IQIM Postdoctoral and Graduate Student Seminar

Friday, September 30, 2022
12:00pm to 1:00pm
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East Bridge 114
Gravitationally induced decoherence vs space-time diffusion: testing the quantum nature of gravity
Jonathan Oppenheim, Professor of Quantum Theory and Royal Society University Research Fellow, University College London,

Abstract: We consider two interacting systems when one is treated classically while the other remains quantum. Despite several famous no-go arguments, consistent dynamics of this coupling exist, and we derive its most general form. We apply this framework to general relativity, and present a theory of classical gravity coupled to quantum field theory. The theory doesn't suffer from the pathologies of the semi-classical Einstein's equation and can be considered as a candidate for a fundamental theory, or one which is effective. If any system is treated as fundamentally classical, the dynamics necessarily results in decoherence of quantum systems and a breakdown in predictability in classical phase space. We prove that a trade-off between the rate of decoherence and the degree of diffusion induced in the classical system is a general feature of all classical-quantum dynamics. When the trade-off is saturated, the quantum state remains pure conditioned on the classical trajectory and the measurement postulate and Born rule is not needed. Applying the trade-off, we find a relationship between the strength of gravitationally-induced decoherence versus diffusion of the metric. This provides an experimental signature of theories in which gravity is fundamentally classical. Bounds on decoherence rates arising from current interferometry experiments, combined with precision measurements of mass, place significant restrictions on theories where Einstein's classical theory of gravity interacts with quantum matter.

Based on joint work with Carlo Sparaciari, Barbara Ĺ oda & Zachary Weller-Davies

Lunch will be provided, following the talk, on the lawn outside the Bridge arcade.

Attendees joining in person must demonstrate that they comply with Caltech's vaccination requirements (present Caltech ID or AWS ID or vaccination and booster confirmation).

Amid the recent increase in COVID-19 cases on campus due to the arrival of the highly infectious BA.2 subvariant in Los Angeles County and a return to routine social activities, the Institute has reinstated its requirement that high-quality masks (surgical, N95s, KN95s, or KF95s) must be worn in all indoor locations on campus.

For more information, please contact Marcia Brown by phone at 626-395-4013 or by email at