czetie: Felgraf: lifeboat: But I thought I've also read that an entangled particle can be manipulated -- you can change the spin, for example -- and the other particle will follow suit instantly. Does this not imply FTL communication? Could you not devise an apparatus that communicates intentional changes to a particle's state?This is the part that's wrong. This *IS* how it works in sci-fi.. but, as it turns out, this isn't how it works out in reality.This is where a lot of the confusion comes from.What he said, except even more so. Suppose you create two entangled electrons, and send one to Alice and the other to Bob. Because electrons are quantum particles they always have a spin of exactly plus or minus 1 (for appropriate choice of units). If Alice measures her electron along the up/down axis and gets +1 in the "up" direction she can confidently state that Bob will get "down" (or -1) from the same experiment, because they are entangled. So far so good.However here's the confusing part where quantum mechanics completely departs from any classical analogy. Alice measuring her electron's spin as up does not cause Bob's electron to have spin down in any classical sense. Bob can still measure the spin of his electron along any axis he chooses. If he happens to choose the same up/down axis -- perhaps by prior arrangement with Alice -- he will certainly get down as his result. But if he chooses some other axis, something totally non-classical will happen. He will measure plus or minus 1 along whatever axis he measures on -- because any measurement of an electron's spin always results in plus or minus one. Alice measures Spin Up, but Bob does not measure Spin Down.Note that this is entirely different from what would happen if we had two classical objects, say two basketballs spinning in opposite directions. If Bob measures the spin of his basketball on some other axis, he will get a number that is less than 1 (and maybe even 0 if he measures perpendicular to the up/dow ...
kahnzo: czetie: Memo to everybody: if your supposed explanation of Schrodinger's thought experiment includes something called a "measurement" (or an "observation"), it's already wrong. And if it includes "wave function collapse", it's doubly wrong.None of these things has any definition or description in quantum theory, they are just useful approximations for describing what is happening in most practical circumstances. But you can't use them to probe the foundations of QM; that requires you to be precise.So what's your preference? Ensemble? Transactional? Many Worlds? Consistent Histories?You can't say "You can't do that!" and then lift your skinny fists like tiny antennas to heaven.The problem doesn't go away because we can do things with more precision. In fact, violations of Bell's Inequality are quite beautifully astounding, but in no way offer a satisfying resolution to the underlying weirdness.
Felgraf: lifeboat: But I thought I've also read that an entangled particle can be manipulated -- you can change the spin, for example -- and the other particle will follow suit instantly. Does this not imply FTL communication? Could you not devise an apparatus that communicates intentional changes to a particle's state?This is the part that's wrong. This *IS* how it works in sci-fi.. but, as it turns out, this isn't how it works out in reality.This is where a lot of the confusion comes from.
Donnchadha: It's really easy to know if the cat is alive or dead -- you look in the damn box.
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