PHOTONS that tick like a clock could allow atomic clocks to be synchronised with unheard-of accuracy. They could be used to help test Einstein’s theories of relativity, and lead to astonishingly precise satellite-based positioning systems.
Today’s atomic clocks are so precise that it is hard to get them to agree on the time. One way to synchronise them is to send a third clock from one to the other, carrying the time of the first so that the second can be reset. But sending bulky atomic clocks is impractical, especially if you’re trying to synchronise orbiting satellites.
One alternative might…
