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Thread: light quanta

  1. #1 light quanta 
    Junior Member tcrosa27's Avatar
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    I'm not sure if this goes here or in the relativity subject but since a photon is a light quanta I picked this topic.

    My problem is I'm told photons have no mass and momentum is mass x velocity. If that is so light has no momentum and therefore no energy according to E2 = (mc2)2 x pc2.

    Can anyone tell me what's "the quantum way" to get light's momentum?

    P.S. -- This is what you learn in high school although it's wrong for it tells you light is "energyless".
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  2. #2  
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    Quote Originally Posted by tcrosa27 View Post
    I'm not sure if this goes here or in the relativity subject but since a photon is a light quanta I picked this topic.

    My problem is I'm told photons have no mass and momentum is mass x velocity. If that is so light has no momentum and therefore no energy according to E2 = (mc2)2 x pc2.

    Can anyone tell me what's "the quantum way" to get light's momentum?

    P.S. -- This is what you learn in high school although it's wrong for it tells you light is "energyless".
    Photon - Wikipedia, the free encyclopedia

    Look under physical properties for definition of energy and momentum.
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  3. #3  
    Administrator Markus Hanke's Avatar
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    Quote Originally Posted by tcrosa27 View Post
    My problem is I'm told photons have no mass and momentum is mass x velocity.
    That is true only for massive objects, not for photons. The photon momentum is defined as

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    Junior Member tcrosa27's Avatar
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    Quote Originally Posted by Markus Hanke View Post
    That is true only for massive objects, not for photons. The photon momentum is defined as

    h is plank's constant, lambda is wavelength but k is what? never mind it's the wave vector I just found it. and I just also found that h with a dash above is the reduce plank constant too.
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  5. #5  
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    Quote Originally Posted by tcrosa27 View Post
    h is plank's constant, lambda is wavelength but k is what? never mind it's the wave vector I just found it. and I just also found that h with a dash above is the reduce plank constant too.
    Yes, that's right.
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  6. #6  
    Senior Member MaxPayne's Avatar
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    That is true only for massive objects, not for photons. The photon momentum is defined as
    Is this equation of momentum applicable to larger objects as well?
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  7. #7  
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    Quote Originally Posted by MaxPayne View Post
    Is this equation of momentum applicable to larger objects as well?
    No, this equation is specifically for photons.
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  8. #8  
    Senior Member MaxPayne's Avatar
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    No, this equation is specifically for photons.
    And for all other mass-less particles?
    Yes, I should vibrate 10^51 times per second for that to be true.The idea is dizzying!
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  9. #9  
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    Quote Originally Posted by MaxPayne View Post
    And for all other mass-less particles?
    Yes, I should vibrate 10^51 times per second for that to be true.The idea is dizzying!
    The only other known and confirmed massless particle is the gluon - I admittedly don't know how to calculate its momentum, but I have a feeling that it would be a whole lot more complicated than for a photon.
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  10. #10  
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    E^2=mc^2 +p^2c^4 applies to everything massive or not. For low velocity where v << c the momentum p is approximately mv.
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  11. #11 Correction! 
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    Quote Originally Posted by Jilan View Post
    E^2=mc^2 +p^2c^4 applies to everything massive or not. For low velocity where v << c the momentum p is approximately mv.
    M^2c^4 +p^2c^4=e^2. That's better!
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