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Is discreteness supplied or demanded?

  1. May 15, 2014 #1
    Recently I learned in E&M the nature of creation of electromagnetic radiation, specifically in the context of an accelerating charge, as a ripple in the field. My quarrel is this:

    We can measure the electric field surrounding a point charge from an infinite number of points; the field is entirely continuous in nature. Where then, does the wavefront transform from a uniform change at all equal radii to separated packets of discrete energy?

    I am missing where the continuous field becomes non-continuous.

    My professor says that the discreteness is a result of the uncertainty principle, but that doesn't explain the mechanism for me.

    Is the detection of a photon perhaps the consequence of a necessary discrete energy absorption from the continuous field, instead of a necessary discrete production?

    I realize I may have several misconceptions, thank you for your patience and insight.
  2. jcsd
  3. May 15, 2014 #2


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    Discreteness comes in when the intensity is reduced. As an example, the lunar laser ranging experiment.. Lasers on Earth are aimed at retroreflectors on the Moon. What starts out as a continuous wave becomes so weak after the round trip that it is received as individual photons.

    "At the Moon's surface, the beam is about four miles wide"... "Out of 1017 photons aimed at the reflector, only one will be received back on Earth every few seconds."
  4. May 15, 2014 #3
    I suppose a laser makes more sense to me because we are directly stimulating emission of discrete energy in the first place (or so I understood). With an accelerating point charge, the old field and the new field could be measured from an infinite number of points, so where does it become discontinuous?
  5. May 16, 2014 #4
    From my conceptual understanding of quantum mechanics, I believe the continuous wave turns into a discrete, pin-point packet of energy once it is observed. Until the observation, the electromagnetic wave is only a wave because it is a continuous, expanding cloud of probability and behaves like a wave. If, for example, the EM wave hit a detector screen, the wave would collapse into discrete particulates of energy. I may be wrong on this, however, but that's my guess if I'm understanding it right.
  6. May 16, 2014 #5


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    There is no difference what the light source is, whether you use a laser, a radio transmitter, an LED bulb, or a candle. You cannot say some are continuous light waves and others are discrete. Light from a laser can be focused in a lens, refracted through a prism, and so on - everything you do with other types of light. Ordinarily you don't notice the discreteness because of the incredibly large number of photons. The laser ranging example quoted 1017.

    Discreteness comes in when the intensity is reduced.
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