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From what I've understood from reading different online sources including PSE*, measurements in quantum mechanics are generally argued to be irreversible (at least, when a macroscopic measuring device is used). This irreversibility is usually ascribed to an increase in total entropy (measured system $+$ measuring device) that occurs when the measurement is made. Quoting from the article "Irreversibility and Measurement in Quantum Mechanics" by Douglas M. Snyder:

Bohr (1935) maintained that quantum mechanical measurement also depended on the interaction between a macroscopic measuring instrument and the physical existent measured. He noted that when a macroscopical physical measuring apparatus is used, there is inevitably some loss of information concerning the measured system due to the resulting physical interaction. [...] For Bohr, once the information is lost in the measurement process, the measurement cannot be reversed.

However, this description only accounts for interaction-based measurements. What could we say about interaction-free measurements, then? (See the Wikipedia article for examples). If there is in those cases no physical interaction between the measuring instrument and the existent measured, by which mechanism could irreversibility be explained?


*See, for instance, this related question or this answer.

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    $\begingroup$ The subject is presumably wrapped up with entanglement. Pun intended, but the comment is serious. $\endgroup$ – dmckee Aug 9 '16 at 21:15
  • $\begingroup$ @dmckee That's what I suspected... $\endgroup$ – David Herrero Martí Aug 9 '16 at 21:22
  • $\begingroup$ There are no interaction free measurements. Period. End of story. None of the examples in Wikipedia are even close, even though they show different levels of intellectual failure by their authors. $\endgroup$ – CuriousOne Aug 10 '16 at 1:14
  • $\begingroup$ @CuriousOne Could you please explain why do you think so? $\endgroup$ – David Herrero Martí Aug 10 '16 at 4:25
  • $\begingroup$ Because in all examples ever given the authors are blatantly ignoring that nature doesn't make a distinction between the field that is being measured and the matter that is measuring it, they are the exact same phenomenon. Quantum field theory doesn't have any self-consistency issues of this kind, they are only created by application of classical matter/quantum mechanical field approximations in non-relativistic QM. $\endgroup$ – CuriousOne Aug 10 '16 at 6:14

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