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Massless photons? Is everything light?

  1. Nov 22, 2014 #1
    Are photons massless? They have energy and can be affected by gravity, so don't they have mass? I've heard plenty of people say that photons are massless, does this just mean that they have negligible mass?
    Also, if only light/photons can move at the speed of light, then wouldn't everything be light considering that, relative to the photons, they aren't moving while everything else is moving at the speed of light? I'm not super knowledgable about physics so I expect I have errors in a few of my assumptions so please correct me, I'm just curious and have been thinking about this.
  2. jcsd
  3. Nov 22, 2014 #2


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    Within the precision of all measurements, and within theories: yes.
    Those are two independent things.
    Experimental results are never exact, but in theory they are treated as exactly massless. And the experimental limits are really good. It would be odd to have a particle with a mass of just 0.00000000000000000000000000000000000000001 the mass of other particles.
    And gluons, and gravity.
    That does not make sense.
  4. Nov 22, 2014 #3


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    Photons do not have a valid reference frame; time does not pass. It is nonsensical to ask what a photon (or anything moving at c) 'sees' or 'experiences'.
  5. Nov 22, 2014 #4


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    You'll find several relevant threads in the relativity FAQ: https://www.physicsforums.com/forums/relativity-faq.210/ [Broken]
    Last edited by a moderator: May 7, 2017
  6. Nov 23, 2014 #5


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    Nothing stops you from adding a mass to the photon. In theory however as well as in experiments, the photons appear to be massless.

    Yes they have energy which corresponds to their momenta, not mass... Gravity doesn't couple only to masses but to the energy momentum tensor. For the photons the last is non-vanishing.

    Up to very high precision photons appear to be very light and experiments also verify that they can be massless. Note: experiments will never tell that the photon has zero mass, but they can verify it within some precision...

    There is no well-defined reference frame where the photon is at rest. That's exactly because they are considered massless...
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