Amidst all the celebrations of Einstein and 1905 it is easy to overlook another anniversary – it is 50 years since the first atomic clock was built by Louis Essen at the National Physical Laboratory in the UK. Atomic clocks have come a long way since then, and the next generation of optical clocks (which are also atomic) threaten to be so accurate that the second will have to be redefined (see Optical clocks). Einstein – who spent much of his time at the patent office worrying about timekeeping – would be impressed.