Wednesday, January 27, 2010

The Unit "Dirac"

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 replied ashamedly that he did not know. My question was a joke borrowed from the following passage in 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.
Reading a new biography of the Nobel-winning physicist P. A. M. Dirac (Ref. 2), 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, but the latter, which exaggerates Dirac's taciturnity too much, is funnier than the former and too good to be discarded.

The relevant description in Ref. 2 is quoted in Ref. 3 under the title "Unit of taciturnity." Surely the unit commemorates Dirac's taciturnity, but it should be called a unit not of taciturnity but of volubility or talkativeness, because the number of words per unit time is smaller for the person of higher taciturnity.

[A similar story was posted earlier in Japanese:]


  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. Unit of taciturnity: The Dirac, Laudator Temporis Acti (January 26, 2010).


  1. When I read the headline, I thought your post was about the Dirac delta function, since it has a unit "area".

    Anyway, interesting post, Ted >:)

  2. To Cold As Heaven,
    Thanks for your kind comment. If I write about the delta function, the title would be "Dirac's Unit-Area Function."
    Ted or tttabata at twitter