# The operator X for the position in QFT

DrDu
Can you name one hermitian operator in non-relativistic QM which is not an observable?
Let c be the anihilation operator of an electron. Then ##c+c^\dagger## is hermitian but unobservable due to charge or univalence superselection. Based on the latter, you can show that any fermionic operator is not observable.

Last edited:
Demystifier
Gold Member
Let c be the anihilation operator of an electron. Then ##c+c^\dagger## is hermitian but unobservable due to charge or univalence superselection. Based on the latter, you can show that any fermionic operator is not observable.
I said an operator in non-relativitic QM. By that I meant an operator acting in the space of a fixed number of particles.

DrDu