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I read an article that referred to the idea that a double slit experiment near the event horizon of a black hole observed by someone inside the black hole creates a paradox because the inside observer can break the interference pattern, thereby bringing information of their presence out of the black hole.

This seems odd to me.

The significant part of "observation" here, as far as I understand, is that it requires interaction. The inside observer can't interact with the particle to measure it's position. He can only detect it's position if something outside the black hole interacted with the particle, which means the superposition would have collapsed whether or not he was there.

Am I missing something?

Edit: Link: https://www.sciencenews.org/article/black-hole-paradoxes-quantum-states

Qmechanic
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Lucca Fehn
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  • As observed from outside, the falling observer, Bob, never crosses the horizon, but remains outside forever. So do the particles from the double-slit. Thus, Bob cannot change anything from inside the horizon for as long as the external universe still exists (which is forever in the Schwarzschild solution). The "paradox" is made up based on typical misconceptions widespread among so-called "scientists". – safesphere May 07 '23 at 07:59

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The Science News article is about a recent talk, which is about a paper which was published last year. In the paper, there's no mention of a double-slit experiment or of a paradox. Glancing through an earlier version of the talk that I found on YouTube, I don't see a double-slit experiment mentioned either. So it was probably made up for the Science News article.

The idea behind the alleged paradox is that even though Bob (the one inside the black hole) doesn't directly interact with the particle, he can work out where it is by looking at its long-range electric or gravitational field. If Bob can affect what Alice sees by doing that, it would allow for faster-than-light communication. If Bob can't affect what Alice sees, but Alice sees an interference pattern, then Alice and Bob end up with both an interference pattern and which-path information, which should be impossible. Therefore (they argue), it must be that Alice sees no interference pattern whether Bob is there or not.

That argument clearly can't be correct, because it would apply to every double-slit experiment, and we'd never see evidence that quantum mechanics is true. There is nothing about the argument that is specific to black holes. The reason you can't send information out of a black hole is the same reason you can't send information faster than light in any other circumstance.

What the argument really shows is that, in cases where Alice sees an interference pattern, it must be impossible for Bob (or anyone) to obtain which-path information by looking at the distant field. There are some previous Q&As on that subject; here's one: Why aren't particles constantly "measured" by the whole universe?

The actual paper makes a different argument with a more plausible conclusion: that information unavoidably leaks into a black hole at a rate (in quanta per unit time) that depends on the black hole mass and distance. That said, it looks like the rate (equation 14) goes to infinity in the infinite-black-hole-mass limit, which would be a problem since that limit is flat spacetime. But I may be misinterpreting it.

benrg
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  • "The reason you can't send information out of a black hole is the same reason you can't send information faster than light in any other circumstance." - While technically not wrong, FTL is non-physical and confusing to non-specialists who may get an impression that it is an actual possibility. A much better way to say exactly the same, except clearer and without causing a confusion, is, "the reason you can't send information out of a black hole is because the interior is in the future relative to anything outside". – safesphere May 07 '23 at 08:10