I have just finished reading Alex Vilenkin's book, Many Worlds in One: the Search for Other Universes. The anthropic principle is a topic that so easily prompts hysteria and overreaction and, since we're going to be mentioning it a few times, I might as well make it clear what I think. It is a perfectly logically possibility that some features of our universe are anthropically determined. If such a possibility is predicted by a theory that is viable in all other ways, then one should take it seriously as long as it can be tested. If one cannot test this particular prediction, then one might find it a compelling argument, but one will never know if it is correct. Vilenkin is one of the world's leading theoretical cosmologists. He has made seminal contributions to the theory of cosmological inflation, the idea that the universe may contain topological remnants from particle physics phase transitions, to quantum cosmology, and to many other central topics. Perhaps most importantly for the ideas in his book, he has been an architect of the idea of eternal inflation, and one of the primary researchers embracing its implications for the anthropic principle. Two things struck me right away when I picked up the copy that Alex's publisher sent to me. First, it is a small book by the standards of popular cosmology writing; just over 200 pages. Second, when you read the author blurb inside the back dust cover, you find it to be just a bare bones description of Alex's track to his Professorship at Tufts University. The lack of author promotion material is another unusual feature, and, to the extent that I know Alex, is an accurate reflection of his modest character. One finds this tone continued through the entire book; it is refreshing and adds to the appeal of the subject matter. Almost exactly half of the book, with the exception of a brief discussion of the future of life in the expanding universe, is devoted to a description of the standard model of cosmology and its inflationary extension. I find it extremely difficult to know whether a given pedagogical approach is useful for the general public, although I think this material is at no more complicated level than other books on related topics. However, I do think that Alex's description is an exceptionally clear and uncluttered account of the material for anyone with an undergraduate physics education. All the more impressive is that this cosmic tour contains enough personal information and anecdotes to give the reader a real feeling for the excitement and camaraderie that is an essential part of being a scientist. Vilenkin's enthusiasm for the subject matter, and his affection for his collaborators and his students is infectious and carries the reader along into the second half of the book. This is where Vilenkin gets into the more speculative material. First comes a description of eternal inflation. In many inflationary models, there is an argument that, due to fluctuations of the inflaton, there will always be regions of the space-time that are undergoing inflation, even as there are other regions in which inflation ceases and, in a subset of these, universes with properties somewhat similar to ours begin. Given our understanding of quantum field theory, and some assumptions allowing one to try to understand the effect that such fluctuations would have on the background space-time, eternal inflation seems like a reasonable aspect of many inflationary models. But it is important to emphasize that this is not yet completely firmly established, and there are subtleties that one might worry about in trying to infer a nonperturbative effect on space-time from a quantum effect. It would be wonderful to see this settled definitively in a quantum theory of gravity, such as string theory. If eternal inflation does indeed take place, then Vilenkin argues for infinitely many realizations of non-inflating patches of the universe, with all possible values of constants of nature. This is then used to argue for an anthropic understanding of, for example, the cosmological constant. Whether or not all this holds true, as I mentioned briefly above, depends on the correctness of inflation, the ultimate nature of the inflaton, and details of the ultimate theory of quantum gravity. To this end Alex devotes a chapter to string theory and, in particular, to the idea of the landscape as an example of how many different vacua, with different values of the cosmological constant might arise. This is, as many blogosphere readers will be aware, a controversial issue, and one that often gives rise to strong emotions. But Vilenkin confines his discussion of it to a chapter (and a few other comments here and there), and presents his central thesis without relying on the landscape. I found this very positive, not because I'm trying to make a deep point about the landscape, but because it makes Alex's treatment more general, and hopefully will allow a wider interested readership access to the ideas without getting caught up in that particularly heated debate. Many Worlds in One is a tour through one of the most exciting areas in modern physics, led by a modest guide, who most certainly has his own firm opinions, but who is much more interested in a serious discussion of the physics than in self-aggrandizement and evangelizing for a particular area. Vilenkin's credentials in particle cosmology are unquestionable, and the book is fascinating as a selected review of his own contributions to the field. Nevertheless, there are a number of comments in the book that I had problems with at one level or another. When Vilenkin first talks about the multiverse implications of eternal inflation he writes (on p.83)
The response I got from other colleagues was also less than enthusiastic. Physics is an observational science, they said, so we should refrain from making claims that cannot be observationally confirmed. We cannot observe other big bangs, nor can we observe distant inflating regions. They are all beyond our horizon, so how can we verify that they really exist? I was disheartened by such a cool reception...
I was a little surprised by this, since it seems to me to be merely a plain statement of what science really means. We get a better idea of what he means on p.91 when he writes
The main objection against it was that it was concerned with the universe beyond our horizon, which is not accessible to observation. But if the theory of inflation is supported by the data in the observable part of the universe, shouldn't we also believe its conclusions about the parts that we cannot observe?
But again, I can't really sign on to this way of looking at it, since I feel that one of the great strengths of science is the willingness to say "we don't know" about things that are untestable, and to have to live with that. Another criticism is that I would have liked to see a more detailed and complete discussion of the assumptions behind the conclusions Alex draws, and what needs to be done to put them on firmer ground or, in fact, refute them. What we have is a section, starting on p.116
SOME WAYS OUT: Many readers are, no doubt, wondering. ... Is there any way to avoid these bizarre conclusions? ... if you are willing to clutch at any straw at all to avoid it, let me offer you a couple of straws.
which is then followed by three short paragraphs - eight sentences - in which loopholes are briefly mentioned. There are certainly other examples of individual comments with which I don't agree, such as (p.151)
The observed value of the cosmological constant gives a strong indication that there is indeed a huge multiverse out there.
but these are small points. I have gone into a little detail about some of the places where I take issue with Alex's claims precisely because I want to be clear about my problems before stating that I actually liked this book very much and would most definitely recommend it to others. The writing is clear, the tone is appealing, and the science is discussed in an honest way, by a master who loves the subject. Vilenkin provides us not only with a fascinating account of the physics, but also with a fun and, in places, humorous picture of life as a scientist. (As a former MIT postdoc, the description of Alan Guth's office had me smiling for a while) In the end, I remain relatively unemotional about, but unconvinced by the arguments for an anthropic understanding of the cosmological constant. Of course, with significantly more theoretical progress that might change, as should always be the case. But whatever one thinks about this issue, Vilenkin's book is a wonderful, likeable and refreshingly ego-free contribution to the popular discussion (there is plenty in there for experts to learn from as well). Even if you want nothing else but an excellent discussion of modern cosmology, it is well worth a read.
