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The Unit “Dirac”

Updated: Jan 17, 2022

About twenty years ago, I asked one of my colleagues what was the unit “Dirac” for. He, a competent physicist, thought about it for a while in earnest and shyly replied that he did not know. My question was a joke borrowed from the following passage from Calder's book [Ref. 1]:

Dirac’s taciturn and retiring behavior are famous; in his days at Cambridge, a unit of volubility called a Dirac meant one word per year.

P. A. M. Dirac on a Mali Republic stamp issued in 2011

Reading a new biography [Ref. 2], written by Farmelo, of the Nobel-winning physicist P. A. M. Dirac, I found a different definition of the unit Dirac, i.e., one word per hour. This definition is more realistic than that in Ref. 1. However, the latter, which exaggerates Dirac's taciturnity, is funnier than the former and too good to be discarded.

The relevant description in Ref. 2 is quoted by Gilleland [Ref. 3] under the title “Unit of taciturnity.” The unit surely commemorates Dirac’s taciturnity. However, its proper name would be “unit of volubility.” The reason is that the number of words per unit time is smaller for the person of higher taciturnity.


  1. N. Calder, The Key to the Universe (Viking, New York, 1977).

  2. G. Farmelo, The Strangest Man: The Hidden Life of Paul Dirac, Quantum Genius (Faber and Faber, London, 2009).

  3. M. Gilleland, “Unit of taciturnity: The Dirac.” Blog post at Laudator Temporis Acti (January 26, 2010).

Note: This short essay first appeared at on January 27, 2010, and has been one of the most viewed posts of the present author (2.56-k views). In the version given here, he improved English.

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Jan 12, 2022

Hello Prof. Tabata. In physics we don't usually define a unit for something that never happens. Even an aeon would be too short. 🤓

Indeed, I had the rare misfortune of experiencing first-hand a full Diracs worth of Dirac.

The mortifying details are described here Dirac Number = 2

Jan 12, 2022
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Thanks, redrooz! It's nice to get a comment from a "Dirac number = 2" person. I have much enjoyed your stories, "Dirac in Australia," "Dinner with Dirac," etc.

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