I recently finished reading Peter Woit's book Not Even Wrong, one of the two books that have received great attention in recent months for their critical examinations of string theory. The other such book is Lee Smolin's The Trouble with Physics, which I reviewed here.
I have been a longtime visitor to Woit's Not Even Wrong blog. That gave me a good idea of what to expect and, indeed, I found the book to be a systematic expansion of ideas discussed on the blog.
In the Preface (p. xii), Woit explains the expression from Wolfgang Pauli that became the title of his (Woit's) book. Woit notes that the phrase "carries two different connotations, both of which Pauli likely had in mind." As Woit continues:
A theory can be "not even wrong" because it is so incomplete and ill-defined that it can't be used to make firm predictions whose failure would show it to be wrong. This has been the situation of superstring theory from its beginnings to the present day...
...there is a second connotation of "not even wrong": something worse than a wrong idea... worse than being wrong is to refuse to admit it when one is wrong.
For all the hype over Woit's book in relation to string theory, it doesn't even take up the topic until about halfway through. The first half presents a history of particle physics, with a heavy emphasis on the interface between physics and mathematics. The standard model of particle physics receives a lot of attention, and there's also a nice early chapter on accelerators and colliders. Woit himself, in both his blog and book, warns about the technical references to mathematical concepts in the book, but I didn't find it to be any worse than other books in terms of mathematical complexity (which is not to say that I understood any more than a little of the math).
A substantial component of Woit's critique of string theory has to do with the potentially huge (i.e., hundreds of thousands, if not hundreds of millions, or more) number of possible scenarios that would satisfy string theory's requirements, but not produce a unique solution characterizing a single universe. Part of the problem stems from the need for extra dimensions and the applicability of something called Calabi-Yau manifolds (discussed in this earlier posting of mine). Writes Woit:
The predictions of heterotic string theory [one form of the theory] strongly depend on which Calabi-Yau spaces one chooses. In 1984, only a few Calabi-Yau spaces were known, but by now hundreds of thousands have been constructed (p. 153).
A few chapters later, he resumes the discussion, in terms of possible background spaces for the universe(s):
There are many, perhaps infinitely many, classes of background spaces that appear to be possible consistent choices, and each one of these classes comes with a large number of parameters that determine the size and shape of the background space-time... [Regarding possible parameters, one of the proposed mechanisms] picks out not a unique value for the values..., but a very large set of values, any one of which should be as good as any other. Estimates of the number of these possible values are absurdly large (e.g., 10^100, 10^500, or even 10^1000 [where ^ stands for raising to a power]) far beyond the number of particles in the universe or anything else one can imagine counting... The possible existence of, say, 10^500 consistent different vacuum states for superstring theory probably destroys the hope of using the theory to predict anything... (pp. 237-239).
Woit also discusses the landscape/anthropic principle, an attempt by some to salvage the string theory conception of innumerable universes. According to the Wikipedia entry linked in the previous sentence, "The idea of the string theory landscape has been used to propose a concrete implementation of the anthropic principle, the idea that fundamental constants may have the values they have not for fundamental physical reasons, but rather because such values are necessary for life."
Also thrown into the mix by Woit is a chapter entitled "The Bogdanov Affair," the larger point of which seems to be that certain areas of physics have gotten so complex that few people can even follow what is being written. In brief, two brothers named Bogdanov managed to publish several articles that appeared to contain little or no merit, but whose prose was so impenetrable that journal referees appeared to let the manuscripts get by. Writes Woit:
Leaving aside the issue of whether the Bogdanovs are hoaxers or really believe in their own work, this episode definitively showed that in the field of quantum gravity one can easily publish complete gibberish in many journals, some of them rather prominent... Why did the referees in this case accept for publication such obviously incoherent nonsense? One reason is undoubtedly that many physicists do not willingly admit that they don't understand things...(pp. 218-219).
Sean Carroll, who blogs at Cosmic Variance, had an entry about a week and a half ago entitled, "String Theory is Losing the Public Debate." He writes:
...there are very good reasons to think that something like string theory is going to be part of the ultimate understanding of quantum gravity, and it would be nice if more people knew what those reasons were.
Of course, it would be even nicer if those reasons were explained (to interested non-physicists as well as other physicists who are not specialists) by string theorists themselves. Unfortunately, they’re not. Most string theorists... seem to not deem it worth their time to make much of an effort to explain why this theory with no empirical support whatsoever is nevertheless so promising. (Which it is.) Meanwhile, people who think that string theory has hit a dead end and should admit defeat — who are a tiny minority of those who are well-informed about the subject — are getting their message out with devastating effectiveness.
As I've noted periodically on the present blog, at least one prominent physicist, Columbia University's Brian Greene (whom Carroll also mentions), has been assiduously arguing for the promise of string theory in various forums. I agree with Carroll, though. I too would like to see more advocates of string theory explain their perspective.