> were perfectly in sync They couldn't stay synced. There's a measurable frequency shift from a few cm of height difference after all. Making a pair of clocks

> were perfectly in sync They couldn't stay synced. There's a measurable frequency shift from a few cm of height difference after all. Making a pair of clocks that are always perfectly in sync with each other is a major step towards Le Guin's ansible! For other readers' info, clock stability is crucial for long-term precision measurements, with a "goodness" measured by a system's Allan variance: https://en.wikipedia.org/wiki/Allan_variance https://en.wikipedia.org/wiki/Allan_variance