
An older version of an ultra-stable ytterbium lattice atomic clock at NIST. Ytterbium atoms are generated in an oven (large metal cylinder on the left) and sent to a vacuum chamber in the center of the photo to be manipulated and probed by lasers. Laser light is transported to the clock by five fibers (such as the yellow fiber in the lower center of the photo). (Credit: Burrus/NIST)
Time, like money, only seems important when it’s running out. But to physicists, time is always a big deal. Relativity tells us that the flow of time depends on the circumstances you’re measuring it in: Clocks tick faster on top of mountains than at ground level, for example, and the faster you go the slower your clocks go. Time depends on space.