Path Integrals and Reality

We define the idea of {\it real path quantum theory}, a realist generalisation of quantum theory in which it is postulated that the configuration space path actually followed by a closed quantum system is probabilistically chosen. This is done a postulate defining probabilities for paths, which we propose are determined by an expression involving path amplitudes and a distance function that quantifies path separation. We suggest a possible form for a path probability postulate and explore possible choices of distance function, including choices suitable for Lorentz or generally covariant versions of real path quantum theory. We set out toy models of quantum interferometry and show that in these models the probability postulate and specific distance functions do indeed give a physically sensible path ontology. These functions can be chosen so as to predict quantum interference for interference of microscopic quantum systems and the failure of interference for macroscopic quantum systems. More generally, they predict interference when the beams are close, and its failure when they are far apart, as determined by the distance function. If taken seriously in its present relatively unconstrained form, real path quantum theory thus motivates experimental tests of quantum interference in all unexplored regimes defined by potentially physically interesting parameters, including the mass of the beam object, the beam separation distance, the beam separation time, and many others. We discuss open questions raised by these ideas.