Brian Gallagher in Nautilus:
Last April, in the famous Faraday Theatre at the Royal Institution in London, Carlo Rovelli gave an hour-long lecture on the nature of time. A red thread spanned the stage, a metaphor for the Italian theoretical physicist’s subject. “Time is a long line,” he said. To the left lies the past—the dinosaurs, the big bang—and to the right, the future—the unknown. “We’re sort of here,” he said, hanging a carabiner on it, as a marker for the present. Then he flipped the script. “I’m going to tell you that time is not like that,” he explained.
Rovelli went on to challenge our common-sense notion of time, starting with the idea that it ticks everywhere at a uniform rate. In fact, clocks tick slower when they are in a stronger gravitational field. When you move nearby clocks showing the same time into different fields—one in space, the other on Earth, say—and then bring them back together again, they will show different times. “It’s a fact,” Rovelli said, and it means “your head is older than your feet.” Also a non-starter is any shared sense of “now.” We don’t really share the present moment with anyone. “If I look at you, I see you now—well, but not really, because light takes time to come from you to me,” he said. “So I see you sort of a little bit in the past.” As a result, “now” means nothing beyond the temporal bubble “in which we can disregard the time it takes light to go back and forth.”
Rovelli turned next to the idea that time flows in only one direction, from past to future. Unlike general relativity, quantum mechanics, and particle physics, thermodynamics embeds a direction of time. Its second law states that the total entropy, or disorder, in an isolated system never decreases over time. Yet this doesn’t mean that our conventional notion of time is on any firmer grounding, Rovelli said. Entropy, or disorder, is subjective: “Order is in the eye of the person who looks.” In other words the distinction between past and future, the growth of entropy over time, depends on a macroscopic effect—“the way we have described the system, which in turn depends on how we interact with the system,” he said.
More here.