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Multiple events caused by a single photon

  1. Jun 18, 2014 #1
    The situation I am considering is that of a single photon directed at a target (say a photographic plate). My understanding is that the wave function for this system (determined by the whole context of the experiment: photon properties, nature of the target, boundary conditions etc.) gives a probability for the photon's interaction with the plate at any pre-specified point on it. Also, I am assuming that when the interaction occurs, energy is transferred to the target (via a chemical change in the emulsion or whatever). Also that energy is conserved, i.e. either the target absorbs the complete photon quantum, or it absorbs it partially, with the balance being released in the form of, say, the emission of a new, lower-energy photon.

    My specific question is now this. Is is possible that a suitable wave function could give a non-zero probability to an event where the energy of the initial single photon is shared between two simultaneous interactions with the plate, resulting in two "spots" on the plate, separated in a space-like way?

    I can't see any reason in principle why such a compound event should not occur. Also, does anyone know whether any experiments have taken place which exhibited this phenomenon?

    Thanks

    Fred
     
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  3. Jun 18, 2014 #2

    ZapperZ

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    What physics are you basing this on? Can you describe exactly how you think that this is possible? As a reference, let's use the case of single-photon sources that we currently have, and all the current experimental detections of that single-photon that we already possess. This includes, but not limited to, neutrino detectors.

    Zz.
     
  4. Jun 18, 2014 #3
    As far as "all the current experimental detections of that single-photon that we already possess", I don't know - thus the question!

    It just occurred to me that there is no reason in principle why a single photon of energy x shouldn't cause two simultaneous, separated events, each requiring an energy of x/2.

    So, to make it more specific, I fire a single photon towards a photographic plate. Is it demonstrably never the case that two separate spots would be found on that plate as a result of the interaction?
     
  5. Jun 18, 2014 #4

    ZapperZ

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    This is what I don't understand, and that is why I asked you to cite specific physics that led to this conclusion. I don't see it. Even when we know that the wavefunction of an electron in an atomic level is spread out all over the place, we still detect it at only ONE position when a position measurement is made.

    So as far as I know, nothing in the physics says that you can conclude what you just did. Thus, I asked what physics did you base this on?

    Not that I now of. If it is true, then you can question all those neutrino measurements, because it is explicitly assumed that a single photon will make only a single spot on a detector, and thus, allowing them to reconstruct the event. You can also question all those which-way experiments that split the path of a photon, because you are essentially arguing that one can get a single photon to be detected in two separate locations.

    Zz.
     
  6. Jun 18, 2014 #5
    In fact I think that answers the practical part of my question, i.e. that what I described never actually happens in practice.

    So what is the basis of the "explicit assumption" you refer to? Is it simply that the phenomenon has never been observed? Or does the QM formalism require this to be the case?

    Fred
     
  7. Jun 18, 2014 #6

    ZapperZ

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    I'm sure you are fully aware that you've avoided answering my question for the second time already. I can only guess that your assertion that "... there is no reason in principle why a single photon of energy x shouldn't cause two simultaneous, separated events... " was not based on any physics, but rather a guess?

    There are two separate issues here: One is "in principal", the other is experimental observation. "in principal" requires a conclusion based on theoretical arguments. You need to show that this is possible, in theory. This is what I had been asking, and what you had not been able to show. The fact that we have superposition principle, and the fact that a measurement of an observable will produce a single value of that observable, is the whole brouhaha over the so-called "measurement problem" in QM! Now THAT, is what is being shown, in principal!

    I augment my argument by showing you more evidence that the "in principal" theory of QM has also been verified, and used, in experiments, mainly with single-photon detections. Things will be a total mess IF what you are arguing is true, and a lot of things won't make any consistent agreement.

    So your are faced with two obstacles: the theory doesn't support your position, and the experimental evidence isn't consistent with your position.

    Zz.
     
  8. Jun 18, 2014 #7
    Right on both counts! :-)

    I'm not propounding anything dramatic here. I was just trying to get someone more knowledgeable than me in the theory to confirm that my "guess", as you put it, is not theoretically possible. As well as that it doesn't occur in practice. You've confirmed the latter. Thanks!

    As to the "in principle" aspect, is it not possible then to say that because of the way the formalism works, such an event is absolutely precluded? I'm not competent to assert that. But I was hoping that someone here with the appropriate knowledge would be able to do so.
     
  9. Jun 18, 2014 #8

    ZapperZ

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    The issue here, as in many cases when something like this is brought up, is more general than you think. This is not just about a photon having two simultaneous positions upon measurements. It has more to do with the idea of (i) the QM description and (ii) the act of measurement. Any one of those have wide, profound ramifications throughout QM. For example, in the Bell-type experiments, you NEVER detect two different states simultaneously upon a single measurement, even though both states are there before a measurement (in a bipartite Bell-type experiment). You NEVER detect a photon going through BOTH slits when you put a detector on both slit openings.

    There are so many of these experiments and observations, it is no longer funny. And it is clear that this is contained in the formalism of QM. This is why once it was realized, and how different it is from our familiar classical world, various "thought" experiments to illustrate the "weirdness" of it came into being, including the infamous Schrodinger Cat.

    Zz.
     
  10. Jun 18, 2014 #9

    Nugatory

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    As far as the formalism is concerned, such an outcome is absolutely precluded. Even trying to describe such an outcome would be almost like trying to work with a four-sided triangle in geometry; no amount of "but what if..." thinking will let you fit it in.

    Of course we chose this formalism because it does a really good job of matching our observations. If conflicting observations were ever to show up and be confirmed, then we'd have to go looking for some other theory that fits both what we've already observed and the new observations. That's about as likely to happen as the sun rising in the west tomorrow (Hey! we have lots of observations of the sun rising in the east, and a pretty good theory as to why it should do so again tomorrow, but we haven't actually made tomorrow's observation yet!) and forcing us to rethink out theories of planetary motion.
     
  11. Jun 18, 2014 #10

    DrChinese

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    ZapperZ is a top resource in his own right. :smile:

    Theory precludes what you are saying more or less by definition. One photon cannot produce 2 blips. On the other hand, if you really want to see the full theory and experiment on this particular point, try this one:

    http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf
    Observing the quantum behavior of light in an undergraduate laboratory

    "While the classical, wavelike behavior of light - interference and diffraction- has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light i.e., photons is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be ..., which violates the classical inequality g(2)(0)>1 by 377 standard deviations."
     
  12. Jun 19, 2014 #11
    I don't doubt it! :smile:

    Thanks all - time to ponder further on all this.
     
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