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Is photon emission spherical or linear?

  1. Mar 18, 2008 #1
    What happens when a single atom emits a single photon?

    Does the photon itself exist as a spherical wavefront that propagates in all directions?

    Or is it ejected as a tiny, "vibrating ball"?

    In other words, is the spherical wavefront of a light source (like a star) just the result of the composite effect of a bunch of linear "rays", or is there something fundamental about spherical EM propagation?

    From http://en.wikipedia.org/wiki/Electromagnetic_radiation --

    What does it mean that "a single photon is sent through two slits"?

    I would also like to know just what kind of "wave" people are referring to when they talk about EM wave-particle duality. The only kind of spatial wave that I can imagine propagates spherically: either through a torsional action (transverse) or a compression-decompression action (longitudinal).

    Are there any sources out there where someone has attempted to give an explicit "geometric face" to the propagation of EM energy, at the most fundamental level?
  2. jcsd
  3. Mar 18, 2008 #2
    This is a matter of quantum interpretations more than anything else. The wikipedia entry hits the nail on the head pretty much, though I think saying the photon "passes through both of them interfering with itself" is, likewise, a matter of interpretation. In Bohmian mechanics, for example, the photon passes through one slit, but the quantum potential passes through both, and that is what causes the interference. The only things we can say for sure are: 1) if we mapped out geometrically, over time, the places where a photon emitted from an atom might be detected, we will see an expanding sphere defined by r=ct, and 2) the photon will ultimately be detected at one and only one point on the surface of that sphere. What the photon "is" before detection cannot be answered to everyone's agreement.

    This is easy. A single photon is a packet of energy defined by E=hf. For any given frequency of light f, you cannot have a unit of light with less energy than hf.
  4. Mar 19, 2008 #3
    AFAIK, the emission is spherical because the photon is in a superposition of states, but once you detect, for example, the emitting atom's recoil, then the superposition collapse and the emission becomes directional (photon and atom are entangled).
  5. Mar 19, 2008 #4


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    Well, first of all this depends on how the single atom emits a photon. If we have stimulated emission, the emitted photon simply follows the direction of the stimulating photon.

    On the other hand there is spontaneous emission. Spontaneous emission is (in very simplified layman's terms) stimulated emission, which uses vacuum fluctuations of the em-field for stimulation. To be more precise the excited state of an atom is no true eigenstate of the system, which consits of the atom and the em-field and must therefore decay. In free space, the em-field is isotropic, which means, that the spontaneous emission can happen in all directions with equal possibility.

    However one can also alter the vacuum state of the em-field. For example if you put a single atom in the middle of a resonant cavity, the spontaneous emission rate will increase (Purcell effect). This is pretty similar to what happens in the Casimir-effect (virtual particles show a discrete spectrum, if they have to fulfill some special boundary conditions). So in this case, the probability distribution depends on the geometry.
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