He describes one extreme of the spectrum of opinion about this as people who think the whole thing is completely non-scientific, giving what he sees as being the two kinds of objections such people make, neither of which he thinks make sense. Since I’m one of these extremists, I think I should try and explain why and exactly what the nature of my objections are, since they’re not exactly the ones Sean mentions.
The first objection Sean attributes to extremists like myself is that of accusing users of the anthropic principle of “giving up” by assigning the parameters of the standard model to a selection effect instead of calculating them. This is very much David Gross’s objection, and while I would agree with it as a socio/psychological characterization of the behavior of Susskind et. al., my own version of this objection is a bit differerent. For any given supposed fundamental theory, some observables will be calculable from first principles, and others will be aspects of the particular state we are in, dependent on the history of how we got here. Given a particular observable, in some fundamental theories it may be calculable, in others environmental. But the theory is supposed to tell us which it is going to be. The standard model tells us that the earth-sun distance is environmental, and that the magnetic moment of the electron is calculable. It is silent about the origin of its 20 or so parameters, and whether they are environmental or calculable. It is one of the first jobs of any theory that purports to go beyond the standard model to give some sort of explanation of where these parameters come from, which of them are in principle calculable and which aren’t.
The problem with the whole Landscape idea is that it is so ill-defined that it can’t even tell you what things are calculable and what things are environmental. You don’t know what the fundamental M-theory is that is supposed to be producing the Landscape and governing the dynamics of how the universe evolves in it. String theorists would probably claim that while they don’t know exactly what the fundamental theory is, they may know enough about it to make conjectures about what the Landscape should look like, at least in certain limiting cases. The problem is that their conjectures not only don’t allow them to calculate anything, they don’t even allow them to determine what is going to be calculable. The problem with string theory is not that it can’t calculate the vacuum energy, it is that it can’t calculate anything. Some string theorists are now using the Landscape picture purely as an excuse to get them out of this embarassing situation. “Not our fault we can’t calculate anything beyond the Standard Model, because maybe nothing beyond the Standard Model is calculable”. If they had a well-defined fundamental theory which exhibited this behavior, one might take them seriously, but until they do, the whole picture is nothing more than an elaborate excuse for failure. A question that should be asked of anyone promoting this stuff: show us using string theory which of the Standard Model parameters are calculable and which are environmental. If they can’t do this they shouldn’t be taken seriously.
The second objection Sean attributes to the likes of me is that we object to the explanatory use of entities that are unobservable in principle, like multiple universes. This isn’t really my objection to the Landscape. If a compelling fundamental theory existed that made lots of correct testable predictions, and such a theory predicted lots of unobservable universes, I’d happily believe in their existence. But, absent such a compelling theory, people who go on about unobservable multiple universes are not behaving very differently from those theologians who supposedly took an interest in angels and pins. Science is about coming up with explanations for the way the world works, explanations that can in principle be tested by making more observations of the world. If you’ve been working on a theory for twenty years and it has totally failed to make any testable predictions, you should admit failure and move on, not engage in elaborate apologetics for why your theory can’t predict anything.