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For the United States Navy ship, see USS Celeritas (SP-665).

Celeritas (Classical Latin: [kɛˈlɛrɪtaːs]) is a Latin word, translated as "swiftness" or "speed". It is often given as the origin of the symbol c, the universal notation for the speed of light in a vacuum, as popularized in Albert Einstein's famous equation E = mc2. In SI units, the speed of light in a vacuum is defined as 299 792 458 meters per second (1 079 252 848.8 km/h).


In the 19th century, V was commonly used to denote the speed of light. Einstein used this notation in his famous 1905 papers.[1] Thus, Einstein originally wrote his most famous equation as m = L / V2 (he used E elsewhere for a different energy).[2] The first use of the letter c as a symbol for the speed of light was in an 1856 paper by Wilhelm Eduard Weber and Rudolf Kohlrausch.[3] Weber used the notation to stand for constant, and it later became known as Weber's constant. At the turn of the 20th century, c was popularised by influential physicists such as Max Planck and Hendrik Lorentz. In 1907, Einstein switched to this notation in his papers.

A 1959 essay by science fiction and popular science author Isaac Asimov[4] is the first reference to c standing for celeritas, though he cited no evidence to support this. It is now standard to see "c is for celeritas". David Bodanis, in his popular science book E=mc2: A Biography of the World's Most Famous Equation, states that "the speed of light has this unsuspected letter for its name probably out of homage for the period before the mid 1600s when science was centered around Italy, and Latin was the language of choice, Celeritas is the Latin word for swiftness."[5]


  1. ^ Einstein, A. (1905). "Zur Elektrodynamik bewegter Körper". Annalen der Physik 322 (10): 891–921. Bibcode:1905AnP...322..891E. doi:10.1002/andp.19053221004.  edit
  2. ^ Einstein, A. (1905). "Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?". Annalen der Physik 323 (13): 639–641. Bibcode:1905AnP...323..639E. doi:10.1002/andp.19053231314.  edit
  3. ^ Weber, W.; Kohlrausch, R. (1856). "Ueber die Elektricitätsmenge, welche bei galvanischen Strömen durch den Querschnitt der Kette fliesst". Annalen der Physik und Chemie 175 (9): 10–25. Bibcode:1856AnP...175...10W. doi:10.1002/andp.18561750903.  edit
  4. ^ Isaac Asimov "C for Celeritas" in "The Magazine of Fantasy and Science Fiction", Nov-59 (1959), reprinted in "Of Time, Space, and Other Things", Discus (1975), and "Asimov On Physics", Doubleday (1976)
  5. ^ David Bodanis, "E=mc2: A Biography of the World's Most Famous Equation", pg. 37, Macmillan (2000)

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