The clocks that used atoms and black holes to stay in sync

A simulation of a black hole from the 2014 film 'Interstellar'.

You're familiar with clocks. There's probably one if you look up just a little, at the upper corner of your laptop or smartphone screen, showing you what time of day it is, allowing you to quickly grasp the number of daytime or nighttime hours, depending on your needs. There some other clocks that are less…