# Speed of Light. What is c? Why use the letter "c'?

by artie
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 P: n/a 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?
 HW Helper Sci Advisor P: 1,276 Often in physics a symbol doesn't stand for anything. For example, Momentum=p, Magnetic Field=B... As far as I know, the "c" is completely arbitrary.
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 Quote by nicksauce Often in physics a symbol doesn't stand for anything. For example, Momentum=p, Magnetic Field=B... As far as I know, the "c" is completely arbitrary.
That's a little strange in this case, because E and m do match up with Energy and mass! The only thing I can think of is c is constant.

Mentor
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## Speed of Light. What is c? Why use the letter "c'?

I believe that c was chosen for celeritas, Latin for "speed". (But c for "constant" works too.) Read: Why is c the symbol for the speed of light?
 P: 69 'c' is completely arbitrary, probably not having anything to do with 'constant' as there are several constants in the universe. 'c' is just another physical constant, just like a mathematical variable.
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 Quote by Doc Al I believe that c was chosen for celeritas, Latin for "speed". (But c for "constant" works too.) Read: Why is c the symbol for the speed of light?
^^^^^^^^^^^^

or that haha
 PF Patron HW Helper Sci Advisor P: 2,532 I saw a cartoon once of Einstein at a blackboard where he had written E=ma2 and crossed it out, then wrote E=mb2 and crossed that out...
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 Quote by Doc Al I believe that c was chosen for celeritas, Latin for "speed". (But c for "constant" works too.) Read: Why is c the symbol for the speed of light?
Excellent!

Since Einstein was German, I had tried to find a German word for "light" that begins with c but it makes more sense that the word would be rooted either in Latin or Greek.

Thanks
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 Quote by Mapes I saw a cartoon once of Einstein at a blackboard where he had written E=ma2 and crossed it out, then wrote E=mb2 and crossed that out...
that is just a joke.
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 Quote by Mapes I saw a cartoon once of Einstein at a blackboard where he had written E=ma2 and crossed it out, then wrote E=mb2 and crossed that out...
wow, i thought i saw that one too, but it had different powers for c. like E=mc0, E=mc1, and finally E=mc2. maybe there was a crossed out E=mc3 on the blackboard.

i guess it's a way to do physics; guess (and see if experiment supports your guess). about E=mc2, no other power would be dimensionally correct if energy and mass are measured in units that do not define c to be one (or dimensionless).
 HW Helper Sci Advisor P: 4,739 Hmm we have derived E = mc^2 at school, so I dont think Einstein just "guessed" that solution.. ;-)
 P: 22 as we know c is the absoulte speed (speed of light) and as i think c^2 is the greatest constant in the physics i dont know exactly how Eisntein found it...
 HW Helper Sci Advisor P: 4,739 why is not c^88643 bigger? Just google "derivation of E=mc^2" or search in textbooks about special relativity
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 Quote by malawi_glenn why is not c^88643 bigger? Just google "derivation of E=mc^2" or search in textbooks about special relativity
i said i dont know how Einstein found this equation....
when E=mc^2 apllicated on the particles...it was right
so the scientists has no reason to change c^2 to c^5345345345
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 Quote by malawi_glenn why is not c^88643 bigger? Just google "derivation of E=mc^2" or search in textbooks about special relativity
my own explanation why E=mc^2

E means Energy, measured by Joule
we all now that F=ma and E=Fr =mar(r is the distance)
so Joule=Newton*Meter=Kg*Meter*sec^-2
for E=mc^2 --> Joule=Kg*(Meter*sec^-2)^2
anybody agree with me??
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 Quote by Adonis my own explanation why E=mc^2 E means Energy, measured by Joule we all now that F=ma and E=Fr =mar(r is the distance) so Joule=Newton*Meter=Kg*Meter*sec^-2 for E=mc^2 --> Joule=Kg*(Meter*sec^-2)^2 anybody agree with me??
That is just an argument from units, why couldn't it be: E = 8*mc^2 ?

You must do the full derivation.

And WHY is c^2 the biggest constant in physics?

i) There are formulas which have c^6 .. aren't that a bigger constant?

ii) c is the constant, c^2 = c*c, i.e the constant c is multiplied with another constant c ...

iii) c is not a real constant, it also have a unit: Lenght/time, so c^2 has units lenght^2/time^2

iv) How can we compare e.g G with c? They have different nummerical values, but they have also different units. Also, you can easy come up with a unit system where G has a bigger nummerical value than c. It is like comparing colour with sound.

/Glenn - 1 term from Masters degree in physics.
 PF Patron Sci Advisor Thanks Emeritus P: 38,412 I'm still unclear as to what the question is! You say you know that "E" is "energy" and "m" is "mass" and understand that "c" represents the speed of light. Are you really only asking why the letter "c" is used for "speed of light? That was not original with Einstein. Maxwell had already used "c" for the speed of light in his derivation of the wave equation from his equations for the Electric and Magnetic fields and I suspect it had been used that way before. My understanding is that it is from "celerity" which is Latin for "speed".
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 Quote by HallsofIvy I'm still unclear as to what the question is! You say you know that "E" is "energy" and "m" is "mass" and understand that "c" represents the speed of light. Are you really only asking why the letter "c" is used for "speed of light? That was not original with Einstein. Maxwell had already used "c" for the speed of light in his derivation of the wave equation from his equations for the Electric and Magnetic fields and I suspect it had been used that way before. My understanding is that it is from "celeritas" which is Latin for "speed".
yes that's right