Carlo Maria Scandolo (University of Calgary), Roberto Salazar (University of Gdansk), Jarosław K. Korbicz (Center for Theoretical Physics, Polish Academy of Sciences), Paweł Horodecki (University of Gdansk)
We investigate the emergence of classicality and objectivity in arbitrary physical theories. First we provide an explicit example of a theory where there are no objective states. Then we characterize classical states of generic theories, and show how classical physics emerges through a decoherence process, which always exists in causal theories as long as there are classical states. We apply these results to the study of the emergence of objectivity, here recast as a multiplayer game. In particular, we prove that the so-called Spectrum Broadcast Structure characterizes all objective states in every causal theory, in the very same way as it does in quantum mechanics. This shows that the structure of objective states is valid across an extremely broad range of physical theories. Finally we show that, unlike objectivity, the emergence of local classical theories is not generic among physical theories, but it becomes possible if a theory satisfies two axioms that rule out holistic behavior in composite systems.