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by fizx 3864 days ago
Say we're measuring spins of entangled particles with random entangled spins.

So the real problem isn't that "when one is up, then the other will magically be up too." That could be accomplished with local hidden variables (e.g. shared seeds on a PRNG, or your examples).

The real problem is that when you measure A in the "up" direction, and then B in the "10 degrees east of up" direction, then B seems to know that you measured A in the "up" direction.

That is to say: B's probability distribution as a function of the direction its being measured is correlated to the direction that A is measured. There's no way to construct an "A-independent" probability distribution of B's results for arbitrary directions. The probabilities won't sum to 1 and still match experimental results.

It's unfortunate that "A up" therefore "B up" is a degenerate case of this reality where classicality actually works, because it leads to confusion.

(Also the reason you can't use this magic to communicate FTL is that you can only ask one yes/no question of each particle, and because B's probability distribution is distorted in a symmetric way based upon A's measurement, you're still going to get a 50/50 response for yes/no questions asked of random entangled particles)

Feel free to comment, as I paste this on every misunderstanding of Bell's Theorem here, and I edit to make more clear each time.

1 comments

What if I have a million A/B pairs, measured in parallel. Each bit is 100 A measurements, 1 for up and 0 for down. Why is this not FTL communication of 10k bits?
Because you can't use it to pass information to someone listening to only a single end of the pair. You have to measure both particles to detect the correlation, or else all you're getting is random noise.

Imagine I'm flashing a light at you on and off, randomly. Some of the flashes aren't random and contain a message. To read the message, or even detect its existence, you have to know which flashes were random, and this is what observing both particles in an entangled pair allows us to do. There is no signal with only a single particle.

It's FTL communication of 10k random bits, whatever that gets you besides a great symmetric encryption key to share between you and the other end.