What is time? This is not an easy question for a physicist, or anyone else for that matter, to answer. In Why Time Flies: a Mostly Scientific Investigation, the New Yorker writer Alan Burdick tries to address this question by examining the science of how humans and other living organisms perceive the passage of time. In our everyday lives, our relationship with time is driven by two things – the daily cycle of darkness and light, and our own internal circadian rhythm. Burdick begins his exploration by delving into the latest research into circadian rhythms in humans, plants and animals, which coincidentally was the subject of this year’s Nobel Prize for Physiology or Medicine. He wonders whether the origins of our internal clock come from ancient organisms’ needs to prepare themselves for the dawn, which would bring the daily onslaught of DNA-damaging solar radiation. He points out that humans are not the only organisms that understand the concept of time wasting. A wide range of animals from rats to fish are able to spend their time wisely by learning the shortest way to complete a task. We may never know how a fish measures time, but until the previous century humans defined days, months and years by how objects move across the heavens. Burdick describes how in the 17th century, seafaring nations invested in astronomical facilities like the Royal Observatory at Greenwich to improve navigation on the open seas by charting the motions of the stars. By the 19th century, however, astronomers were becoming aware that different people will perceive the motion of stars in different ways – something they called “human equation”. While this discovery was bad news for those wanting more accurate astronomical measurements, it was a boon for physiologists studying how humans perceive events and react to them. Burdick explains that our best measure of time – co-ordinated universal time (UTC) – is a running average of the time kept by a collection of atomic clocks worldwide. The best clocks in this group undergo a process called “steering”, whereby their accuracy is refined regularly by comparing them to UTC. So, it is true that time does not stand still, at least in terms of how we define it today.