Joe Polchinski’s contribution to the ongoing Munich meeting has now appeared on the arXiv, with the title String Theory to the Rescue. Evidently he’s not actually to be at the meeting, I’m not sure how his paper will be presented.
It’s pretty much the usual hype about the string theory and the multiverse, with untestable ideas about quantum gravity the only topic. The one innovation is that it contains a calculation: the probability of a multiverse is 94%:
To conclude this section, I will make a quasi-Bayesian estimate of the likelihood that there is a multiverse. To establish a prior, I note that a multiverse is easy to make: it requires quantum mechanics and general relativity, and it requires that the building blocks of spacetime can exist in many metastable states. We do not know if this last is true. It is true for the building blocks of ordinary matter, and it seems to be a natural corollary to getting physics from geometry. So I will start with a prior of 50%. I will first update this with the fact that the observed cosmological constant is small. Now, if I consider only known theories, this pushes the odds of a multiverse close to 100%. But I have to allow for the possibility that the correct theory is still undiscovered, so I will be conservative and reduce the no-multiverse probability by a factor of two, to 25%. The second update is that the vacuum energy is nonzero. By the same (conservative) logic, I reduce the no-multiverse probability to 12%. The final update is the fact that our outstanding candidate for a theory of quantum gravity, string theory, most likely predicts a multiverse. But again I will be conservative and take only a factor of two. So this is my estimate for the likelihood that the multiverse exists: 94%.
This is not to say that the multiverse is on the same footing as the Higgs, or the Big Bang. Probability 94% is two sigma; two sigma effects do go away (though I factored in the look-elsewhere effect, else I would get a number much closer to 1). The standard for the Higgs discovery was five sigma, 99.9999%.
My problem with a lot of the West Coast theorists is that I don’t seem to have the same sense of humor, so often I have trouble telling when they’re joking (does anyone know when Andrei Linde is joking?). Here though it seems quite clear that Polchinski is pulling the leg of the philosophers gathered to hear him speak. My calculations show that the chance the above text could be a serious contribution to a philosophy of science conference cannot be above .1%.
The other evidence that something comical is going on here comes from some of the over-the-top claims about the virtues of string theory. In particular, we’re told
A remarkable feature of string theory is that the dynamics, the equation of motion, is completely fixed by general principle. This is consistent with the overall direction of fundamental theory, describing the vast range of phenomena that we see in terms of fewer and fewer underlying principles. Uniqueness would seem to be the natural endpoint to this process, but such theories are truly rare…
Indeed, when I assert that the equations of string theory are fully determined by general principle, I must admit that we do not yet know the full form of the equations, or the ultimate principle.
Polchinski is quite right that it is “remarkable” to know that you have a unique theory, with unique equations, fixed by a general, ultimate principle, but you don’t know what the theory is, what the equations are, or what the principle is. I’m curious to hear from people at the conference what gets the bigger laughs: this or Kane’s argument that the unknown theory predicts a 1.5 TeV gluino (unless it’s not found, in which case it doesn’t).
Update: See here for the latest coverage of the meeting from Massimo Pigliucci, who comments at one point:
[Yet another string theorist. I must say, there does seem to be a stacking of them at this conference, and no experimentalists have been invited either]
So strange how the public perception of string theory has been warped by a few contrarian voices. Good topic for some future PhD thesis.
and that Polchinski’s new paper
lays out the case for string theory, and how unexpectedly successful it’s been.
I’m assuming by “contrarian voices” he’s referring to Polchinski. That string theory has been “unexpectedly successful” and the multiverse is a 94% sure thing is a highly contrarian point of view among physicists.