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Why the speed of light?

  1. Apr 24, 2004 #1
    Hey all, we are all familiar with the equation [tex]E=mc^2[/tex]
    Where energy equals mass times the speed of light squared...

    My question is...why? I mean, of course it works, but why the speed of light?

    My theory so far has been that since light is pure energy, its speed is used as a sort of constant when it comes to measuring energy. But...why the speed? Why not some other property. I dunno, its jsut causing some cognitive dissonance right now, It seems like speed shouldn't be in there.

    Please don't go out of your way to tell me "oh yes it should, because that equation works"...I'm not making any claism that its false at all.
  2. jcsd
  3. Apr 24, 2004 #2


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    It comes out of relativity, where the speed of light plays an important role. The covariant formula for energy in relativity is [tex] E^2 = p^2c^2 + m^2c^4 [/tex] where p is the magnitude of the momentum and m is the invariant mass. If you take the momentum to be zero, meaning you are in the rest frame of the mass you are working on, then the famous formula results when you take square roots.
  4. Apr 24, 2004 #3
    and it also predicts the existence of antimatter, E=+-mc^2
  5. Apr 24, 2004 #4
    Just how would that :confused: equation :confused: have anything to-do with anti-matter.

  6. Apr 24, 2004 #5
    What if mass is varying? E*dE=mc^4*dm and then integrate?
  7. Apr 24, 2004 #6

    You mean Integrate E*dm = mc^2 dm?
  8. Apr 24, 2004 #7
    I think you mean negative energy.
  9. Apr 25, 2004 #8
    Integrate the original function.
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