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Quantum Mechanics postulates that the act of observation affects the behavior of the observed object.The most common example of this feature is the fact the unobserved object(e.g. a photon) behave like a wave in a superposition of states while when observed it starts to behave like a particle with a defined state. Now my question is : It's also correct to say the act of observation rally define the object's proprieties? It's correct to say that different choices made by the experimenter about which observable will be observed leads to different states and properties of the observed object ? I often read something similiar but i want to clear up my confusion about this so i will avoid any misunderstanding.

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Yes it is. The book "Quantum Theory and Pictures of Reality" (Schommers et al.) states that it is incorrect to regard a certain propriety of quantum objects as a property of the quantum objects itself; but as an attribute which must be assigned both to the quantum system under study and the experimental arrangement. So, answering your question, the observer can choose (changing the experiment) which physical quantity he want to observe. In fact the act of measuring the physical observable $A$ ( which is associated to a self-adjoint operator operator $\hat{A}$) is equivalent to decompose the physical state of your system in the eigenstate of your operator, which eigenvalues may be different of those corresponding to another physical quantity $B$ (which you can measure with another experiment).

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  • $\begingroup$ Yes this is pretty much what i understood too..and the fact that in some sense the way we observe object (or measure quantities if you want) determines the evolution of the system(if it's also correct to say this) is for me one of the most fascinating parts of QM $\endgroup$ – Andrea Scaglioni Aug 24 '18 at 13:53
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    $\begingroup$ It is a little remark, but don't think it's correct to say that the observation determine the evolution of the system, but only it's initial state. The evolution, after the measurement, follows the Schr\"odinger equation. However, I agree with you; "the role of observer " in QM phenomena is very fascinating $\endgroup$ – LadyOfShalott Aug 24 '18 at 14:31
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    $\begingroup$ Sorry,my bad..."evolution of the system" it's not the correct way to say what i had in mind in this context..i know that after the measurament the system follows the Schrodinger equation with its probabilistc outcomes...So the observer's choice on which observable is to be measured doesn't really fully determine anything on the evolution of the system but i guess it's fine to say that it still has a causal role on the system final state. $\endgroup$ – Andrea Scaglioni Aug 24 '18 at 14:54
  • $\begingroup$ Sorry if i open again the discussion after months but i had a little little doubt while reading again your answer...when you talk about eigenstates and eigenvalues ,it isnt trivial to say that different observables lead to different eigenvalues ?just indeed because they're talking about different physical quantities?just as i get different values for example for the base and height of a triangle just because those are two different thing? $\endgroup$ – Andrea Scaglioni Oct 31 '18 at 20:29
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How to interpret quantum mechanics is a matter of ongoing debate. What you ask would possibly have been answered positively by Heisenberg and Bohr. But few serious physicists, especially no experimentalists, would really hold the view that things somehow only come into existence by observing them.

  • Think about cosmology: We investigate a time in the universe where no one was there to measure or observe anything, still the predictions of our physical theories work quite well.

  • If measurement/observation is given a too fundamental role in the formulation of your theory, you will forget something very important: Measurements disturb a system and the real value will never be the one you measure. This is totally clear to any experimentalist. If you now claim that only observation makes something real...well then, how to even talk about measurement errors? Why should one give up the classical intuition that there was something there which you then measured?

  • If an experimentalist does anything, they are used to talking about real things going on: "I have put an electron in this trap." Seriously saying that this is all nonsense since we have not observed or measured it yet would lead to the impossibility of talking about nature.

  • Can we even try to give a precise definition of measurement or observation? Shouldn't quantum mechanics also describe what's going on inside a measurement apparatus?

Therefore, the fact that measurements (or observation) play such an important role in the basic formulation of quantum mechanics leads to the so-called measurement problem. Several formulations of quantum mechanics without this problem were also developed, e.g. de Broglie-Bohm theory.

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  • $\begingroup$ Sure, but observation & measurement don't necessarily imply a conscious observer. $\endgroup$ – PM 2Ring Aug 27 '18 at 12:18
  • $\begingroup$ Yes i know...but i wasn't saiyng That observation" brings thing into existence" in a literal way....but it defines the object'state...and Yes...the observer doesn't need to bene conscious. $\endgroup$ – Andrea Scaglioni Sep 10 '18 at 11:32

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