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Generally Quantum mechanics divides a system what is to be observed and an observer. This is generally taken to be some human being. But why restrict it to such? Why not a particle?

Is there a good physical reason or philosophical reason for this to dismissed as not sustainable?

I'm thinking here specifically of the Copenhagen interpretation, or of its modern incarnation, consistent histories. I understand that decoherence in consistent histories completely replaces the idea of the wave collapse in the Copenhagen Interpretation.

Essentially, the idea of observed system and an observer is supplemented with an environment, which on the face of it seems entirely natural. The idea of decoherence comes from statistical physics.

I'm suggesting that a particle that acts like an 'observer' needs to 'know' what state the observed system is in to 'know' how to react to it. Decoherence resolves the superposition of states in the observed system to a probability mixture.

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    $\begingroup$ Quite possibly related: 1, 2 ("An important aspect of the concept of measurement has been clarified in some QM experiments where a single electron proved sufficient as an "observer" — there is no need for a conscious "observer""), 3, 4, 5 $\endgroup$
    – Řídící
    Commented Jul 28, 2013 at 15:17
  • $\begingroup$ Only one question. Isn't an electron in its own rest frame a potential observer? $\endgroup$
    – 71GA
    Commented Jul 28, 2013 at 21:52
  • $\begingroup$ @Gugg that Wikipedia quote seems awfully exaggerated. The research it references proves weak measurements and can be interpreted as disproving the need for a conscious observer, but it doesn't go anywhere near saying that a single electron can act as an observer. $\endgroup$
    – Malabarba
    Commented Jul 28, 2013 at 22:21
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    $\begingroup$ (To the original question ) The formal answer is no, because observers are defined as conscious. The practical answer is no, because decoherence of the wave function requires interaction with a very large system (and evidence has been showing that collapse just might arise as an approximate consequence of that interaction). $\endgroup$
    – Malabarba
    Commented Jul 28, 2013 at 22:30
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    $\begingroup$ How about this as an alternative to the question: What is the simplest system that can be an observer? Can such system be "unconscious": for example, has anyone thought up a quantum observer automaton, analogously with the finite state machine Maxwell Daemons considered by Bennett and others. Or is anyone doing research into such constructions? This would seem to be a wonderful research topic, if in the unlikelihood that someone isn't doing it already. $\endgroup$
    – Selene Routley
    Commented Jul 29, 2013 at 0:19

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This depends on the placement of the Heisenberg cut. The placement of Heisenberg cut is arbitrary (but certain choices would be called different interpretations of quantum mechanics). Everything beyond the Heisenberg cut (on the side of Heisenberg cut opposite to the observed system) can work as the observer.

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Since this question has come up to the front again, I think it involves the navel gazing of people discussing philosophy of quantum mechanics.

In particle physics , all the experimental results on which the standard model is validated happens with "particles in --- particles out" , to get the crossections and decays. The primary interaction is the observer. Certainly it is not the data banks where the interaction's measurements are held, nor the physicists that gather the thousands and millions of interactions to check the quantum mechanical probability distributions for crossections and decays.

In the interaction region there are only particles and fields, no human observers observe each event as it happens, it is the motion of the incoming particles, protons on protons, that creates the interactions at LHC.

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Whether a particle can be an observer or not is partially a question of which interpretation of QM is being considered and partially a semantic question (how we define “observer”).

The viewpoint you describe in the question is closest to Rovelli’s relational quantum mechanics.

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I think that in QM one usually refers to the observer as a human, since is a human who reads the result on the screen of the experimental device but, fundamentally, for anything more. When a human observe a quantum mechanical system, it does through fields and particles, not with his mind. The fact that a human observe spin up or down in a Stern-Gerlach experiment is indeed the fact that the electron interacts with a magnetic field and then it passes through a screen. Where is the human? Of course, there exist a philosophical question: if the world exists if there isn't anybody looking at it. I suppose the answer could depend on who answers the question, but of course I don's think that human beings have any special role in the universe and, of course, not in the physical laws. So i'd say definitely no, QM doesn't need humans.

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    $\begingroup$ You can formulate this much more succinctly: a measurement requires irreversibility (otherwise there can be no lasting record). That way all mention of humans can be avoided. $\endgroup$
    – CuriousOne
    Commented Jul 21, 2016 at 21:36
  • $\begingroup$ @CuriousOne You cannot possibly form a testable theory of Nature which is both useful and does not at least implicitly involve humans running experiments. $\endgroup$
    – DanielSank
    Commented Jul 22, 2016 at 8:52

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