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Quantum physics question

  1. Jan 15, 2009 #1
    So, EPR shows that once something like the spin of a particle is determined, the wave function of a particle can be collapsed instantaneously but i have a question about this. Say there is an observer on a planet 5 light years away from you (you are on earth) and he makes an observation that collapses the wave function of a particle by observing it. You have a super telescope and you can see him quite well from earth. You can also see the particle in question and are running a remote analysis on it. One day you see your friend on the planet 5 light years away collapse the wave function of the particle by observing that it say... has an up spin. So you say "the wave function was collapsed today because we have observed that the particle has a certain spin" but the wave function wasn't collapsed today, it was collapsed 5 years ago! So even though the observers on earth had to use quantum physics to measure the probabilistic state of the particle up to the point at which we saw the observer observe it, there was an EXACT answer to what that state was because it has already been measured. even thou we said that the particle had x probability of having an up spin... it didn't, it had a 100% probability of having an up spin because that's what it has been for 5 years. Thats kinda like saying before i rolled the dice i thought i had a 1/6 chance of landing a 4 but actually i had a 100% chance of landing a 4 because thats what i got. So, does a wave function collapse all together? or just from observer to observer?
  2. jcsd
  3. Jan 16, 2009 #2
    The particles have correlated spins only up to time of simultaneous measurement. After that both spins are uncorrelated again. But it does not mean that they are forever an up spinner and a down spinner.
  4. Jan 16, 2009 #3
    Just from observer to observer. Probability statements (wavefunctions) don't describe the physical states of the object systems (detectors, dice, etc.). They describe what's known about them by a particular observer at a particular time.
  5. Jan 16, 2009 #4
    see, my big problem is that with the example i gave, there wouldnt be x% that some particle is a certain way, there would be a 100% probability because thats what happened but we just dont know that yet because we are five lightyears from the observation. Wouldnt that mean that in any point in time and space any particle IS in a particular state wether we have or havent measured it or can or cant calculate it? I know what im saying goes against QM but just thought i'd share my little thought experiment.
  6. Jan 16, 2009 #5
    Your question is valid, but here we have the problem again, that nature is trying to cheat as long as it can. For you it is already determined what the spin would be, because you have measured the particle already, and then nature makes sure, that the other observer on the other star will get a result consistent with you measurement. It doesn't matter who collapses first. But just maybe there is some kind of EPR like paradox that makes it possible to determine when "the electron knows" what state it has to be in. So far I am not aware of one and it doesn't seem to exist.
  7. Jan 16, 2009 #6
    Its not that i don't understand the basic concepts behind QM even though i am technically a "layman" i.e. I'm only in grade 12 doing grade 12 physics but i do a lot of my own research because I'm deeply interested in the way things work. Its just that no matter how much i research and study QM and no matter how convincing the evidence is, i still find a little voice in the back of my head saying "nothing is completely random" :P
  8. Jan 17, 2009 #7
    Conceptually, the idea that the universe or some part of it is in a particular state at a particular time even when it isn't being observed doesn't go against QM. QM isn't a description of the deep nature of reality, even though the models and equations of motion, as well as the experimental designs and results, do suggest some more or less reasonable inferences wrt underlying physical causes. QM is a means of assigning probability values to instrumental behavior.

    It wouldn't make any sense to say that nature itself is random. QM probabilities and the randomness of quantum experimental results don't refer to physical states of some deep natural reality that might underly the instrumental behavior.
  9. Jan 17, 2009 #8
    I have some sympathy with that - I wonder to how many decimal places it is random? Probably quantized IMO. Have not given it much thought yet so I am out on a limb as are you. (We are not really allowed to speculate much in this particular forum I believe). I have a suggestion but it would be deleted for being speculative. Email me to find out.
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