A decade ago, two books for educated, lay readers attempted to poke holes in string theory: Peter Woit's Not Even Wrong (which I reviewed here) and Lee Smolin's The Trouble with Physics (which I reviewed here). While I can't claim with certainty that no books were published in this genre over the past decade that attempted to argue for string theory, I at least did not see any. That changed with the 2016 publication of Why String Theory?, by Joseph Conlon. I recently finished reading Conlon's book and I review it here.
The greatest potential promise many physicists see in string theory is its (theoretical) ability to link the heretofore incompatible areas of general relativity/gravity and quantum mechanics, so that the equations of one theory do not yield nonsensical answers at the scale of the other theory. Conlon, however, contends that, "very few [people associated with string theory] actually work on quantum gravity or have their main interests in quantum gravity" (p. 228).
Conlon also shows a sense of humor regarding the lack of direct empirical evidence for string theory. Chapter 7 is entitled "Direct Experimental Evidence for String Theory," and the entire text of the chapter consists of this one sentence: "There is no direct experimental evidence for string theory."
What, then, are Conlon's reasons for urging the study of string theory? A major claim of his is that string theory offers conceptual and analytic tools to physicists and mathematicians working outside quantum gravity, which can help these scholars in their own domains. As one example, notes Conlon, string theory "... came to the field theorists and showed them how it could be used to compute quantities of interest to them... string theory was able to show them a way to solve their own problems and on their own terms" (p. 230).
Conlon weaves some technical topics into the various chapters, such as the "AdS/CFT Correspondence," which make the reading very challenging in places. I think the larger arguments in the book come through clearly, however. Also, I found some of Conlon's general impressions about physics and science to be very interesting.
One is what he refers to as "gold" and "silver" standards for theories. The gold standard "has always been to make predictions in advance of the experiment, and then to see these predictions verified." Conlon notes further that, "All research involving quantum gravity lacks this gold standard" (p. 234). The silver standard "is to make predictions for theoretical problems, for which the answer is not known in advance but can be checked using very different techniques." By that, for example, he means reaching the same mathematical solution after coming at the problem from very different approaches. Conlon pronounces the silver standard to have been "abundantly satisfied" with string theory, but not for a competing theory, loop quantum gravity.
Another interesting fact, mentioned early in the book, is that the last theorizing in particle physics to be awarded the Nobel Prize (in 2004) was originally published in 1973 (independently by Politzer, and by the team of Gross and Wilczek on asymptotic freedom in the strong force). A Nobel has been awarded more recently than 2004 for theoretical particle physics (i.e., the 2013 prize to Peter Higgs and Francois Englert), but their research predated 1973. Conlon contends that the Standard Model of particle physics, which "originally appeared a temporary ersatz construction that would be good for a few years..." (p. 4), has unexpectedly withstood all experimental tests since the 1970s, and consequently has directed a great deal of research toward further refinements of the Standard Model.
Much remains to be learned, about quantum gravity and many other areas of physics. And, as Conlon argues in his book, string theory is a valuable tool for these diverse pursuits.
