Optical clocks could redefine the second

Physicists have devised a way to synchronise the ticking of two clocks through the air with extreme precision, across a record distance of 113 kilometres, using precise pulses of laser light. The feat is a step towards redefining the second using optical clocks — timekeepers that are 100 times more precise than atomic clocks. Hyper-precise clocks could have a role in testing the general theory of relativity and revealing subtle changes in gravitational fields.