A femtosecond is the SI unit of time equal to 10^-15 or ¹/_{1,000,000,000,000,000} of a second. That is one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second as a second is to about 31.71 million years; a ray of light travels approximately 0.3 µm (micrometers) in 1 femtosecond, a distance comparable to the diameter of a virus.