Sandro Donadi: Gravity-related quantum collapse: where are we?

The quest to unify quantum theory and gravity is still ongoing. The standard and by far most widely studied approach consists in quantizing gravity. An alternative perspective proposes to modify quantum mechanics, as in the Diósi–Penrose (DP) model, which introduces a gravity-related spontaneous wave function collapse. The model makes different predictions from those of quantum mechanics, therefore it can be tested experimentally.

After a brief introduction to the DP model, I will discuss possible experimental tests, focusing in particular on experiments conducted at the Gran Sasso National Laboratories, aimed at detecting radiation emitted as a consequence of spontaneous collapse. These experiments place an upper bound on the strength of the collapse mechanism, which we compare with a lower bound derived from the requirement that macroscopic systems are well localized. We also discuss how the spectrum of radiation emitted at energies lower than those considered so far is highly model dependent across different collapse models.

Finally, we present more recent results showing that, if gravity is classical, it must necessarily induce spatial collapse, which is accompanied by unavoidable diffusive effects. We derive a lower bound on this diffusion and show that it is always present, even for well-localized systems. This result allows one to explore the quantum versus classical nature of gravity without the need to create large-mass quantum superpositions.