“Today’s physicists rarely debate what time is and why we experience it the way we do, remembering the past but never the future. Instead, researchers build ever-more accurate clocks. The current record-holder, at the Joint Institute for Laboratory Astrophysics in Colorado, measures the vibration of strontium atoms; it is accurate to 1 second in 15 billion years, roughly the entire age of the known universe. Impressive, but it does not answer ‘What is time?’”