I recently happened upon a physics related blog and discovered that there has been an active debate within the physics community for the past few weeks, months and years about string theory and its relation to multiverse concepts.
The blog is "Not Even Wrong", http://www.math.columbia.edu/~woit/blog/, by Peter Woit, a mathematician and string theory skeptic at Columbia. His blog takes its name from his opinion about string theory: that it should not even be considered a part of physics, since it has failed for decades to make any testable predictions. Recently things have heated up with the publication of a new book by Leonard Susskind, the so-called "father of string theory": "The Cosmic Landscape: String Theory and the Illusion of Intelligent Design". Depending on who you read, the book either finally exposes the intellectual bankruptcy of string theory, or else displays its incredible mathematical beauty, elegance and power. The New York Times book section recently had a scathing review of the book, which has triggered much of the current discussion. At this time the most recent blog post is here: <http://www.math.columbia.edu/~woit/wordpress/?p=348> and it links to others, but the topic is far from exhausted. The reason for such contrasting opinions seems to come down to multiverse models and anthropic reasoning. Apparently modern formulations of string theory do not describe a single universe. Rather, there are a nearly infinite (or actually infinite, by some interpretations) set of possible parameters, all of which could represent a universe. Our observable universe is just one example of those. Susskind basically adopts Tegmark's level 2 multiverse to propose that all of the other string theory universes exist within an overall framework of cosmological inflation. The fact that we see a particular set of parameters is explained on anthropic principles, that in the other universes either there are no people, or they observe different parameters. To say that this reasoning is unattractive to many physicists would be an understatement. Anthropic reasoning seems to be one of the great dividing issues within the physics community. Anti-anthropicists see it as basically a matter of giving up on physics. If all universes and all sets of physical laws exist, then there is no possibility of explaining them, no deep underlying principles, and no scientific theory involved. It is just a matter of cataloging observations and putting them into some semblance of order. Further, no powerful predictions are possible since there are a virtually infinite number of models which match any set of observations. To these physicists, any physical constant or property whose value is left up to the anthropic principle is one which has departed from the realm of physics. On the other side there is a small but growing number of workers who accept anthropic reasoning for at least some constants. I'm not sure exactly why they are OK with it, but I get the impression that it comes down to the fact that there seem to be many physical constants that are fine-tuned for life. The alternative is that God created the universe specially for mankind, which is problematic for many well known reasons. (Apparently Susskind goes to some lengths in his book to discuss this issue.) I think another reason is that there are so many physical constants, and they seem so arbitrary, that the chance that they will just pop out of some simple theory is becoming more and more remote, so that these physicists have pretty much given up on that. Then there is the fact that the level 2 multiverse (separate universes as bubbles within a background of cosmic inflation) is pretty much compatible with modern day physics, so there is a place for multiple universes. Apparently Hawking is something of an anthropicist, and there is some discussion of this on the blog at <http://www.math.columbia.edu/~woit/wordpress/?p=347>. I think that part of the reason why multiverse models are seen as nonphysical is because physicists do not recognize or accept the concepts we have explored of "measure". When Susskind writes about the so-called "landscape" of string theory, each point representing a different model, he and everyone else assumes that each point is equally probable, has equal measure. Given such an assumption of uniform measure, it would indeed be difficult or impossible to make predictions. In many of our discussions of multiverse models, we have explicitly or implicitly included the notion of measure, that some universes would be more "common" or more "prominent" than others. This is often linked to extensions of Occam's Razor, so that universes with relatively simple models would have higher measure than those that are more complex. Kolmogorov complexity is one way of defining a measure with this desirable property. This provides, in principle, a quantitative way of evaluating the measure of different universe models. Applying the anthropic principle we then derive the result that we should live in a universe which is about as simple as possible (i.e. has as high measure as possible) consistent with the evolution of life. One could then look at all of these string theory models, where no doubt some of them are very much simpler than others. The simpler ones would be the universes where we are most likely to live, if life is possible at all in them. Therefore the (near) infinity of possible models should not immediately be cause for dispair, but rather there is reason to hope that a model that defines the universe we actually live in may well be relatively simple, within the reach of a practical research program. Of course there are no guarantees of this. It could be that even though the complexity of our universe is small compared to infinity, it is still large compared to what human research could hope to achieve. We really don't have much basis to decide at this point, other than that the various attempts to explore relatively simple string theory models have all failed to describe a universe like ours. They do reveal universes with particles and force fields, but not ones that closely resemble our universe. And usually these hypothetical universes instantly explode or otherwise behave in a manner hostile to life. Of course, this level of analysis reminds us that there is no reason a priori that a variant of string theory has to be the correct model. As we know from Tegmark and Schmidhuber, among others, it is easy to generalize into a much wider set of possible universes. String theory has the nice feature that it creates a well-defined "particle zoo", i.e. a set of particles with particular masses, charges and other properties; and it is perhaps not so easy to pull that out of other models. But there are undoubedly at least some other ways to do it. And the truth is, we don't even know that the existence of particles as we observe them is truly as fundamental as string theory assumes. Particles could be an illusion, a manifestation of some deeper underlying phenomena. Unfortunately I think that in fairness we still have to classify most of these multiverse concepts as philosophy more than physics. Although we can come up with some in-principle predictions - such as that we will probably never discover a universe model that is much simpler than our own but which would evolve life - I don't know of any predictions that one could make based on anthropic reasoning that could be tested today. Physics is a science, and that means it needs to work with theories that can be tested and disproven. We are a long way from being able to come up with any experiment that a working physicist in his lab could run to see whether multiverse models are correct. (And no, quantum suicide doesn't count!) I also get the impression that Susskind's attempts to bring "disreputable" multiverse models into "holy" string theory is more likely to kill string theory than to rehabilitate multiverses. Perhaps I am getting a biased view by only reading this one blog, which opposes string theory, but it seems that more and more people are saying that the emperor has no clothes. If string theory needs a multiverse then it is even less likely to ever be able to make physical predictions, and its prospects are even worse than had been thought. A lot of people seem to be piling on and saying that it is time for physics to explore alternative ideas. The hostile NY Times book review is just one example. Hal Finney
