Time flows: steady, unstoppable, invisible. The Greek philosopher Heraclitus imagined time streaming through existence, an inexorable medium of change. We understand it scientifically as a hidden background evident in the synchronized beat of clock hands or more precisely in the attosecond pulsing of beryllium ions. But our ordinary perception of time suggests that it is inconstant, elastic, rhythmic. Time flies; time drags on.

In moments of intense engagement—when conversation flows, when work gets done or when “fun” seems such an inadequate description—time moves in great, condensed chunks. By contrast, at a traffic light or in an anxious waiting room a plodding second hand expands time, marking ever widening, frustrating intervals. Time crawls, spreads out, and rises to thwar...