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B About the mass of photons

  1. Sep 8, 2016 #1
    this is the first time im learning about photons and i have asmall doubt.
    if photons are massless then how come light dosent escape from blackholes.
    or if they do have mass the by the eqn m=m0/(1-V2/c2)then theyll have infinite mass and turning on a torch may be like getting hit by a bullet train.so how do photons actually exist.with mass or without it?
  2. jcsd
  3. Sep 8, 2016 #2


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    Because they still exist in spacetime and thus have to follow the appropriate paths through spacetime. A black hole bends spacetime in such a way that ALL paths through spacetime lead further into the black hole.

    That's a bare minimum explanation, and if you want to know more I encourage you to look into relativity.

    They have no mass, but they still have energy and momentum. Einstein's full equation is: e2 = m2c4 + p2c2

    The term which includes mass is zero for photons, but the second is not, and reduces to: e=pc
  4. Sep 8, 2016 #3


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    Photons are quantum objects, elementary lumps of energy of the electromagnetic field. As of such, they have a hard time sharing classical particle properties (like position). If one still adopts a quantum particle interpretation of the uni-photon states, then these objects are massless, i.e. zero invariant mass.
    When one forcibly puts photons in black holes, they do escape, as far as I know.
  5. Sep 8, 2016 #4
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