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Physics Faq clarifiaction - light duality

  1. Sep 22, 2009 #1
    There's a very useful FAQ entitled IS LIGHT A WAVE OR A PARTICLE?

    It carefully warns against confusing a "particle" of light with a regular particle. I have a funny feeling I may be doing this... hence the need for a clarification!

    When considering light as a wave; should I consider that the quanta of energy contained in the wave oscilates back and forth at the wavefront?

    I guess the other way of asking the question would be should I consider that a photon moves in a wave like motion along a straight line, the width of which would be the amplitude of the wave?

    Or - as I suspect - am I confusing the hell out of myself!!

    Thanks in advance
  2. jcsd
  3. Sep 22, 2009 #2


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    If you are going to treat light as a wave, then treat as a wave, end of story. The classical picture of electromagnetic wave theory is incompatible with any particle theory. Any particle theory is incompatible with a wave theory. Any wave or particle theory is incompatible with quantum theory. Light is propagated by energy quanta called photons that have wavelike properties, the behavior of which is described by quantum electrodynamics. Quantum mechanics does not treat light as a wave or as a particle but something inbetween. Whenever you decide to treat light as one or the other (wave or particle) you are making a classical approximation and you should not try to intermix the two.

    So classically, light is an electromagnetic wave, no photons, that is propagated as time-varying electric and magnetic fields. The energy of the light is contained in the fields of the electromanetic waves. In quantum electrodynamics, light is propagated as energy quanta, photons. These photons behave like waves but that does not mean they propagate like waves. I do not think we can make any real assumptions on how they propagate. The properties that make photons wave-like are not directly observable. We can observe the consequences of the wave nature but not the wave nature directly. In reality, we can detect individual photons and their properties like spin and energy. The other observables we can detect are the electric and magnetic fields.

    We can easily see the wave properties of the electric and magnetic fields. We can place a probe in the fields and observe the phase dependence and wave propagations. We cannot do this with photons. In addition, we talk of wave motion, we do not mean that the particle or fields are oscillating in space. That is only true of mecanical waves like sound waves. Sound waves propagate by displacing matter and we can see the displacement of the medium. But electromagnetic waves do not displace matter to propagate (though they can displace matter if it is charged or polarizable for example). The wave's amplitude and phase will oscillate but do not think that this represents some motion or perturbation of space. Likewise, the wave nature of the photon is not some physical movement of the photon as it moves through space. Rather, the wave-like property has to do with the fact that we describe the photon by functions that map out the probability of finding the photon at a given place in space with specific properties of energy and spin. These functions, called wavefunctions, behave like waves. They have phase dependencies and can constructively or destructively interfere. But these do not provide direct indication on how the actual particle will behave physically, they indicate what we will find when we make measurements.

    So does the photon move in a perfectly straight line through a vacuum or does it oscillate from side to side along an average line of motion? Who knows, as far as I know, quantum mechanics does not say how the photon gets from point A to point B, it can only comment on the probability that it will move from point A to point B under circumstances X. Do note though, that if we were to measure the photon constantly as it traveled from A to B, then that would be additional constraints on our X, it would alter the path of motion of the photon so that the final behavior going from A to B is different than if we made no measurements along the path.
    Last edited: Sep 22, 2009
  4. Sep 22, 2009 #3
    Thanks - that puts me back on the right path!

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