Highlighting the usefulness of string theory

► Witten replies: In my article I tried to explain in a succinct way some of the exciting highlights of string theory, and I assumed for readers only a basic comfort level with Feynman diagrams and general relativity. The points in question should be widely understandable, but I am not sure where they have been explained in quite as elementary yet substantive a way as I aimed for.I certainly did not claim that everything has been understood; there are plenty of unsolved problems, as George Chapline points out, and that is one reason that the subject remains exciting. It was not possible in a short article to explain all the fascinating things that have been discovered and the many interesting ways that string theory interacts with other topics in physics and mathematics. Some of that has been covered in the past in other articles in Physics Today (see, for example, the article by Steve Giddings, April 2013, page 30, and the Quick Study by Hong Liu, June 2012, page 68).I have worked on the specific subject of twistor theory quite a lot, as Chapline probably realizes. Actually, one reason that I suspect string theory is on the right track is that when critics have had good ideas—whether involving black hole entropy, noncommutative geometry, or twistor theory—those ideas have tended to be absorbed into string theory.I regret that Peter Hansen did not find my article compelling, and I hope other readers thought otherwise. Many circumstantial clues suggest that string theory is on the right track. If that is the case, it is reasonable to hope that it will become clear, probably through a combination of theoretical and observational progress.© 2016 American Institute of Physics.