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Light Waves

  1. Jan 12, 2005 #1
    If an moving electric charge produces light waves what is it is oscillating?
     
  2. jcsd
  3. Jan 12, 2005 #2

    DB

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    Do you mean what is oscillating in a light wave?
     
  4. Jan 12, 2005 #3

    pervect

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    A charge moving at a constant velocity doesn't produce electromagnetic radiation (light waves) - only an accelerating charge does that.
     
  5. Jan 12, 2005 #4

    JesseM

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    The strength of the electrical field E and the magnetic field B, whose force vectors are at right angles to each other in an electromagnetic wave. See the animation on this page.
     
    Last edited: Jan 12, 2005
  6. Jan 12, 2005 #5

    Garth

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    Thanks for that link.

    A question, follow an oscillation along the 'x' axis with your cursor. From our perspective a photon is a 'wave packet', but what is it from the photon's perspective?

    Garth
     
  7. Jan 12, 2005 #6

    DB

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    well, pervect and jesse have covered ur question.
    Jesse cool animation :biggrin:
     
    Last edited: Jan 12, 2005
  8. Jan 12, 2005 #7

    JesseM

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    When people talk about particles as wave packets, they're not talking about waves in the electromagnetic field, but waves in the quantum wavefunction which assigns different probabilities to different possible positions the particle could be found when you measure its position. That's how it works in nonrelativistic quantum mechanics anyway...but to understand electromagnetic waves in quantum terms, you'd need to get into the quantum field theory of electromagnetism, quantum electrodynamics or "QED", which is a lot more complicated than either classical electromagnetism or nonrelativistic quantum mechanics. I'm not sure if there are situations in quantum field theory where particles can be treated as wave packets, it may not work that way.
     
    Last edited: Jan 12, 2005
  9. Jan 12, 2005 #8
    I don't think so. It is a simple question. With a water wave water is oscillating, with sound air is oscillating. So what is the deal with light?
     
  10. Jan 12, 2005 #9

    JesseM

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    The answer is that you shouldn't take the analogy with soundwaves in water or air too literally. In the past people imagined that light was a soundwave in an invisible substance called the "luminiferous aether", and that the explanation for why light always travelled at the same speed regardless of the velocity of the source was the same as the explanation for why soundwaves travel at the same speed regardless of the source velocity (for instance, the sound from a car moving towards you doesn't arrive any faster than the sound from a stationary car at the same position). But soundwaves only travel at the same speed in the reference frame of the substance they're moving through--if you have a velocity v relative to the rest frame of the air, then you will see sound waves move away from you at the speed s+v in one direction and s-v in the other. People thought the same would be true of the aether, that the laws of classical electromagnetism would only give precisely correct predictions in the aether's rest frame. Based on this, you'd think that when the earth was at different points in its orbit it'd have different speeds relative to the aether, so the velocity of light measured from earth might change slightly. But all attempts to discover such an effect failed. Then Einstein showed that it is actually possible for observers in all reference frames to measure the speed of light to be c in all directions, provided that each observer sees other observers' rulers shrink and their clocks slow down when they are in motion relative to the first observer. Since experiments continue to show that the laws of physics seem to work the same way in all reference frames defined in Einstein's way, physicists have dropped the idea that electromagnetic waves should be understood in analogy with soundwaves in a medium, and instead they just think of them as waves of changing strength in the electromagnetic "force field" which fills all of space.
     
    Last edited: Jan 12, 2005
  11. Jan 12, 2005 #10

    Garth

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    I wasn't talking about a massive particle but a photon, which nevertheless, like everything else, has a "wave/particle" duality. It is a 'packet of electromagnetic energy', but can it be described in the fully relativistic frame of reference? (With no extended time dimension)

    Garth
     
  12. Jan 12, 2005 #11

    JesseM

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    Photons only exist in quantum theory, they are not wave packets in the classical electromagnetic field. In quantum mechanics, both photons and massive particles may be in a state involving a wave packet in their quantum wavefunction, but that does not mean these particle are just wave packets in the wavefunction (in other states their position wavefunction may not involve a wave packet at all, like with an electron in an atom).
     
  13. Jan 12, 2005 #12
    Then it is the force field that is oscillating.
     
    Last edited: Jan 12, 2005
  14. Jan 13, 2005 #13

    Garth

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    The photo-electric effect?
    I have said above photons exist in a wave/particle duality like everything else!

    Garth
     
  15. Jan 13, 2005 #14

    JesseM

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    The photo-electric effect is part of quantum theory--it can't be explained properly using classical electromagnetism. I agree that wave/particle duality applies to photons in quantum theory, but the point is that there is no wave/particle duality in classical electromagnetism, so a wave packet in the classical electromagnetic field is not also treated as a particle.
     
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