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When is a photon a photon?

  1. May 22, 2008 #1
    When is the term "photon" valid?

    This question seems to have taken on different forms in a separate post:
    a) Do photons exist in empty space (vacuum)?
    b) Do photons exist in the absence of interaction (with matter)?
    c) Are photons called photons only when they are exhibiting particle-like behavior?
    d) Are electromagnetic waves photons?
    e) ...

    There may be subtleties to these questions, but lets try to cover the straightforward answers first.

    Please review marlon's and ZapperZ's FAQ topic "Is light a particle or a wave"
     
  2. jcsd
  3. May 22, 2008 #2
    My take is that photons exist continuously between emission and absorption/annihilation regardless of interaction with matter, else statements like "photons travel (in a vacuum) at the speed of light, c" would have no meaning.
     
  4. May 23, 2008 #3
    Furthermore, quantum electrodynamics, quantizing the electromagnetic field, assumes photons exist whenever the em field exists, so, by definition, even between source and detector. However, since, in my opinion, we should always talk about measurable things, in physics, then I have doubts about the physical meaning of photons between source and detector, but it's only my opinion of course.
     
  5. May 23, 2008 #4

    ZapperZ

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    Are water waves H2O molecules?

    Zz.
     
  6. May 23, 2008 #5
    measurement alters what's being observed

    Cool. What if I turn the statement around and have doubts about the physical meaning of photons at the source and detector instead?

    As an analogy, nitroglycerin might really be described with a calm image of an oily liquid. However the imagery and measurements that we get upon arrival at the (clumsy) detector is completely different because the original item is no longer nitro, but dramatically transforms into spent fuel because of the detector.

    If we look at the endpoints, aren't we really often measuring the destruction of a photon rather than the photon itself?
     
  7. May 23, 2008 #6

    ZapperZ

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    You need to look at the issue surrounding Bell-type experiments and "local realism" (or lack thereof) within the context of QM.

    Furthermore, everything that you know of, really, is based on what you measured/detected/observed. It isn't restricted only to "photons".

    Zz.
     
  8. May 23, 2008 #7
     
  9. May 23, 2008 #8
    Help?
    Are you suggesting to ponder a sort of "undefined-ness" before detection as opposed to unknown?

    Okay, knowledge vs theory.
    Our typical observations of a grain of sand are gentle enough not to alter the grain all that much. On the other hand if we are in a dark room detecting nitro with a swinging hammer we're more likely to detect an explosion than to detect an oily liquid. The hammer, when it doesn't detect nitro tells us something, about say the limited size of the nitro in its stable state. When the hammer does detect nitro, it gives us a very different image that we shouldn't extrapolate backwards as the interpretation of the stable state.
     
  10. May 23, 2008 #9
    The photon represents the quantized interaction between EM field and matter, so it's at least present at source and detector by definition.
    But you *are* able to detect nitroglicerin oil with the appropriate instruments; with photons it's all another story because you are'nt even theoretically able to detect "the original item". Can you see the difference? :smile:
     
  11. May 23, 2008 #10

    ZapperZ

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    No such thing. This isn't about knowledge versus theory, because "theory and knowledge", at least valid ones, are based on what we can verify emprically!

    You have missed the point. Everything that you think you know is based on your knowledge of a set of properties and characteristics of that entity. Think about it. These properties and characteristics are based on what you have measured/observed/detected of these entities. It has nothing to do with what you just described here.

    Zz.
     
  12. May 23, 2008 #11
    I'm a little confused. The above statement on its own would lead me to think that the photon is defined only during the interaction. I don't think that's what you are trying to say though.
     
  13. May 23, 2008 #12
    I'm not sure I'm catching the nuances of your statement. In some sense, it's like all of our instruments are like hammers to the photon, no?

    If we aren't able to even theoretically detect the original item, which is my point too, then aren't we on the same page when I try to represent that the "pure" thing really only exists when we, or matter itself, aren't tampering/interacting with it? I'm pretty sure I'm not representing your thoughts, but I'm not sure what's different still.
     
  14. May 23, 2008 #13
    I'm not sure how I missed the point Zz. I hope it didn't sound sarcastic.

    From where I stand, a theory isn't just a list of measured facts, but it's reasoning that fills in the blanks to explanation of facts. There can be multiple good and competing theories supporting the same verifiable facts. In this sense, knowlege and fact are different than theory, no?

    I thought you and lightarrow were emphasizing that technically, the only facts we have are measurements, and whatever else goes on between the measurements was sort of in a black box that we could only technically theorize about.

    Thinking that I got your point, I started to draw out that the measurement of the facts was a two-edged sword--it alters the thing we are measuring and it can be difficult to subract out the effects of the measurement in order to create a theory of what happens in the black box.

    Not sure where the disconnect is.
     
    Last edited: May 23, 2008
  15. May 23, 2008 #14

    ZapperZ

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    Illustrate this with an example. For instance, how are Maxwell equations a "reasoning"?

    Zz.
     
  16. May 23, 2008 #15
    How about this: Maxwell made mathematical reasoning to modify Ampere's law. Maxwell's equations and Newton's mechanics both agreed with observations made at the time. Maxwell's reasoning, and not necessarily measured observation, filled in some blanks that were unobservable at the time. The blend of facts and reasoning made predicitions whose facts could only be measured later.

    Sound okay?
     
  17. May 23, 2008 #16

    ZapperZ

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    No, because none of what you said can be backed up. Maxwell equations really are nothing more than a set of mathematical description. It "reasoned" nothing! In fact, one can easily describe them as phenomenological, which means that to put it crudely, it is nothing more than a well-defined description of empirical observations. It says nothing on why the E and B fields are that way, i.e. no reason!

    Again, show a theory that actually satisfy what you are claiming that it can do.

    Zz.
     
  18. May 23, 2008 #17
    I thought your point was that we know nothing more than what we observe.

    The mathematical descriptions are not the same as empirical observations. Empirical observations wouldn't generally use an "equals" sign, but would put bounds on the experimental error. As we get better measuring techniques, we often recheck basic theories for any undiscovered news that the mathematical descriptions didn't show.

    We can take mathematical descriptions, combine them and conjure up physical situations that have never before been observed. In this case, it isn't the reason why things happen, but it is still reasoning.

    Maybe an example could be that a ...body stays in motion... We haven't actually observed this "law" to be true--nobody has observed a body for an infinitely long time, but it seems reasonable enough to use in our equations.

    Are we still on different pages? I do want to understand the photon better, so...
     
  19. May 24, 2008 #18

    ZapperZ

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    Our set of observations allows us to formulate (i.e. put in mathematics) how a system will behave under the same condition wihtout having to redo the whole experiment. What's what a phenomenology is - to describe the behavior of a system. It "reasoned" nothing the way you claim a theory would do.

    Newton's first law is a description of a behavior of an object under (or without) a force. I could say that no one as verified that it will work for all forces over an infinite amount of time, yet, we know the description has worked so far (your house was built using that premise). But did this "reasoned" anythiing? Look at the original premise that you made that I objected to.

    If you want to understand photons better, learn QM and QED, not by claiming something a theory isn't.

    Zz.
     
  20. May 24, 2008 #19
    Nobody here was trying to make claims about the definition of the word "theory" as a tool for understanding photons. That doesn't make sense to me. Additionally, statements like "It has nothing to do with ...", and "...none of what you said..., and "...nothing the way you..." are a bit suspiscious.

    I'll try to stick with the dictionary definition of theory and hope you will too. Note that there are differing degrees of certainty to the word theory ranging from simple conjecture to scientific theory whose statements are well-backed by data and which are generally accepted. Many definitions will use the word "explain" in the definition. I may have used the word "reason". I'd almost bet that we agree that there are limits to what is meant by "explain" or "reason" in this context.

    Not sure what premise you were talking about. Was it that measurement interferes with the subject being measured? I thought we agreed on that.
     
    Last edited: May 24, 2008
  21. May 24, 2008 #20

    ZapperZ

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    Here's what you said earlier:

    You have continued to neglect to illustrate where such "reasoning" occurs. A valid theory does a lot of describing. I've given Maxwell Equations as an example, which you appears to have completely misunderstood. Did Gauss's law "reasoned" something? Or does it simply "describe" how the E-field looks like, given a charge distribution. Tell me where it "reasoned" it. The same could be said with Quantum Mechanics. Does it "reasoned" with you which outcome you would measure in a superposition of states? Or does it simply tell you all the possible outcome you would measure?

    Your idea of what "knowledge" is is also strange. A set of facts or observation does not make a knowledge. That's stamp-collecting. It is the theory that provides a "frame" to understand how these facts and observations are connected together. What separates phenomenological theory versus a well-defined theory is that the latter could derive the former via First Principles. So a valid, well-defined theory IS, for all practical purposes, knowledge!

    This thread is diverging into philosophy and anymore of this will probably push it into that forum.

    Zz.
     
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