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Why the photon has no mass?

  1. Jan 10, 2017 #1
    We know that the photons are massless particles. The travelling speed of the photon is equal to the speed of the light(C). The Einstein's law of relative mass is says when a particle travels at the speed of light, then it's mass becomes infinity (i.e. highly increases). The photons are also energy particles which travels at the speed of light. Then why the photons have no mass????
     
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  3. Jan 10, 2017 #2

    ZapperZ

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  4. Jan 10, 2017 #3

    Orodruin

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    Relativistic mass is an antiquated concept not used by most physicists.

    The mass physicists will generally talk about is the invariant mass (since we do not use relativistic mass, we normally just call it "mass").
     
  5. Jan 10, 2017 #4
    Do the photons have Higg's field?
     
  6. Jan 10, 2017 #5

    Orodruin

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    This is a nonsense statement. Particles do not "have" Higgs field. They may in general have interactions with the Higgs field that results in a mass when electroweak symmetry is broken, but the electromagnetic field does not.
     
  7. Jan 10, 2017 #6
    If a particle with non-zero rest mass is accelerated from rest its mass increases as its speed increases.

    A photon does not need to be accelerated to go at light speed. Its rest mass is zero. Any particle with rest mass zero will go at light speed without being pushed.
     
  8. Jan 10, 2017 #7

    Orodruin

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    No it does not. Again, relativistic mass is a deprecated concept. The only mass physicists talk about is invariant mass. We just call it "mass".
     
  9. Jan 10, 2017 #8
    Some empathy for the newbs here, every textbook I have ever seen that introduces relativity defines relativistic mass as a brute fact of nature.
     
  10. Jan 11, 2017 #9

    vela

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    This hasn't been my experience. I just checked all of the textbooks I have, and none mentions relativistic mass. Some are from when I was in college 30+ years ago, and some are from classes I've taught during the past few years. Griffith's book on particle physics, I recall, mentioned the idea only to ridicule it.

    Pop-sci books, in contrast, seem to go out of their way to emphasize the idea to make relativity sound really cool and mind-blowing. If you were to say the energy of the particle increases without bound as its speed approaches that of light, it doesn't sound too exciting. But if you say its mass increases, readers find it intriguing, and the authors know this. (At least this was true of the books I read in the past; I don't know if they still do it. Given how often the question comes up here, I'm guessing they still do.)
     
  11. Jan 11, 2017 #10

    Orodruin

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    Basically this. Regardless, it should not need to be repeated in a thread with 7 posts.
     
  12. Jan 11, 2017 #11
    Q: Why does a photon have no mass?

    A: Because it's light.
     
  13. Jan 11, 2017 #12

    Nugatory

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    Hmmmm..... Should I "like" this post, or should I give @Jehannum a warning for it? There's nothing in the forum rules about bad puns....
     
  14. Jan 11, 2017 #13
    Photons don't need to be light and light may have mass.
     
  15. Jan 11, 2017 #14

    Orodruin

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    Who has not heard that one before? But I don't work here any more and I am only mildly suffering from parking-guard-on-vacation-in-less-organized-country-syndrome.
     
  16. Jan 11, 2017 #15

    vela

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    Well, you could always do both. But I think you should give Jehannum a pass for the proper use of the it's as opposed to its.
     
  17. Jan 11, 2017 #16

    Orodruin

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    That's funny, just yesterday I berated my PhD student for getting it wrong the other way around in his thesis draft.
     
  18. Jan 11, 2017 #17

    PeroK

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    At least it was only an apostrophe and not a catastrophe.
     
  19. Jan 11, 2017 #18
    Just to add a little bit on what Orodruin and Vela have already wrote to you: in case you are thinking to the equation:
    E = m*c^2,
    E = body's energy, m = body's mass, this equation doesn't tell you anything in this case, because it's correct *only for not moving bodies*, that is, it is valid only in a frame of reference where the body's momentum is zero; so it couldn't be more incorrect for photons...
    The correct equation, that is, the one which is always correct in special relativity, is:
    E^2 = (mc^2)^2 + (cp)^2 (1)
    p = momentum.
    We know from classical electrodynamics that light's momentum p is equal to cp. Then, from (1) you see that m must be zero.

    In general, every particle which has finite energy and which travels at light speed must have zero mass.

    --
    lightarrow
     
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