Title: quantum mechanics - Observer effect, do this mean literally someone or just any interaction with other matter? - Physics Stack Exchange
Headings: 
Observer effect, do this mean literally someone or just any interaction with other matter?
5 Answers
5
Not the answer you're looking for? Browse other questions tagged quantum-mechanics measurement-problem observers or ask your own question.

Content: This implies, e.g., that it is impossible to assign values to what is called complementary observables (such as momentum and position as mentioned above). In the Copenhagen interpretation, it is only allowed to ask question regarding the probability for observing outcomes of a specific choices of measurement. Since the quantum state reflects the amount of information available it naturally changes when a measurement is performed, since we now know more... This is similar to Bayes theorem from regular probability theory, where a gain in information is reflected in an updated, changed, probability distribution for our unknown information. Thus, in this view, the quantum state is not so much a property of the system, but a reflection of the observers knowledge about the system. If you have heard of the Bayesian approach to classical probability theory, this is quite similar. 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.