Over the holiday break, I read Lee Smolin's book, The Trouble with Physics. I've already alluded to the book a couple of times on this blog -- citing a New Yorker magazine review of it in the posting immediately below the present one and linking to the audio of a radio interview with Smolin and Brian Greene in my August 19, 2006 entry -- so I don't think a detailed review on my part is needed.
Rather, I just want to share, briefly, a few of my major reactions to Smolin's book. First, I feel Smolin provides very clear explanations of a number of concepts in string theory, supersymmetry, etc. For example, regarding Edward Witten's famous "M-Theory" Second Superstring Revolution, in which Witten integrated the five major string theories extant at the time, Smolin explains the use of dualities to bring the theories together, in a very readable manner.
Second, from my position of having no technical training or expertise in the subject matter, just an educated layperson's interest, the book portrays string theory and supersymmetry (together known as "superstring theory") as having a fairly tenuous foundation. Even having read a number of critical writings in the past, Smolin's book has enhanced my skepticism of superstring theory. Three areas of concern are as follows:
(a) String/M theory, as I interpreted Smolin's writing, pertained to "the five consistent superstring theories in ten-dimensional spacetime," but there were "millions of variants in the cases where some dimensions were wrapped up" (p. 129). Smolin had earlier defined a mathematically consistent theory as one that, "...never gives two results that contradict each other... [and] all physical qualities the theory describes involve finite numbers" (p. 112, footnote). Ten spacetime dimensions refer to time, plus nine dimensions of space (six beyond the three we see). Further, Witten "did not actually present a new unified superstring theory; he simply proposed that it existed and that it would have certain features" (p. 129).
(b) Another concern involves the apparent malleability of the equations in superstring theory. Smolin notes that, "...the original standard model has about 20 free constants we have to adjust by hand to get predictions that agree with experiment. The [Minimally Supersymmetric Standard Model] adds 105 more free constants. The theorist is at liberty to adjust them all to ensure that the theory agrees with experiments... There turn out to be many different ways to tune the dials to ensure that all the particles we don't see are so heavy that they're as yet unseeable" (pp. 75-76).
(c) In addition, the generality of many calculations is in question. Regarding work following the second revolution, "To get any results, we had to choose special examples and conditions. In many instances, we were left not knowing whether the calculations we could do gave results that were a true guide to the general situation or not" (p. 146).
Another direction taken in an attempt to advance string theory is its potential applicability to black holes. However, such attempts "all suffer from a general problem, which is that whenever they stray from the very special black holes where we can use supersymmetry to do calculations, they fail to lead to precise results... So we are left with the same dilemma that afflicts so much research in string theory: We get marvelous results for very special cases, and we are unable to decide whether the results extend to the whole theory or are true only of the special cases where we can do the calculations" (p. 190).
The first 250 or so pages of the book, focusing on substantive theoretical physics, would probably be more than sufficiently filling (using the metaphor of "food for thought") for most readers. However, another 100 pages of more epistemological discussion follow, which leads to my third major reaction.
Smolin's perceptions of how science in general, and university-based science in particular, do operate and should operate, are reasonably interesting, as is his call for the support of unconventional thinkers. What struck me the most, however, was a possibility raised by Smolin. Perhaps what is hampering the string-theory enterprise to integrate general relativity (gravity) and quantum mechanics is that the traditionally accepted conceptualizations of relativity and quantum mechanics are themselves flawed.
After discussing some lines of inquiry questioning relativity, Smolin writes:
So much for questioning relativity. What if quantum theory is wrong? This is the soft underbelly of the whole project of quantum gravity. If quantum theory is wrong, then trying to combine it with gravity will have been a huge waste of time... (pp. 316-317).
Whether the foundations of theoretical physics will ultimately be rocked to this extent is unknown. But Smolin asks his readers to remain open-minded to a variety of possibilities.