• Physics and Astronomy Colloquium

October 3, 2025 3:30 PM - October 3, 2025 4:30 PM
PAIS 1100

Host:

Paco Salces

Presenter:

Yanbei Chen (Caltech)

Video Recording

Testing the quantum nature of gravity in the laboratory has become an exciting frontier. A leading proposal is to demonstrate quantum entanglement between massive objects mediated solely by their Newtonian gravitational interaction. At the same time, “classical” models of gravity—where a classical field couples to quantum systems—provide valuable intermediate experimental benchmarks, often more accessible than entanglement tests. Two distinct approaches arise depending on how classical information about quantum matter is obtained. In the Schrödinger–Newton approach, the gravitational potential is determined by information accessible only through the experimentalist’s measurement devices. This produces a nonlinear modification of the Schrödinger equation, analogous to quantum feedback control, where measurement outcomes are continuously fed back onto the system. The resulting self-gravity effect takes place at a much shorter time scale than mutual gravity between two objects, due to the concentration of a solid object's mass near lattice sites.  By contrast, in universal monitoring models, one assumes that the universe itself continuously “measures” all matter. This leads to gravity-induced decoherence, connects naturally to collapse theories (e.g., the Diósi–Penrose model), and links to entropic gravity proposals, where gravity emerges as a thermodynamic effect of many underlying degrees of freedom. Finally, I will emphasize that apparent entanglement can still arise within classical frameworks unless experiments enforce causal separation, underscoring the subtlety of probing the quantum–classical boundary of gravity.