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rkn
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Hi,
Lately I've been reading up on quantum entanglement, Bell's inequality, and EPR experiments. My question is fundamental, but I'm asking it here because I haven't yet read an explanation that answers this question (that I recall).
Take this typical description of what goes on in an EPR experiment (http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/bell.html):
"Let's suppose that we get a green light [one of two possible outcomes, red or green- e.g., spin up or spin down], so we know electron A is pointing in direction 3. Because of the way the electrons are entangled we now also know that electron B is pointing directly away from direction 3. This is where the spooky action at a distance comes in. The fact that we chose to measure electron A in this particular direction not only affected that electron, it also forced electron B to be pointing exactly towards or away from this direction! So what results do we get at detector B? If detector B is set to position 3 then it will definitely flash green. (Remember that green means opposite things at the two detectors.)"
To say that electron A "forced electron B" to point in the opposite direction means that, at some point after the entangled particles got split apart, nothing in their previous entangled state could be determining how they will be detected later at the detectors.
If this were not the case, then A doesn't "force" B into any position- B's position is the opposite of A's due to the fact that the sum of their spins must = 0 since they were just split apart from a coupled state. On this view, once you detect A's position, precisely what one would expect is for B to be detected in the opposite direction. ??
But clearly this is not the correct understanding, since "spooky action at a distance" entails that at some point the particles, after splitting apart, are no longer causally/deterministically "attached" to their previous state.
Can someone explain why the particles (photons, electrons) are not determined by their previous states- that is, by being coupled and then split apart? Especially since some sort of determinism must be operative if physicists know that, even after being split apart, their spins will = 0 (thus knowing the spin of one, you can know the spin of the other)?
I'm guessing also that what must come into play here is the fact that two different measurements can be made at each detector- one along the x-axis and one along the y-axis (I don't know what this mathematical abstraction represents in reality). Such that when Alice takes a measurement on the x-axis, somehow that "spookily" affects even the y-measurement at Bob's detector (in what way his detector is affected after Alice makes a measurement I know not).
Eek, I don't understand what's going on here (clearly). Can someone please explain what exactly is "spooky" about these experiments? Some enlightenment would be greatly appreciated! Please be gentle on my non-physicist mental capabilities.
Lately I've been reading up on quantum entanglement, Bell's inequality, and EPR experiments. My question is fundamental, but I'm asking it here because I haven't yet read an explanation that answers this question (that I recall).
Take this typical description of what goes on in an EPR experiment (http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/bell.html):
"Let's suppose that we get a green light [one of two possible outcomes, red or green- e.g., spin up or spin down], so we know electron A is pointing in direction 3. Because of the way the electrons are entangled we now also know that electron B is pointing directly away from direction 3. This is where the spooky action at a distance comes in. The fact that we chose to measure electron A in this particular direction not only affected that electron, it also forced electron B to be pointing exactly towards or away from this direction! So what results do we get at detector B? If detector B is set to position 3 then it will definitely flash green. (Remember that green means opposite things at the two detectors.)"
To say that electron A "forced electron B" to point in the opposite direction means that, at some point after the entangled particles got split apart, nothing in their previous entangled state could be determining how they will be detected later at the detectors.
If this were not the case, then A doesn't "force" B into any position- B's position is the opposite of A's due to the fact that the sum of their spins must = 0 since they were just split apart from a coupled state. On this view, once you detect A's position, precisely what one would expect is for B to be detected in the opposite direction. ??
But clearly this is not the correct understanding, since "spooky action at a distance" entails that at some point the particles, after splitting apart, are no longer causally/deterministically "attached" to their previous state.
Can someone explain why the particles (photons, electrons) are not determined by their previous states- that is, by being coupled and then split apart? Especially since some sort of determinism must be operative if physicists know that, even after being split apart, their spins will = 0 (thus knowing the spin of one, you can know the spin of the other)?
I'm guessing also that what must come into play here is the fact that two different measurements can be made at each detector- one along the x-axis and one along the y-axis (I don't know what this mathematical abstraction represents in reality). Such that when Alice takes a measurement on the x-axis, somehow that "spookily" affects even the y-measurement at Bob's detector (in what way his detector is affected after Alice makes a measurement I know not).
Eek, I don't understand what's going on here (clearly). Can someone please explain what exactly is "spooky" about these experiments? Some enlightenment would be greatly appreciated! Please be gentle on my non-physicist mental capabilities.
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