Time really does move faster on Mars, but the breakthrough that confirms it did not come from a spacecraft’s stopwatch.

Tracking time is one of those things that seems easy, until you really start to get into the details of what time actually is. We define a second as 9,192,631,770 oscillations of a cesium atom.

Time travel has shifted from pure fantasy to a serious, if highly constrained, topic in modern physics. The equations that ...