Douglas’s article is mainly a series of excuses for why string theory can’t predict anything. He begins with an historical analogy, comparing the present state of string theory to that of quantum mechanics in the period 1913-1926, and the Bohr model of the hydrogen atom to N=4 supersymmetric YM and AdS/CFT. This kind of analogy has some rather obvious problems: it took 13 years to get from the Bohr model to the complete theory of QM, whereas the idea of string unification has been around for about 30 years, with not the slightest sign of success. More importantly, the Bohr model of the hydrogen atom gave a fairly accurate prediction of the spectrum of the hydrogen atom, so it was clear that there was something very right about it. On the other hand, N=4 supersymmetry YM doesn’t predict anything that corresponds to the real world. In a footnote, Douglas writes:
A similar analogy was made by David Gross in talks given around 2000. However, to judge from his talk here, he now has serious reservations about it.
but I’m not sure what reservations of Gross this refers to.
Douglas’s second analogy is a “chemical” analogy, basically pointing to the fact that deriving bulk properties of solids from the underlying many-particle Schrodinger equation is difficult if not impossible. Again, this isn’t a very good analogy. The problem isn’t that you have a simple theory whose ground state is hard to identify because of the numbers of degrees of freedom, like in condensed matter physics. The problem instead is that the ground state of the superstring doesn’t look like the real world. Douglas describes the current situation as:
Perhaps all this is a nightmare from which we will awake, the history of Kekule’s dream being repeated as farce.
He goes on to give the standard argument for the landscape, referring to it by the novel name of the Weinberg-Banks-Abbott-Brown-Teitelboim-Bousso-Polchinski et. al. solution to the cosmological constant problem, but notes that:
On further developing these analogies, one realizes that we do not know even the most basic organizing principles of the stringy landscape.
and proceeds to discuss a little bit two topics that he hopes might be related to this problem, but I don’t see any evidence for this. One topic is an abstract proposal for a metric on the “space of all CFTs”, the other is the classification of D-branes on a Calabi-Yau in terms of a derived category.
He goes on to discuss his recent work with Denef on computational complexity, which indicates that even if our universe corresponds to some local minimum in the landscape, there is no hope of ever identifying which one it is and actually computing anything about the real world. In his concluding section, he admits that there is no evidence of any simple structure in the landscape, and argues that maybe this is just the way the world is:
Still, our role as physicists is not to hope that one or the other picture turns out to be more correct, but to find the evidence from experiment and theory which will show us which if any of our present ideas are correct.
The problem with his invocation of the role of experimental evidence is that he has just finished making an excellent case that there is no hope at all of ever getting any such evidence for string theory. He seems to have completely abandoned without comment his project of the last few years of counting vacua in hopes of making statistical predictions, and is left with not a single idea about how one can ever hope to get a prediction of any kind out of this framework. In this kind of circumstance, standard scientific practice is to acknowledge that this is a failed project and go on to something else. Douglas not only refuses to acknowledge the failure of string theory, he doesn’t anywhere even mention the possibililty that the underlying idea might be wrong.
Yesterday’s hep-th preprints included another landscape one, Probabilities in the Landscape by Alexander Vilenkin. Vilenkin shows no more evidence of having a viable way of ever making a physical prediction than does Douglas, but like Douglas invokes experimental confirmation at the end of his paper, then closes with one prediction that is sure to be accurate:
It seems safe to predict that we will hear more on this subject in the future.
For some more entertaining reading, take a look at A Comment on Emergent Gravity by Waldyr A. Rodrigues Jr. Rodrigues says that he gave as an exercise to his students to find the errors in Jack Sarfatti’s recent arXiv posting Emergent Gravity. Sarfatti had been blocked for many years from posting on the arXiv, but the arXiv moderators recently relented, perhaps because this paper includes a section at the end about the landscape. Susskind’s recent book is one of only two references in the paper. Sarfatti’s paper has gone through six drafts, three of which are on the arXiv, and the latest of which incorporates some of the objections from Rodrigues. Rodrigues however concludes his paper thus:
Sarfatti’s paper we regret to say, is unfortunately a potpourri of nonsense Mathematics. The fact that he found endorsers which permitted him to put his article in the arXiv is a preoccupying fact. Indeed, the incident shows that endorsers did not pay attention to what they read, or worse, that there are a lot of people with almost null mathematical knowledge publishing Physics papers replete of nonsense Mathematics…
A careful reading of [Sarfatti’s paper] shows that his hypotheses are completely ad hoc assumptions, since in our view no arguments from Physics or Mathematics are given for them. Summing up, we must say that Sarfatti’s claim to have deduced Einstein’s equations as an emergent phenomena is a typical example of self delusion and wishful thinking.
Sarfatti will be giving the closing talk at the 22nd Pacific Coast Gravity Meeting, to be held this coming weekend at the KITP in Santa Barbara.