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Ideal measurements in quantum mechanics can be described formally as instantaneous changes of the state vector or, more generally, of the density matrix W. By its lack of covariance, this formalism is not very adequate for relativistic quantum theories such as local field theory. For the latter, we propose a modified formalism, according to which a field measurement in a space-time region C changes the field state W in the future and side cone of C. This proposal is justified by physical considerations.
Hellwig et al. (Thu,) studied this question.