lkm0

No worries, explaining helps my understanding as well.

The new thing you're mentioning is a common observation about observers: if there are two cats in the box, should they be |dead dead> + |alive alive> or is one measuring the other and they've collapsed to a classical state? This depends on a person being outside the box; the scenario is called Wigner's friend. It's been used to show many other unintuitive aspects of quantum physics.

Hidden variables are more about finding a way to derandomize quantum physics. The state |00> + |11> can collapse to |00> or |11> when measured. It appears to be that this randomness is baked into reality (God, meet dice, etc), and many people find this a bit distasteful, because it means measurements propagate faster than light (even if Alice and Bob can't communicate that info). If it turns out the theory is incomplete and there are missing/hidden variables, then it means whether it was |00> or |11> was determined from the start, before the measurement was made, and there's no faster-than-light interaction. Experiments around Bell's inequality prove that hidden variables don't exist, unless we start getting into scenarios that are as weird as entanglement itself. So the cat is truly alive and dead at the same time, and superposition is physically happening; this property is called quantum contextuality and is related to Bell/etc but still another discussion.

The important distinction in this case is about FTL existing but not being able to exploit it for FTL communication.

Yes, Alice knows what Bob's measurement is going to be, but only if they agreed beforehand on what state to prepare ("let's make the |00> + |11> state"). In that case no information traveled because what Alice learned came from the previous discussion (which presumably happened in the break room, sublight speed). If they prepared a random state, then there is no way for Alice to know what the second spin is only using measurements on the first.

One of the "coping mechanisms" is to say that information cannot travel faster than light, even if entanglement is propagated instantaneously and measurement acts faster than light. If Alice and Bob share an entangled pair of spins and Alice measures it, Bob has gained no information until Alice communicates (sublight speed) her result. No new information is shared at measurement time. This is known as the no-communication theorem.

"IS QUANTUM PHYSICS CORRECT AND COMPLETE?" is reductive, there are more quantum denominations: Copenhagen, QBism, RQM, consistent histories, Many-worlds, Bohmian mechanics, and various spontaneous collapse people. I might've missed a few.

One application that sounds pretty interesting would be the creation of wikidata pages for anything. Plug a topic/word/concept/historical event in, take a bunch of wikidata properties, rephrase them as questions with the choices being the existing property values. Then feed it to LLMs or something. Does that make them more reliable? Probably not.

Immensely disappointed; I expected an actual cookbook.

I've always wanted to do this! I've scraped Gutenberg and tried some clever ways to get the first line, but I always got so much noise. Perhaps it's a good time to try again!