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One has to wonder about photon interaction, and when to think of them

  1. Jun 2, 2010 #1
    One has to wonder about photon interaction, and when to think of them as a particle and when as simply a wave. A friend of mine told me to think of them as a wave, because they are without mass. But why should I view it that way, when even theoretical mass is mass, such as weak force. So how might one think of them? And what about this:


    Energy of one photon:

    By this logic, and by no means do I claim it to be infallible, photons do have a theoretical mass inversely proportional to its wavelength and multiplied by the constant (1.37951014×10^-23) which I dub, were it to have any scientific ground to it, Demitri constant.
  2. jcsd
  3. Jun 2, 2010 #2


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    Re: Photons

    You make a mistake that is so common, we have a FAQ entry on it. Please review the FAQ thread first in the General Physics forum.

  4. Jun 2, 2010 #3
    Re: Photons

    the full equation is E^2=(mc^2)^2 + (pc)^2 the mass of a photon is zero so it then
    becomes E^2=(pc)^2 then E=pc hc/(lambda)=pc
    then (lambda)=h/p which is the debroglie hypothesis .
  5. Jun 2, 2010 #4
    Re: Photons

    Photons have a mass identically equal to zero. The equation E mc^2 cannot be applied to massless particles such as photons. The short explanation is that photons have momentum, but no mass.

    The longer explanation has to come with the derivation of E = mc^2. As Cragar said, the actual equation for a massive particle is,

    [tex]E =\sqrt{p^2c^2 + m^2c^4}[/tex]

    This can be rewritten as,

    [tex]E = mc^2\sqrt{1+\dfrac{p^2}{m^2c^2}}[/tex]

    We can Taylor expand this in terms of p^2/m^2c^2,

    [tex]E = mc^2(1+ \dfrac{1}{2}\dfrac{p^2}{m^2c^2}-\dfrac{1}{8}(\dfrac{p^2}{m^2c^2})^2+...)[/tex]

    When we assume a small momentum (in nonrelativistic cases the momentum is always much smaller than the mass times c), we can just take the first two terms,

    [tex]E = mc^2+ \dfrac{p^2}{2m}[/tex]

    One can recognize the second term as the formula for kinetic energy. However, in the nonrelativistic limit we recover the peculiar mc^2 term, which seems to be momentum-independent. This is why we say tha E = mc^2. Note also that in the derivation, we must assume that p << m, and this certainly isn't true for a massless particle. E = mc^2 only works when you understand that a particle can have momentum but no mass.

    Hope this helps!
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