# Measurement

## Idea

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.

## References

Revised on April 1, 2014 01:19:38 by Urs Schreiber (185.37.147.12)