Whether gravity must be quantized remains one of the biggest open problems in fundamental physics. Classical-quantum hybrid theories have recently attracted attention as a possible framework in which gravity is treated classically yet interacts consistently with quantum matter. Schemes based on completely positive dynamics satisfy most formal consistency requirements and enable a systematic treatment of quantum backreaction, but they also give rise to features that challenge conventional physical intuition, such as the breakdown of conservation laws. To illustrate this issue, we consider a qubit interacting with a classical particle and demonstrate that the corresponding hybrid system violates angular momentum conservation despite the rotational symmetry of the underlying equations of motion.
Hotta et al. (Wed,) studied this question.
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