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Are photons undivisible ?

  1. May 4, 2005 #1
    I would like to know some answers to related questions. I would like to know how we know that a photon is the smallest part of energy ? Can we divide a photon in more than 1 piece of energy ?
    Do our current instruments identify only a sub-set ? Is a photon immediately destroyed after observation or converted ?
     
  2. jcsd
  3. May 4, 2005 #2

    Meir Achuz

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    1. Y
    2. N
    3. N
    4. Most of the time, but not necessarily. The photon may be scattered and not absorbed.
     
  4. May 4, 2005 #3

    SpaceTiger

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    Meir Achuz, would you really say we can design an instrument to detect a photon of any energy (in response to the "subset" question)? Perhaps I misunderstood his question, but it seems to me that there could be wavelengths either too large or too small for our detectors to pick up.
     
  5. May 4, 2005 #4
    One hundred years ago, people do not know the answer of the question. The fact that a photon carries a part of energy that could not be divided to smaller parts is known in 1905 by explanation of photoemission.(maybe from Planck,but not clear)
    Photoemission told us that the electron can whether get the whole energy of photon
    or none of it, but never part of it. Later Compton scattering and other experiments
    all supports this fact. So people believe that a photon carries the basic part of energy.

    In the other hand, people try to test whether photon has structure inside it through
    high-energy colliders, and find no structure within the 100 GeV level.
     
  6. May 4, 2005 #5

    Meir Achuz

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    There is no wavelength too small to be detected.
    There might be a longest wavelength, because the energy would be too small to be detected. But very low energies can be detected. For instance, the radiation from your body is readily detected by "night vision" scopes.
     
  7. May 4, 2005 #6
    By the way, today I happened to have read this, I think this is helpful to you.
    Berkeley Vol.4 Chap.4 Sect.3.
     
  8. May 4, 2005 #7

    SpaceTiger

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    The radiation from our body is IR radiation, much longer wavelength than the longest we can detect. Even radios can go up to wavelengths of a kilometer or more, but I've never heard of a theoretical upper limit to it, so I don't see any reasonable way we could detect, for example, 1 Hz radiation.

    I'm not sure why you say there's no lower limit, because even pair production rates drop off as you raise the energy high enough and as far as I know, such regimes haven't been tested.
     
  9. May 4, 2005 #8

    Claude Bile

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    Optical Parametric Osciallators (or OPOs) are able to convert one photon into two photons, whose energies sum to the energy of the original photon. For example, OPOs can be used to convert a 250nm photon into two 500nm photons.

    So, in a sense, the energy of the original photon is being divided.

    SpaceTiger, taking the Fourier transform of an electronic signal will yield its spectrum, from 0 Hz upwards (though the noise near 0 Hz is rather large), though I suspect you may be referring to individual photons rather than radiation in a macroscopic sense.

    Claude.
     
  10. May 4, 2005 #9

    SpaceTiger

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    That's part of it, but I'm also referring to the fact that antennae are extremely insensitive to wavelengths much longer than their own length. If the signal was strong enough, however, I suppose that, in theory, you could detect such low-frequency radiation.

    EDIT: One would also have to consider the limit induced by integration times: that is, you can't reliably detect a signal at frequencies below 1/(integration time).
     
    Last edited: May 4, 2005
  11. May 4, 2005 #10
    Thanks guys,
    Can I say that general consensus is that photons can be divided ? Irrelevant of the fact that we speak about individual photons or large numbers of photons making up an electromagnetic signal we can detect with our instruments ?
    This makes room for a sub-set of quantum mechanic behaviour in my mind. And also could create difficulties with some proposed quantum computation theories.
    If photons can be divided then surely the quantum behaviour of the new photons would be related just like entangled particles. Has this sort of thing ever been tried ?
    Does anyone know ?
     
  12. May 5, 2005 #11

    ZapperZ

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    How in the world did you arrive at THAT conclusion based on the responses you got in this thread?!!

    And note that the photon that enters an OPO is NOT the same photon that is chopped up into two. If the physics of non-linear optical response is THAT easy, we would have a big party already.

    Zz.
     
    Last edited: May 5, 2005
  13. May 5, 2005 #12
    Pair production is a quantum unitary process: e.g. a 20 MeV photon divides into nineteen 1.022 MeV increments of pair production plus a residual 570 KeV daughter. Just a thought! Cheers, Jim
     
  14. May 5, 2005 #13

    arivero

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    Hmm yes, the subtle point is that cklein asked
    and it seems that people interpreted "morethan1 = two".

    But of course a photon can polarise the vacuum, and the electron/positron pair has time to launch another photons. Still, the global process should preserve energy as well as linear and angular momentum.
     
  15. May 5, 2005 #14
    Am I wrong in the assumption that the process of detection goes hand in hand with 2 things, namely:

    1) Electrons are produced, which are the only means of being registered on our instruments.

    2) "Time" has to pass to process all the interactions of events. I refer to "Arivero's" last comment.

    Both the fact that we can only observe indirectly and that we have to wait, so to speak, leads me to think that "ZapperZ" is a bit quick in judgement.

    There is still a way to go to the party, but yes there is room for that philosophy in my mind.

    The difference between doing science rather than talking about science may well lead to people being too busy doing to note that there is room to talk about a lot more then the current weather conditions in science. New experiments come from new challenges and new thoughts.
     
  16. May 5, 2005 #15

    ZapperZ

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    So how did *I* end up as being "quick in judgement"? I asked you how you end up with your conclusion based on what you were told in this thread? Unless I missed something, most were along the flavor that "photon is not divisible". The ONLY possible deviation from this is Claude's example of the OPO. And I cautioned you or anyone who have not looked into the details of the OPO that you will be hard-pressed to convince anyone that what is going on in the non-linear crystal is photon being split into two.

    Therefore, I asked how you end up concluding from what you have been told here that the "general consensus is that photons can be divided"? Where did this come from?

    Why is this relevant here? Were you trying to pick on my signature line?

    Zz.
     
  17. May 5, 2005 #16

    Claude Bile

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    I should have said that the energy from a 250nm photon can be used to produce to 500nm photons, it is not the 250nm photon itself that is being divided. (Note that I did say in my original post that it was the energy of the photon being divided, not the photon itself.)

    Note too that this is the end result, it does not pay homage to the complexity of the nonlinear processes, as ZapperZ correctly points out, it is much more than photons being chopped in two.

    P.S. cklein, I just noticed that the title of the post should be 'are photons indivisible', you dont want to lose credibility needlessly through poor grammar!

    Claude.
     
    Last edited: May 5, 2005
  18. May 6, 2005 #17

    vanesch

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    I don't think that what you write is the general consensus ; rather the opposite. However, there are some poetic liberties that you can take if you want...
    Let's go back to the basics, to what is a photon, in QED.
    What happens when one quantizes the free classical EM field (that is, when one applies the procedures to turn a classical prescription into a quantum prescription) is the following. The classical free EM solutions (modes) turn out to be equivalent to quantum mechanical oscillators with the same frequency, which have discrete energy and momentum levels. The energies are given by hbar omega (n + 1/2) where n is an integer, and the omega is the omega of the mode of course. This means that each EM mode can have energy levels with fixed steps ; so it is as if there were n "beables" with constant energy hbar omega and associated momentum in that mode. These beables are called photons. They start out as a book-keeping device and turn out to start to lead a physical life of their own, but remember that a photon is just a "step" on the quantization ladder of a given EM mode.
    As such, it cannot be divided: there are no energy levels in between two steps in a given mode.

    However, what can happen is that modes interact (through charges). This means that one mode A goes down one quantum level, and two other modes B and C go up one quantum level each. Does that mean that the original photon was "devided" ? Strictly applying the formalism, it is not: you first annihilate one photon in mode A, and then you create a photon in mode B and in mode C. But, as I said, if you insist on a certain poetic liberty you could say that the A photon has been "split" in a B and a C photon. But it is not the most natural explanation from a formal point of view. There, the most natural view is the annihilation and the creation (it is literally done that way with annihilation and creation operators).


    cheers,
    Patrick.
     
    Last edited: May 6, 2005
  19. May 6, 2005 #18

    DrChinese

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    Maybe this will help...

    Photons ARE fundamental quantum particles. Like electrons, they do NOT have an internal structure. In that sense they are indivisible.

    Protons and neutrons are not fundamental quantum particles in the sense that they have an internal structure composed of 3 quarks.

    For a good reference on the quantum nature of the photon: Observing the quantum nature of light...
     
  20. May 10, 2005 #19
    Thanks guys, I get the message. As for the spelling, English is not my first language, and I agree it does not look credible to use the wrong spelling.

    I still think we rely a lot on our limited instrumentation and the theories attached to the instruments we use. I design instrumentation for a living and am always aware of shortcommings in many ways.
    Hope that explains my point of view.
     
  21. May 10, 2005 #20

    ZapperZ

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    And let me explain MY point of view. As an experimentalist, I always am aware of the limitation of the instruments, and to what extent I can confidently say or deduce about the results that I get. However, there is still a difference between what is limited by the accuracy, resolution, or the capabilities of our instruments, and what is limited in principle based on our current state of knowledge.

    There is no "general consensus" that a photon can be chopped up the way you are thinking of. Most (if not all) of the responses you got in this thread have indicated that.

    Zz.
     
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