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One question about h*f=m*c*c

  1. Sep 13, 2009 #1
    f is the frequency of a particle.
    In the theory of relativity, energy equals m*c*c.
    In the theory of Photoelectric effect, energy equals h*f
    If the particle keeps still.
    m is the rest mass of the particle.
    In this case the frequency is 0 and h*f is 0
    However m*c*c isn't 0.
    It's a little strange.
  2. jcsd
  3. Sep 13, 2009 #2
    I think E=hf is just for particles with 0 rest mass such as photons. So m*c*c is 0.
  4. Sep 13, 2009 #3


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    Well... actually, you can say [itex]E = hf[/itex] for massive particles too (although the formula is usually used for massless particles, true) The reason this works is that the formula for energy is actually
    [tex]E^2 = p^2c^2 + m^2c^4[/tex]
    If you set [itex]p=0[/itex], then you get [itex]E = mc^2[/itex] - but only then. The popular version of Einstein's formula applies only to particles at rest.

    In this case [itex]E = hf[/itex] is used to define the frequency of a particle.

    I refer you to Wikipedia: http://en.wikipedia.org/wiki/Matter_wave for more information.
  5. Sep 13, 2009 #4
    Well, the mass can be alternatively defined as the rest frequency of a wave. [tex]f_0 = m c^2 / h [/tex]. De broglie postulated that every massive particle has an internal clock whose frequency is fixed by the mass, and actually this intrinsic periodicity of the particles has been observed in a recent experiment [ http://www.ensmp.fr/aflb/AFLB-301/aflb301m416.pdf [Broken] ]
    Last edited by a moderator: May 4, 2017
  6. Sep 13, 2009 #5
    From the full formula given by diazona you can see that when mc^2 is zero but the particle still has energy it must all be in E = pc.
    Thus the particles with no rest mass have momentum, frequency, and energy. having no rest mass they must travel at c.
  7. Sep 14, 2009 #6
    I think when the mc^2 is zero, p must be zero. So the E=pc must be zero.
  8. Sep 14, 2009 #7
    It seemed have answered my question. I will read the paper first.
    Last edited by a moderator: May 4, 2017
  9. Sep 14, 2009 #8
  10. Sep 15, 2009 #9
  11. Sep 16, 2009 #10
    Zong Xiaobo -I think when the mc^2 is zero, p must be zero. So the E=pc must be zero.

    No ! A photon transports energy, it has momentum p = hf/c
    You know that light has momentum everytime you sit on the beach.
    E = hfc/c = hf.
  12. Sep 16, 2009 #11
    I think when a photon travel at a speed of c. The invariant mass of a photon isn't zero.
    So mc^2 isn't zero.
  13. Sep 16, 2009 #12
    That statement is not only not logical - if the invariant mass is able to vary, then it's not invariant - but it's just wrong in physics.
  14. Sep 17, 2009 #13
  15. Sep 17, 2009 #14
    It seemed that I misunderstand what you said.
    when you mentioned mc^2, m is the rest mass.
    I always consider m as the relativistic mass.
  16. Sep 17, 2009 #15


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    You might want to read an entry in the FAQ thread in the General Physics forum.

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