Gravitationally-induced quantum effects
By we denote physical phenomena in which gravity modifies the standard quantum behavior in ways not fully captured by relativistic quantum field theory. Such effects are expected either when quantum systems probe spacetime curvature or when gravity and quantum mechanics are treated within a unified framework, independently of the microscopic realization of quantum gravity.
In the strong-gravity regime, quantum effects arise near causal horizons and regions of large spacetime curvature. Hawking radiation [Hawk75] revealed the thermodynamic nature of black holes, motivating studies of quantum corrections to black hole entropy and consistency conditions that promote such astrophysical objects to probes of quantum gravity. Beyond thermodynamic aspects, strong gravitational backgrounds can also induce genuinely quantum correlations in the gravitational field itself, for instance through the production of squeezed graviton states in primordial and astrophysical environments.
In the weak-gravity regime, interacting quantum systems may be described via modified Schrödinger dynamics. Motivations for such modifications include collapse models [BLSS13], which introduce gravitationally-induced decoherence in order to address the measurement problem, but also semiclassical gravity, according to which the gravitational field remains classical. Predictions arising from these frameworks can be tested using matter-wave interferometers that delocalize massive quantum systems into spatial superpositions across the interferometer arms. Along this line, gravitationally-induced phase shifts were first observed with a neutron interferometer in the celebrated COW experiment [CoOvWe75]. More recently, similar techniques have revealed the gravitational Aharonov–Bohm effect, demonstrating that gravity acts on quantum systems through the metric rather than as a Newtonian force. Complementary experiments using large-molecule interferometry further constrain collapse models and [gravitational decoherence]/docs/2ndlevel/Gravitationally-induced_decoherence.qmd) [FGZK19].
A research field closely related to the one discussed above investigates gravity-mediated entanglement between distinct quantum systems. Indeed, quantum mechanics predicts that the mutual gravitational coupling of two spatially-delocalized systems is capable of generating entanglement between their positional degrees of freedom. Inspired by Feynman’s proposal at the Chapel Hill conference [Berg57], several Gravity-mediated entanglement tests have been recently put forward. In these works, it is argued that the observation of quantum correlations stemming from the gravitational interaction would constitute evidence for the quantum nature of gravity, since a purely classical field could only mediate local operations and classical communication (LOCC) and would therefore be unable to generate entanglement. The generality and robustness of this conclusion, however, remain the subject of active debate.
Beyond entanglement, quantum gravity effects may also manifest as deviations from the standard gravitational interaction, motivating experimental tests of the equivalence principle with quantum systems, precision measurements of short-range gravity, proposed tests of quantum time dilation and local Lorentz symmetry breakdown.