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So I am wondering over the Copenhagen Interpretation how can a decoherence or measurement ever happen. If in the beginning of the universe everything was in a state of coherence being in superposition of many states then all the interaction that would happen would only result in more superposed quantum states. If so, how can quantum decoherence ever happen if things are continuously in superposition ? For example if I observe or measure the spin of entangled particles my state will then be in superposition of whether yes or no I actually look at the particles and then more superposition state will appear... if we want to introduce decoherence in superposed states then the medium that makes the measurement has to be decoherent or consequently more superposition will arise. Given this how could quantum decoherence happen in the beginning of the universe if everything was actually coherent (in superposition state)

Edit: I think I may have mixed the concepts decoherence with collapse which in this case I am referring to collapse because I am also referring to the Copenhagen Interpretation

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  • $\begingroup$ What is the. difference between a "state" and a "superposed state"? $\endgroup$
    – WillO
    Commented Jul 17, 2018 at 4:32
  • $\begingroup$ By single state I mean eigenstate which has only a single value for example the cat is dead but superposition state is dead and alive at the same time $\endgroup$
    – user198045
    Commented Jul 17, 2018 at 4:34
  • $\begingroup$ But every state is an eigenstate of some operator (e.g. the. projection onto it) and of course every state has a single value (i.e. itself). $\endgroup$
    – WillO
    Commented Jul 17, 2018 at 4:38
  • $\begingroup$ So do you mean that there is no real superposition ? $\endgroup$
    – user198045
    Commented Jul 17, 2018 at 4:40
  • $\begingroup$ Every integer is a sum of integers. Does it follow that there is "no real addition"? $\endgroup$
    – WillO
    Commented Jul 17, 2018 at 4:41

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Decoherence doesn't eliminate superpositions. Decoherence is a result of information spreading from the interfering system to other systems:

https://arxiv.org/abs/1212.3245

This process suppresses interference, it doesn't eliminate branches of the wave function. As such, whether the early universe is decoherent or not is totally irrelevant since decoherence doesn't eliminate branches. If you want a physical process that eliminates other branches, you have to modify quantum mechanics along the lines of spontaneous collapse theories or something like that.

There is a large literature on how to understand quantum mechanics without invoking collapse. See this paper and references therein:

https://arxiv.org/abs/1111.2189

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  • $\begingroup$ Yes I think I may have mixed the concepts "collapse" and "decoherence" I think the two are pretty related. So in this case if we talk about collapse of the wave function how can it ever happen if everything was in superposition. $\endgroup$
    – user198045
    Commented Jul 17, 2018 at 17:25
  • $\begingroup$ My understanding is that dynamical collapse models suppress most of the possible outcomes to have low probability rather than completely eliminate them: arxiv.org/abs/1501.05778.I don't take the Copenhagen or statistical interpretations as viable options at all because they don't make specific statements about what is happening in reality, which makes them untestable and prevents them from explaining anything. $\endgroup$
    – alanf
    Commented Jul 18, 2018 at 12:47
  • $\begingroup$ I don't think the existence of multiple versions of each system is a problem, see "The Fabric of Reality" and "The Beginning of Infinity" by David Deutsch. And even if it was a problem, having multiple possibilities a little bit is like being a little bit pregnant. $\endgroup$
    – alanf
    Commented Jul 18, 2018 at 12:47
  • $\begingroup$ Yes you are right this is kind of how I was thinking because the universe does not make sense until there is a collapse which I thought was pretty weird for a universe to evolve whiteout collapse... $\endgroup$
    – user198045
    Commented Jul 18, 2018 at 15:05

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