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Thread: Do photon contain/carry more than energy, momentum and angular momentum?

  1. #1 Do photon contain/carry more than energy, momentum and angular momentum? 
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    Noether theorem says that with symmetries come conservation laws and so because of time, translation and rotation symmetry, EM field itself guards energy, momentum and angular momentum conservation.
    While atom deexcitation there clearly appears energy and (orbital) angular momentum difference, so there should be created EM field configuration carrying this difference.
    Photon's angular momentum is usually imagined as something only "quantum-transcendental", but in fact it is very real angular momentum.
    For example Richard Beth in 1936 has measured the tiny reaction torque due to the change in polarization of light on passage through a quartz wave plate: Phys. Rev. 50, 115 (1936): Mechanical Detection and Measurement of the Angular Momentum of Light
    Here is nice video of rotating macroscopic object using circularly polarized light - at about 20 second the polarization was switched to opposite one:


    Is optical photon something more than just EM wave carrying energy, momentum and angular momentum?
    If not - what more? Other than EM interactions? Some electric/magnetic moments?
    Is it just a "twist-like wave"? - like behind marine propeller, but this time in viscosity-free environment and so does not dissipate - can travel undeformed for years (is soliton) from a concrete single atom to anther one ...
    One would say that because of spin conservation, it has also "spin 1" - e.g. due to electron changing spin from -1/2 to +1/2 ... but isn't it just 180deg rotation - twist again? Especially that in opposite to other particles with spin, photon doesn't have magnetic dipole moment...
    Another question: why it has momentum? Is it that it was just required for this kind of waves or maybe there is some momentum change required for atom deexcitation itself?


    Last edited by Jarek Duda; April 19th, 2012 at 03:58 AM.
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  3. #2  
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    Hello,
    you are right that photon carrys just energy, momentum and angular momentum. It however interacts through its magnetic and electric field with eletric or magetic dipole of say atoms. Its happening through standart means like stern-gerlach term in Pauli eq. and such. It carrys momentum because it gains nonzero "mass" through relativistic means (it has nonzero energy and is traveling at speed of light). In that video is probably some dipole which interacts with EM field of photons.
    I hope Im right correct me someone if Im not.


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