EPR Paradox

  1. I saw this Einstein, Podolsky, Rosen paradox in one of my textbooks and it is very interesting. The paradox simply challenges the Copenhagen (Orthodox) Interpretation of Quantum Mechanics by using the constancy of the speed of light.

    It simply states that if we observe a pi meson decay into a positron and an electron, then wait long enough that

    {delta}x >> ct

    and measure the spin of the electron, we are going to collapse both the wavefunction of the electron and the positron. If we find the electron spin to be +1/2 the positron spin will be -1/2.

    Thus it is claimed that then information will be carried over much faster than the speed of light which is not possible, so they conclude that the electron and the positron had definite spins prior to the measurement.

    What do you make of that?

    P.S. I would also be glad if someone would provide me a good source showing how Schroedinger came up with the Schroedinger Equation, I have looked at a bunch of books and could not find one, I am really curious about that.

    P.S. How do you implement latex code in your messages???

    Thanks.
     
    Last edited: Mar 12, 2008
  2. jcsd
  3. DrChinese

    DrChinese 5,625
    Science Advisor
    Gold Member

    Welcome to PhysicsForums!

    The EPR paper was the first in a series of 3 critical papers on this subject. It raised questions, but did not provide firm answers that were likely to change opinions. No experimental tests were proposed.

    The second was the 1964 paper by J.S. Bell called "On the Einstein Podolsky Rosen paradox" which demonstrated that the EPR explanation was inconsistent with Quantum Mechanics: No physical theory of local Hidden Variables can ever reproduce all of the predictions of Quantum Mechanics. An experimental test was imagined to resolve the issue.

    The third part was a series of experimental tests of the CHSH version of Bell's Inequality (derived from Bell's paper) which provided results consistent with Quantum Mechanics, but inconsistent with EPR.

    You might be interested in this page from my web site:

    EPR, Bell and Aspect: The original References
     
  4. I'm not sure exactly what kinds of responses you're looking for, but the EPR issue inidcates to me that there is something fundamental about the relationship between quantum mechanics and spacetime that we do not fully understand.
     
  5. This video may be an answer to your question:



    However i have another question regarding this youtube vid:



    Its the double slit experiment and shows that when we observe particles the wavefunction collapses and behave as particles. Some philosophers say it has to be an intelligent being observing the particle in order to collapse the wave function. But this wouldnt make any sense to me. In the video they show an eyeball watching the particles as a way of measuring it. However I would picture it as a device interfering with the particles, and showing the measurements on a screen.

    So the question is: If a person does not look at the screen to see the results of measurements, would the particles still create an interference pattern or behave as particles when measured right before they pass the slits?

    If answer is the wavefunction still collapses those philosophers would be wrong because no intelligent being observed its position.
     
    Last edited by a moderator: Sep 25, 2014
  6. I've never heard any reputable physicist say they believe that. Most theories I've heard either don't attempt to define wavefunction collapse, or hold something along the lines of a wavefunction collapse being caused by a thermodynamically irreversible interaction. Anyone who thinks that there needs to be an intelligent being involved is, for lack of a better word, nuts.
     

  7. I am sorry to say this but neither my question is about the double slit experiment nor these videos have any truth value in them. One of the clips are from "what the bleep do we know" (a movie) and the other is an animation made by infamous Dr. Quantum "Fred Allen Wolf". These guys are Zen Cultists and they have no idea of what they are talking about and all of it is just bullcrap.

    Collapse of the wavefunction is just the localization of a probabilistic ensamble whose physical properties are defined by the Schroedinger Equation, it has nothing to do with consciousness or looking. But measurement is a different process because measurement picks out a value (by disturbing the system thus changing the physical constitution of the system) thus collapsing the probability of other values rather than the observed to zero.
     
    Last edited by a moderator: Sep 25, 2014
  8. Even though those clips have nothing to do with your question, I'm glad I got a chance to watch another one. I still see absolutely nothing wrong with those movies if they are considered as intended to a general or young audience. They clearly DO know what they are talking about. That second clip is a dumbed down version of the first chapter of volume III of the Feynman lectures.
     
  9. Well the first video does indicate that all the particles are connected to eachother, even at distant positions. This may be how the positron knows to pick the oposite spin state of the electron after a pi meson decay without having to be predetermined.

    I agree that the videos are decieving. Crazy philosophers make crazy ideas based on the deceptive/missinterpreted arguments from the dr quantum vid.

    A proof of that these philosophers are wrong, is if the wavefunction collapses without being observed/perceived in the mind. That is all im asking.

    In my physics book it just says a measurement causes it to collapse, but what is that? A force from a charged particle? If it is the force from a charged particle, why isnt a wave function collapsed all the time while traveling through air? How long does the collapse last?
     
    Last edited: Mar 12, 2008
  10. It may be similar, but they are right out of target, by showing an eye looking at a particle and basketballs jumping around the space. If you want to make a young audience interested
    you should at least mention the disturbance of the system and not get into the consciousness part.

    The basketball argument shows complete "n" basketballs occupying the whole space and than collapsing into only one basketball???

    The videos thus are neither raising interest nor showing the truth, Feynman's "Nature of Physical Law" for example is directed towards a general or young audience (it has the double slit of volume III too) and it is not misleading.

    Misleading creates pseudo-science and pseudo-science is no more tame but dangerous.
    eg. scientology.
     
  11. OMG. Now you're comparing that to scientology!! This is paranoia.

    You wanted a video directed to kids to mention "disturbance of the system"? First of all, the uncertainty principle doesn't care whether the system was disturbed at all so your point isn't even valid.

    Anyone who thinks that video is malicious or dangerous really needs to chill out and stop thinking themselves to be the Magisterium of all things quantum.
     
  12. I read this from one of Heisenberg's books once, I do not remeber the exact quotation but still I will try to put it in my own words:

    "What quantum mechanics taught us is that we can only talk about the interactions of two physical systems, because an isolated standing alone system need to be interacted by another one to "look" (measure, observe whatever) inside it but when this interaction occurs both systems are disturbed by their effect on each other and in the end we can only talk about this interaction"

    I will give a crude example of collapse. Lets say I am running on a straight line and your eyes are closed and you have a catapult. You estimate that I am confined in a 100m line running back and forth and you start throwing rocks at me. When one rock hits me and sends you a signal that it hit me at +25.3m on the x axis, you have collapsed my wave function on top of this +25.3 at the instant. But I am disturbed too and after a while when I recover from the concussion I will continue running along the line again and if you wait long enough the only knowledge you have about me will be that I am confined in a 100m line. Now if the effect of the disturbance was "big" say you broke my leg my wavefunction will be different than the first 100m confining function.

    Now this maybe a bit misleading too but think of this as picking a photon by a geiger counter, it clicks at a certain point but if the interaction between the counter and the photon somehow reduced or increased the total energy of the photon it will behave differently later on.
     
    Last edited: Mar 12, 2008
  13. Doc Al

    Staff: Mentor

    For someone who obviously hasn't watched this crackpot movie, you're quite the fan. Go and rent it! (As childish as those clips might have seemed, the movie is not directed towards kids.)

    Enough already.
     
    Last edited: Mar 12, 2008
  14. Well i like the vids, but the thing with the eyeball makes it unnecessary missleading. That part has convinced many non physicists that things dont exist before we see them etc.
     
  15. JesseM

    JesseM 8,490
    Science Advisor

    As I understand it, basically any thermodynamically irreversible interaction with the outside environment will cause the quantum system to behave as if it has been "measured", although the details of what variable the environment will act as if it is measuring will depend on the details of the interaction (in most cases I think the environment acts like it's measuring position).
     
  16. Thanks, now i understand it :)
     
  17. I haven't seen the whole "crackpot" movie. I've only seen those two youtube clips. I'm really immensly curious what is so bad about it, but I'm reluctant to rent it becuase I fear I will continue to not see what the big deal is and just get myself more worked up.
     
  18. JesseM

    JesseM 8,490
    Science Advisor

    Those "Dr. Quantum" clips aren't actually in the movie, I guess they were DVD extras or something. The one on the double-slit experiment didn't strike me as too bad except for at the end where they implied a conscious observer was needed to collapse the interference pattern, and the clip on entanglement isn't so bad except that "do something to one and the other responds instantly" has the potential to be pretty misleading, making people think entanglement could be used for FTL communication or something (and the part about everything still being entangled since they were together at the big bang would be controversial, although it could be seen as correct in the many-worlds interpretation). But the actual movie is a lot more new-agey then these Dr. Quantum clips, and I think that one clip on the double-slit experiment basically conveys more meaningful physics info than the entire movie.
     
  19. Ok.

    Well here's my point. As a lawyer and ex-poly sci guy I can nit Schoolhouse Rock's song about how a bill becomes a law till I'm blue in the face, but at the end of the day it's a cute cartoon that conveys mainly correct information in an entertaining way. Sure the Dr. Quantum thing takes some liberties but not outrageous ones, and the liberties it does take are designed to spark more interest or wonder in the subject, NOT to mislead. After all - what do they possibly have to GAIN by making people think that "everything is connected" or that an "eyeball" implies human intervention. It is utterly harmles and might actually do some good.

    As for this You Don't Know Bleep film, if these Dr.Q clips aren't even in the film then I really don't understand why everyone's in an uproar over it. I thought the uproar was over the Dr. Quantum stuff. I'm slightly relieved it's not.

    Do people here really think that anything that doesn't cover a subject comprhensively and technically perfectly should be banned literature? If so we need to throw out every elementary or even high school science book because none of them can live up to that standard and still be comprhensible or remotely interesting to their audience.
     
    Last edited: Mar 12, 2008
  20. Doc Al

    Staff: Mentor

    The "Dr Quantum" clips were certainly in the version of the movie I saw. The double slit cartoon was the best part of the movie! The problem I have with those clips in the context of the movie was that they were used to provide the illusion of "scientific support" for the many outrageous crackpot claims made in the movie. I discussed these claims in a previous thread--and even provided links to detailed reviews of the content of the film.
     
  21. Well it is the case that the liberties they take make a huge difference. Dr quantum basically tells that you have to perceive a particle with your mind to collapse the wave function, and that is very strange and what makes ppl think its an interresting movie. They gain a lot of viewers with this cheat, and it leads to retarded theories which quantum physics in reality does not support. People think that the quantum indetermacy is connected to the mind. If they had explained it correctly it would be obvious that the particle position is decided by hidden variables.
     
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