Consider a system consisting of a collection of identical particles. In classical mechanics, it is, in principle, possible to continuously monitor the position of each particle as a function of time. Hence, the constituent particles can be unambiguously labeled. In quantum mechanics, on the other hand, this is not possible because continuous position measurements would disturb the system. It follows that identical particles cannot be unambiguously labeled in quantum mechanics.
- Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)