THE philosopher Augustine of Hippo once wrote that he felt he knew what time was, so long as no one asked him. Fast forward 16 centuries and the picture has hardly changed, says physicist Carlo Rovelli. Time is "perhaps the greatest mystery", he says. "At the most fundamental level we currently know of, there is little that resembles time as we experience it." The passage of time - a uniform, universal flow that transports us inexorably from a past we cannot revisit to a future we cannot know - is perhaps the most fundamental experience of our existence. Yet our best theories suggest that it is not real. Time doesn't flow, and past, present and future cannot sensibly be defined. There isn't even one single time that governs the order of events. Rovelli, who works at Aix-Marseille University in France, is just one of many physicists hunting for a better answer. As they do, a new generation of experiments is giving hope that we can probe the nature of time more searchingly than ever