Why Time Flies: A Mostly Scientific Investigation. By Alan Burdick. Simon & Schuster; 320 pages; $28.

TIME is such a slippery thing. It ticks away, neutrally, yet it also flies and collapses, and is more often lost than found. Days can feel eternal but a month can gallop past. So, is time ever perceived objectively? Is this experience innate or is it learned? And how long is “now”, anyway? Such questions have puzzled philosophers and scientists for over 2,000 years. They also began to haunt Alan Burdick of the New Yorker. Keen for answers, he set out “on a journey through the world of time”, a lengthy trip that spans everything from Zeno’s paradoxes to the latest neuroscience. Alas, he arrives at a somewhat dispiriting conclusion: “If scientists agree on anything, it’s that nobody knows enough about time.”

Humans are apparently poor judges of the duration of time. Minutes seem to drag when one is bored, tired or sad, yet they flit by for those who are busy, happy or socialising (particularly if alcohol or cocaine is involved). Eventful periods seem, in retrospect, to have passed slowly, whereas humdrum stretches will have sped by. Although humans (and many animals) have an internal mechanism to keep time, this turns out to be as reliable as a vintage cuckoo clock. “It’s a mystery to me that we function as well as we do,” observes Dan Lloyd, a philosopher and time scholar at Trinity College in Hartford, Connecticut.

St Augustine, a fourth-century philosopher and theologian, was the first to recognise time as a property of the mind, an experience of perception and far from absolute. His insight turned what had been a subject of physics into one of psychology, and it informs much of the work of later scientists. In the mid-1800s William James, a philosopher and psychologist, noted that the brain does not perceive time itself but its passage, and only because it is filled in some way. He grew baffled by efforts to quantify the present, observing that any instant melts in one’s grasp, “gone in the instant of becoming”.

Of all interior clocks, the circadian is perhaps best understood. Nearly every organism has a molecular rhythm cycle that roughly tracks a 24-hour period. In humans all bodily functions oscillate depending on the time of day. Blood pressure peaks around noon; physical co-ordination crests in midafternoon; and muscles are strongest at around 5pm. Night-shift workers are not as productive as they think they are. Cataclysms of human error, including accidents at Chernobyl and aboard the Exxon Valdez, all took place in the small hours, when workers are measurably slowest to respond to warning signals. Long-distance travel often makes a hash of the body’s “synchronised confederacy of clocks”, disrupting not only sleep but metabolism. The jet-lagged body recovers at a rate of about one time zone per day.

Mr Burdick spent quite a lot of time on this book, beginning it just before his twin sons were born and finishing it when they were old enough to suggest titles. It reads like a discursive journey through a vague and slippery subject, a thoughtful ramble across decades and disciplines. Although the study of time has yielded few firm conclusions, one lesson is poignantly certain: most people complain that time seems to speed up as they get older, in part because they feel more pressed for it. “Time”, writes Mr Burdick, “matters precisely because it ends.”