So OK, we have a Fock space of some particles of one kind. Then we have energy operator that acts on the states and returns some value. The action of this operator on the whole space is derived from the behaviour on single particle states. Now my question: how can we measure the energy of individual particles? The energy operator only gives us the sum of all energies. We would have to define [tex]n[/tex] energy operators, one for each particle. How to do this so they not interfere with each other?