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Mass and Energy Differentiation

  1. Nov 3, 2012 #1
    Hello friends:

    My Question:
    A massive object cannot move at the speed of light. Photons can move at the speed of light because they are massless. However, since energy and mass are equivalent, due to Einstein's famous equation E^2=(m(c^2))^2+(pc)^2, mass is energy by a conversion factor. And photons are just quanta of electromagnetic radiation and are therefore energy as well. Since both a massive object and a photon contain some amount of energy, what differentiates the two which allows a photon to move at the speed of light and while a massive object cannot? Is it in regards to the Higgs field?

    Thank you for your time
  2. jcsd
  3. Nov 3, 2012 #2


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    Staff: Mentor

    I would not say "mass is energy". I would say that "mass has energy". But leaving that aside as a semantic quibble, the converse is not true in general. You cannot always say that "energy is mass." (I would rather say, you cannot always say that a particle's energy comes from its mass.)

    In general, a particle's energy comes from both its mass and its motion.

    For a massive particle in motion, its energy comes both from its mass and its motion, via Einstein's famous equation.

    For a massive particle at rest, its energy comes from its mass alone. Take Einstein's equation and set p = 0.

    For a massless particle (photon), the energy comes from its motion alone. Take Einstein's equation and set m = 0.
  4. Nov 3, 2012 #3
    There was a time when you could. We used to painfully distinguish 'rest mass' from 'relativistic mass' carefully sprinkling m0 everywhere throughout the texts.
    Nowadays mass (rest mass) is mass and that's it. A photon has zero mass by definition.
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