distler at 5:06 PM | Permalink | Followups (3)

## March 16, 2008

### Fiscally Conservative

No doubt, as this political season progresses, we will hear all sorts of dire warnings about the terrible things that will happen to the Economy, if a Democrat is elected to the Presidency. Surely, he (and, yes, it will be a he) will follow in the footsteps of his predecessors, raise taxes, boost spending, and drive the Economy into the ground. If you want good stewardship of the Economy, vote for the Republican, as they are — after all – the party of business.

Back in 2000, in my pre-blog era, I decided to compile a few statistics to test out these assertions. Now that 2008 has rolled around, I’ve updated my spreadsheet to include the George W Bush era, and answer a few of the objections to the previous, not-widely-circulated, version.

I decided to look at two items: budget deficits and real GDP growth. The historical data goes back to 1930. And I did the simplest possible thing: separate out the time-series into Republican and Democratic Administrations, computing the average annual budget deficit, and the average annual real GDP growth for each.

Now, there are several immediate objections you could raise.

• There are, to be sure, lots of exogenous factors which influence economic performance. In any given year, one can ascribe performance to something other than who occupies the White House. But that’s where the law of large number comes into play. If you average over many years, these exogenous factors should cancel out. The longer the historical baseline, the more likely it is that you’re seeing a real “inter-party” effect.
• That said, the Great Depression and World War II were truly singular events with a dramatic effect on these averages. In 1932, real GDP contracted 13%. Perhaps it’s unfair to blame the Great Depression on the Republicans. By the same token, real GDP contracted 11% in 1946, in the great post-War contraction. It would be equally unfair to blame that on the Democrats. On the deficit side, the cost of waging WWII was extraordinary. The On-Budget Deficit in 1943 was an eye-popping 30.8% of GDP. For both of these reason, you might not want to take the first two rows, in each of the tables below, too seriously.
• Less obvious, but equally salient, you probably should assign the performance during the first year of each Administration to the previous one. Arguably, the economic policies of the Administration only really begin to kick in its second year. I’ve presented the data both ways.
Posted by distler at 4:27 PM | Permalink | Followups (12)

## March 15, 2008

### Phun with Rails

Content-management systems that produce well-formed XHTML are not exactly thick on the ground. When it comes to Wiki software, there’s my branch of Instiki and, … umh ….

Which means that Instiki gets more than its share of attention from those interested in the question of whether XHTML is suitable for the Web.

Philip Taylor has been tireless in poking holes in various peoples’ XHTML implementations. Recently, Philip found a pair of issues in Instiki. Both were quickly fixed, but they illustrate my general maxim that any instance of a well-formedness issue is very likely an XSS issue as well.

Of the two issues that Philip found, the more serious one had to do with the author IP Address displayed at the bottom of each wiki page, next to the author’s name. What could be dangerous about an IP Address?, you ask. Well, in this case, it’s generated using Rails’s request.remote_ip method. And that, in turn, uses the HTTP Client-Ip header, if one has been set.

Install, say, Firefox’s Modify Headers extension, and you can set the Client-Ip header to whatever the heck you want. As Philip ably demonstrated, this can make the targeted page ill-formed, but it can equally-well be used to inject an XSS attack.

Arguably, Rails itself should take care that this method returns an actual IP address, rather than arbitrary garbage, but it’s easy enough to fix at the application level.

require 'resolv'
def remote_ip
ip = request.remote_ip
logger.info(ip)
ip.gsub!(Regexp.union(Resolv::IPv4::Regex, Resolv::IPv6::Regex), '\0') || 'bogus address'
end

Anyway, the bottom line is: if you’re using my branch of Instiki, please upgrade immediately to version 0.14pre(MML+).

If you’re using the main branch of Instiki, I have committed the requisite fixes to SVN Source Tree and contacted the maintainer (twice). Presumably, he will roll out a security update.

#### Update (3/17/2008):

Matthias has released new version of the main branch of Instiki, with the fixes rolled in.
Posted by distler at 9:43 AM | Permalink | Followups (4)

## March 10, 2008

### Exceptional F-Theory.

I’ve been reading Beasley, Heckman and Vafa’s recent 125 page opus, hoping to get through it before the promised Part II comes out.

F-theory is the fancy name for Type IIB string theory with 7-branes. If we compactify on $B$ (for compactifications down to 4 dimensions, we’re interested in $B$ a complex 3-fold), the 7-branes are wrapped on divisors in $B$. The complex IIB coupling, $\tau$, has monodromies as we circle those divisors and, viewing it as the modulus of an elliptic curve, we get the total space of an elliptically-fibered Calabi-Yau 4-fold, $X\to B$.

Except for the case where one has only D7-branes (and orientifold O7 planes), $Im(\tau)$ cannot be taken to be uniformly large. So perturbative string theory techniques are not applicable. General configurations of 7-branes are hard to study, except in some special cases.

The interest, here, is to study a local model for a wrapped 7-brane, or perhaps a pair of 7-branes intersecting transversally, and study the local physics from the point of view of the twisted SYM theory living on the brane.

Posted by distler at 9:34 AM | Permalink | Followups (1)

## March 9, 2008

### Battery

Having dithered during the last (cheaper) round, I signed us up for wind power. As a result, our fuel charge (one component of our electric bill) is now pegged at a small premium to the current rate, but is guaranteed for the next 14 years.

In other electrical news, …

My iBook’s battery’s slow, inexorable decline had reached the point of 25 minutes of use, before it would put the machine to sleep. So I decided to get a replacement.

Alas, I have two complaints

1. The new battery is rated at 5000 mAH. But, when you plug it in, you find its ‘current’ capacity is 4400 mAH. The manufacturer says that it will reach its rated capacity after a few discharge/recharge cycles. I have not found this to be the case, and have given up trying.
2. Related, but much more annoying: the OEM batteries, when they get low, issue a dialog box, warning you that they are running low, and then a few minutes later, they put the computer to sleep. With this battery, there’s no warning dialog box and, rather than going to sleep, the computer abruptly shuts down, losing all your work.

After having Tech support walk me through the usual useless magical incantations, I sent back the first battery as defective, but when the second battery exhibited exactly the same problems, I threw up my hands, and decided I needed to learn to live with it.