The Order of Time

In this far-reaching text rife with references to poets, artists, and philosophers, as well as scientists, theoretical physicist Rovelli (Seven Brief Lessons on Physics) takes readers through the current scientific understanding of time, stating that “we inhabit time as fish live in water.” Rovelli begins with a look at why time, Rainer Maria Rilke’s “eternal current,” only flows forward. Humans can see the past but not the future, he writes, because of how heat flows, from hot to cold. He states that “only where there is heat is there a distinction between past and future,” using as an example a film of a rolling ball gradually slowing, due to heat-producing friction; if run backwards, the film becomes absurd. Entropy, “the quantity that measures this irreversible progress of heat in only one direction,” provides the direction of “time’s arrow.” Meanwhile, the human perception of simultaneity, the idea of “now” in two different locations, is an illusion, an insight that Rovelli calls “perhaps the greatest and strangest of Einstein’s discoveries.” In considering time, Rovelli also explores quantum time, loop theory, and the nature of memory. As much philosophy as physics, this accessible study introduces the complex questions behind the perception and study of time. (May)