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In principle, any interaction between two physical systems is a measurement, whereby each system measures the state of the other. In practice, we break the symmetry by treating one system as the real physical system $S$ of study and the other system as merely a measuring apparatus $A$ with the sole purpose of measuring $S$. If the properties of $A$ are more directly observable than those of $S$, then this is useful.
Last revised on April 1, 2014 at 01:19:38. See the history of this page for a list of all contributions to it.