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Comment by friendzis | Hacker News Reader

My go-to ELI5 explanation for entanglement: suppose you take an object, cut it along non-symmetry axis (e.g. cut a bill), place both halves in different envelopes. Upon opening one envelope (measuring) you immediately know what is in the other envelope. Not because of "spooky action at a distance", but rather due to prior knowledge of the system[s].

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That is not correct though, at least not in the Copenhagen interpretation.

The reason is that the quantum state is supposed to be a complete representation of the system right now. It doesn't quantify our ignorance, it should actually correspond completely to the physical state of the system (check the PBR theorem for a strong justification of this)

Therefore, according to Copenhagen, the state really is collapsed for the other system upon measurement of the first. There is spooky action at a distance, because the entangled state does not encode our lack of information or ignorance, it encodes an actual physical thing.

The most common example is two entangled spin systems A and B in an initial state:

|psi> = 1/sqrt(2) (|upA upB> + |downA downB>)

if you then measure the spin of A and find it to be |upA>, you then know system B is in |upB>

BUT it wasn't |upB> all along, because |psi> before measurement is the complete physical state of the quantum system. |psi> is not an epistemological construct, it is an ontological one. It does not correspond to our lack of knowledge of the system, but the actual physical system.

so god doesn't following best practices and uses global interdependent variables. maybe our universe was a PoC and the Universe 2.0 is going to be well refactored version.

Not quite, entanglement is indeed spooky action at a distance given bell’s inequality theorem showed that it can’t be local hidden variables (prior knowledge)

Also not quite, since that depends on your interpretation of quantum mechanics. For example, in the Many Worlds Interpretation there is not spooky action at a distance, since there is no wavefunction collapse in the first place, i.e. measurement does not cause a projection operator to occur, but instead occurs through unitary decoherence. Therefore, no spooky action at a distance (although it is still non local)

Yeah, each valid interpretation is consistent with the observations, but each is weird in its own way (usually some form of non-local, sometimes more out there like superdeterminism).

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