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Photon characteristics

  1. Dec 12, 2014 #1
    [Moderator's note - split off from this thread as a new topic.

    Well its probably off the topic in this thread [Moderator's note - you got that right], but perhaps its the way the statement above is structured.

    Can one really observe a photon to start?

    My understanding is , when a photon is absorbed by a detector or your eye, it no longer exists.
    If it is reflected no information passes on , and the photon continues on its merry way , unaltered at the same frequency.

    If absorbed however , information bits /energy from the source Atom(or particle for a better choice) has been transferred to the Absorbing Atom(or particle).

    Have we actually "observed" a photon here , I think not.
    The photon is the carrier of information , perhaps the information itself.

    And when it has been absorbed we gain some information about what created it.
    We can manipulate its properties and path, but we have not observed it in "flight".

    Perhaps the correct wording is , a photon can only be emitted/absorbed once.
    Which leads to more deeper issues , when teleporting an entangled photon.

    OR Perhaps the wording should be , a photon can only be "experienced" once when being absorbed.
    I'm not sure about the correct semantics? Its a matter of interpretation really.
    Unfortunately those interpretations can't all be correct , and they have deeply profound implications which differ in how wee see the world.

    If I roll a billiard ball along the table , I receive the information about the ball's path through the absorption of photons it reflects during its trajectory.
    So I can extrapolate its future path using Newton's laws .

    I am observing the ball in "flight" through continuous measurements/observations.
    In the microscopic world we '"experience" a photon when consuming It.
    Perhaps that is why we are still silent on how it gets from source to detector.
    Last edited by a moderator: Dec 12, 2014
  2. jcsd
  3. Dec 12, 2014 #2


    Staff: Mentor

    Of course you can as a simple google search will show eg:

    It would be a rather strange view photomultiplier's are not observing photons and for some reason those that build and use them are fooled.

    Last edited: Dec 12, 2014
  4. Dec 12, 2014 #3


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    No. A beam of light being reflected is not photons hitting a reflecting surface and rebounding; it is photons being absorbed by the surface and new photons moving in the other direction being emitted. You need some knowledge of quantum electrodynamics (for a non-technical introduction, try Feynman's "QED: The strange theory of light and matter") to see how the microscopic absorption and emission of photons can produce the macroscopic phenomena of classical optics.

    That's a fair summary of why quantum mechanics has nothing to say about the path of an undisturbed photon. However, you're still making an assumption, namely that the photon has a path, it just so happens that we can't "observe" it. That assumption usually comes from a mental model of light as a stream of photons flowing by in the same way that river is a stream of water molecules flowing by, and that model is very wrong - a photon is not to light as water molecules are to water or grains of sand are to beaches.

    The distinction you're making between "experience" and "observation" is, as Bhobba points out, not going to get you very far. In all of science, but especially in quantum mechanics, we use the word "observation" to describe any interaction that allows us to infer the existence of a phenomenon; it's not helpful to reserve the word "observation" specifically for those inferences that we draw from light interacting with the observed object.
    Last edited: Dec 13, 2014
  5. Dec 13, 2014 #4

    Could you elaborate , on this , I cant 'seem to wrap my head around the above statement.

    1. When a photon hits a half silvered mirror , and does not pass through , is a "new" photon always emitted in the direction of the reflected angle?
    And the incident photon was absorbed?

    2 . When we detect photons streaming in from the direction of the sun for example , those photons may have gone through many iterations
    of absorption an re emitting due to possible collisions with stellar gas, so they are not the "Original " photon emitted?

    3. When a photon passes through a polarizer , is a "new photon" emitted?
  6. Dec 13, 2014 #5
    [QUOTEJohan0001 said:
    I did however read in DennisN 's link above that "A photon can only be observed ONCE" which is quite a profound statement in my view.

    Why you think that photons are generally destroyed when you detect them is profound I cant quite follow.][/QUOTE]

    Yes bill , I understand the photoelectric process detailed above.

    I would still not agree that we have "observed" the incoming photons in " flight" striking the photocathode .
    We have measured the effect of the incoming photons striking the Photocathode.
    One can obviously deduce that incident photons did cause the electron ejecting .
    But to observe the photon , I still think not.
    It may sound like semantics , but to me there is a huge difference.

    Just to summarize
    Would it be inline with QED if I said , before a photon is incident onto a surface it is in a superposition of states, it then collapses into a "pure" state
    during interaction , and then at reflection it recombines into a superposition of states again.
  7. Dec 13, 2014 #6


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    Whether it is the "same" photon or not after reflection is a matter of taste. All that matters is the quantum state before and after reflection, which can be a single photon state.

    In the quantum formalism, the state collapses only when an observation or detection is made. An observation or detection is a classical or macroscopic record.

    Although photons are usually destroyed when they are detected, it is possible to detect photons without destroying them: http://arxiv.org/abs/1311.3625.
  8. Dec 14, 2014 #7


    Staff: Mentor

    Well actually QED doesn't say that's what's happening - but that another story.

    I cant quite follow why you think it has anything to do with QM being silent on whats going on when not observed.

    Hmmmm. I think you are not quite understanding what observe in QM means. It does not mean shining light on something and from what's reflected you know something about what you are shining light on. What it means is something has occurred you can assign a number to. In the photomultiplier its easy to count the number of clicks that occur in a given time period.

  9. Dec 15, 2014 #8
    An observable


    I prefer the latter definition quoted , although the boundary between classical/macroscopic and microscopic is becoming a grey area.

    I personally don't think the formalisms of observables and events are well defined at all.
    Just assigning a number to an event and recording it somewhere , well it sounds like , "shut up and calculate" syndrome.

    Observables and events must be considered in some collective ensemble(System) to get the macroscopic reality we experience.
  10. Dec 15, 2014 #9


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    It is possible to "observe" a photon without destroying it. A good example is in cavity/circuit QED using a non-linear resonator, it is then possible to detect the photons in one mode of the resonator by probing another mode, i.e. the interaction is "indirect" and does not -as far as I understand- destroy the number state in the first mode.

    (mathematically this is described by something similar to the usual J-C Hamiltonian, but with a Kerr term which makes it non-linear)
  11. Dec 16, 2014 #10
    The "absorption-emission" process which you mentioned is called scattering, it is not a real absorption and emission. When reflecting from a surface the photon undergoes basically the same process as the electrons in the Davisson-Germer experiment: they scatter off many layers of atoms and macroscopically one recovers the reflection law.
  12. Dec 19, 2014 #11


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    The problem with this topic is that the word "observe" is poorly defined by the OP.

    Start with something that you consider to be a valid observation. Argue why you consider this to be a valid observation. Then describe and compare our "observation" of a photon and why you think there is a difference. I see many hints here where the concept of "observation" and "detection" are being intertwined.

    Often people trip themselves with using the classical notion of observing something, because the prejudice of our classical world often infringes into our definitions without us realizing it.

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