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Photon dimensions

  1. Feb 13, 2007 #1
    Sorry for opening up the discussion again, but I joined late.

    anantchowdhary's question is an extremely valid one, even though most commentators claimed to not understand the question.

    A photon has a wavefunction. The standard interpretation is the wavefunction gives the probability that the photon is at a specific location at a specific time.

    As it travels at c, there is always an area where it can not be (probability is 0). What is the size of the area where it can be? It is also an electromagnetic wave (maybe wavelet or wavicle is a better word). How many cycles of the wave makes the photon? What is its length (in time or in space), and through which hole will it fit? The energy in a single cycle of an EM wave is always the same (Plank's constant) - no matter the frequency. When absorbed or emitted, the energy is transferred to/form motion. Is the energy transferred in the head of the wavefunction, the tail or anywhere in between depending on the probability?
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  3. Feb 13, 2007 #2


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    But what you wrote has nothing to do with the "size" of anything, much less, a photon.

    Again, the very basic and fundamental question here has never been answered: where exactly is there a definition of the size of a photon in our current physics?

    I'm not talking about speculating or conjecture here. I'm talking about standard, accepted values such as those you find in CODATA or CRC Handbook. Where exactly in all the properties of a photon is there a definition of its size?


    P.S. There's a reason why we close certain threads. It's because it keeps going around and around in circles. Unless there is a definite answer to at least the question that I asked, this thread may end up the same way.
  4. Feb 13, 2007 #3


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    There's another issue with calling the extent of the wavepacket "the size of the photon".

    This is the fact that the wavefunction is not a function in physical space. It's a function in configuration space. This doesn't show up until one considers multi-photon systems.

    The relevant issue here is sometimes called "wavefunction realism". Google scholar finds

    Life in Configuration Space - group of 5 »
    PJ Lewis - The British Journal for the Philosophy of Science, 2004 - philsci-archive.pitt.edu


    which appears to be sensible and talk about these issues in spite of being published in microsoft word. (It also appears to be peer reviewed, an important point for PF discussion).

    see also http://bjps.oxfordjournals.org/content/vol55/issue4/index.dtl

    The author argues that there is a sense in which wavefunction realism can be made to work, but that it's not as simple as thinking of a "photon" as being a wave in a 3d space.

    If you really want to talk about his in more depth, you should probably try the quantum mechanics forum - this gets rather deep quickly, and it doesn't have much to do with relativity. I know enough to know that there are problems with thinking of the wavefunction as "real" in 3d space, but you'd be better off talking with someone else about the fine details.

    You could also try the philosophy forum, though in this case I'd guess that you'd have the best luck in the QM forum.
  5. Feb 13, 2007 #4


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    I have seen the size of a photon listed as lambda/2pi. I don't know where that author got the value but I noted it because I had never seen it before. It seems to me that the size depends on frequency and I look to the Fourier transform as the analog. Where is the photon? If the waveform is continuous in time then one point location can be assigned but as one narrows the wavelet in time, the space dimension grows. Now, I don't know that enough is known about the nature of a photon to answer this question. (It seems a bit harsh to close the question because the answer is unknown, however, IMO.)
  6. Feb 13, 2007 #5


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    I would like to see that, considering that ALL of the standard reference sources that I have mentioned made zero assessment on the size of a photon.

    The issue here isn't that it is unknown. The issue here is that such a property was never defined for a photon! It is like asking how salty is blue? You are asking for a property that was never attached to the entity being asked.

    Now remember that there was never any question on the "size of light" during the classical EM wave scenario. There is a size of the light pulse, but this is arbitrary depending on when you truncate that light source. So such a property is meaningless. Yet, people seem to think it is OK to ask for the size of a photon. Why? Because they have the impression that a photon is an ordinary particle in real space simply because we have attached the word "particle" to a photon. Yet, a photon is a "quanta of energy". It was never defined as a classical particle that has a definite boundary in real space. That is why I asked if blue is salty. We are asking for the characteristic of something that was never defined to have it in the first place!

  7. Feb 14, 2007 #6
    Does a photon travel in a wave?Arent the particle and wave theory different?Excuse me if i dont understand
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