Among my last several postings, the main focus has been string theory and criticisms thereof. One problem I said I would discuss further was the theory's lack of falsifiable predictions. Fortunately, I learned recently that a New Yorker magazine article from early fall provides a substantial discussion of this topic, so I can now draw upon the article (thanks to Matt Russell for showing me the magazine issue).
The New Yorker article reviews two books that are critical of string theory, Lee Smolin's The Trouble with Physics and Peter Woit's Not Even Wrong (the latter having a very active blog associated with it). In reviewing the books, the New Yorker piece manages also to succinctly summarize a century of research -- on general relativity, quantum mechanics, and the standard model of particle physics -- leading up to today's controversies. Some key quotes from the article are as follows:
Today, more than a decade after the second revolution, the theory formerly known as strings [the article alludes to the more recent "M theory"] remains a seductive conjecture rather than an actual set of equations, and the non-uniqueness problem has grown to ridiculous proportions. At the latest count, the number of string theories is estimated to be something like one followed by five hundred zeros...
In their books against string theory, Smolin and Woit view the anthropic approach as a betrayal of science. Both agree with Karl Popper’s dictum that if a theory is to be scientific it must be open to falsification. But string theory, Woit points out, is like Alice’s Restaurant, where, as Arlo Guthrie’s song had it, “you can get anything you want.” It comes in so many versions that it predicts anything and everything. In that sense, string theory is, in the words of Woit’s title, “not even wrong.”
[A brief description from Seed Magazine of the aforementioned anthropic principle and the associated concept of a landscape is available here. A Scientific American article that fleshes out these ideas in greater depth is introduced here; this article can either be purchased online or looked up at a library.]
In his book The Theory of Almost Everything, on the standard model, George Mason University physicist Robert Oerter notes that there actually are some empirical predictions that follow from string theory (pp. 273-274). These predictions involve very massive particles, fractionally charged particles, deviations in how gravity behaves for very small and large objects, and light from distant galaxies -- and all have failed to be confirmed by observation.
Oerter asks, "With so many negative experimental results, why do physicists continue to be excited about string theory?" His answer: "The experiments that have been performed rule out some versions of string theory, but there are literally billions of other versions that could still be true."