BOULDER • Every second in a small laboratory room in Boulder, a green light flashes. Within the webs of yellow wire and shelves of computer systems, this green light represents the passage of time.

Time doesn't come from your phone. It doesn't come from your watch either. It comes from atoms oscillating in a vacuum — billions of times a second — inside a system that never stops checking itself.

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.