How long is a jiffy?

When someone tells you they'll do something in a jiffy, you understand roughly what they mean: soon, shortly, before long. It is the kind of promise that is more social lubricant than hard commitment. But jiffy, at least when used by scientists, actually has a technical definition (or rather, several of them, depending on who you ask).

The word has been in English since the middle of the 18th century as a casual term for a brief, unspecified moment. In many of the early uses it appears to have been used by sailors, or in a nautical setting. The first dictionary to make note of it was compiled by Francis Grose, who was known to have spent time in waterfront taverns in his attempts to capture the slang of the era.

Blast ‘em, I see I shall be unrigg’d in a jiffy, if I don’t close haul.
— Bartholomew Bourgeois, The Disappointed Coxcomb, 1765

I no sooner give you leave to step upon the gunwale, than whip you are up at the Mast-head in a jiffy.
— The Times; or, a Fig for Invasion, 1797

It will be done in a jeffy; it will be done in a short space of time, in an instant.
— Francis Grose, A Classical Dictionary of the Vulgar Tongue (3rd ed.), 1796

From the middle of the 18th century up until the middle of the 19th jiffy retained this sense of “a moment.” However, in recent years a variety of scientists have adopted the word as a specific unit of measurement.

Some physicists use jiffy to designate the time it takes light to travel one femtometer; this is one quadrillionth of a meter, a unit of measurement used for things smaller than atomic nuclei. If you find it difficult to picture how short this is, that is entirely the point. A jiffy in this sense is so vanishingly small that no human experience could be meaningfully described by it.

Electrical engineers, not to be outdone, adopted the word for their own purposes. In alternating current systems, a jiffy corresponds to the duration of a single cycle: roughly 1/60th of a second in North America, where AC power runs at 60 Hz. Computer scientists have used it too, though less consistently, to refer to intervals anywhere from one to ten milliseconds, depending on the operating system. The lack of consistency across (and within) disciplines is one of the reasons why we define this word as imprecisely as we do: “a very short period of time.”

There is something pleasant about the fact that jiffy, a word born in the slang of 18th-century sailors, ended up pressed into service by particle physicists and electrical engineers. The everyday speaker who promises to return in a jiffy is, without knowing it, invoking timescales that range from the subatomic to the vaguely brief. Whether that counts as scientific literacy or cheerful obliviousness probably depends on how quickly they actually come back.