Time is inextricably woven into what might be the most fundamental goal of physics ?" prediction. Whether they're studying cannonballs, electrons or the entire universe, physicists aim to gather information about the past or present and project it forward to catch glimpses of the future. Time is, as Nobel Prize winner Frank Wilczek put it in a recent episode of Quanta's The Joy of Why podcast, "the master variable under which the world unfolds." In addition to prediction, physicists face the challenge of understanding time as a physical phenomenon in its own right. They develop ever-sharper explanations of the most obvious feature of time in our daily lives: that it flows inexorably forward. And recent experiments showcase more exotic ways in which time can behave under the established laws of quantum mechanics and general relativity. As researchers deepen their understanding of time in these two cherished theories, they encounter puzzles that seem to bubble up from mur