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Quantum weirdness of photon behavior

  1. Oct 16, 2009 #1
    As a chemist, I'm accustomed to thinking of electrons in the wave/particle dualistic sense and their energy orbitals in terms of quantum mechanical wave equations and probabilities. But I would like some input on the following thought experiment relating to photons and the wave behavior of light.

    If we place a light source in the center of a room and flick it on briefly, all walls, floor and ceiling are illuminated. Jillions of photons. But what if our light source were designed to allow only a single photon of energy to be released? My thinking is that, given that the energy released is a quantum mechanical wave, that wave propagates from the source at the speed of light in all possible 3-D directions. Theoretically, illumination can occur anywhere in the room where an object might be struck by that photon. However, the very first "contact" with an object causes the wave equations to collapse and the photon at that instant exhibits particle behavior, and that one miniscule spot, no matter where we decided to put the object, is the only spot illuminated, and because of our random choice of location for the object, that becomes, de facto, the "direction" of the wave/photon from the source.

    Is this an appropriate understanding of light's wave behavior?
  2. jcsd
  3. Oct 17, 2009 #2


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    Yes. An emitted photon has the potential to take a path to many different spots even though it ends up only going to one. When it is absorbed somewhere in the room, there is collapse, and all other paths are now the ones not taken. So an absorption "here" means no absorption "there". (The collapse is instantaneous as best as can be determined.)
  4. Oct 17, 2009 #3
    If you put a photon detector at some place in the room, there is a *probability* that the detector will detect it, which is calculated from the wavefunction of the photon (and depends on the size of the detection region). I'm not sure what you mean by "very first `contact' with an object...".

    The answer to your question is yes, with some caveats. First you have to choose an "interpretation of quantum mechanics" to work with. E.g. if you take a consistent histories interpretation (or similar framework), you can say the following:

    Between emission and detection, the photon doesn't *generally* have a precise location, so the direction of the photon's path may not be described with a linear path. E.G. if we happen to have a double slit apparatus set up between the detector and the source, you can't trace back a precise path for the photon from the detector, through one of the slits and back to a particular part of the source. But in your particular setup, without anything going on but emission and detection, it is consistent to say the source had a probability of emitting in some direction, and if your detector picks up the photon, it is consistent to trace back the path of the photon.

    Another framework for the interpretation of quantum mechanics would say that, no, you can't say the photon had any precise path, being fundamentally described by a wavefunction, until a detection occurs, causing a *physical* collapse of the wavefunction.
  5. Oct 17, 2009 #4
    No, I don't think you can make this claim. Single photons from a point source can be focused by a lens. If the photons chose a certain direction for their trajectory at the moment of emission, then the lens wouldn't work. The lens only works because the light is radiated in all directions at once.
  6. Oct 18, 2009 #5
    Thanks for your responses. The double slit experiment and the lens focusing phenomenon both help "illuminate" the 3D wave characteristic of even a single photon of light. From this behavior I conclude that I can repeat my single photon emission experiment 1000 times, and position my detector anywhere in 3D space around the source, and regardless of where I place the detector, I will always get a positive response showing a single photon striking the detector.

    I can't help but feel that we are missing some fundamental understanding of our physical universe, leaving us forced to accept wave/particle dualism as a make-shift (and hopefully temporary) description of observed phenomena. Maybe when we discover how to merge our understanding of relativity and gravitational fields into quantum mechanics (or vice versa), we will be able to merge these two phenomena into a single cohesive concept.
  7. Oct 18, 2009 #6


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    There are experiments that clearly show that a light wave consists of exactly one photon (which is I think one of your points). For example:


    The only thing I might disagree about is the idea that we don't have a fundamental understanding of the wave and particle nature of light. That dualism is well described by the Heisenberg Uncertainty Principle.
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