Agreed - as an experimentalist specializing in precision gravitational measurements, their lives just got worlds harder. Now they're not fighting challenges wi

Agreed - as an experimentalist specializing in precision gravitational measurements, their lives just got worlds harder. Now they're not fighting challenges within their instruments, but they have to ensure that they know each clock's depth in Earth's gravitational potential. Atomic clocks are small and elegant things with a pristine heart. They're now irreducibly sensitive to a very messy world. Earth tides (dirt, not water) are larger than centimeters. With further improved precision, they'll be sensitive to small changes in the locations of local gravitational sources, even big ones at long range. In our gravity lab in Seattle, we see the effects of annual variation in water table. All of these things must be properly characterized in order to compare two clocks that are not co-located. These are good problems to have, and are inherent to measuring time at this precision, but they're very daunting. Not insurmountable, but often frustrating.