Most interesting physics, The EPR Paradox

Discussion in 'Physics & Math' started by Cazzo, May 10, 2008.

  1. Cazzo Registered Senior Member

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    I'm not an expert at physics, but this is the most profoundly interesting thing I've heard of in physics : The EPR Paradox

    From the article linked :
    "The EPR paradox draws on a phenomenon predicted by quantum mechanics, known as quantum entanglement, to show that measurements performed on spatially separated parts of a quantum system can apparently have an instantaneous influence on one another. This effect is now known as "nonlocal behavior" (or colloquially as "quantum weirdness" or "spooky action at a distance"). In order to illustrate this, let us consider a simplified version of the EPR thought experiment put forth by David Bohm."...


    So basically, it sounds like entangled quanta can instantaneously "communicate" with each other; when one's acted on -> the other one reacts instantly, no matter how far apart they are. Right ?

    Could one extract from this "non locality" that the universe exhibits a reality which implys EVERYTHING is actually in one spot, and distance is an illusion ?

    The only other way I could think of explaining it is if the universe is being run by something like a (very advanced) computer program. The 4D universe is the program running, and the code behind it could have objects any distance effect one another instantaneously. :shrug:
     
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  3. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Wrong. Sorry.

    The idea is that the entangled particles live on the same waefunction. The wavefunction itself carries no ``information''. So the idea of two ends of the wavefunction communicating is not right---there can be no exchange of information because there IS no information.

    Specifically, let's say that you and I set up an experiment to measure the spins of an entangled particle. I live in New York and you live in California. Let's imagine pions or something being made in nebraska, so that when the pion decays, you get one photon of one spin, and I get another photon of another spin, so that the two spins add together to make a spin zero. So when I measure a photon, I see it has one spin, and i KNOW that you measured another photon with the opposite spin.

    When cased this way, it's clear that no information can be communicated between us. I measure spin up, but in order to see what YOU measured, I have to call you on the phone, or something.

    Further, we have to know that it was a pion decaying in the first place! That means that someone has to call me and you on the telephone to TELL us that we should be ready to detect a pion.

    This is actually not as far fetched as it sounds, but I feel that these kinds of questions aren't scientific. And either way, it doesn't really follow from your previous statements.
     
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  5. Cazzo Registered Senior Member

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    Ohhh, I knew this was some deep stuff. :bugeye:

    Leaving out the "communication bit", isn't it correct to say if one of the entangled particles is acted on, the other one reacts instantaneously ? (observed or not observed) ?
     
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  7. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Hmmmm maybe not. It's not a reaction, per se. If it was a reaction, then it would be non-local

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  8. Vkothii Banned Banned

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    Doesn't one observer know that the entangled spins have been observed, or the "other end" has a known spin state, so has a 50/50 chance of guessing which it is?

    I remember a Alice/Bob scenario where the remote party can determine that this has happened, but only knows with a probability of 0.5 what's been observed.

    Except the whole thing has to be pre-arranged before Alice and Bob part company, to transmit the signal, and the distance between them still affects what they can arrange to do, given the known states of these entangled photons.

    The pre-arranged bit is like having a long telegraph wire, that gets connected briefly, so the message is: "there's a connection", but no further information is available on the quantum entanglement line. You have to use ordinary speed-of-light photons for that.
     
    Last edited: May 10, 2008
  9. temur man of no words Registered Senior Member

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    The two pions are on the same wavefunction, but when you measure the spin of one of the pions, isn't the collapse of the wavefunction occurring instantly also for the other pion?
     
  10. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Sure. The collapse of the wavefunction is instantaneous. But both photons live on the same wavefunction. It's the wavefunction that tells the photons how to behave, not vice versa.
     
  11. fadingCaptain are you a robot? Valued Senior Member

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    How can a wavefunction collapse instantaneously regardless of distance? If I measure a photon in Alpha Centauri, its entagled partner's wavefunction INSTANTLY collapses even if its here on earth. This is the most befuddling thing in physics.
     
  12. extrasense Registered Senior Member

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    On the face of it, it is impossible.
    I think such claim is result of mistaken interpretation.
     
  13. Vkothii Banned Banned

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    Not if you consider that the entangled photons are two halves: of the same wavefunction, with identical but opposite (in sign) quantum "numbers": spin, direction, frequency.

    Distance, if you're a photon, doesn't come into it, a photon is an excitation with an indefinite "lifetime". Photons don't decay or "dissipate" - but they do stretch, along with space itself (cosmic expansion); obviously the distance between two entangled photons is zero distance, as far as photons are concerned. Or the wavefunction is the connection, and it's represented as two exactly equal but "opposite" photons, travelling in exactly opposite directions (of course, the direction of a photon can be changed as it propagates).

    The change of direction implies that entanglement is immune to the direction of either of the pair of photons - although when the original photon "splits" into two equal halves, they must propagate at 180 deg to each other.
    Subsequently the wavefunction "ignores" changes in the direction of propagation (and so the distance) of either of the entangled pair.
     
    Last edited: May 16, 2008
  14. granpa Registered Senior Member

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    I hear people constantly say that entangled particles dont communicate instantly over great distances but they never give any reason for this. In fact their thinking seems circular to me. 'we know that particles dont communicate ftl because we know that ftl is impossible'. but what if it is possible. As the op pointed out:

    if the universe is being run by something like a (very advanced) computer program...and the code behind it could have objects any distance effect one another instantaneously

    there seems, therefore, to be no reason to just assume that ftl is impossible. instant communication to any point in the universe would mean that there is in fact an absolute time but relativity doest say that absolute time doesnt exist just that we cant determine what it is without flt.
     
    Last edited: May 16, 2008
  15. Pete It's not rocket surgery Registered Senior Member

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    There's difficulty in pinning down what, exactly, is communicated.
    For the simple notion of entangled photons, there's no more apparent communication involved than if the photons were spin up / spin down right from the beginning of their journey. Of course one will be up and the other will be down, and of course you won't know which is which until you measure it.


    Further investigations (the Aspect experiment) make it clear that it's not quite that simple, but the point remains... what (if anything) is being communicated?
     
  16. granpa Registered Senior Member

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    dont know about photons but if you change the spin of one entangled particle doesnt that change the spin of the other?

    if entanglement is just the fact that when produced 2 particles have opposite spin then the whole concept seems rather trivial. even physicists arent that dumb. there must be something more to it than that.
     
  17. Vkothii Banned Banned

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    You don't "change" the spins, you "observe" them.

    If you observe say the plane (or circular) polarization of one of an entangled pair of photons, that means you know the polarization of the other half (of the wavefunction). At the "other end", an observer can determine that a measurement has been made, is all, not what the measurement was.
     
  18. granpa Registered Senior Member

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    the question was what if you change the spin?
     
  19. Vkothii Banned Banned

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    Again, you only "measure" spin.

    A photon of light has an unknown polarisation until it's measured. You can't "measure" an unpolarised photon - you have to polarise it with a filter of some kind.

    The filter "measures" polarisations of incoming photons, and allows those with the same plane polarised "direction" through. IOW you can't tell anything about photon polarisation until you polarise some photons.
     
  20. granpa Registered Senior Member

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    I specifically said that I wasnt referring to photons.
     
  21. Pete It's not rocket surgery Registered Senior Member

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    Most interactions (changing the spin, for example) would break the entanglement.
    I wish I understood the Aspect experiment - I think it's the definitive one that showed that something very spooky is going on, but I don't have a good handle on what that spookiness is.
     
  22. Vkothii Banned Banned

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    I have a question about the question: how do you "change" the spin of a particle (that isn't a photon), like an electron or a proton, say?
    How do experimenters go about changing particle spins exactly, or what do they need to do?
     
  23. draqon Banned Banned

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    so this entanglement...how exactly can it be used in teleportation?
     

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