This paper grew out of the issue of wave function collapse that occurs in the Copenhagen interpretation.  Something like wave function collapse must occur as a single particle wave function measured in one place cannot give a measurement in another location as that would mean that wave function is not single particle.  For the classic case of measurement of a photon by a photodetector, if there is any case where measurement is unambiguous, this is it.  Single photon experiments are done all the time.  Instantaneous action over all space is not invariant under Lorentz transformations. Instantaneous collapse can only occur in one coordinate system.  

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This paper is a proposal for an experiment to detect this coordinate system using entangled photons.