An Einstein-Podolsky-Rosen argument based on weak forms of local realism not falsifiable by GHZ or Bell experiments

Abstract: The Einstein-Podolsky-Rosen (EPR) paradox gives an argument for the incompleteness of quantum mechanics based on the premises of local realism. A general view is that the argument is compromised, because EPR's premises are falsified by Greenberger-Horne-Zeilinger (GHZ) and Bell experiments. In this paper, we present an EPR argument based on premises not falsifiable by these experiments. We propose macroscopic EPR and GHZ experiments using spins $S_\theta$ defined by two macroscopically distinct states. The analyzers that realize the unitary operations $U_\theta$ determining the measurement settings $\theta$ are devices that create macroscopic superposition states. For a system with two macroscopically distinct states available, macroscopic realism (MR) posits a predetermined outcome for a measurement $S_\theta$ distinguishing between the states. Deterministic macroscopic realism (dMR) posits MR for the system prior to the interaction $U_\theta$. Weak macroscopic realism (wMR) posits MR for the system after $U_\theta$, at the time $t_f$ (when the system is prepared for a final "pointer" measurement), the outcome of $S_\theta$ not being changed by interactions that might occur at a remote system $B$. The premise also posits that if the outcome for $S_\theta^A$ of a system $A$ can be predicted by a pointer measurement on a system $B$ defined after the interaction fixing the setting at $B$, then the outcome for $S_\theta^A$ is determined at this time. The GHZ predictions negate dMR but are consistent with wMR. Yet, an EPR paradox arises based on wMR for the set-up proposed by Schr\"odinger, where one measures two complementary spins simultaneously, "one by direct, the other by indirect" measurement. We revisit the original EPR paradox and find similarly that an EPR argument can be based on a weak form of local realism not falsifiable by GHZ or Bell tests.