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According to some interpretations of quantum mechanics, in our daily life we perceive classical reality because we, as the observers, collapse the wave function to perceive the environment around us. So in essence we, the observers play a crucial and central role in the universe, in a way, we help create the reality.

But what can be said about the unobserved part of the universe, the part from which light has yet to reach us? Are the particles beyond the observable universe (beyond the 96 billion miles diameter range) in a state of super position because they have not been observed yet? Or did the wave function collapsed because the particles in the observed universe somehow interacted with the unobserved universe? And what about the parts of the universe that we will never observe?

closed as off-topic by Alexander S King, Dave, James Kingsbery, Rex Kerr, Keelan Dec 16 '15 at 22:41

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  • "While this question may be related to philosophy or occur in a philosophical context, the question itself doesn't seem to be about philosophy, and is therefore not a good fit for our site." – Dave, James Kingsbery, Rex Kerr, Keelan
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    This seems to be a physics question more than a philosophy question. – Alexander S King Dec 16 '15 at 18:51
  • This seems a good moment for xkcd.com/1240. – Keelan Dec 16 '15 at 22:42
  • @Keelan Physics does not concern itself with how observers create reality, unobservable parts of the universe, or metaphysical speculations in quantum mechanical interpretations. I doubt Physics SE can do much for this question, I also do not see the relevance of the link. – Conifold Dec 16 '15 at 23:05
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    @AlexanderSKing I initially did post this question in the physics SE, but got a response saying that its more of a philosophical question since physics cannot explain what it can never see. – opensourcegeek Dec 17 '15 at 5:58
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    @JoWehler you're basically (almost) always voting to leave open or to reopen. I don't think you need to leave a comment every time. – Keelan Dec 17 '15 at 9:19
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According to modern physics, all of the universe is quantum mechanical (or quantum theoretic rather), but macroscopic universe, observable or not, is largely classical. That observer's consciousness collapses the wave function is a more poetic version of the dated view of the Copenhagen interpretation advanced by Heisenberg and Wigner in 1950s. The work on decoherence in 1990s showed that observers are not unique in this respect, their role is the same as that played by the environment in their absence. It is a bit anticlimactic, but consciousness plays no special role in limiting macroscopic effects of quantum behavior, if it plays any role at all:"Continuous monitoring enforces environment – induced superselection... Schrodinger cats, Wigner's friends, and, generally, all of the systems which are in principle quantum but sufficiently macroscopic will be forced to behave in accord with classical mechanics as a result of the environment - induced superselection".

Zurek gives a striking example of what would have happened to the Solar system if observers were indeed the ones collapsing the wave function:"According to the Schrodinger equation, less than a billion years after its formation the behavior of the solar system should be flagrantly non-classical, with the quantum states of celestial bodies spread over dimensions comparable with the sizes of their orbits, and with the planetary dynamics no longer in accord with the laws of Newton!" The reason is similar to the Schrodinger cat's predicament of subsisting in a superposition of dead and alive states until an observer opens the box. But as Zurek points out "in the case of the cat it was possible to entertain the notion that the (admittedly preposterous) final superposition of dead and alive cat could be avoided if the process of measurement was properly understood. This “way out” is no longer available in the case of celestial bodies we are discussing".

What plays the observer's role in the case of the Solar system is the interplanetary gas, it is constantly "monitoring" its state and the correlations this introduces quickly destroy quantum superpositions of coherent states turning them into decohered classical states:"for a planet of the size of Jupiter a chaotic instability on the four million year timescale and the consequent delocalization would be easily halted even by a very rarefied medium (0.1 atoms/cm3, comparable to the density of interplanetary gas in the vicinity of massive outer planets)".

  • I've always felt that decoherence side steps the issue of wave function collapse more than it explains it. Decoherence doesn't explain explicit cases of observer induced collapse ( like the double slit experiment). – Alexander S King Dec 16 '15 at 20:38
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    @Alexander S King Double slit collapse is not observer induced, or rather QM is agnostic on the issue. The "Heisenberg cut" of collapse can be placed anywhere at the detector, the screen, observer's retina or observer's consciousness, QM predictions are the same. It was Heisenberg's fancy to get consciousness involved. Decoherence theory (interpretation neutral) shows that its purported role can be played not only by measuring apparatus, but by any classical system. But as von Neumann noted, "the cut can be moved but it can not be removed", and decoherence does not help with that. – Conifold Dec 16 '15 at 22:55
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    @opensourcegeek I do not know who the downvoters are, so I can only speculate, but quen_tin described in another thread some reasons for philosophical distaste of quantum theory by many contemporary philosophers philosophy.stackexchange.com/questions/30637/… They may be shared by users on this site. Voting on SE is not regimented so users do not need articulated reasons for it, it can be based on sentiment, personal preference or "gut feeling". – Conifold Dec 18 '15 at 20:58
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    @Conifold I happen to come across this article dailymail.co.uk/sciencetech/article-3107996/…, which seems to confirm that the universe is indeed an illusion and is entirely dependent on the observer which i thought might be the reason for the downvote. What do you think of it, eager to hear your words. :) – opensourcegeek Dec 19 '15 at 13:27
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    @opensourcegeek It'll probably disappoint, but this article describes an old experiment performed with more precision. One of the experimenters says "If you choose to believe that the atom really did take a particular path or paths then you have to accept that a future measurement is affecting the atom's past". Unfortunately, most physicists choose not to believe that, or to liken atoms to billiard balls that take paths. So what they mean by "illusion" is just that matter is made of parts not behaving like daily objects we are used to, and when we try to force it on them they seem like ghosts. – Conifold Dec 22 '15 at 1:16
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The idea that we, conscious observers, collapse the wave function is a specific interpretation of quantum mechanics (proposed for a time by Wigner) and not the most consensual one, because it implies dualism and is close to anti-realism (it's not very far from saying that all reality is mental).

If this interpretation were true, then everything outside our past light cone would be in a superposition of state, unless there are other conscious observers at other places in the universe. Even if unobserved particles interacted with observed one in the past, they would have evolved into superpositions since then.

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A single microcosmic particle is suitably described by a quantum mechanical equation like Schroedinger equation or Dirac equation. The equation is a differential equation for the time development of the state function, in general termed psi-function. The time development is deterministic, but the state function is considered a probability.

The collapse of the wave function is the brutal name for the fact, that observations on the mesocosmic level terminate the unitarian development of the state function and project the state function into one distinguished state, often belonging to a discrete set of possible states. This model (Copenhagen interpretation) solves the problem how to apply quantum mechanics, but it does not explain why nature acts according to these two incompatible descriptions.

In the last decade the effect of decoherence seems to establish a smooth transition from the microcosmic to the mesocosmic description. Continous interactions of the particle with its environment reduce and finally erase the possibility of the state function to interfere with itself and to realize different possibilites. Apparently, this kind of interaction is not bound to the interaction with a human observer. Hence decoherence also holds for particles traveling through the interstellar space, which is not empty.

For a gentle introduction to decoherence see the chapter "Decoherence and Quantum Reality", p. 208ff in Greene, Brian: The Fabric of Cosmos (2004).

For a textbook with contributions from several experts see Giulini, D. et al.: Decoherence and the Appearance of a Classical World in Quantum Theory (1996).

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