In the answer here and on the wiki article and many other articles it is mention

Question

In the answer here and on the wiki article and many other articles it is mentioned that if one of 2 entangled particles is measured their state collapses according to the Copenhagen interpretation.

Lets take the example from the EPR paradox article, which mentions a positron and an electron occupying quantum states and becoming entangled. There are two observers, Alice and Bob.

In state I, the electron has spin pointing upward along the z-axis (+z) and the positron has spin pointing downward along the z-axis (-z).

In state II they are opposite.

Alice now measures the spin along the z-axis. She can obtain one of two possible outcomes: +z or -z. Suppose she gets +z. According to the Copenhagen interpretation of quantum mechanics, the quantum state of the system collapses into state I.

Meaning if Bob measures the spin, he will get -z.

My question is, what happens if Bob or Alice measure the spin along the z-axis again, will it remain z+ for Alice and z- for Bob or can it change between measurements?

Explanation / Answer

Yes, if you measure the spin again and assuming the absence of a magnetic field, the measured value of jz of a particular electron will be the same as it was after the latest measurement of the same quantity

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