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The size of a photon

  1. Oct 28, 2006 #1
    A good case can be made for the hypothesis that all photons are the same size and have the same spatial distribution (energy density/energy) of mass-energy as the electron.

    It runs like this: We all accept that an electron in motion can have a De Broglie wavelength ranging from, say, 10^-15 m to 10^50 m, and that throughout this range the size of the electron is unchanged. I can find no experimental results that conflict with the assumption that this is true for the photon as well. The photons produced in electron-positron annihilation must, at least momentarily, have the same mass-energy density as their parent particles. There is no evidence that it changes as they move apart. At any speed the kinetic energy density of an electron is proportional to its rest mass energy density. In any emission of part of this energy as a photon this will at least initially have the same spatial distribution.

    Can anyone cite any experimental results that conflict with this hypothesis?

    Phil Gardner
     
  2. jcsd
  3. Oct 29, 2006 #2

    ZapperZ

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    Just think: you are saying a radio wave, that can have wavelength of the order of miles, has the same "spatial distribution" (which you haven't defined to what that means) as a gamma ray that has a wavelength in angstroms. Would a diffraction experiment change your mind?

    Note your bastardization of "de Broglie" wavelength". It is NOT defined to any extent as the SIZE of the object. If it is, then you have something mysterious here, because the size of an electron can somehow depends on how fast it is moving - and no, this is not the relativistic effect either! A QM wavefunction that you use does not give you a "size".

    Zz.
     
    Last edited: Oct 29, 2006
  4. Oct 29, 2006 #3
    Does an electron have a size?
     
  5. Oct 29, 2006 #4

    jtbell

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    No indication that the electron has a finite size has been found in high-energy scattering experiments. A quick Google search gave me an upper limit of about [itex]10^{-19}[/itex] m for the radius of the electron, from an experiment at CERN. I don't know if this is the most recent figure.

    (That is, IF the electron has a finite radius, it has to be smaller than this.)
     
  6. Oct 29, 2006 #5
    The classical radius is about (10^-15)m - but electrons are not clumps of matter - so looking for a size is an abstraction. They behave differently in different experiments
     
  7. Oct 30, 2006 #6
    Which physics principles would this violate?
     
  8. Nov 2, 2006 #7
    the electron is a point particle - it has no volume. to discuss the "size" of a photon is meaningless. you cannot state with certainty anything about a photon in between the time it is emitted and the time it is absorbed.

    btw, an earlier post talked about slowing down the speed of light in lab experiments - IMO, that is a hogwash. C is a constant in a vacuum - and all of spacetime is a vacuum in between any two given particles. any artifact of measurement which indicates a variance in C is being misled by absorption/re-emission actions, and is representative of merely an apparent slowing of C.
     
  9. Nov 2, 2006 #8

    ZapperZ

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    It violates the physics principle of understanding the principle FIRST before using it.

    Zz.
     
  10. Nov 2, 2006 #9

    Lol. Do you ever have a good day zapper?
     
  11. Nov 3, 2006 #10
    If this were true for all photons, then what creates the track in a bubble chamber at the passage of an highly energetic gamma ray?
     
    Last edited: Nov 3, 2006
  12. Nov 3, 2006 #11
    This principle states that an electron has no size, or that is point-like? Explain this principle, please.
     
  13. Nov 3, 2006 #12

    ZapperZ

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    No. If you look at what you underlined from my quote, it is the "deBroglie" wavelength, which was the issue in that quote. Check the deBroglie wavelength and see how it changes with the velocity of an electron.

    Zz.
     
  14. Nov 3, 2006 #13

    jtbell

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    The gamma-ray photon itself does not create a track in a bubble chamber. What does create a track (or rather, tracks) is the [itex]e^{+}e^{-}[/itex] pair that the photon produces.

    I saw lots of these while working on a bubble chamber experiment as a graduate student many years ago.
     
  15. Nov 3, 2006 #14
    I know how it changes with electron's velocity. I intended that if the "size" of an electron would be related with its wavelenght, and so, would depend upon its velocity, I don't understand why this should be so strange, or impossible. I think there already are many strange things in QM and this would't certainly be the most strange one.
     
  16. Nov 3, 2006 #15
    Ok, but you mean that an highly energetic photon is completely "invisible" until it hits a nucleus (creating a pair), and is only possible to determine its direction? Or (if it has enough energy) it can interact in a point (Compton or pair production) and in another one and so on, so that we can trace its path?
     
  17. Nov 3, 2006 #16

    ZapperZ

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    It isn't strange for QM, it is strange for someone wanting to find the SIZE of something based on our pre-defined understanding of what a "size" is. If you tell me how you'd measure the size of something, then that's another issue. However, to say that "Oh, let's use the deBroglie wavelength" as the size, then you've defined something entirely new without bothering to define and justify it FIRST.

    This is ignoring still the fact that the wavefunction defined by the deBroglie wavelength has nothing to do with a "size". What operator would you associate this with? The position operator is just that, a position, not a "width" of the object. That is what I meant when I said that it is a bastardization of the deBroglie wavelength - concluding something in ways it wasn't meant to be used.

    Zz.
     
  18. Nov 3, 2006 #17

    jtbell

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    We were dealing with energies above about 1 GeV, up to several tens of GeV, and the bubble chamber's working substance was a liquid helium-neon mixture, so pair production was by far the dominant photon interaction mechanism. We could get the momentum and energy by measuring the electron and positron tracks (in a magnetic field). At those energies, the energy and momentum taken up by the recoiling nucleus are negligible.

    At much lower energies Compton scattering would become important, but all you'd see would be the recoiling electron. I don't know if you could see enough successive Compton-scattering events from the "same" photon to get a usable track.
     
  19. Nov 4, 2006 #18
    If you assume that QM is a complete descrition of reality, I don't undesrstand why you work as experimentalist; I expected from you to talk about experimental limits of a particle size, or experimental limits in measuring a particle size, as jtbell did. Why does an electron leave a track in a bubble chamber? Why does high momentum (little wavelenght) electrons go straight after a slit in a screen, while low momentum ones form a spread diffraction pattern? I mean, I know how QM describes all this, but does QM, for example, gives the value of a nucleus size? Or we need something more, for example nuclear force theory, (assumed those values could be determined theorically)?
     
    Last edited: Nov 4, 2006
  20. Nov 4, 2006 #19
    Since the least energy required to form a couple is 0.511*2 MeV, it's strange that a several tens of GeV photon produces only one pair in a chamber. How is this explained? However, with higher energy, do you think it would be possible to have more than one interaction from a single photon?
     
  21. Nov 4, 2006 #20

    ZapperZ

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    You are making an incoherent argument here. What does what you said have anything to do with my response so far? I was addressing YOUR question to me regarding the deBroglie wavelength and how it was misused. You didn't ask me for a treatise on what an electron is. I didn't know that this is what this thread has become, because it certainly is NOT part of the OP. So if you wish to hijack this thread to be on the "size of an electron", then make one!

    BTW, ALL electrons, be it "high momentum" or "low momentum" can make diffraction patterns. Whoever told you that the high momentum electrons "go straight" through?

    There's a lot of very confusing disconnected statements in your questions. Why don't you create a thread and ask ONE question at a time?

    Zz.
     
  22. Nov 4, 2006 #21
    ZapperZ, I will start a new thread about an electron size; however:

    When, talking about a possible relation between De Broglie Wavelenght and what an electron "size" could be, you wrote that this would be "mysterious" because the electron "size" would then depend on the electron's velocity. I then replied to this asking: "which physics principles would this violate?", because, for what I know now, it wouldn't be such an impossible idea; on the contrary, it would seem reasonable to me, in the sense that it would explain better (to me) why particles, electrons in this case, have a particle-like and a wave-like behaviour at the same time, and so we go to the second point: the difference between geometrical and wave optics, and the same is with electrons, is substantially done by the value of photons or electrons wavelenght, in comparison with the other dimensions implicated in the experimental apparatus.

    That is (and I am perfectly aware that you know all this much better than me) energetic electrons, with "normal" experimental dimensions of slit and screen's distance (let's say 0.01 millimiters wide the slit and 1 meter the screen distance to the slit, but of course they would be different from these values) behave, almost exactly, as if they were a beam of particles, or, said in another way, their wave-like behaviour is very difficult to reveal in this case. You know very well that the angular dispersion of a wave is proportional to lambda/d (d is the slit's width), so, with a very little value of lambda (energetic electrons) the angular dispersion is pratically zero. I simply intended this, nothing else.
     
    Last edited: Nov 4, 2006
  23. Nov 4, 2006 #22

    ZapperZ

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    But this is certainly different than saying the energetic electrons do NOT have any diffraction! All I need to do is make a smaller equivalent of a slit.

    Furthermore, I can easily do this using atomic spacings using solid crystals. We do this all the time in bragg scattering/reflection! I can show you diffracton and interference effects from low energy electrons all the way to "high" energy electrons in the MeV range!

    Zz.
     
  24. Nov 4, 2006 #23

    jtbell

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    Pair production by its nature is a destructive process. In QED, the basic interaction vertex involving electrons/positrons and photons has two electron/positron lines and one photon line. You can have an electron coming in and an electron + photon coming out. Or a positron coming in and a positron + photon coming out. Or a photon coming in and an electron + positron coming out. Or a photon + electron coming in and an electron coming out. Etc. Some of these have to involve a virtual particle because they can't conserve both energy and momentum if all the particles are real.
     
  25. Nov 4, 2006 #24
    I agree with you, infact I haven't said it; even if, I admit, it could have been interpreted in this way, when I said "they go straight". But, since you are more used to "physical meanings" than a mathematician, I believed you could understand what I intended: that what really makes the difference in physics, sometimes is nothing else than values. For example, I could say that a truck can go through a 0.01 millimeter wide slit, but probably everyone (included you) would take me as mad.

    Of course, and not only with electrons, but with trucks as well.
     
    Last edited: Nov 4, 2006
  26. Nov 4, 2006 #25
    Ok jtbell. I was just wondering if it's possible to track a single photon's path in a chamber, with multiple interactions, whether they are pair productions or Compton scatterings or both, since, from a book I have, a figure seems to show exactly this (Introductory Nuclear Physics - Kenneth S. Krane - John Wiley & Sons_1987. Chapter 7=Detecting Nuclear Radiations-paragraph 7.6=Energy Measurements-fig.7.24, page 221).
     
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