TITLE: How to reconcile active and passive observation in quantum physics?
QUESTION [0 upvotes]: The two-slit experiment is a classic example of how measurements can affect the behavior of particles. This seems reasonable because, to my knowledge, the measurement is "active" in that it adds energy to the system. But how is this reconciled with macroscopic observations (such as looking at Schroedinger's cat) that are "passive" in that they are collecting information already there without affecting the system?

REPLY [1 votes]: Well Schrödinger's cat is considered to be not "passive", otherwise the paradox is moot.
The best explanation we know is decoherence theory. That is we disregard environmental micro-states that are macroscopically the same, in this interpretation one gets the density matrix $\rho = \frac 1 2 \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}$ for the states "cat alive" and "cat dead" without any observation. That is classical probability is created from quantum probability by decoherence.
The bottom line is, that the cat is coupled strongly to the environment at all times, while the particle in the double slit experiment is not (unless we measure it, which requires some strong interaction).
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