Orthodox quantum mechanics intrinsically requires an observer - since the only connection from the mathematical formalism to physical reality is through the measurement axiom (probability of observing something given via Born rule, etc.). Otherwise, no assumptions about physical reality are made, and QM enables (maybe even encourages -- e.g. Bell's theorem) one to take a non-realist position, namely that there exists no independent (of observation) reality out there.

Given such a position though, what meaning do claims about the distance future such as "The universe will end in a heat death" have? By their very premise, there can be no possible observers left at that time - but then, who is doing the measuring? Or, put differently, if there is no possible way to observe a prediction, then according to a non-realist view such a prediction should be meaningless.

Are we then to assume that all such predictions about the distant future when there will be no observers left, are made in a realist context? But even then, strictly speaking, if there is no one left to verify the prediction, it is untestable - and so in some sense epistemologically meaningless (and is in the same equivalence class as claiming, say, that "the Earth is supported by a giant invisible turtle").

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    $\begingroup$ Orthodox quantum mechanics intrinsically requires an observer - I don't think this is necessarily true. Your favoured interpretation of QM may require an observer to collapse superpositions, but this is not true of all interpretations. $\endgroup$ Dec 22 '13 at 19:38
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    $\begingroup$ In QM an observer can be almost everything not just a person. Anyway, most of the things science describes are not directly observed, so your deduction is wrong. $\endgroup$
    – jinawee
    Dec 22 '13 at 19:40
  • $\begingroup$ The universe itself is doing the measuring. As @jinawee pointed out in the response above, decoherence can happen (and usually does - I am not aware of any experiment allowing us to observe it as it happens, yet) way before an actual person does the measuring. As such, other boson, fermions, atoms, or "almost everything" in the universe can, and does, the "observation", long before and after us humans. $\endgroup$ Dec 23 '13 at 1:18
  • $\begingroup$ For a good (IMO) explanation of QM and conscious observers, see physics.stackexchange.com/questions/48383/… $\endgroup$ Dec 23 '13 at 2:35
  • $\begingroup$ @JohnRennie : Right, but I was specifically talking about non realist interpretations - which do require a subjective observer to collapse the superpositions. $\endgroup$
    – user114806
    Dec 23 '13 at 4:00

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