Comments on proposed gravitational modifications of Schrödinger dynamics and their experimental implications

We discuss aspects of gravitational modifications of Schrödinger dynamics proposed by Diósi and Penrose. We consider first the Diósi–Penrose criterion for gravitationally induced state vector reduction, and compute the reduction time expected for a superposition of a uniform density cubical solid in two positions displaced by a small fraction of the cube side. We show that the predicted effect is much smaller than would be observable in the proposed Marshall et al mirror experiment. We then consider the 'Schrödinger–Newton' equation for an N -particle system. We show that in the independent particle approximation, it differs from the usual Hartree approximation applied to the Newtonian potential by self-interaction terms, which do not have a consistent Born rule interpretation. This raises doubts about the use of the Schrödinger–Newton equation to calculate gravitational effects on molecular interference experiments. When the effects of Newtonian gravitation on molecular diffraction are calculated using the standard many-body Schrödinger equation, no washing out of the interference pattern is predicted.