The cosmic landscape
Peter Woit has blogged (and no doubt many others will also blog) about Leonard Susskind's new book The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. Before I go any further I must confess that I have not yet read Susskind's book, so I will confine my comments to standard background material, and I will write later on about the specific details that Susskind includes in his book.
To summarise, Susskind says that string theory does not make a useful number of testable predictions, but it does predict a mind-bogglingly large number of vacua (each of which has its own laws of physics) so by random chance it can give rise to a universe in which the laws of physics are indeed as observed.
This last property is called the "cosmic landscape", because there is a landscape of alternative vacua, which allows the weak form of the anthropic principle to bear fruit. What this means is that we observe the universe to be exquisitely fine-tuned for our existence, because it is one of the mind-bogglingly large number of alternative vacua that happens to allow the development of observers like us. So there should be no big surprise that this fine-tuning is observed by us, because otherwise we would not be around to make the observations. The only universes that can be observed by their occupants are precisely the ones that are tuned in such a way that observer-occupants can exist in them, and for which the universe must therefore appear to be fine-tuned from their own point of view.
Note that the anthropic principle is not circular reasoning, because any system that can make measurements of its own behaviour (i.e. which has dynamics which allows its various parts to interact) must necessarily have certain correlations between its measuring parts (A) and its measured parts (B), so A will express "surprise" that B (i.e. what A observes) has certain properties that are (seemingly unexpectedly) related to the properties of A. This is a generalisation of what the phrase "fine tuning" means.
Another way of looking at this is using Bayes theorem, which provides a uniquely rigorous and consistent way of calculating what correlated entities "know" about each other. If the properties of the observer-occupants (A) are correlated with the laws of physics (B), then it follows using Bayes theorem that each can make predictions about the other. However, B => A is uncontroversial, whereas A => B is the weak anthropic principle. People who like B => A but not A => B are implicitly denying Bayes theorem, which means they are happy to reason inconsistently. Enough said?
Despite the logical consistency of the weak anthropic principle many physicists hate it, and denounce it as unscientific. Susskind is not one of these people; he accepts the "landscape" as a fact, and follows up its logical consequences where it is a physical framework for invoking the anthropic principle. However, he attracts the venom of those who hate the anthropic principle, because they perceive it as pulling the rug out from underneath science. This is because if the particular point which you occupy on the "landscape" has to be determined by experimental observation, rather than by theoretical prediction, then they claim that science has no predictive power, because the laws of physics are fixed by experiment rather than predicted by theory.
Just how arrogant can they get? Of course the laws of physics fixed by experiment rather than predicted by theory! That is what science is about, after all. The role of theory is to interrelate the results of experiments where these results are correlated with each other. That is still possible in a universe whose laws of physics are determined by experiment, which is how scientists have been working all along. The problem is that the goal (dream?) of string theorists is to assign a much larger role to theory than to experiment, so only a few basic properties (or perhaps no properties at all!) need to be determined by experiment, and then string theory is used to fill in the rest of the details. It is tough luck if the properties of the universe are such that that the predictive power of theory is smaller than string theorists would like it to be.
If the properties of the universe are correctly described by string theory (this may indeed be the case), and string theory predicts a whole "landscape" of alternative laws of physics, then the string theorists have to work with that, and they will have to measure what they cannot calculate. There is nothing wrong with that. It is called doing science, rather than doing philosophy. It seems to me that the string theory zeolots who do not like the "landscape", and who pour scorn on the weak anthropic principle, are trying to start a new type of philosophy that prefers only (or mainly) to calculate rather than to observe and calculate.
String theorists should get out more, and they should do science rather than philosophy. The weak version of the anthropic principle is science, but asserting that the laws of physics have to be derivable from first principles is philosophy.