Wave function collapse is postulated in standard quantum mechanics to explain how the only possible result of the measurement of a physical quantity is one of the eigenvalues of the corresponding observable. However, standard quantum mechanics does not explain how this nonunitary process occurs; it only states its probability. In the frame of objective collapse theories, which modify the Schrödinger unitary propagation to account for collapse, Diósi and, independently, Penrose proposed that gravity induces collapse within a time inversely proportional to the gravitational self-energy of the quantum superposition.