Celeritas
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Celeritas 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 exactly 299,792,458 meters per second (1,079,252,849 km/h).

Origins of the c notation

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 a 1856 paper by Wilhelm Eduard Weber and Rudolf Kohlrausch.^[3] Weber used the notation to stand for constant, and it later become 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.

How c came to stand for celeritas

It is thought that Weber originally intended the letter c to stand for "constant" rather than celeritas.^{citation needed} 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.^{citation needed}

It is now standard to see "c is for celeritas" stated as fact, although some continue to question the origin.^{citation needed} 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 centred around Italy, and Latin was the language of choice, Celeritas is the Latin word for swiftness."^[5]

References

1. ^  A. Einstein, Annalen der Physik, Band 17, Seite 891-921, 1905
2. ^  A. Einstein, Annalen der Physik, Band 18, Seite 639-641, 1905
3. ^  R. Kohlrausch and W.E. Weber, "Ueber die Elektrizitätsmenge, welche bei galvanischen Strömen durch den Querschnitt der Kette fliesst", Annalen der Physik, 99, pg. 10 (1856)
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)

External links

• http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html
• [6] download 1905 papers in original German