Position Measurement for a Relativistic Particle:   Restricted-Path-Integral Analysis
  Measurements of the position of a relativistic particle is considered in the framework of the Restricted-Path-Integral (RPI) approach. The amplitude describing such a measurement is shown to be exponentially small outside the light cone of the space-time point corresponding to the measurement output, in a qualitative agreement with the Hellwig and Kraus' postulate of relativistic state reduction. Theory of the measurement including the probability distribution for different measurement outputs is suggested. It is shown that correct theory does not exist (for arbitrary initial states) if the error of the measurement is less than the Compton length. The physical reason is that the picture of measurement is destroyed in this case by pair creation. 
Michael B. Mensky (P.N.Lebedev Physics Institute, Moscow, Russia) and   Horst von Borzeszkowski (Institut fuer Theoretische ...