# E=mc² redux

1. Jul 27, 2008

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

(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.

Last edited: Jul 27, 2008
2. Jul 27, 2008

### Integral

Staff Emeritus

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.

3. Jul 27, 2008

### Mentz114

It means 'swiftness'. 'Celer' means 'fast' or 'swift'.

M

4. Jul 28, 2008

### lightarrow

Yes. Even in italian, "celere" means "fast, rapid".