Simon Singh reports on a new generation of clocks that are 1,000 times more precise than existing atomic timekeepers. Optical clocks use laser light, and are so accurate that it would take them over 100 billion years to lose just one second. Not only could these clocks allow global positioning systems to increase their accuracy from metres to within a centimetre but they might also mean the basic unit of time itself, the second, has to be redefined. Producer Pam Rutherford EMAIL: material.world@bbc.co.uk