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June 29, 2007


My laptop is on the fritz, so there will be no further updates from Strings 2007. Which is a pity, because there have been some very cool talks since my last posting.

Update (7/04/2007):

Thanks to those who speculated on the nature of my laptop’s ailment. On Friday evening, Kelly Stelle came to my rescue, equipped with an iPod1 with a copy of MacOSX on it, a variety of cables and connectors, and a Swiss Army knife. We tried everything to resuscitate the machine. Nothing worked, until we opened it up and removed the 1GB RAM module. It then booted just fine, with the 128MB of RAM on the motherboard. Unfortunately, there’s not much you can do with 128MB of RAM. So a real fix had to wait until my return. I flew back on Monday, arriving home at 2 am on Tuesday. At 1 pm, the new memory module arrived, I installed it, and my machine was happy again.

1 The cool thing about the old-style FireWire iPods is that you can actually carry around an entire System and boot a Macintosh off of them.

Posted by distler at 5:48 AM | Permalink | Followups (6)

June 27, 2007

Remains of the Day

It would be rather useless for me to simply list the talks for the rest of Tuesday, as that information is already available.

Still, as sort of Rorschach test, it’s sorta useful to group them thematically.

Posted by distler at 1:44 AM | Permalink | Post a Comment

June 26, 2007

Witten on 2+1 Gravity

The first talk of the morning was by Witten. I’ll give him his own post.

Posted by distler at 5:50 AM | Permalink | Followups (2)

June 25, 2007

Strings 2007, Part 1

Well, the idea of live-blogging Strings 2007 kinda evaporated with the flaky wireless in the lecture hall (keeps going down every few minutes), and the fact that the place is just packed (there are over 450 participants, and the room seems to be standing room only). I guess it’s all for the best; I think I’m better when I write in complete sentences.

Posted by distler at 10:30 AM | Permalink | Followups (9)

June 17, 2007

Bloggy Matters

I’ve been too busy, lately, to do much posting. But there are a couple of blog-related matters I thought were noteworthy. So here goes …

Posted by distler at 11:07 PM | Permalink | Followups (10)

June 6, 2007

Tunnelling branes

It’s well-known that, in certain respects, low-energy brane dynamics can differ markedly from naïve field theory expectations. An important example is D-brane inflation, where the DBI action allows much steeper potentials to be compatible with inflation.

There’s a very interesting recent paper which argues that the DBI action, similarly, modifies the Coleman-de Luccia tunnelling rate, governing the decay of the false vacuum by bubble nucleation. The main effect is that the DBI action modifies the domain wall tension

(1)T=dϕV 0(2fV 0) T = \int d\phi \sqrt{V_0(2-f V_0)}

where the potential has a true minimum, V(ϕ )=V V(\phi_-)=V_- and a local minimum, V(ϕ +)=V +V(\phi_+)=V_+. V 0(ϕ)=V(ϕ)+V +/f(ϕ) V_0(\phi) = V(\phi) + V_+/f(\phi) f(ϕ)f(\phi) is the warp factor, which appears in the Euclidean DBI action as S E=2π 2ρ 3dρ(1f(ϕ)1+f(ϕ)ϕ˙ 21f(ϕ)+V(ϕ)) S_E = 2\pi^2 \int \rho^3 d\rho \left(\frac{1}{f(\phi)} \sqrt{1+f(\phi) \dot{\phi}^2}- \frac{1}{f(\phi)} +V(\phi)\right) Dot represents derivative with respect to ρ\rho and we’ve taken an O(4)O(4) symmetric ansatz. In the absence of warping, f(ϕ)=α 2f(\phi)={\alpha'}^2. In the limit f0f\to 0, we recover the usual thin-wall formula. But as V 0f2V_0 f\to 2, the tension can be much smaller than expected.

In the absence of gravity, the decay rate per unit volume Γ/Ve π 2Tρ¯ 3/2=e 27π 2T 4/2ϵ 3 \Gamma/V \sim e^{- \pi^2 T \overline{\rho}^3/2} = e^{-27\pi^2 T^4/2\epsilon^3} where ϵ=V +V \epsilon = V_+-V_- and the radius of the bubble is ρ¯=3T/ϵ\overline{\rho}=3T/\epsilon. So (1) can have a huge effect on the lifetime of the false vacuum.

Gravitational corrections modify ρ¯\overline{\rho} ρ¯=3TM pϵ 2M p 2+3T 2(V ++V )/2 \overline{\rho} = \frac{3 T M_p}{\sqrt{\epsilon^2 M_p^2 +3 T^2 (V_++V_-)/2}} and contribute an “extra” gravitational contribution in the exponent, just as is the usual case. But the qualitative effect remains: the lifetime of these metastable open string vacua can be *much* shorter than the naïve CDL result.

Posted by distler at 12:31 PM | Permalink | Post a Comment

June 5, 2007

Bit Flip

I got a frantic email from John Baez, Saturday night. Evidently, our MovableType installation had suddenly gone haywire, and various CGI actions were producing Perl errors, rather the desired results.

It sometimes happens that my mucking about with the software produces untoward side-effects. But that wasn’t the case here. I’d been out for the evening, and hadn’t touched anything.

Eventually, I tracked down the problem. The file, in which the Perl error occurred had changed. A single occurrence of “>” had become a “?”. Changing it back cured the problem.

Mind you:

  • The file is writable only by root.
  • The mtime of the file had not changed.
  • There were no other changes to the file, or to anything else in our MT installation.
  • There were no signs of intrusion or system compromise.

Now, “>” is the byte 00111110 and “?” is the byte “00111111”. So a single bit had changed. This was quite enough to send our MovableType installation into upheaval. As far as I can tell, it happened without the intervention of human hand.

I’ve read about cosmic rays causing bit-flips. Seemed like a rather unlikely risk. Now I’m not so sure …

Posted by distler at 11:29 PM | Permalink | Followups (9)