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Photon Self-Interference

  1. Dec 22, 2004 #1
    Has anyone ever heard of the double slit experiment being described as a photon interacting with itself? I've seen people mention this "explanation" but it doesn't sound very QM to me.

    Does Feynman say this verbatim somewhere?

    Thanks

    Pmb
     
  2. jcsd
  3. Dec 22, 2004 #2
    To say that the photon "interferes" with itself sounds perfectly correct to me. You have one photon and two slits. This one photon then has two amplitudes (one for each slit). These two amplitudes for this one photon then interfere.

    But to say that the photon "interacts" with itself ... well, I think that that word choice is inappropriate.
     
  4. Dec 22, 2004 #3

    ZapperZ

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    Read

    L. Mandel, Rev. Mod. Phys., v.71, p.274 (1999).

    There is a clear difference between single-photon interference (which is the explanation for the 2-slit observation) and 2-photon interference. Again, most people are confusing the "focus" of the issue here. It really isn't the photon that is the major cause here. It is the superposition if it's PATH! The photon is merely the carrier of this effect! It means that you can replace the photon with ANYTHING that exhibit the identical deBroglie wavelength and the same superposition. That's why you can get this with photons, electrons, protons, neutrons, buckyballs, etc. etc.

    Zz.
     
  5. Dec 22, 2004 #4
    To me the double slit simply determines the probability amplitude of a photon stiking the screen at a particular location. The "Photon interacts with itself" description simply gives me the willies! :yuck:

    Thanks folks

    Pete
     
  6. Dec 22, 2004 #5

    ZapperZ

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    Not if you get rid of that classical confinment of "particles".

    Most people's inability (or refusal) to comprehend quantum phenomena is primarily due to the ingrained view of what the world should be based on what is familiar, which is the classical world. It is utterly arrogant for us to force the microscopic world to fit to this description. We are forcing onto it concepts that are simply vaguely applicable, and then we complain when the results make no "sense". What gives?

    Zz.
     
  7. Dec 22, 2004 #6

    Nereid

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    Oh you'd really enjoy some of the discussions we've been having in the Philosophy section Zapper! :devil:
     
  8. Dec 22, 2004 #7

    ZapperZ

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    I'll be right over as soon as I have a lobotomy! :)

    Zz.
     
  9. Dec 22, 2004 #8
    Who says I am unable to or refuse to comprehend quantum phenomena? I understand it/comprehend if just fine thanks. I took two full courses of quantum in grad school. That doesn't mean I like a particular description/interpretation. Especially in this case. I don't see this as a photon interfering with itself. I see the slit as a quantum well and the well determines the probabilty of measuring the position a photon lands on the screen behind the double slit. There is no need for me to invoke images of a photon interfering with itself and that's the way I see it. That does not mean I don't comprehend QM.

    Pmb
     
  10. Dec 22, 2004 #9

    ZapperZ

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    Then don't use a slit. Use a beam splitter, or a SQUID as done for Cooper pairs interference. The results are identical. If you think your slit is a quantum well, then you need to explain how your quantum well translates to all these setups that give the same phenomena.

    After you have done that, then you should then address how an electron in an H2 molecule "interferes" with itself in two different but overlapping potential wells to create the bonding-antibonding states.

    Zz.
     
  11. Dec 22, 2004 #10
    Yes. I'm well aware of that. Thank you.

    The same way that the Hamiltonian of a given system determines the possible quantum states. Different systems may have the same Hamiltonian. There is a Hamiltonian associated with the slit. The possible wave functions are then determined by that Hamiltonian and the probability that a particle will be measured at a particular location will depend on the the wave function.
    Hamiltonian -> Probability distribution. The rest is all flowery tails. However if you can provide a measurement which indicates that what I've said, i.e. Hamiltonian - > wave function - > Probability density etc is wrong then I'm all ears.

    What observation are you saying is made where a photon/particle interfers with itself and what are you saying that physically means?

    Pete
     
  12. Dec 22, 2004 #11

    ZapperZ

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    Then show me the Hamiltonian for the slit and especially wher you incorporate the "potential well". You might want to submit this for publication, since Marcello has already derived this using photon interference.[1] And then justify how the similar Hamiltonian can be used in SQUIDs interference pattern. I'd like to see where the "potential well" is for that one.

    When a photon passes through only one slit, and we know it passes through only that slit, you have no interference pattern. When it is allowed to pass through both slits, we have interference pattern. Same thing with the SQUID. If I detect the cooper pair going through one arm of the SQUID loop, I see no Fraunhoffer current pattern. But if I leave it alone, I get that pattern. More same thing, this time with H2 molecule. If I confine the electron only to one of the H atom, I detect no change in the ground state. But if I let them overlap, then the electron will occupy both atoms simultaneously to produce the "interference" effect that results in the bonding-antibonding state.

    And I have no idea what you want by "physically means". "Meaning" of things tend to be a matter of tastes. We might as well argue about favorite colors.

    Zz.

    [1] T.V. Marcella Eur. J. Phys. v.23 p.615 (2002).
     
  13. Dec 22, 2004 #12
    Since the idea is the same with an electron I'll refer to electrons instead. The Hamiltonian for such a potential as a slit is not easy to express. Its an infinite potential at the wall of the slit and zero potential where the slit is open.

    I'm not certain if you understand the term "potential well" in the way I've intended it here. For an example of a potential well see diagram at
    http://www.geocities.com/physics_world/qm/harmonic_oscillator.htm

    A potential well for a slit would have infinite walls and zero potential inside the slit.
    I agree.
     
  14. Dec 22, 2004 #13

    ZapperZ

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    Then your potential well is the physical confinment within the slit. This then is inconsistent with, for example, the SQUID experiment. Each branch of the SQUID loop can be made as thick as one care to. It doesn't affect the nature of the interference pattern, so the pattern has nothing to do with physical confinemnt. In fact, we KNOW how quantum well/quantum wire states behave since we can already fabricate those nowadays. These things do not show the interference pattern of.

    Zz.
     
  15. Dec 22, 2004 #14
    The quote is due to Paul Dirac (The Principles of Quantum Mechanics, 1930):

    "Each photon then interferes only with itself. Interference between two different photons never occurs."

    But different radio transmitters and lasers do interfere...
     
  16. Dec 22, 2004 #15
    Maybe I'm wrong, but I think you can deduce from this statement the main idea behind the the bose-einstein and fermi-dirac statistics. That's shortly why you don't have the exclusion principle for bosons.

    Please don't mix the photon with the superposition principle acting in the electromagntic waves. I think to use this picture you have to use the quantum field theory too.

    I think the overall discussion comes from trying to picture the quantum mechanics properties in a language herited from the classical world. It's easier to use math. (maybe I'm a fool, but really cant picture the wave function unless as an integral of the action of the infinte possible paths, wich I really cant visualize) Try to use the "is like a" expression and apologize for not giving the right picture and you don't need this kind of fighting. Otherwise we can have in mind the others difficulties to picture their ideas.
    tank you
     
    Last edited: Dec 22, 2004
  17. Dec 22, 2004 #16
    ??
    The double slit interference pattern is the same for photons and electrons.
     
  18. Dec 22, 2004 #17
    The physics is the same, yes. I.e. there is an interference pattern in both. See Feynman lectures V-III if you have them.

    Pete
     
  19. Dec 23, 2004 #18

    reilly

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    What exactly does it mean for a photon to interfere with itself? As far as I can figure, it can happen only as a result of a nonlinear light-light interaction, and that's easy enough to rule out as a possible explanation. However, the self interference of waves is quite another matter, and it provides the basic classical explanation for diffraction and interference -- the path length inerference discussed by Zapper, above.

    Is there not a dilemma? The quantum and classical fields in diffraction and interference phenomena are, in practice identical, -- same momentum, angular momentum, same energy and momentum density. The mathematial struture of the interference is identical for both quantum and classical fields. Classically the energy density at the detector shows the standard interference effect, for example, for two slits. The actual photon density is energy density/(hbar omega), and for free fields this classical = quantum is rigorous.

    The best thing to say is; the photon is governed by a probabiliy density , which is governed by interference phenomena. Photons will interfere with themselves only in extreme high energy circumstances in which they undergo a nontrivial self interaction, and very likely photons at such extremes no longer exist as such.

    Under normal circumstances and with normal the meaning of the following words, photons do not, in fact cannot interfere with themselves. The description of photons, however, does show the familiar classical interference effects.

    There's always a danger of confusing the THING and its NAME. Could not be more true here: the description involves interference. But, there's no physical way for the photon to interfere with itself. Fields interfere, photons do not. Same is true for electrons.

    Regards,
    Reilly Atkinson
     
    Last edited: Dec 23, 2004
  20. Dec 23, 2004 #19
    Ok Reily, but electron has a spin and spin can interact with the field or the way around. Isn't it a possible way to begin a description of a self-interaction of the electron with itself via the field existing around it? Thank you for explainations concerning this subject; I am actually trying to understand this Papapetrou forces which seem to be an illustration of my question.
     
  21. Dec 23, 2004 #20
    Thanks Reilly. That's exactly what I had in mind. :tongue:

    Pete
     
    Last edited: Dec 23, 2004
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