Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mass of a photon tear me up!

  1. Sep 29, 2004 #1
    well while im assuming im just an idiot and this is wrong check it out...
    [tex] \lambda = \frac{h}{p}[/tex]
    [tex] c = f \lambda[/tex]
    [tex]p = \frac{hf}{c}[/tex]

    [tex] \frac{c}{f} = \lambda = \frac{h}{p}[/tex]

    [tex]\frac{c}{f} = \frac{h}{mv} [/tex] v = c for a photon..

    [tex] m = \frac{fh}{c^2}[/tex]->[tex] m = \frac{E}{c^2}[/tex]

    while this is most likely hugely flawed it brings me to a question...if momentum of a photon is the above 3rd orginal equation, the momentum would increase as the frequency increases, and sence any EM radiation v = c the "mass" of the photon would have to change in a classical view p = mv so...whos to say that photons of all frequencies must have the same mass? let me know what everyones think'n!
    Last edited: Sep 29, 2004
  2. jcsd
  3. Sep 29, 2004 #2


    User Avatar
    Science Advisor

    Momentum of the photon is not mv. Energy is hf:
    Since for a photon, m=0,
    p is not mv except at velocities small compared with c.
  4. Sep 29, 2004 #3
    i did not use the momentum for a photon in the formula, i merely used it for the therotical question...and the equations i did use do deal with mass, i was merely useing it possibley show that the mass of a photon could increase with increase in frequency...
  5. Sep 29, 2004 #4
    It does increasy, at a rate of x times 0.
  6. Sep 29, 2004 #5
    prove it (0x = m)
  7. Sep 29, 2004 #6


    User Avatar
    Science Advisor

    Mass is zero. Phymath, you are being inconsistent. First you say
    , then when I point out the flaw, you defend it.
  8. Sep 29, 2004 #7
    the flaw u pointed out is incorrect, and i will defined it until an undeniable proof is given to me. if i told i might be wrong but 2 + 2 = 4 and u say no its 5 im suppose to assume ur correct? no i need to see ur proof...
  9. Sep 29, 2004 #8
    I don't think anybody does. Just remember that a photon has no rest mass, but it does have relativistic mass. This relativistic mass is dependent on the photon frequency. Just because [tex]m = \frac{E}{c^2} = \frac{hf}{c^2}[/tex]. That's all you need to show this.
  10. Sep 29, 2004 #9
    so photons have relativistic mass, which is in that form which i said it was up above, and also showed that i realized that was all i had to do above, so are u telling me everyone but me has always known photons DO have mass just no rest mass?....
  11. Sep 30, 2004 #10
    Anyone, as in any physicist who has done special relativity (SR), and has thus learned to distinguish "the two types of mass."

    In classical physics, there is only "one type of mass", and it the one that is equivalent to the rest mass in relativity. So for someone who hasn't gone through SR yet, photon mass is 0 and nothing but 0 and it is ok to think that way for 99.9999% of humanities problems.
  12. Sep 30, 2004 #11


    User Avatar
    Science Advisor

    The thing called "relativistic mass" is a stopgap measure to help those who have learned Newtonian mechanics transfer their intuition over to SR. Those who have just learned SR, and little else, need it. Those, like me, who work with relativistic particles on a daily basis don't need it. There is only mass ("rest mass") and momentum. In the absence of other energy terms, coming say from field potentials, they add in quadrature to give energy. You can take this energy, divide by c^2 and call it "relativistic mass" if you want, but then it is nothing more than a re-naming of energy (since c never changes).
  13. Sep 30, 2004 #12
    I totally agree. I wouldn't see any advantage of talking about the relativistic mass of a photon over its energy or momentum. Thanks for the precision krab.
  14. Sep 30, 2004 #13


    User Avatar
    Science Advisor
    Gold Member

    It's quite common to make the substituion c = 1 (no units), so then the distinction between energy and relativistic mass disappears completly.
  15. Oct 1, 2004 #14
    I was first taught 'relativistic mass' and later converted to the concept of that there is no such thing as 'relativistic mass'. And I firmly believe this (thought I think this can be just a matter of personal preference).

    for photon, the very definition of 'relativistic mass' would be
    So what's the big deal about photon have increasing 'mass' (kinetic energy basically) as function of frequency? It does...
  16. Oct 30, 2004 #15

    Who cares!!!!!!!!!!!!!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook