A new paper by Horava and Keeler came out a few days ago, with the whimsical title M-Theory Through the Looking Glass: Tachyon Condensation in the E_8 Heterotic String. Luboš Motl gave it a thumbs up, as do I for the use of only one copy of E8. The abstract is as follows:
We study the spacetime decay to nothing in string theory and M-theory. First we recall a nonsupersymmetric version of heterotic M-theory, in which bubbles of nothing -- connecting the two E_8 boundaries by a throat -- are expected to be nucleated. We argue that the fate of this system should be addressed at weak string coupling, where the nonperturbative instanton instability is expected to turn into a perturbative tachyonic one. We identify the unique string theory that could describe this process: The heterotic model with one E_8 gauge group and a singlet tachyon. We then use worldsheet methods to study the tachyon condensation in the NSR formulation of this model, and show that it induces a worldsheet super-Higgs effect. The main theme of our analysis is the possibility of making meaningful alternative gauge choices for worldsheet supersymmetry, in place of the conventional superconformal gauge. We show in a version of unitary gauge how the worldsheet gravitino assimilates the goldstino and becomes dynamical. This picture clarifies recent results of Hellerman and Swanson. We also present analogs of R_\xi gauges, and note the importance of logarithmic CFT in the context of tachyon condensation.
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While I realize that when the word "tachyon" comes up in these theories it applies only to the vacuum before symmetry breaking (or something like that, this is all over my head), I've always found it interesting that I've been driven to believing in preons that are tachyons.
One way to think about tachyon condensation is in terms of Hawking radiation. Imagine that you have a D-brane and its anti-D-brane separated by an open string. In the limit that these branes approach each other, they will eventually coincide, and a tachyon mode arises as the lowest mode of the open string connected the branes. A potential for the tachyon arises, and the sliding down from a local maximum to a local minimum is called tachyon condensation.
The value of the potential at the minimum is lower than its value at the maximum by twice the brane tension, corresponding to brane-antibrane annihilation and ultimately resulting in the emission of closed-string quanta. These quanta form the Hawking radiation of non-extremal black holes.
Ah! I was wondering if you'd like this paper. Thanks for the recommendation - er, though I'm not sure how far I'll get through it! Does this mean that even mainstream M theory is leaving behind the SUSY shackles?
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