Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why the c in mc^2?

  1. Dec 25, 2009 #1
    In the mass-energy equivalent equation, E=mc^2, why is it related to the speed of light?
  2. jcsd
  3. Dec 25, 2009 #2
    That is the formula for rest energy predicted by Special Relativity.

    From the postulates of SR (Invariance of c, physical laws are the same in all inertial frames) one can derive the equations describing energy:
    [tex]E_{tot}=\gamma mc^2[/tex]
    [tex]E_k=(\gamma -1)mc^2[/tex]
    which lead to the conclusion that a mass has a rest energy [tex]E_0=mc^2[/tex] which is called the mass-energy equivalence.

    So the the short answer is that it follows from the postulates of SR.

    For a more in depth description detailing how the equation is derived, see
  4. Dec 25, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper

    Merry Christmas!

    He DeadCat_86! Have a bouncy Christmas! :smile:

    e is energy, which is ML2/T2, while m is of course just M,

    so the conversion factor must have dimensions of L2/T2, ie it has to be a velocity squared …

    and c is the only non-arbitrary velocity for such a general equation! :wink:
  5. Dec 25, 2009 #4


    User Avatar
    Science Advisor

    Re: Merry Christmas!

    Dimensional analysis saves the day yet again!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why the c in mc^2?
  1. Why C in E=MC^2 (Replies: 11)

  2. Why does E=mc^2 (Replies: 9)