Time keeps getting away from us. It has a fraught relationship with our subjective experience — sometimes slowing down or dragging on; and at other times speeding up, with years seeming to fly by as we get older. Our brain cells are even known to distort our sense of time when we're tired. In physics, time gets even more complicated, incorporated into theories of relativity — the experience of the clock's ticking being relative to the observer and their place in space — and entropy, which gives time its unidirectional flow. Meanwhile quantum mechanics indicates time can move both forwards and backwards, at least inside quantum systems. In this excerpt from "On Time: The Physics That Makes th...