# Time Scales: How Short Is A “Femtosecond”

Post on Timezone Public Forum 11/29/97

How short is a femtosecond?

Moving downward on the scale of time, ultra fast science passes quickly beyond human experience. One second is a familiar and manageable piece of time. It’s not hard to create a second’s worth of light by turning a flash light on, then quickly off. What’s harder is to comprehend that in that second the light has gone 186,000 miles – three quarters of the way to the moon.

From one second, let’s move on to other time scales:

One thirtieth of a second is the time it takes human eyes to react to light. Project each frame of a home movie for one thirtieth of a second, and viewers, unable to distinguish separate frames, see continuous motion. Light, during the time one frame is projected, travels 6,200 miles. If you climb aboard a light beam in Chicago, you’ll be in Tokyo in the blink of an eye.

One microsecond— a millionth of a second — is the duration of the light from a camera’s electronic flash. Light that short freezes motion, making a pitched ball or a bullet appear stationary.

One nanosecond— a billionth of a second — is the speed at which transistors in today’s computers turn on and off to represent the ones and zeros of binary logic and arithmetic. It is a time-duration so short that light, which can speed seven times around Earth in the second between our heartbeats, travels only one foot.

One picosecond— a trillionth of a second — is a spot of time from the domain of molecules. Light, traveling for one picosecond, would barely make it across the period at the end of this sentence. Only with a laser that generates picosecond light pulses can scientists freeze the short-duration motion of molecules and produce images of what goes on at the molecular level. Used in this way, the picosecond laser is comparable to a strobe, which can freeze the motion of a sprinter’s stride in time-lapse photography.

One femtosecond— a quadrillionth, or million billionth, of a second — is a thousand times shorter than the picosecond snippets of time in which molecules react. Light, in one femtosecond, goes only far enough to traverse about 1,000 silicon atoms.