Relational physics with real rods and clocks and the measurement problem   of quantum mechanics
  The use of real clocks and measuring rods in quantum mechanics implies a natural loss of unitarity in the description of the theory. We briefly review this point and then discuss the implications it has for the measurement problem in quantum mechanics. The intrinsic loss of coherence allows to circumvent some of the usual objections to the measurement process as due to environmental decoherence. 
Rodolfo Gambini and Jorge Pullin