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What Really is a photon?

  1. Nov 15, 2006 #1
    A light source emits a weak beam of low frequency (I will explain later why low frequency) light, so that a revelator "ticks" once every, let's say, minute. Sometimes in physics books or in scientifical magazines, it's explained this way: "one single photon is emitted from the source every minute, travels to the revelator and it's revealed".

    Question: How can we say that, in this case, a spatially localized particle travels from the source to the revelator? A way to prove it would be to put...another revelator in the middle of the route, so destroing the photon (here is why low frequency and hence low energy); so we are left with the same question: What is the photon from the source to this new revelator?
    What could the photon be if not the "click" of the revelator?
  2. jcsd
  3. Nov 15, 2006 #2
    From my understanding, QM says one thing about the photon: The probability of a photon being created at the source, combined with the probability of being detected at the detector. Nothing more.
  4. Nov 15, 2006 #3
    You should read the Foreword in Loudon's book "The Quantum Theory of Light". It really directly addresses the issue of what a photon "is" quite well, I think.
  5. Nov 15, 2006 #4

    I was able to find this foreword in amazon's online reader for those of us whose bank account has an expectation value of $0.
  6. Nov 15, 2006 #5


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    It is meaningful to talk about a single photon. But it is not always meaningful to describe it as local point. The Heisenberg Uncertainty Principle (HUP) proscribes limitations that must be considered on the description.

    Sometimes it is *convenient* to think of a photon as particle moving in space, but this is not rigorously accurate per the HUP.
  7. Nov 15, 2006 #6
    Well, to sorta of summarize the Loudon for others, photons aren't "particles" at all. The only way to know where they are is by their interactions with matter, as they don't warp space-time or anything like that. So the detector measures a "click" and this corresponds to, say, a photon exciting an electron out of an atom and producing a small current. In the strictest sense, a photon is just an excitation of the local E&M field (U(1) gauge field), and then the "click" is one of the excitations of the gauge field dropping down and transferring the energy to a massive particle.

    Asking "where's a photon?" is in many ways analogous to asking "where is a phonon?" They're both collective excitations, one of them more abstract than the other, but they can be thought of in a loosely analogous way for the purposes of a physical picture.
  8. Nov 15, 2006 #7


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    A friendly tip, as I suspect that English isn't your native language: the appropriate word here is "detector."

    In fact, I don't remember ever seeing the word "revelator" before your postings, so I did a Google search for it. I got references to a rock band, an album, and some kind of software. So apparently the word exists, but it's simply not used in a physics context.
  9. Nov 16, 2006 #8


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    If a photon couldn't be detected in terms of its position, wouldn't that make it difficult for energy, in the form of photons, to create a gravitational field?

    By the way, my old copy of Landau & LLiippsshhiitzz says that a position representation does not exist for photons but more recent work shows that if you suppose the photons are circularly polarized, you can make a position rep.
  10. Nov 16, 2006 #9
    Thank you for your tip jtbell!
  11. Nov 16, 2006 #10

    lambda here is actually lambdabar in the original paper and is: [tex]\hbar[/tex]/p and u is the elic's radius.
    Last edited: Nov 16, 2006
  12. Nov 16, 2006 #11
    Do these guys think that the polarized photon really propagates in helix mode or they derived the physical point from their model of representation of the trajectory?
    [tex]\sqrt{2}c[/tex] and some other things are pure abstraction,and speculation one shouldn't connect with reality.
    Last edited: Nov 16, 2006
  13. Nov 16, 2006 #12
    First of all, GR is not my strong point, but I'm not sure that photons exert a gravitational field.

    Also, just out of curiosity, what is your reference for being able to localize a photon? I'd be interested in reading the article.
  14. Nov 16, 2006 #13
    I should like to think of the photon as it is in its own frame, that is, outside of, or perhaps containing all of, our space and time. Photons at light speed do not experience time or space in the sense we know it at all. So I guess the local constraints of detection of a photon are entirely a property of the detector, and have nothing to do with the idea of a photon in its own frame. So it is really meaningless to ask what a photon really is. It would be just as meaningless, perhaps, but much more accessible, to ask what is an observer.

    It seems to me that the detector responds to the presence or absence of a photon at a place and time entirely dependent on the construction limits of the detector, which in turn depend entirely on the limits of the observer. For example, it would be an internally consistant logic to suppose that the photon is not moving at all, but that the observer is expanding to meet the photon, which in this logic does not move at all. We play the experience of the photon like a needle on an old fashioned record player plays the music. In what sense does the music exist upon the vinyl disk? The ridges and furrows, like the photons, do not move up and down, but the detector, racing across them, moves up and down, so reconstructing the music for the listener.

    In short, we cannot know what a photon really is, but only what it does in our experience. We can only be concerned, as scientists, with measurables like frequency and mass, and have to forgoe the pleasure of laying down answers to existential questions, like what is a photon, or what is it, really, to be human.

    happy holidays,

  15. Nov 16, 2006 #14


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    Hi Richard! Good to hear from you again. Persumably by "in its own frame" you mean a conceptual frame, not a physical intertial one, since as you're of course aware, the photon (within SP at least) doesn't have one of those.

    That said, my usual test of "what a photon is" speculations is: "Is it consistent with the representation as plane waves travelling through space (which is the standard QM representation of a free photon)?" And your approach fits very nicely with this as I see it.
  16. Nov 16, 2006 #15
    Hi Tehno, you posted

    Do these guys think that the polarized photon really propagates in helix mode or they derived the physical point from their model of representation of the trajectory?
    and some other things are pure abstraction,and speculation one shouldn't connect with reality.

    what makes you imply that photons, like everything else (that is in movement), does not propagate in an helical fashion ?

    Edit: actually, should have written the whole (like everything else that is in movement) in paranthesis.


    Last edited: Nov 16, 2006
  17. Nov 17, 2006 #16
    But, since we are in a physics forum (and not a pylosophy one), all the times we use the word "photon" we must give it a physical meaning, otherwise, we shouldn't use it at all.
  18. Nov 17, 2006 #17
    I think the following issue of OPN Trends (published by Optical Society of America) may be very useful for your understanding of photon.

    OPN Trends—The Nature of Light: What Is a Photon?
    Guest Editors, Chandrasekhar Roychoudhuri and Rajarshi Roy
    Vol. 14, No. 10, Page 49
    October 2003

    1 The nature of light: what is a photon?
    Guest Editors: Chandrasekhar Roychoudhuri and Rajarshi Roy
    2 Light reconsidered
    Arthur Zajonc
    6 What is a photon?
    Rodney Loudon
    12 What is a photon?
    David Finkelstein
    18 The concept of the photon—revisited
    Ashok Muthukrishnan,Marlan O. Scully, and M. Suhail Zubairy
    28 A photon viewed from Wigner phase space
    Holger Mack and Wolfgang P. Schleich

    “All the fifty years of conscious brooding have brought me no closer to the answer to the question: What are light quanta? Of course today every rascal thinks he knows the answer, but he is deluding himself.”
    ---Albert Einstein

    We are today in the same state of “learned ignorance” with respect to light as was Einstein...
    Last edited by a moderator: Nov 17, 2006
  19. Nov 17, 2006 #18
    What makes them think that they propagate in helical fashion,no matter how the wave carrier is polarized?:smile:
    If you carefully think over the paper ,you will see that that component of "trajectory" is completelly impossible to be verified by any kind of measurment or recovered by a detector .
    Last edited: Nov 17, 2006
  20. Nov 18, 2006 #19
    Is there really nobody here who can give a succinct English definition of what a photon is?
  21. Nov 18, 2006 #20


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    It was already given I think. A photon is an excitation of the free electromagnetic field. More in detail it is the one-particle state in the Fock representation of the electromagnetic field.

    A different thing is the localization as a property of the photon. The notion of localization and even the notion of wavefunction as it is known in non-relativistic quantum theory cannot be extrapolated to quantum field theory. However, localization is a property that is not necessary for the definition of a particle.
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