Sufficiently clever experimentalists also pulled this off in about 20 meters in 1959, the clever bit being ditching the clocks entirely: https://en.wikipedia.

Sufficiently clever experimentalists also pulled this off in about 20 meters in 1959, the clever bit being ditching the clocks entirely: https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment