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What is quantum entanglement?

  1. Dec 5, 2004 #1
    What is quantum entanglement? why is it "spooky"
  2. jcsd
  3. Dec 5, 2004 #2
    Its been a very long time since I've thought about these things so I'll try to recall what I can - Consider a system of two particles. Measurements on the system effects the entire system, not just part of the system, i.e. not just one particle. Thus, for example, if you have a system of two particles which is in a quantum state for which the spins must always be in opposite states when you measure them then the spin of one particle must be measured to be the opposite of the spin of the other particle and hence the spin of the other particle is determined by the measurement of the spin of the first particle. Since the particles were not in a particluar state before the measurement then eacb particle acquired the particular spin upon measurement. Thus if two particles are seperated by a finite distance and you measure the spin of one then instantaneously the spin of the otherone is determined, i.e. the spin of the other particle falls into a particular, measureable, state. This requires the cause to travel instantaneously and thus faster than the speed of light, contrary to what one would predict from relativity. This means that one can choose a frame of reference in which one measurement occured before the other and you can also choose a frame of reference in which the opposite is true - thus causality gets all messed up.

  4. Dec 5, 2004 #3
    so ur saying that the other particle takes the opposite spin, when measured, but because the uncertianty principle the spin has not already been determined until u measure it? see i don't understand why this is odd, if i were to spin a ball in space (no external forces) and leave when i come back why would it be odd for me to look a the ball as still spinning? like why is measuring it odd?
  5. Dec 6, 2004 #4
    You missed the point. I was talking about two particles. Not one. If you were to flip the light switch on in your room to turn the light bulb on and at the exact same time a light came on at the exact same time in a room 20 light years away as a directy result of you tossing that switch then would you think that was odd?

  6. Dec 6, 2004 #5

    This is a misconception.

    Suppose you have two ensembles each provided with an entangeled state that you describe. Both ensembles are described by precisely the same density matrix. Thus, there is no conceivable measurement that one obersver can do that will distinguish the two ensembles. Also, there is no way the observer 1 can determin what measurement observer 2 performed. The message is unreadable. For example the outcome will be 50% chance of spin UP and 50% change on spin down...

  7. Dec 6, 2004 #6


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    Hey! It is just my imagination or you have returned? Welcome again, boy!!!
  8. Dec 6, 2004 #7
    To elaborate a bit. Let's call observer 1 Alice and observer 2 Bob.

    So both Alice and Bob have many copies of the entangeled state in question. Suppose Bob measures either all of his spins along the z-axis(1) or along the x-axis(2). Then he "calls" Alice and tells her the results of his measurements but without mentioning what he has done. So Bob only gives results : first spin up, second spin down...But he does not tell along what axis... Now Alice performs either (1) or (2) on her spins. If both Alice and Bob measured along the same axis, Alice will find that every single one of her outcomes will agrees with what Bob has sent to her. But if they measured along a different axis, Alice will find no correlation between her results and Bob's. About half of her measurements agree with Bob and the other half won't.

    So Alice does have a way to distinguish Bob's two preparation methods (measuring along x or z -axis), but there is no faster then light communication because Alice had to receive Bob's phonecall before she could perform her test.

  9. Dec 6, 2004 #8
    Whether entanglement is "spooky" or not is actually controversial. :smile:

    The traditional way of looking at experiments is that values are in a superposition until measurement and this makes it appear that one particle deciding a value when measured is influencing the other particle to choose the opposite value since they always match up this way. This gets called "nonlocality" and the matching-up effect can appear to travel faster than light and even instantaneously.

    Einstein sarcastically called this "spooky-action-at-a-distance".

    More recent work by some very accomplished physicists, Gell-Mann and Hartle amongst them, disgrees with the faster-than-light interpretation of the results, however, and they state that that faster-than-light influences are a misinterpretation and don't exist.

    So the spookiness of nonlocality in entanglement is controversial, but most physicists probably aren't aware of the more recent work in this area as interpretation is not really important as calculations work out the same whichever way physicists look at it.

    What isn't controversial is "nonseperability" and that we can't talk about the properties of one particle which is entangled without mentioning the properties of any other particles it is entangled with.

    "Entanglement" is just another way of saying "correlated".
    Last edited: Dec 6, 2004
  10. Dec 6, 2004 #9
    Don't think of quantum entanglement as too much like everyday correlation though...
  11. Dec 6, 2004 #10
    Then the interesting question is: when can we say that two systems are NOT entangled?
  12. Dec 6, 2004 #11
    ok I understand the idea that measuring one influences the other at a great distance, however say in a pair production in space, the two particles move away from each other same mass diffrent charges, both with some spin that is undecided and unknown until measured. What I don't get is sence with no external forces on the system how is it odd that measuring one means the opposite on the other, wouldn't we expect that when they are created to just remain the same? Or are you trying to show me that when its made its both spinning up and down but measuring it directly forces the other one to be the opposite in an instant. Obviously I have very little quatum education. sorry...
  13. Dec 7, 2004 #12
    Doesn't this come back to the way quantum statistics differ from everyday statistics? In the traditional interpretation, the superposition of states is measurably different from one being up and one being down. Both have to be considered up and down at the same time. It seems "spooky" that measuring one then has an effect on the other one as well, regardless of distance. There are other interpretations I know, but I fancy Einstein might have found them a bit "spooky" too.
  14. Dec 7, 2004 #13
    That sounds like what I understand it to mean, that the usual way of looking at it is that both particles match up when measured for the same property but which matching set is going to happen isn't decided until one particle is measured.

    It's a bit like you'll get A+ and B- or A- and B+ but it isn't decided which matching A and B you'll get until measurement.
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