Electromagnetically Charged Wave Light

  1. I was wondering if light when it's in wave form (seeing as when it's in wave form it's an electromagnetic wave, energy alternating from electric to magnetic in a continuous pattern) has an electromagnetic charge, and if so, what would it be?
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    The EM wave itself has a vector for each field component that oscillates back and forth, energy is not moving from the magnetic to the electric field and back again. I'm not sure you can apply a charge to the wave, and even so photons themselves are electrically neutral.
     
  4. vanhees71

    vanhees71 3,508
    Science Advisor
    2014 Award

    The electromagnetic field itself carries no charge. The deeper reason for this is that it is an Abelian gauge field. This is different for the fields mediating the strong and weak interaction which are non-Abelian gauge fields which carry a charge, but that's more a topic for the quantum section of this forum.
     
  5. Thanks for that, helped a lot. :)
     
  6. Matterwave

    Matterwave 3,845
    Science Advisor
    Gold Member

    A much simpler answer than delving into QFT, would be that EM waves in vacuum obey the source free Maxwell equations which tells us that the total charge in any volume element is 0.
     
  7. Has anyone actually proved that light waves don't have an electromagnetic charge, and if they have, how?
     
  8. jtbell

    Staff: Mentor

    When sunlight shines on you, do you become electrically charged? When you turn on a light bulb in a room, do the walls of the room become electrically charged?
     
  9. Yes. If you send light through an electric or magnetic field of any arbitrary strength, you will see no bending of the path of light.
     
  10. They could, but very unnoticeably. Not only that, but with the example of humans and walls, the electric charge would discharge very quickly.
     
  11. Drakkith

    Staff: Mentor

    No, the amount of light hitting you from a standard light source would VERY quickly charge you. You might get around this by saying light is both + and - charged, but we can do experiments with single photons and we can honestly say they are not charged. Plus I'm not sure what would even happen if photons were charged. Upon absorption they are gone, so whatever charge they may have had is also gone. For the charge to remain you would need a charged particle to be created. This doesn't happen. (Particle-antiparticle creation from high energy photons is different)
     
  12. Exactly, upon absorption a photon is gone, so the wall remains no longer charged. Hence what I've previously said, if a photon did have a charge, it would discharge unnoticeably quickly. Thanks Drakkith, but how have experiments that prove an individual photon is neither negatively or positively charged been carried out? What are they? I thought of a possible one;

    Key:
    ¡ = Torch __
    |-|+| = Bar magnet
    _ = Piece of card ( |-|+| )
    ( ) = Magnetic field boundaries
    ^ = Light beam ^
    ^
    ¡

    The idea is you'd reverse the bar magnet and see which time more light hit the card. If light has no charge, the amount of light hitting the card would be the same each time.
     
  13. Drakkith

    Staff: Mentor

    No, the term discharge means something entirely different than how you are using it. A normal discharge is a movement of charges. Charges being electrically charged particles. The charge does not disappear, it simply moves, unlike your description of a charged photon.

    I don't usually take this tone, but...are you serious? A bar magnet deflecting light onto a card? The Large Hadron Collider has multiple detectors that use multi-ton magnets to deflect charged particles. Light is not affected by these fields. High voltage and magnetic sources are routinely worked with around the world on a daily basis, including in plenty of very sensitive experiments in a number of areas such as Quantum Mechanics, Optics, and even basic Medical MRI's.

    Furthermore, for light to be charged it would undermine the standard model of particle physics thanks to charge conservation violations. (Charge conservation is an integral part of the LHC collisions, of which over 1 TRILLION have taken place since it started up. Not to mention the dozen+ other major colliders we've had in the past 60 years or so.) The number of experiments involving light, charged particles, and EM fields is so vast I can't even imagine it. If light were charged, we would know about it.
     
  14. Good point. Sorry, I was thinking of electrons when I said discharged. Photons would be an entirely different matter.
     
  15. Remember also that if photons are charged, then they would emit other photons during travel. This does not happen, we cannot see light coming from a beam of light passing in front of our eyes unless that light is scattering off something, like dust. Charged light carriers would make Electromagnetism a nonlinear force, like gravity.
     
  16. Recently I read in a book in my school library, that mentioned photons can in a way be polarised, but seeing as it's only a intermediate library, there was no resource explaining how this is possible. So a, is the book correct, and b, how is that possible?
     
  17. sophiecentaur

    sophiecentaur 13,583
    Science Advisor
    Gold Member

    The term 'polarised' has two distinct meanings when applied to electric charges and with the direction of the fields in an EM wave. This may account for the confusion. Look them up and you will see.
     
  18. Born2bwire

    Born2bwire 1,776
    Science Advisor
    Gold Member

    To wit, the polarization in light has to do with direction of the plane in which the electric field vector oscillates as a function of time. For example, linearly polarized light is when the plane is constant in time while elliptically or circularly polarized light has the plane rotating as time passes.
     
  19. The book is correct. Light polarization just has to do with the way fields are oriented in a light wave. Polarized sunglasses, for example, take advantage of the fact that light reflected off of surfaces on the ground reflect toward you with predominantly one polarization, and the glasses are designed with microscopic slits to phase out this polarization of light, and significantly reduce glare. If you were to take two polarized glasses, and turn one at 90 degrees from the other, you would theoretically block out all light.
     
  20. sophiecentaur

    sophiecentaur 13,583
    Science Advisor
    Gold Member

    And the same word 'polarisation' refers to separation of charges. Is this where the (OP) idea that a photon could be charged has come from?
     
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?