## September 26, 2008

### Planet Hopping

The eagle-eyed among you may have noticed something subtly different about Planet Musings. I got quite annoyed that the existing software (Venus) was unable to handle my own Atom feed. Apparently, the Universal Feedparser is weak, and easily confused by posts like this one.

I needed something better. So, despite Sam’s diffidence, I decided to upgrade to Mars. I made a few changes to the code (which I dutifully passed on to Sam), and it seems to be functioning as well or better than Venus, without the embarrassment of it choking on my own posts.

Do let me know if you encounter any glitches. In the case of the Red Planet, I’m rather more willing to dive into the code, to fix them.

#### Update (10/5/2008):

I was going to point to the URL of the Git repository for my branch of Mars, but Sam has been so expeditious, in merging my changes into the main branch, that that seems unnecessary. Up till now, I’ve been using BZR for for my DVCS needs. This is my first experience with Git and, overall, I’m favourably impressed.
Posted by distler at 12:21 AM | Permalink | Followups (2)

## September 22, 2008

### 3D Mirror Symmetry

Nick Proudfoot was here last week talking, in the Geometry and String Theory Seminar, about his work with Braden, Licata and Webster. I gave a talk two weeks ago, laying out the physics background. And, since it is probably of more general utility, I thought I would reproduce some of it here.

Our subject is “Mirror Symmetry” for $D=3$, $\mathcal{N}=4$ supersymmetric gauge theories (8 real supercharges).Much of what I have to say is based on the papers of Intriligator and Seiberg and de Boer et al.

Posted by distler at 10:25 PM | Permalink | Followups (4)

## September 15, 2008

### Tidbit

One concept emphasized in Harry Collins’s talk at the Science in the 21st Century Conference was that of “tacit knowledge,” something brought home to me, later in the week, in a conversation with Rob Myers. He and collaborators noticed that, in AdS/CFT backgrounds which admit (at most) $\mathcal{N}=2$ supersymmetry in the UV, curvature-squared terms are allowed in the effective bulk supergravity, and that this modifies the lower bound on the ratio of shear viscosity to entropy density, $\eta/s$, in the boundary theory. They found $\frac{\eta}{s}\geq \frac{1}{4\pi} (1-4\lambda_{\text{GB}})$ where $\lambda_{\text{GB}}$ is the coefficient of the Gauss-Bonnet density in the bulk effective action. They also argued that

(1)$\lambda_{\text{GB}}\leq \frac{9}{100}$

because, otherwise, one finds acausal behaviour in the boundary SCFT.

## September 14, 2008

### My Dinner with Garrett

I had a college chum named David. David knew how to live well. He had a perennial twinkle in his eye, as he recounted his last escapade, or told you about his plans for his next. David finished his Senior Thesis a half year early and then spent most of the Spring Semester following the Grateful Dead around the country1.

I was reminded of David, when I sat down recently to lunch with Garrett Lisi. Garrett is one of those free spirits whom many of us (with comparatively humdrum lives) find charming to be around. Garrett seems to also have charmed the folks at FQXi. He received a grant in 2007 to develop a “Theory of Everything” which, as it turns out, has no chiral fermions (and could not possibly have any). Which is fair enough. Most ideas in physics don’t work out, and the only way to find out what works, and what doesn’t, is to try. So it makes total sense to fund the attempt. It’s not so clear to me that another grant, to “further develop the ‘$E_8$ Theory’” makes sense, but luckily, I’m not on the FQXi selection committee. More remarkable, yet, was that he said he’s organizing a Workshop about his “theory”, and was trying to ascertain whether I would be worth inviting (I suspect the answer was “no”).

So I asked Lisi how he intended to further develop a “Theory of Everything” which, it was already known, could not contain chiral fermions. He said that he was still hoping to obtain chiral fermions (somehow or other) and that complex $E_8$ was one possibility. Another possibility had something to do with abandoning the whole notion of Lie algebras, but I’m not sure what’s left of the “$E_8$ Theory” then.

I should point out that, in many ways, $E_{8\mathbb{C}}$ is much simpler. Proving that a Lisi-esque theory, based on one of the noncompact real forms of $E_8$, cannot contain chiral fermions requires either a somewhat ugly brute-force calculation, or a cleverer, but slightly indirect argument. In the case of complex $E_8$, even a brute force calculation takes only about a page.

So, in the interest of reducing my carbon footprint (and at the cost of boring my readers) …

Posted by distler at 11:39 PM | Permalink | Followups (7)

## September 10, 2008

### Blogging is the New Black

David Berenstein joins the blogosphere.

I gave a talk, entitled Blogs, Wikis, MathML: Scientific Communication in a New Century at the Science in the 21st Century conference. The sound quality is a little dodgy, and the synchronization of the video with the slides could be better but, all in all, it’s not half bad.

Posted by distler at 12:03 AM | Permalink | Followups (10)

## September 9, 2008

### A Soft Pion Theorem

I’ve been meaning to write a longer post about Arkani-Hamed et al’s paper on recursion relation and $\mathcal{N}=8$ supergravity. I keep getting distracted by other things, and besides, there’s a beautiful result (of much broader interest) buried there, that deserves to be highlighted. It’s a “soft pion theorem” that we all should have learned about in grad school, but didn’t.