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Single particle interference

  1. Dec 13, 2006 #1
    Believe it or not I'm having trouble understanding quantum physics. If we take the double slit experiment and one photon is sent through the slits at a time an interference pattern is formed. From what I've read the particle goes through both slits and interferes with itself. Now when it says interfere is it taking in a general sense or does it mean that the particle will hit itself to interfere with itself?

    I also don't understand why we just assume that the particle is going through both slits and then interferes with itself. It's just as crazy to say that the particle is only going through one slit and then takes two directions at the same time hitting different positions on the screen which just so happen to cause an interference pattern. I think the attitude quantum physics takes that it can't be proved wrong so it must be right type of attitude is very poor. If we can't measure the state that the particle is in then we should only conclude that it makes an interference pattern but we shouldn't start pretending that we know what is happening to the particle before it hits the screen.

    I hope you can at least clear a little bit of my confusion. Thanks in advance.
     
  2. jcsd
  3. Dec 13, 2006 #2
    You have to stop thinking about the electrons and photons as particles. They are quantum particles, and exhibit both wave like and particle like properties, and this enables them to interfere with each other like in the double slit exp.

    Quantum physics does not try to say anything about what happens to the particles before they hit the screen, because this is what quantum physics is all about! In the double slit experiment we cannot say anything about which slit the particle went through, and still have an interference pattern on the screen. If we make a measurement and determine which slit the particle went through, the wave-particle duality is determined (to be a particle, because we know it's position at some instance) and an interference pattern will no longer form.
     
  4. Dec 14, 2006 #3
    The way out might be to step back and drop the assumption that QM applies to any single
    particle. QM certainly applies to an ensemble of identically prepared particles for it gives
    excellent predictions for probabilities (many repetitions of the same experiment). At the same time QM cannot predict where exactly a single photon in the double slit experiment will hit the screen.
    This attitude is called the statistical interpretation of Quantum Mechanics. The wave
    function there describes ensembles of identically prepared quantum objects and not
    an individual quantum object.

    The theory of single quantum objects is still to be discovered.

    Good question, anyway!

    Cheers!
     
  5. Dec 14, 2006 #4
    Ofcourse QM applies to any single particle at the atomic scale. What are you saying ?

    So ? What is that supposed to mean ?


    ANY "attidude" of QM should NOT contradict with it's basic fundaments.

    Oooh, no, is this the single particle stuff again ? I already explained you why you were WRONG making such claims.

    HERE IS SOMETHING FOR YOU. Read the second and third paragraph on the Quantum Computer. You might actually read the very opposite of what you have been saying in this and the other thread with respect to a particle's self interference/interaction.

    Greets
    marlon
     
  6. Dec 14, 2006 #5
    You are barking on the wrong tree. I am not interested in your opinion anymore.

    Good luck!
     
  7. Dec 14, 2006 #6
    Thanks for the mature answer but we have an obligation to correct mistakes. Otherwise this forum is not what it should be. Both Reilly and myself have asked you specific questions and clarifications in the "Statistical interpretation" thread to which you never answered. Both of us have explained you where you are making mistakes. Again, i have given you a CalTech reference where you can read the exact opposite of what you are claiming.

    To be clear, you need to stop selling stuff that violates the very fundaments of physics. Such behaviour will not remain unnoticed, i assure you.

    regards
    marlon
     
  8. Dec 14, 2006 #7
    Smiley10:” Believe it or not I'm having trouble understanding quantum physics.”

    Read R. Feynman Lectures. If you still do not understand, leave quantum physics.There are plenty interesting problems in other areas.
     
  9. Dec 14, 2006 #8
    LOL

    Err, there is one problem though...Most of the physics area's that have direct applications to today's industry are based upon QM (semiconductors, electronics in general, spintronics, photonics, etc etc). So the OP really needs to understand what is going on.

    TO THE OP : HERE'S A SIMPLE EXPLANATION OF SINGLE PARTICLE INTERFERENCE AND QUANTUM INTERFERENCE. BOTH ASPECTS ARE FUNDAMENTAL TO QM SINCE 'SUPERPOSITION' IS A DIRECT CONSEQUENCE OF THIS BEHAVIOUR

    Greets

    marlon
     
  10. Dec 14, 2006 #9

    vanesch

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    Or, to quote the same Feynman: "nobody understands quantum mechanics" :biggrin:
     
  11. Dec 14, 2006 #10
    What kind of advice is that? Did you hear it from some one yourself?

    If you think you understand QM then this place is just made for you to share this blessing.

    Cheers!
     
  12. Dec 14, 2006 #11
    Thanks for the replies everyone.

    This does make a lot of sense and if I didn't have the answers to my questions I'd be satisfied with that. But I still can't help being curious about how the particle side of light could work (or even if it's possible to work).

    From the link that Marlon gave it's reasonable from the experiment to assume that the particle can in fact be in two different places at the same time, at least as long as the detectors are in fact detecting a single photon since I don't think this is mentioned. The photon does seem to take a random path but where the photon hits the screen isn't completely random. We know that a photon will never hit the dark spots. But still the photons appear to take a random path so at some point the photon must be heading at this direction of the dark spot so the photon must somehow be interferred. Now at this point I haven't got a clue what the hell interferes with the particle. I've read that the photon interferes with itself but this sounds pretty crazy. What appears to be two photons in fact isn't they're the same matter so it just doesn't seem right that when the same matter hits together that momentum will be transferred (or will it?).

    I'm not entirely sure on the photon taking a completely random path and I'll have a read up on hidden variables, bell's theorem etc.

    I haven't read the lectures but I'll have a look at them and I'm glad that someone caught onto my very subtle joke in the first sentance of my first post.

    Thanks again, at this point I'm just making discussions and I believe I have a good enough understanding to write about it in my coursework :wink: . Feel free to keep the thread going.
     
  13. Dec 14, 2006 #12

    reilly

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    Given more opportunities like this thread, Marlon and I might write identical posts, or, more likely, ones that say much the same thing.

    Now, one of the reasons that QM is difficult to grasp -- weird, contradictory, "shut up and compute", "but we really don't understand it" -- is that the phenomena that led to QM are strange, weird -- blackbody radiation, photoelectric effect, atomic spectra, electron and neutron diffraction, pair production, spin, and on and on.

    For the double slit business, first review basic probability, preferably discussed in terms of events. That will cure you of any concerns about predictions of single events. For all practical purposes, the theory of electromagnetic wave diffraction and electron diffraction are very similar -- the diffraction occurs primarily from restriction of the impinging wavefront, just like Huygens told us a few years back.

    Interfere with itself? In my view, that's a personal choice. In fact, given the broader behavior of waves governed by Maxwell or Schrodinger, I see no good reason to formulate such a controversial notion. It only serves to muddy and confuse something that is basically straightforward, provided you accept that the phenomena of particle diffraction does occur . Then the point is to accept reality and figure out a theory that can explain how such diffraction occurs. QM does just that, and has done so for almost a century. Also, contrary to some contentions that have appeared in this forum, the vast majority of physicists accept QM -- albeit sometimes in different flavors.

    As Dorothy said, "Toto, we're not in Kansas anymore."

    Regards,
    Reilly Atkinson
     
  14. Dec 15, 2006 #13
    Vanesch:” Or, to quote the same Feynman: "nobody understands quantum mechanics".

    You are right. But I don’t know what he meant: everything in QM or QED only.

    Marlon:” Most of the physics area's that have direct applications to today's industry are based upon QM (semiconductors, electronics in general, spintronics, photonics, etc etc).”

    If he going to be engineer, why he need understanding?

    Zbyszek:” What kind of advice is that? Did you hear it from some one yourself?”
    Smiley10:” I haven't read the lectures but I'll have a look at them and I'm glad that someone caught onto my very subtle joke in the first sentance of my first post.”

    What is the matter with you? Jokes are forbidden?
    You react with anger on my criticism of L.E. Ballentine. Is he your close relative? Before you wildly attack S.L.Adler (without justification).
    You like Ballentine version of statistical interpretation? I never said that it is not legitimate (as well as other interpretations).
     
  15. Dec 15, 2006 #14

    vanesch

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    The situation is the following: in "ordinary" QM, the formalism is entirely understood ; in quantum field theories (such as QED), even the formalism is not entirely understood, but well enough to do some calculations. In other words, we know that the calculations that are done, are mathematically somehow unsound, but this can be explained away by saying that it must be an approximation to something else (effective field theories).

    But Feynman wasn't talking about the formalism, he was talking about the "physical meaning". This is what is not understood, although several attempts with varying degrees of success have been invented. Nobody really knows what the mathematical objects in a quantum theory actually represent.

    Some claim that it is just a mathematical tool which gives you statistical outcomes of experiments (in other words, that one shouldn't look for any physical meaning) - fine, but they can't come up with an explicit underlying physical mechanism ! Some (Bohr, with Copenhagen) claim that there IS no explicit physical mechanism, that all there is, is "statistics". This is essentially the "standard" Copenhagen interpretation: the quantum-mechanical formalism links statistically setups and outcomes of a "classical" macroscopic world, and there is no underlying explanation for this link. The formalism of quantum mechanics simply allows you to calculate the probabilities, but doesn't represent anything physical, because there IS nothing physical at that scale.

    Others (such as me) claim that the formalism of quantum theory is to be taken seriously, and that it represents genuine physical quantities. These views are "many worlds" views, because you cannot avoid that way, to make a distinction between "the physical state" and "observed reality by an observer", which is so terribly weird.

    Others think that the quantum formalism has something real to it, but that there is also an explicit "projection" mechanism. However, this usually introduces some clashes with relativity.

    Still others think that there's something fundamentally wrong with the quantum formalism, although it makes correct predictions in many cases, for an ununderstood reason.

    This is a discussion that goes on now for almost 80 years, and is usually referred to as the "measurement problem" of quantum theory. However, and that is the nice part: you don't need to think about all this to get the formalism working in practical cases, and in any case, it is a good idea to learn very well the formalism before delving into these issues.

    So, the practical attitude to adopt when learning the formalism of quantum theory, is simply this: "quantum theory is a mathematical model which allows you to generate statistical predictions for outcomes of experiment, but for which no evident physical interpretation is known." From the moment that you try to do so, you delve into the problems of the measurement problem, which haven't really been resolved since about 80 years. This is what Feynman meant.
     
  16. Dec 15, 2006 #15
    Is this another joke of yours? I have hard time telling whether you are joking or lying.

    For those who would like to check on the moral integrity or the sense of humor (pick one)
    of Anonym please read post #111 under the wave packet description thread
    https://www.physicsforums.com/showthread.php?t=142399&page=8

    I believe I have provided quite detailed justification.
     
    Last edited: Dec 15, 2006
  17. Dec 15, 2006 #16

    vanesch

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    If this is going to turn into a flame war, then this thread will be locked and warning points issued.
     
  18. Dec 15, 2006 #17
    Do I have a right to defend my reputation on this forum? You see, attacking somebody's
    work without justification is one of the lowest things a scientist can do. I am being accussed
    of doing just that.
    Is it a flame war for you, vanesch? In this world, I mean.

    Cheers!
     
  19. Dec 15, 2006 #18

    vanesch

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    The informal warning was not only directed at you, be reassured. Try to argument your position in a neutral way, or just don't argue, but try to avoid remarks of a personal nature which might trigger other remarks of the same kind and result in a silly flame war which is interesting for nobody to read. This counts for all of the antagonists. Let's keep it level-headed here...
     
  20. Dec 15, 2006 #19
    Reilly:” Marlon and I might write identical posts, or, more likely, ones that say much the same thing.”
    “Interfere with itself? In my view, that's a personal choice. In fact, given the broader behavior of waves governed by Maxwell or Schrodinger, I see no good reason to formulate such a controversial notion.”

    Marlon:”Particles, in QM, ALWAYS exhibit wavelike properties : THAT IS THE ENTIRE POINT OF THE DUALITY. This duality does NOT give us an "either wave or either particle like" picture.’
    “Self interaction IS an essential part of QM.”
    “But i just want to be clear here : the particle/wave duality does NOT say that particles are waves or something like that. this duality just states that at the atomic scale, particles exhibit wavelike behaviour like in the double slit experiment. This ALWAYS happens for each kind of particle like electrons photons, etc etc. Wavelike behaviour is a general term that denotes all the types of interference, diffraction, etc etc...
    We need this duality because we look at QM through "classical eyes".”

    I identify the opposite points of view.

    Smiley10:” I've read that the photon interferes with itself but this sounds pretty crazy.”

    Otherwise you should to accept that a single particle may do statistic with itself.
    Make your choice what is more crazy.

    In the double slit the photon/electron wavefront hit the lossless beamsplitter simultaneously. The lossless beamsplitter is the macroscopic device which do not perform the measurement. No violation of the special relativity take place. Each slit may be considered as Huygens secondary source as usual. The measurement performed at the detector screen (the macroscopic device) cannot produce two outcomes simultaneously. It require the collapse of the wave packet in time, otherwise the requirements of special relativity will be violated (A. Einstein,1928).
    In addition, the macroscopic physics is the theory of certain events. Translation into the language of the functional analysis say: the classical physics is a dispersion free physical theory (J. von Neumann in Foundations for example). It require the collapse of the wave packet in space.
    You may see more detailed discussion in the wave packet description session with Ueit,Reilly and Zbyszek. But you should read Feynman (at least double slit and Stern-Gerlach) before.
     
  21. Dec 15, 2006 #20
    strangely aggresive thread over something fundamental to understanding QM. haven't these issues been thrashed out already?

    maybe we should all "shut up and calculate", which i take to mean try and understand better the mathematical models
     
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