Shake (unit)

Short description: Informal unit of time equal to 10 nanoseconds

A nuclear fission reaction. The process takes about 1 shake.

A shake is an informal metric unit of time equal to 10 nanoseconds, or 10−8 seconds.[1] It was originally coined for use in nuclear physics, helping to conveniently express the timing of various events in a nuclear reaction.

## Contents

* 1 Etymology
* 2 Nuclear physics
* 3 See also
* 4 References

## 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 (1840);[2] however, the phrase was already part of vernacular language long before that.[3]

## 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 (i.e. the typical time for each neutron to cause a fission event, which releases more neutrons) is of the order of 1 shake, and a chain reaction is typically complete by 50 to 100 shakes.[4]

## See also

* Barn, a companion unit of cross-sectional area created by the same people, for the same general purposes, at the same time (the measured value of nuclear-reaction cross section was larger than expected, hence deemed "as big as a barn").
* List of humorous units of measurement

## References

1. ↑ Rowlett, Russ, ed. "[Letter S"]. University of North Carolina. http://www.unc.edu/~rowlett/units/dictS.html.[Letter]+''S''&rft.atitle=&rft.pub=[[Organization:University+of+North+Carolina|University+of+North+Carolina]]&rft_id=http://www.unc.edu/~rowlett/units/dictS.html&rfr_id=info:sid/en.wikibooks.org:Physics:Shake_(unit)">
2. ↑ Richard Harris Barham (1840). "The Babes in the Wood". The Ingoldsby Legends. p. 191. https://archive.org/details/ingoldsbylegend02ingogoog/page/n212. "I'll be back in a couple of shakes." Also on page 212 ("A Row in an Omnibus") "in a brace of shakes" and on page 247 ("The Lay of St. Alois") "in a couple of shakes." But the phrase appeared in print before Barham; see for example James Edward Alexander (1833). Transatlantic Sketches. p. 284. https://archive.org/details/transatlanticsk00unkngoog/page/n320.
3. ↑ Elyse Bruce (2011). "Two shakes of a lamb's tail". http://idiomation.wordpress.com/2011/01/28/two-shakes-of-a-lambs-tail/.
4. ↑ Cochran, Thomas B. (10 April 1994). "Hydronuclear Testing or a Comprehensive Test Ban?". Natural Resources Defense Council. p. 4. http://docs.nrdc.org/nuclear/files/nuc_04109401a_122.pdf. "The period, 10−8 seconds, turns out to be a convenient unit of time, and it was defined during the Manhattan Project as one ‘shake’."

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Time measurement and standards

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Obsolete standards|

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Time in physics|

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Horology|

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Calendar|

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Archaeology and geology|

* Chronological dating
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Astronomical chronology|

* Galactic year
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Other units of time|

* Flick
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Related topics|

* Chronology
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* Decimal time
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* Timekeeper

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