David Mermin’s musings about that elusive Now (Physics Today, March 2014, page 8) are a welcome indication that physicists are beginning to ask questions about time. Physics treats time as a coordinate in spacetime in which a point can represent an event. But a person’s Now doesn’t figure in that scheme—it has no special status, yet it is what each of us directly experiences.
The disjunction between directly experienced time and abstract physics time was first and famously pointed out by French philosopher Henri Bergson.1 On 6 April 1922, Bergson and Albert Einstein both attended a meeting of the Philosophical Society of Paris, convened to honor Einstein’s work. There, the distinction between what Einstein called psychological time and physicist’s time was discussed.2 Mermin’s note reminds us that 92 years on, there is still no accommodation.
Mermin’s Now, however, is not Bergson’s. Mermin’s Now can be depicted as a point in his world line, and that sort of depiction is exactly what Bergson objected to. Treating time like space, he thought, was a deep error. What one was conscious of was not a point-like Now but rather a duration, in which the Now was a multiplicity; it was not a single thing that could be recorded as a point on a world line but a mutual penetration of past and present and a hint of the future.
Nor could one person’s Now be exactly the same as another’s. Different memories, perceptions, temperament, and so on make one’s Now unique. Physics cannot deal with the unique and has yet to understand consciousness, so a meaningful accommodation between Bergson’s duration and the point-like Now of physics is still some way off.
I’m grateful to Mermin for telling us what he thinks about Now. He alludes to deep issues.
