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Thread: I don't understand it!

  1. #1 I don't understand it! 
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    E=mc^2
    and
    E = hf
    so
    mc^2 = hf

    It means, that a photon has a mass. Isn't a photon massless?


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  3. #2  
    Brassica oleracea Strange's Avatar
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    That is only half the story. The full equation is: E2 = p2c2 + (mc2)2

    Where p = momentum; so particles with zero mass (photons) have energy given by E = pc.

    Edit: A nice little presentation about this here: http://www.wlu.ca/physcomp/pc242/Phys242Lec5.pdf
    (As it points out, the E2 = mc2 version is the classical approximation - when velocity is much less than c)


    ei incumbit probatio qui dicit, non qui negat
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  4. #3  
    Reptile Dysfunction drowsy turtle's Avatar
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    (But yes, photons do have mass. What they don't have is rest mass)
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  5. #4  
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    Quote Originally Posted by drowsy turtle View Post
    (But yes, photons do have mass. What they don't have is rest mass)
    .. does photon attracted to gravity because it really has mass OR do we say so because having 'mass' fits the equation??
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