Of all the units of physical science,one second is perhaps the most mysterious.Unlike a meter,we cannot see it.Unlike a kilogram,we cannot hold it in our hands.Unlike a volt,we have no nerves to sense it.rnYet a second is the most precisely quantified unit we have[1].This year,metrologists-scientists who study measurement-announced the first transportable clock that can measure time to 18 decimal places.To put this in perspective,a clock running with this precision since the Big Bang would have lost or gained less than half a second.