When we measure a quantum particle, its wavefunction collapses to an eigenstate. This is not a unitary transformation.
However, measuring a particle neccesarily involves the particle interacting with other particles (from the measurement device). These particles also have wavefunctions. During the measurement of the particle, these wavefunctions are also influenced.
If we consider the composite wavefunction of the particle + measurement device, is time evolution then unitary during the measurement?
