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2023/03/22(Wed)
15:00 -16:00
七樓研討室7th Floor(P7F)

視訊演講 Video Seminar [ Meeting Link / ID: 2518 531 3604 / Password: gcMQM9pP8r9 ]

Title

Quantum Fluctuations in Gravity: Gravitational Cat State, Graviton Noise and Stochastic Gravity

Speaker

胡悲樂教授 (美國馬里蘭大學基礎物理學中心和聯合量子研究所) Prof. Bei-Lok Hu (Maryland Center for Fundamental Physics and Joint Quantum Institute, University of Maryland, USA)Abstract

The intersections or unions of gravity (G), quantum (Q) fields and quantum information (I) have sparked many interesting new directions of research in fundamental physics in the last decade, black hole information and wormhole physics are amongst the better known examples. In this talk I want to present two classes of low energy quantum fluctuation phenomena amenable to earth-environment laboratory or outer-space tests. One is gravitational cat state, representing quantum entanglement in gravitating systems; the other, graviton noise, signifying the quantum nature of perturbative gravity. In a quantum description of matter a single motionless massive particle can in principle be in a superposition state of two spatially-separated locations. This superposition state in gravity, or gravitational cat state, would lead to fluctuations in the Newtonian force exerted on a nearby test particle. The centerpiece is the energy density correlation, corresponding to the noise kernel in stochastic semiclassical gravity theory, evaluated in the weak-field nonrelativistic limit. It may come as a surprise that such a theory originally developed to describe quantum field effects in black holes and the early universe is actually needed for the description of gravitational entanglement in laboratory settings. For the graviton noise problem, we consider the effects of gravitons and their fluctuations on the dynamics of two masses using the Feynman-Vernon influence functional formalism. The Hadamard function of the gravitons yields the noise kernel acting as a stochastic tensorial force in a Langevin equation governing the motion of the separation of the two masses. The fluctuations of the separation due to the graviton noise are then solved for various quantum states including the Minkowski vacuum, thermal, coherent and squeezed states, generalizing previous results of Parikh et al. We end with a discussion of what would constitute a demonstration of the quantum nature of perturbative gravity, and a comment on the prospect of detecting these fluctuations in primordial gravitons using interferometers with long baselines in deep space experiments.

Language

演講語言 (Language): in English