Does an observer modify the observed?

What many people misunderstand is that in quantum theories the “state” of a system is not a property of the system itself but rather of how it appears to an observer.

There are actually at least two stages to the observation process. One is when the system of interest interacts with the much more complex system of a measurement apparatus whose precise quantum state is too complex for the observer to keep track of and so has to be expressed as a statistical mixture. This can have the effect of causing the combined system, in which the observed subsystem was initially in a pure “coherent” superposition state (with interference still being possible between different possible observed eigenvalues), to end up very close to a statistical mixture in which each possible measured value of the observed quantity has a well defined value with no interference between them. This “decoherence” process can be caused by interaction with any sufficiently complex system (even, as Viktor Toth notes, just a brick) and it does modify the observed (as does any interaction with anything – even just another simple quantum system). But it still leaves the actual value of the observation unspecified. The “collapse” process, which identifies which particular value has occurred, only happens in the mind of the observer whose conscious experience corresponds to just one of many possible histories of the universe. But this doesn’t modify the observed – at least no more than it modifies everything in the universe that is dependent on that observed value. (For example if we are in a room together and I see a red flash then the you that I see will also see a red flash, but if you see a blue flash then the I that you see will also have seen a blue flash.)

Source: (1001) Alan Cooper’s answer to In the quantum mechanical idea that the observer modifies the observed, can the observer be an insect? – Quora

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