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Comment by gus_massa | Hacker News Reader

I really don't understand this sentence:

> So Anne takes her bit, A, and puts it through her handy entanglement-decoding machine, which spits out an answer: either B or C.

(Note: I have a Ph.D. in Math and half a major in Physics that include a few quantum mechanics curses (for example, with the Sakurai book))

What is a entanglement-decoding machine? You cant use entanglement to send information. Just repeat after me: You cant use entanglement to send information. This is one of the most common misunderstandings about entanglement.

When you make the measurement and get the collapse, you only get a random value (with a weighted probability). It's not possible to use that to get information about B or C.

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half a major in Physics that include a few quantum mechanics curses

I'm sure you meant "courses" but I like it more the way you wrote it.

The article doesn't say that information from B or C is useful. All it explains is only 2 particles can be entangled with each other. So either A is entangles with B or it is entangled with C, not both. Which means that either Anne's reality is correct, or yours. The whole thing about the decoding machine is just described to oversimplify it for the benefit of the masses.

Near the bottom of the article:

> One clue might lie in Anne's decoding machine. Figuring out which other bit of information A is entangled with is an extraordinarily complicated problem. [...] In 2013 they calculated that, even given the fastest computer that the laws of physics would allow, it would take Anne an extraordinarily long time to decode the entanglement. By the time she had an answer, the black hole would have long evaporated, disappearing from the universe and taking with it the threat of a deadly firewall.

The "machine" is an oversimplification, but I don't understand the equivalent experiment that can distinguish between B and C.

Also, there are more complicated situations. You can entangle 3, 4, ... or more particles http://en.wikipedia.org/wiki/Multipartite_entanglement

Usually the discussions are only about entanglement of 2particles, because that is weird enough, but the math for more particles is just a little more complicated.

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