Atomic clocks measure time by counting the oscillations of laser light tuned to an atomic transition of known frequency. Optical clocks, the most sophisticated variety of atomic clock, probe optical transitions in ultracold atoms and can measure time with an uncertainty approaching 1 part in 1018. (Olympic athletes can be timed to the millionth of a second.)
Among their many applications, optical clocks can be used to measure Earth’s gravitational potential: According to general relativity, clocks raised to higher potentials have their atomic transition frequency ν increased by Δν = ν ΔU/c2, where ΔU is the change in potential and c the speed of light. Until recently the best optical clocks had always operated within the friendly confines of a controlled laboratory environment. Now an international group led by Christian Lisdat of the National Metrology Institute of Germany has developed an...
