Download Gravitational Cat State and Stochastic Semiclassical Gravity*

Gravitational Cat State and Stochastic Semiclassical Gravity*
Professor Bei- Lok Hu (胡悲樂教授)
Maryland Center for Fundamental Physics and Joint Quantum Institute,
University of Maryland, College Park, U.S.A.
We investigate the nature of a gravitational two-state system (G2S) in the simplest setup in
Newtonian 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 central quantity of importance for this inquiry is the energy density
correlation. This corresponds to the noise kernel in stochastic semiclassical gravity theory [1,2],
evaluated in the weak field nonrelativistic limit. In this limit, quantum fluctuations of the stress
energy tensor manifest as the fluctuations of the Newtonian force. We describe the properties of
such a G2S system and present two ways of measuring the cat state for the Newtonian force, one by
way of a classical probe, the other a quantum harmonic oscillator. Our findings of general interest to
quantum and gravitation theory include: (i) mass density fluctuations persist even in single particle
systems, of the same order of magnitude as the mean; (ii) a classical probe generically records a
non-Markovian fluctuating force; (iii) a quantum probe interacting with the G2S system may
undergo Rabi oscillations in a strong coupling regime. This simple prototypical gravitational
quantum system could provide a robust testing ground to compare predictions from alternative
quantum theories [3], since the results reported here are based on standard quantum mechanics and
classical gravity. A preliminary feasibility study of experimental setups for preparing and probing a
gravitational cat state can be found in [4] which contains some references to recent work in this
area.
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*Joint work with Charis Anastopoulos. Based on Charis Anastopoulos and Bei-Lok Hu, “Probing a Gravitational
Cat State” Class. Quant. Grav. 32 (2015) 165022 [arXiv:1504.03103] and [4]
[1] B. L. Hu and E. Verdaguer, “Stochastic gravity: Theory and Applications”, in Living Reviews in Relativity 11
(2008) 3 [arXiv:0802.0658]
[2] B. L. Hu, Gravitational Decoherence, Alternative Quantum Theories and Semiclassical Gravity, Invited talk at
the Second International Conference on Emergent Quantum Mechanics, Vienna, October 3-6, 2013
[arXiv:1402.6584]
[3] Angelo Bassi, K. Lochan, S. Satin, T. P. Singh and H. Ulbricht, “Models of wave-function collapse,
underlying theories, and experimental tests”, Rev. Mod. Phys. 85, 471- 527 (2013)
[4] M. Derakhshani, C. Anastopoulos, B. L. Hu, Probing a Gravitational Cat State: Experimental Possibilities
[arXiv:1603.04430]