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E=MC^2 question about the M

  1. Oct 1, 2011 #1
    E=MC^2 question about the "M"

    So, energy equals mass times the speed of light squared.

    Energy can determine speed, because the energy is transferred into the 3 spacial dimensions.

    My question is with E=MC^2 itself, Energy of light would equal zero mass times the speed of light squared, right? But that would mean light had zero energy. Care to help explain? I really wanna figure this out, and I'm reading the Elegant Universe right now so it would help to understand this before going in too much further.


  2. jcsd
  3. Oct 1, 2011 #2


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    Re: E=MC^2 question about the "M"

    This question actually comes up a lot. [itex]E = m_{0}c^{2}[/itex] is only valid for particles in their rest frames (the [itex]m_{0}[/itex] is the rest mass). A photon has no rest frame so the equation does not apply to it. In general, [itex]E = \sqrt{(m_{0}c^{2})^{2} + (pc)^{2}}[/itex] so for a photon [itex]E = pc[/itex] where p is the momentum.
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