Over many centuries, man’s obsession with time has led to increasing accurate ways to measure it. As the precision of techniques and instruments has increased over the years, so the units used to define time have become ever smaller.

The minute and the second have given way to the SI units of the millisecond, the microsecond, the nanosecond, the picosecond, and the femtosecond, which is the the length of time that light takes to travel the diameter of a virus.