Title: quantum mechanics - Observer effect, do this mean literally someone or just any interaction with other matter? - Physics Stack Exchange
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Observer effect, do this mean literally someone or just any interaction with other matter?
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Content: In order to try to answer you original question "what is an observation", the question can usually be resolved by realizing that any measurement action always entails the interaction of your measurement apparatus (another quantum system) with the system you are measuring. The subsequent measurement on the apparatus then modifies the original quantum system (since we extract information indirectly). What is perhaps less obvious is, that if someone were to measure our system in the way, and not tell us the result, the quantum system state of our original system is still modified. Simply the possibility of someone having measured our system (in the sense that information is extracted from it to another system) requires us to take the measurement into account. This has something to do with the complementarity mentioned above, and is a distinguishing feature of quantum mechanics opposed to classical probability theory. Thus any kind of interaction with an external system can be viewed as a kind of observation of our own system - the modification of the quantum system state depends on whether or not this other system is measured. So I might have answered you question by moving the measurement away from the original system to another system (the measurement apparatus). The point is, that to some extent, it is not too important whether or not th