# Who are “we”, and what counts as a “question” in consistent histories?

If the preferred basis in quantum mechanics and/or choice of consistent histories in consistent histories is arbitrary, and can only be determined by the "questions we ask", just who exactly is this nebulous "we", and what counts or doesn't count as a "question"?

This is a dead serious question.

PS: Suppose we are closed (i.e. no external interactions) quantum simulations in a mega quantum computer. We are simulated without being measured or observed or decohered in any way at the "substrate level" of our programmer/sysadmin gods. Then, these gods decide to unsimulate and uncompute the entire simulation. Suppose we asked some questions before being "uncomputed". Are these questions wiped out after uncomputation? These "gods" don't know what we asked or observed precisely because they avoid observing/measuring us in any way.

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You might be dead serious, but none of these sentences are understandable to me. – hwlau Jan 8 at 10:44
@hwlau the first sentence contains a good serious question which has a well defined answer from considering quantum mechanics, the second paragraph starting with PS: confuses me a bit too ... – Dilaton Jan 8 at 11:33

## 1 Answer

The consistent histories approach to quantum mechanics doesn't require the word "we" or any similar philosophical word to be defined. Instead, it defines what a history is (a sequence of projection operators at different times) and what it means for pairs of histories to be consistent (an orthogonality condition of a sort). That's enough. The dynamical maths of quantum mechanics can then be used to calculate the probabilities of different histories.

People's formulations only used the word "we" associated with a set of consistent histories for a particular set of consistent histories that (incidentally) "we" or one of us could find relevant or helpful to do planning of anything etc. However, the rules of the consistent histories approach do not require any consistent histories to be "relevant" or "important" (for anyone or anything, whether it's "we" or "them" or anything else), so there's no need to define "us".

The consistent histories approach, as well as any meaningful Copenhagen-like interpretation of quantum mechanics, rules out the possibility that we're a "classical simulation" of the quantum system (and I guess you meant a "classical" simulation). Our world is genuinely and fundamentally quantum and this fact is indeed very important for the consistent histories formalism to make sense. A classical simulation always envisions just one set of observables, one sets of questions that fundamentally make sense. But it's a genuine and true feature of quantum mechanics that is made plain obvious in the consistent histories approach that there can be many ways to choose the set of consistent histories and none of them is "objectively better" than others.

The closest question to yours that could be a genuine concern is how the consistent histories approach guarantees the consistency between the conclusions of different observers – different sets of consistent histories. But it does guarantee that. As long as two sets of consistent histories contain some questions that may be answered in both sets, the mathematical formalism guarantees that the predicted probabilities will match independently of which set of consistent histories we choose. That's ultimately guaranteed by the "consistency condition", after all.

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 I have moved this discussion over to this chat room, please continue there :) – Manishearth♦ Jan 8 at 14:12 Apologies, Manishearth, I won't because my continued answers here weren't really addressed to Kumar himself or herself. They were addressed to those whom I believe would learn something out of the comments, i.e. other visitors of this page. I think it's a waste of time to continue in infinite discussions with one person who isn't willing to understand. – Luboš Motl Jan 8 at 14:32 @LuboMotl: No need to apologize -- in this case you couldn't have done much (the user was a one-rep user, so he can't access chat unless a moderator lets him; so moving the discussion to chat yourself was out of the question) except flag the post asking for one of us to help (though the post is autoflagged at 20-ish comments). You don't have to continue the infinite discussions, stepping away is a perfectly valid response. If the user keeps troubling you after you have stepped away, flag asking for us to deal with it :) – Manishearth♦ Jan 8 at 14:42 Regarding They were addressed to those whom I believe would learn something out of the comments, i.e. other visitors of this page. -- when you want to address page visitors, edit the stuff into your answer somehow. Comments are (a) mostly hidden, and (b) liable to deletion at any time -- so it's better to incorporate your comments into the post itself. After all, that is the primary use of comments-- to get a post improved. – Manishearth♦ Jan 8 at 14:44 Hi @LubošMotl Lumo, maybe it would help if you could add a link to your nice slightly technical TRF article about consistent histories into this answer ... ;-) – Dilaton Jan 8 at 21:02