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

  1. Jun 9, 2008 #1
    Speed of Light. What is c? Why use the letter "c'?

    In the equation E=mc2

    E is energy

    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. jcsd
  3. Jun 9, 2008 #2


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    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.
  4. Jun 9, 2008 #3
    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. Jun 9, 2008 #4

    Doc Al

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    c for celeritas

    I believe that c was chosen for celeritas, Latin for "speed". (But c for "constant" works too.) Read: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html" [Broken]
    Last edited by a moderator: May 3, 2017
  6. Jun 9, 2008 #5
    '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.
  7. Jun 9, 2008 #6

    or that haha
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  8. Jun 9, 2008 #7


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    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...
  9. Jun 9, 2008 #8

    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.

    Last edited by a moderator: May 3, 2017
  10. Jun 10, 2008 #9


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    that is just a joke.
  11. Jun 10, 2008 #10


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    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).
  12. Jun 11, 2008 #11


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    Hmm we have derived E = mc^2 at school, so I dont think Einstein just "guessed" that solution.. ;-)
  13. Jun 12, 2008 #12
    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...
  14. Jun 12, 2008 #13


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    why is not c^88643 bigger?

    Just google "derivation of E=mc^2" or search in textbooks about special relativity
  15. Jun 12, 2008 #14
    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
  16. Jun 13, 2008 #15
    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??
  17. Jun 13, 2008 #16


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    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.
  18. Jun 13, 2008 #17


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    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".
  19. Jun 14, 2008 #18


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    yes that's right
    Last edited by a moderator: Jun 14, 2008
  20. Jun 14, 2008 #19
    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 that`s right.... so where`s the problem???
    c is specified unit (at least in caccum)
  21. Jun 14, 2008 #20


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    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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