Shake (unit)


A shake is an informal metric unit of time equal to 10 nanoseconds, or 10−8 seconds. It was originally coined for use in nuclear physics, helping to conveniently express the timing of various events in a nuclear reaction.

Etymology

Like many informal units having to do with nuclear physics, it arose from top secret operations of the Manhattan Project during World War II. The word "shake" was taken from the idiomatic expression "in two shakes of a lamb's tail", which indicates a very short time interval.
The phrase "a couple of shakes", in reference to the measurement of time, may have been popularized by Richard Barham's Ingoldsby Legends ; however, the phrase was already part of vernacular language long before that.

Nuclear physics

For nuclear-bomb designers, the term was a convenient name for the short interval, rounded to 10 nanoseconds, which was frequently seen in their measurements and calculations: The typical time required for one step in a chain reaction is of the order of 1 shake, and a chain reaction is typically complete by 50 to 100 shakes.