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

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.

Contemporary physics has learned to describe the universe with unprecedented precision. Yet when it tries to say what time is, it stumbles again and ...