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Photon Mass Question.

  1. Apr 12, 2008 #1
    I know that Photons have no mass at all. Although I read it was something insanely small like 1.1×10−52 kg.

    So because it still has some small mass, and the fact that there are so many photons.

    If it was possible to get all the photons in the Universe in one spot say the size of a cup. Would the whole lot weigh anything?

    thanks. :)
  2. jcsd
  3. Apr 12, 2008 #2


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    I think that you might be confusing photons and neutrinos. The latter were always thought to be massless, until experiments showed that they do in fact have a very negligible rest mass. Photons have none.
  4. Apr 12, 2008 #3


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    Photons do have mass via a technicality - mass = [tex]\frac{hf}{c^2}[/tex] which of course is that famous equation rephrased.

    They don't have an INVARIANT MASS, which is what everyone talks about - I think?

    If you got all the photons in the world and "compressed it into a cup" then you would most probably create particles or it might collapse into a black hole if the radius is small enough.

    I don't think it makes sense to localise photons like that though.
  5. Apr 12, 2008 #4
    Yer I read that neutrinos have very small mass.

    But if photons have no mass at all, then I would just assume all the photons in the universe in one small spot would still be total 0 mass. You could pick the cup with ease.
    Kinda hard to wrap my brain around.
  6. Apr 12, 2008 #5


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    this might put me in a minority position here at PF, but i would not add the emphasis or affirmation "at all" with that statement. in fact i would go the other way and add a qualification:

    Photons have mass. "Relativistic mass" or "inertial mass" that is [itex] m = E/c^2 = h\nu/c^2[/itex]. but, if they travel at the same speed as the wavespeed of light (which is "c"), their "rest mass" or "proper mass" is zero.

    that 10-52 kg figure is a maximum rest mass (from some weird experiment i know little to nothing about) and if photons had that rest mass, they would be moving at a speed ever so slightly less than the wavespeed of light.
  7. Apr 12, 2008 #6


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    How about a perfect sphere of some finite radius, inside being mirrored, containing every photon in the universe except for those required for observation and measurements. Does that have mass greater than the shell?

    What would happen if you started rotating it? Etc...
  8. Apr 13, 2008 #7

    Shooting Star

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    The photons do not have any rest mass. What you had read is an attempt by the present experimental methods to confirm that very fact. Due to experimental limitations, that value is the present upper limit we can set on the rest mass of a photon.

    However, as mentioned by the others, there is an effective mass of hf/c^2. Note that this mass depends on the frequency of the photon, and is not a constant.

    Let's stick to the standard model -- the rest mass of a photon is zero.

    The number of photons is not conserved. At every moment, photons are produced and absorbed by matter, not necessarily in equal numbers.

    The energy or frequency of a photon is frame dependent. So, it is actually not a very meaningful statement to collect all the photons in the Universe. By bringing them to your frame from another, either their energy will change or they'll simply vanish by interacting with matter. Also, there may be parts of the Universe beyond an event horizon, from where anyway we cannot bring anything.

    However, in a thought experiment, by putting in sufficient amount of thought light in a shell whose inside was a perfect mirror, the whole thing can be made to weigh anything. :surprised
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