# Does unitarity apply in between measurements?

Sorry if this is a silly question (engineer here), but I was wondering if the math in particle physics assumes that unitarity applies even between measurements. In other words, I take it that the evolution of quantum states is governed by an operator that ensures the probability of all possible events adds up to 1 at all times. What I'm wondering -- is there an operator for which this does not apply at all times but still gives measurement results with probability between 0 and 1?

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You are asking if you can violate unitarity in intermediate steps? THe answer is yes, using ghosts. –  Ron Maimon Sep 2 '12 at 22:16

The physical wave functions are functionals of all fields, annihilating the BRST operator $Q$. These evolve unitarily according to Schroedingers equation. The Hamiltonian is the generator of the time translations of the corresponding unitary representation of the Poincare group defined on $Ker~Q$. There is no room for a separate non-unitary dynamics of some states - unless they are unphysical states without any relevance. –  Arnold Neumaier Sep 3 '12 at 17:55
yes, but the full state space is physically irrelevant, as only states annihilated by $Q$ can be prepared or observed. Therefore they are also the only states that exist at finite time, i.e., between measurements. - The others are just mathematical constructs. They may appear in perturbative calculations, but not in the dynamics. –  Arnold Neumaier Sep 4 '12 at 7:09