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How could a photon travel at lightspeed if it has mass?

  1. Jun 29, 2013 #1
    I've heard that photons have a mass. If a photon were to be stationary, would it have mass? If not, then where does the mass of a photon come from?

    I know that if an object has stationary mass (forgive me if this isn't the correct term), then it takes an infinite amount of energy in order to achieve light speed.

    I suppose that the mass of the photon is actually energy which is acquired somehow?
  2. jcsd
  3. Jun 29, 2013 #2


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    Staff: Mentor

    Where did you hear/read this? I'm not aware of any results indicating that photons have a nonzero invariant mass (a.k.a. "rest mass"). All I've ever seen are upper limits.
  4. Jun 30, 2013 #3


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    Staff Emeritus
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    This thread is a good example of WHY it's less confusing to say "photons have energy" than "photons have relativistic mass". These two statements mean exactly the same thing, but I believe one is significantly less confusing to the lay person and significantly less likely to lead to confused followup questions and other assorted confusions.
  5. Jun 30, 2013 #4


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    It was explained to me, on my first internet science forum, on March 13, 1997, thusly:

    hmmm...... I wonder if anyone has kept track of how many times this question has been asked.

    hmmm.... It sure took me a long time to get a humour award.


  6. Jun 30, 2013 #5
    photons always move at 'c' locally.

    as noted, photons have energy [and momentum], not mass.

    The term you seek is 'invariant mass', also commonly called called 'rest mass'. The 'infinite energy' is an imprecise way to explain that mass cannot travel at speed 'c'...only slower. There is not sufficient energy anywhere to make that happen. A simpler view is that any observer [which requires mass] sees local light at speed 'c' no matter how fast they are going.

    So even a really fast observer still sees light buzzing by at good old 'c'....In other words, if you were able to accelerate and move at, say, 0.7C, which is possible, you would still observe light passing you by at the usual 'c'.
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