Build an atomic clock anywhere in the universe, and it will always “tick” at the same rate as all of the other atomic clocks. So, how do we know atomic clocks are consistent? For example, if you started up a bunch of modern cesium atomic clocks just after the Big Bang, they’d all agree to within about a minute and a half today. It turns out that the length of the day is substantially less consistent than the groups of atomic clocks we have. But the rest of us mostly use atomic clocks for GPS, and even GPS doesn’t require that many.