Why Is 'c' Used for the Speed of Light in E=mc²?

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Discussion Overview

The discussion centers around the choice of the letter 'c' as the symbol for the speed of light in the equation E=mc². Participants explore the historical context and linguistic origins of the symbol, as well as its significance in physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that the letter 'c' was first used in a 1856 paper by Weber and Kohlrausch, where it stood for constant, later becoming known as Weber's constant.
  • Another participant mentions that 'c' derives from the Latin word 'celeritas', meaning 'swift', and emphasizes the double meaning in the context of relativity.
  • A participant provides historical context, referencing Ole Christensen Romer's early estimate of the speed of light based on astronomical observations.
  • Some participants argue that the choice of symbol is not crucial to understanding the nature of light or electromagnetic radiation.
  • There is a clarification regarding the meaning of 'celeritas', with one participant asserting it means 'swiftness' and another noting the Italian equivalent "celere".

Areas of Agreement / Disagreement

Participants express differing views on the importance of the symbol 'c' in representing the speed of light, with some downplaying its significance while others emphasize its historical and linguistic roots. The discussion remains unresolved regarding the broader implications of the symbol's choice.

Contextual Notes

Some statements rely on historical interpretations and linguistic definitions that may vary. The discussion does not resolve the significance of the symbol in relation to physical concepts.

Blackadder
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For my first thread allow me to answer a question posed by 'Artie' who requested the the following,

Unfortunately the thread was locked so I had to start a new thread just to answer this question.

(Quote Artie:)In the equation E=mc2

E is energy

m is mass

What is c? I know that c is the speed of light, but why use the letter c? What does c stand for? (Unquote)

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. Weber used the notation to stand for constant, and it later become known as Weber's constant.

Lower case 'c' stood for celeritas as was stated previously but the Latin celeritas actually means 'swift'. Plank, Lorentz, and ultimately Einstein used the letter 'c' and in Eistein's case, his theory of relativity demanded the speed of light in a vacuum be 'constant' thereby finessing a double entendre to the symbol.

The first quantitative estimate of the speed of light was made in 1676 by Ole Christensen Romer whose estimate of approximately 136,000 miles per second based on astronomical observations of the moons of Jupiter was in the ballpark but about 26% too low. Although for a first shot it was magnificent.

It took me some time to find this Forum and I have some interesting questions on the Big Bang and the origin of the universe and my own hypothesis on the 'Intelligent design imbroglio'.

Looking forward to exchanging ideas and some good debates.

The Blackadder
 
Last edited:
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What's your question?

It really is not important what symbol is used to represent a physical quantity. If your question has to do with the nature of light/ electromagnetic radiation then I recommend that you read the FAQs on this site.

If you want to know why the symbol c is used then read the thread you are referring to. There is really not much else to say.
 
... the Latin celeritas actually means 'swift'.
It means 'swiftness'. 'Celer' means 'fast' or 'swift'.

M
 
Mentz114 said:
It means 'swiftness'. 'Celer' means 'fast' or 'swift'.
Yes. Even in italian, "celere" means "fast, rapid".
 

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