taken into account, this effect might disappear and therefore not be present in the scenario described by Penrose. A second problem, also arising in Penrose's proposal, is the origin of the Born rule. To solve the measurement problem, a mere explanation why a wave-function collapses to, e.g., a dot on a screen is not enough. A good model for the collapse process also has to explain why the dot appears on different positions of the screen with probabilities that are determined by the absolute-value squared of the wave-function. Although it might be possible that a model based on Penrose's idea could provide such an explanation, there is no evident way how the Born rule could arise naturally from it. Finally, since the gravitational potential is linked to the wave-function in the picture of the Schrödinger–Newton equation, the wave-function must be interpreted as a real object. Therefore, at least in principle, it becomes a measurable quantity. Making use of the nonlocal nature of entangled quantum systems, this could be used to send signals faster than light, which is generally thought to be in contradiction with causality. It is, however, not clear whether this problem can be resolved by applying the right collapse prescription, yet to be found, consistently to the full quantum system. Also, since gravity is such a weak interaction, it is not clear that such an experiment can be actually performed within the parameters given in our universe (see the discussion about a similar thought experiment proposed by Eppley and Hannah). See also Nonlinear Schrödinger equation Semiclassical gravity Penrose interpretation Poisson's equation References Gravity Quantum gravity Equations Nonlinear partial differential equations Schrödinger equation