Bell Nonlocality in Classical Systems Coexisting with other System Types

The realistic interpretation of classical theory assumes that every classical system has well-defined properties, which may be unknown to the observer but are nevertheless part of reality and can in principle be revealed by measurements. Here we show that this interpretation can in principle be falsified if classical systems coexist with other types of physical systems. To make this point, we construct a toy theory that (i) includes classical theory as a subtheory and (ii) allows classical systems to be entangled with another type of systems, called anti-classical. We show that our toy theory allows for the violation of Bell inequalities in two-party scenarios where one of the settings corresponds to a local measurement performed on a classical system alone. Building on this fact, we show that measurements outcomes in classical theory cannot, in general, be regarded as pre-determined by the state of an underlying reality.