As a teen, and into my twenties, I used a shortwave radio to listen to WWV and set my watch. To this day it bothers me to find a clock more than 5 seconds off. So imagine my excitement at learning that clocks may be coming which are so accurate that the time of day can depend on how high up the wall they’re mounted.
Mind-boggling as such effects are, they are apparently not detrimental to our well-being. For optical clocks, they could be. To tell the time consistently, all clocks need to be at a known height relative to Earth’s “geoid”, an imaginary surface that links points at which the gravitational field has the same strength. But the height of this geoid varies over time at any given place by up to 20 centimetres, because of effects such as tectonic movements, glacial melting and changes in ocean levels, and varying atmospheric pressure. Changes of that magnitude could wreak havoc with any attempt to establish a global time standard at an accuracy of 1 part in 1018 or better.
