What is time? It ticks by, moment by moment, day after day. But it’s so baked into our lives that we don’t think much about it, unless we’re late for an appointment or suddenly notice those gray hairs in the mirror.

Then the physicists weigh in, and time suddenly seems more complicated. Einstein tells us that it can expand and shrink, while the second law of thermodynamics, which says that the universe is growing more disordered, seems somehow to be tied to time’s “flow.”

Carlo Rovelli has devoted most of his career to pondering the enigma of time. An Italian physicist working at Aix-Marseille University in France, Rovelli is known among scientists for his pioneering work on loop quantum gravity — a theory that attempts to unite quantum mechanics with Einstein’s theory of general relativity. The wider public, meanwhile, has come to know him as an eloquent, even poetic writer on scientific topics.

Rovelli’s 2014 book, "Seven Brief Lessons on Physics," sold more than a million copies around the world. Now he’s out with a new book, "The Order of Time," which zeroes in on that most peculiar dimension: time.

Recently, MACH spoke with Rovelli about the nature of time, whether it has a beginning and if humans might one day master time travel. The interview has been edited for length and clarity.

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