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Originally Posted by CraigS
Hmm .. thanks Rob and Steven;
It seems one needs to be careful about what one reads on this subject.
Wiki's opening paragraph on entanglement is kind of where I was coming from in my last post to Steven, (although, I have read this interpretation elsewhere):
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Quantum entanglement, also called the quantum non-local connection, is a property of certain states of a quantum system containing two or more distinct objects, in which the information describing the objects is inextricably linked such that performing a measurement on one immediately alters properties of the other, even when separated at arbitrary distances.
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However, further down it says:
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When each of the particles in the entangled pair is measured in the same way, the results of their spin measurement will be correlated. Measuring one member of the pair tells you what the spin of the other member is without actually measuring its spin
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There's a big difference between saying one's spin is correlated with the other, and saying that the measurement itself alters the other's spin.
So which one should I believe ?
Cheers |
Craig,
I think the first quote is a bit vague and the term properties is perhaps not the best description.
The first quote is similiar to my previous post. Before a measurement is made the entangled state is a superimposed state of various possible outcomes. Measuring a state of a particle, also changes the state of the unmeasured partcle. You can't have the measured particle in a particular state and the unmeasured particle still in a superimposed state. Both particles are either in a particular state or both are in a superimposed (unmeasured state). The measurement doesn't have to be spin. It could be for example energy or angular momentum.
The second quote refers to a particular property of spin. For fermions the state is either spin up or spin down. In the entangled state the fermion pair have opposing spins. This is due to the Pauli exclusion principle.
When a measurement is made, if the measured particle is spin up the other particle must be spin down (or vice versa). No measurement of the spin state of the second particle is required if the spin state of the measured particle is known. The spin state of each particle will always be opposite.
Hope this clarifies things.
Steven