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Photon Mass Zero?

  1. Apr 5, 2006 #1
    Photon Mass Zero???

    Ok right to the point, I've learned that a Photon since it is moving at c has a mass zero, but yet i've also heard that with a sensitive enough scale, that you can measure the force of sunlight or other light as photons...very small force, but still a force
    A measureable force, must have a mass associated with it.
    F=ma - classical or not, still true...

    and if theres a mass with a velocity c, then....
  2. jcsd
  3. Apr 5, 2006 #2


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    Photons have zero rest mass. However, they do have energy and momentum, so they can exert force. Only zero rest mass things can travel at the speed of light.
  4. Apr 5, 2006 #3


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    To add to this, consider Newton's second law, "'An applied force is equal to the rate of change of momentum'."
  5. Apr 5, 2006 #4


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  6. Apr 5, 2006 #5

    Meir Achuz

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    I need at least ten letters for the answer.
  7. Apr 5, 2006 #6


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    actually Bart, [itex] F = m a = m \frac{dv}{dt} [/itex] is not precisely true, but is very close at slow speeds relative to the speed of light when the mass of the object remains constant. the accurate expression is:

    [tex] F = \frac{dp}{dt} = \frac{d(mv)}{dt} = m \frac{dv}{dt} + v \frac{dm}{dt} [/tex]

    due to the product rule of differentiation. so if you had a body moving at a constant velocity, v, and somehow its mass was increasing (thus increasing its momentum) a force would be required to keep it going at that constant velocity.

    that being said, i am of "the other side" which maintains that Photons have mass, but no rest mass. . differentiating between "mass" in general and "rest mass" seems to be out of vogue of late but the reality of that difference exists. the reason there can be no rest mass is that if any particle moving at c did have rest mass, its relativistic mass would be infinite at speed c, and photons do not have infinite mass.
  8. Apr 5, 2006 #7
    sorry for my ignorace... I'm still second year physics, and just really trying to have a FEEL and Intuition for Physics....lol....
    Also a lot is based on the fact that c is a constant... but some physicist contend this... that c may not be constant, and Einstein might be wrong
    I'm really not trying to piss you guys off or seem like a know it all , that might not know anything...lol....
    I also read that Hawkings retracted a couple thoeries or papers, in light of new evidence and reconsideration... and I'm sure they were accepted as LAW....

    In my last year of highschool, I had to do a research paper for english and being a physics student I picked, Trying to say the speed of light was not a constant....
    Two papers were my foundation, one from "I think" cambridge and MIT...
    1. made light travel through a cesium gas tube faster than c, almost 300 times faster, and,
    2. made light travel at 26mph in a gas tube....
    Also, another paper said that maybe at the time of the big bang the speed of light was 300 times faster...

    So I went on to say that the speed of light depends on the energy in the medium that it travels... Big bang, Very energetic and very fast
    And now the universe has cooled and the speed has slowed down to the current speed of c......
    And Another textbook actually had a graph that the speed of light has change since the bigbang, and for the couple hundred years that we have been able to measure it, the universe hasnt cooled measureably.....

    SOrry for this being SOOO long.....
  9. Apr 6, 2006 #8

    Claude Bile

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