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

1. ### artie

0
Speed of Light. What is c? Why use the letter "c'?

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?

2. ### nicksauce

1,275
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.

3. ### artie

0
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.

5. ### AzonicZeniths

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.

69
7. ### Mapes

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...

8. ### artie

0
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

9. ### malawi_glenn

4,726
that is just a joke.

10. ### rbj

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).

11. ### malawi_glenn

4,726
Hmm we have derived E = mc^2 at school, so I dont think Einstein just "guessed" that solution.. ;-)

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...

13. ### malawi_glenn

4,726
why is not c^88643 bigger?

Just google "derivation of E=mc^2" or search in textbooks about special relativity

22
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

22
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??

16. ### malawi_glenn

4,726
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.

17. ### HallsofIvy

40,657
Staff Emeritus
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".

18. ### FNL

13
yes that's right

Last edited by a moderator: Jun 14, 2008

22
i said first....im not sure of it...but maybe c^2 is the
biggest physical constant
and tell me about these formulas which have c^6... i hav no idea abt them...
about c is not a real constant?? i didnt understand
...it is a unit thats right.... so wheres the problem???
c is specified unit (at least in caccum)

20. ### malawi_glenn

4,726
Can you please spell better?

The thing is that we can only compare things that have equal units. For example 3Joules - 1 Joule = 2 Joule. But what is 3Joules - 5m/s ? And what is biggest between 300 000 000m/s and 1.626*10^-34 J*s ?

Using your argument, why is not h^-1 bigger than c^2? If we only look at the figures, using SI-units, h^-1 = 1.51*10^33 ...

It is only meaningful to compare quantities with the same units. c.f. pi vs. natural number e, or 2000m/s with 10m/s.

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