  We analyse a gedankenexperiment previously considered by Mari et al. that
involves quantum superpositions of charged and/or massive bodies ("particles")
under the control of the observers, Alice and Bob. In the electromagnetic case,
we show that the quantization of electromagnetic radiation (which causes
decoherence of Alice's particle) and vacuum fluctuations of the electromagnetic
field (which limits Bob's ability to localize his particle to better than a
charge-radius) both are essential for avoiding apparent paradoxes with
causality and complementarity. We then analyze the gravitational version of
this gedankenexperiment. We correct an error in the analysis of Mari et al. and
of Baym and Ozawa, who did not properly account for the conservation of center
of mass of an isolated system. We show that the analysis of the gravitational
case is in complete parallel with the electromagnetic case provided that
gravitational radiation is quantized and that vacuum fluctuations limit the
localization of a particle to no better than a Planck length. This provides
support for the view that (linearized) gravity should have a quantum field
description.
