Bricmont replies: I find it odd, as the reviewer of a book, to be criticized because I did not discuss a theory that was not mentioned in the book. But I’ll answer Robert Griffiths point by point.
In Bohmian mechanics (BM), it is simply a mathematical fact, noted by John Bell in chapter 14 of reference 1 of the Griffiths letter, that in the delayed double-slit experiment Griffith describes, particles cannot cross a symmetry plane but instead bounce back from it.1 Offering a different theory in which particles follow a different trajectory does not refute that fact.
In standard quantum mechanics (QM), particles do not have trajectories; that fact was emphasized by, among others, Richard Feynman2 and Lev Landau and Evgeny Lifshitz.3
The theory by Griffiths, based on the idea of “consistent histories,” is therefore not standard QM but is instead, like BM, an attempt to complete QM, by adding histories that consist of real events that occur independently of any measurements made on the quantum system (a measurement means an interaction with that system that may affect what would happen to it in the absence of measurements). Unfortunately, Griffiths’s attempt runs into contradictions, as shown in particular by Sheldon Goldstein in his two-part feature for Physics Today (March 1998, page 42, and April 1998, page 38).
Griffiths misses the first step in Bell’s proof of nonlocality: the Einstein-Podolsky-Rosen (EPR) dilemma that perfect correlations between distant events cannot be explained unless one supposes either that some form of action at a distance occurs or that the events are predetermined by antecedent causes. The latter assumption is not a “classical” one; it is one part of the EPR dilemma. But Bell showed with his inequalities that the assumption leads to a contradiction. Hence, nonlocality follows. For more details, see, for example, references 1 and 4 and Travis Norsen’s book.
