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We study how meaningful physical predictions can arise in nonperturbative quantum gravity in a closed Lorentzian universe. In such settings, recent developments suggest that the quantum gravitational Hilbert space is one-dimensional and real for each -sector, as induced by spacetime wormholes. This appears to obstruct the conventional quantum-mechanical prescription of assigning probabilities via projection onto a basis of states. While previous approaches have introduced external observers or augmented the theory to resolve this issue, we argue that quantum gravity itself contains all the necessary ingredients to make physical predictions. We demonstrate that the emergence of classical observables and probabilistic outcomes can be understood as a consequence of partial observability: physical observers access only a subsystem of the universe. Tracing out the inaccessible degrees of freedom yields reduced density matrices that encode classical information, with uncertainties exponentially suppressed by the environment's entropy. We develop this perspective using both the Lorentzian path integral and operator formalisms and support it with a simple microscopic model. Our results show that quantum gravity in a closed universe naturally gives rise to meaningful, robust predictions without recourse to external constructs.
Biography：
Tomonori Ugajin is an Associate Professor in the Department of Physics at Rikkyo University, Japan. He received his Ph.D. in Physics from the University of Tokyo in 2014, where he also obtained his M.S. in 2011. He earned his B.S. from the Tokyo Institute of Technology in 2009. Before joining Rikkyo University in 2023, he served as an Assistant Professor at the Yukawa Institute for Theoretical Physics, Kyoto University. His research focuses on holography, black holes, and closed universes from the perspective of quantum information theory.
Tencent Meeting：471-945-450
Shanming Ruan