Testing the limits of quantum mechanics

Why do classical macroscopic systems not obey the quantum superposition principle? The falsifiable theory of spontaneous localization provides an explanation  for this strange nature of the quantum-to-classical transition. The theory agrees with quantum mechanics for small systems,  and with classical mechanics for large systems, but differs from them both in the mesoscopic range. We review the physics of spontaneous localization, and the experimental efforts to test it. Tests include molecular and cold atom interferometry, as well as non-interferometric tests with mesoscopic optomechanical systems such as levitated nanospheres. We also propose that  spontaneous localization is responsible for the emergence of spacetime, and we predict the novel phenomenon of quantum interference in time.