According to Einstein’s theory of relativity, Earth’s gravity causes the frequency of an atomic clock to depend on its altitude. Consequently, the height differ

According to Einstein’s theory of relativity, Earth’s gravity causes the frequency of an atomic clock to depend on its altitude. Consequently, the height difference between two remote clocks can be determined by measuring their difference in frequency. At the level of measurement uncertainty achieved in the latest work, clock comparisons could resolve centimetre-sized height differences. Therefore, clocks could provide new tools for long-term environmental monitoring of, for example, ice sheets or ocean levels.