Michael Dine from Santa Cruz was here at Columbia this afternoon to give a talk on “Branches of the Landscape”. His talk more or less corresponded to his recent paper with the same title. He’s following the philosophy pioneered by Michael Douglas of trying to look at the statistics of KKLT vacuum states, fixing the observed values of the cosmological constant and electro-weak breaking scales. The hope is that the distribution of supersymmetry breaking scales one gets would allow one to in some sense predict what this scale will be.
Dine finds three disconnected “branches” of the landscape, sets of vacua with different properties. The bottom line is that on two of them you have various problems getting something that looks like the real world, but you can do some kinds of counting. But on one of the branches you get lots of states with badly broken supersymmetry and the vast majority of states are in a region where there seems to be no hope to analyze what is going on. You can’t even say whether the number of these states is finite or infinite. So, he isn’t able to get the sort of prediction he and others were hoping for, but intends to keep working in this area nonetheless, with various ideas of what to try calculating. To me, he didn’t seem to have even a glimmer of a hope of ever getting even the vaguest sort of prediction out of any of this.
He did say that the landscape is now the only idea on the table for getting physics out of string theory. Brian Greene was in the audience and somewhat objected to this. Brian’s point of view appears to be the more traditional one that people should just try and cook up vacua with as many features as possible close to the Standard Model, and that once they’ve got such a thing it will have other implications for physics that can be checked. It seems to me that that kind of work has been going on for more than twenty years with no sign of success, but Brian still believes this will ultimately work out. Dine’s ideas for the future are converging somewhat with Brian’s older point of view. He seems to be giving up somewhat on the idea of counting all vacua in the Landscape, instead thinking about counting vacua satisfying some chosen conditions, e.g. being on one of his three branches. So he may be getting back to the older idea, looking at complicated constructions with some set of conditions imposed on them to make them look like the Standard Model, then hoping to extract something new, perhaps in terms of probability distributions rather than the more specific predictions people used to hope for.
Of course I find this whole thing pretty bizarre, since it’s horrifically ugly, and appears to me to have not the slightest hope of success. It’s discouraging that I don’t see any way of having a rational discussion with the people doing this. They are motivated by a hope that somehow, some way, they will find amidst this complicated mess the Standard Model, in some context that allows them to predict something else. As far as I can tell this is the purest of wishful thinking. They aren’t claiming to find anything encouraging, but they are pressing on, and convincing an increasing number of people to join them. One hopes that sooner or later they’ll get tired of this and move on to something more promising.