Hi all. I'm having trouble 'getting' entanglement. We were shown the EPR paradox in a lecture once, and I didn't get it. We've also been shown quantum computing, and there was something (can't remember the details) that required an understanding of entanglement, and I didn't get it. It's a little embarrassing to be at third year level and not get it. I've been trying to figure it out by reading stuff about it. Here is one main aspect I don't understand: http://en.wikipedia.org/wiki/EPR_paradox I don't get why we would expect Bob to get an answer with absolute certainty when measuring x-spin. Prior to measuring Alice's z-spin, Bob's x-spin is unknown, and therefore + or - is equally likely. AFTER measuring Alice's z-spin, the same is true. So measuring Alice's z-spin has no effect on Bob's x-spin. Only on his z-spin. If the z component of both Alice and Bob's particle are determined at creation - and opposite - then when we measure Alice's z-spin, we know Bob's. If we then measure Bob's x-spin, it will have a fifty fifty chance of being + or -. The way I have stated it, does NOT imply non-locality, therefore I am misunderstanding some aspect. Can anyone explain what I am missing? (EDIT: I should point out that I realise that it is understood that the spins are not determined on creation, but actually at measurement, I just don't understand how this conclusion has been arrived at. ie How does the observation that the x-component of B is random suggest that it 'knows' about the measurement made on A?) (EDIT 2: Also, I don't understand what it means that B's x-spin is "out of bounds". It says that you can measure it and it has equal probability of being measured in each state. So how is it out of bounds? If you measure A's z-spin and then B's x-spin you know the x and z spin of both particles, violating HUP, so again I am missing something.) (yet another EDIT: From http://en.wikipedia.org/wiki/Quantum_entanglement Suppose I put one red and one blue marble in a black felt bag. I shake the bag around to mix them up. Blindfolded, I remove one marble and put it in another black felt back. I give one bag to Alice. If I open my bag, I have a fifty-fifty chance of getting either red or blue. If, however, I ask Alice to look in her bag and she tells me she has a blue marble, I know with 100% certainty that my marble is red. I open my bag and, indeed, it is red. Were the marbles entangled? Has the measurement on system A altered the outcome on system B? No. The outcome is the same as in the quote, but the interpretation is different. What am I missing?