  Roger Penrose and the author share the proposal that the spatial
superposition $|x_1\rangle+|x_2\rangle$ of a massive object collapses into its
localized components $|x_1\rangle$ or $|x_2\rangle$ with the characteristic
time $\hbar/E_\Delta$ where $E_\Delta$ is the gravitational self-energy excess
of the superposition versus the localized states. Underlying arguments of such
radical departure from standard quantum mechanics and different derivations of
the rate equation are briefly recapitulated and discussed.
