Classical and quantum framing of the Now

Mermin replies: Understanding the Now was not the main purpose of my March 2014 commentary . My primary point was that a new way of resolving the puzzles and paradoxes of quantum mechanics, called QBism by Christopher Fuchs and Rüdiger Schack, was also able to settle that longstanding problem in classical physics. After my earlier commentary on QBism (Physics Today, July 2012, page 8 ), I was concerned that many of the letters about it (December 2012, page 8 ) had sounded the same theme: that the letter writer’s own way of looking at quantum mechanics was already perfectly satisfactory. I hoped that by applying QBist thinking to a strictly classical problem, I could disengage those readers from their favorite interpretation of quantum mechanics and help them think about QBism on its own merits. I recently emphasized in Nature ¹ that QBism sheds light on classical physics, too, but there I discussed quantum applications of QBism as well as classical applications (“CBism”).

The letters here all address only the problem of the Now but not the fact that I deal with it through a classical application of QBism. While I’m disappointed that they say nothing about QBism or CBism, I’m pleased that they all agree that the problem of the Now is indeed a problem. Not everybody does.

Berge Tatian and B. K. Ridley both criticize me for taking the Now to be a point. But I don’t. I call the Now an event “whose duration and location are restricted enough that it can usefully be represented as a point in space and time.” I say that “the events we experience are complex, extended entities” and that “to represent our actual experiences as a collection of mathematical points … is a brilliant strategic simplification, but we ought not to confuse a cartoon … with the experience itself.”

Ridley’s comment “Nor could one person’s Now be exactly the same as another’s” suggests that I say it could be. What I do say is “The commonality of my Now and your Now whenever we are together requires that our Nows must coincide at each of two consecutive meetings.” “Commonality” or “coincide” mean only that our two private Now experiences happen at a common place and time, not that they are identical. Indeed, the personal experiences of different people are incomparable, except through the imperfect medium of language.

I agree with James Hartle on much in the paper he cites, but we have important differences. He takes spacetime to be objective and fundamental; I take it to be an abstract tool used by an agent to organize her experience. He uses the notion of a point in spacetime uncritically; I regard it as an approximate representation of an agent’s spatially and temporally extended experience. He takes an agent’s experience to be an objective property of the agent, like the contents of a register. I take an agent’s experience to be private and self-evident to that agent and to be the fundamental basis for her inference of an external world; the experience of each agent plays a special role for that—and only that—agent, analogous to the special role played by “the classical domain” in the quantum mechanics of Lev Landau and Evgeny Lifshitz.

Rudolf Peierls wrote to John Bell in 1980, “In my view, a description of the laws of physics consists in giving us a set of correlations between successive observations. By observations I mean … what our senses can experience. That we have senses and can experience such sensations is an empirical fact, which has not been deduced (and in my opinion cannot be deduced) from current physics.” ² If “us” is expanded to “each of us,” then nobody has ever put QBism and CBism more concisely than that.