On the Foundational Assumptions of Modern Physics

Abstract

General relativity and the standard model of particle physics remain our most fundamental physical theories enjoying robust experimental confirmation. The foundational assumptions of physics changed rapidly during the early development of these theories, but the subsequent challenges of their refinement and the exploitation of their explanatory power turned attention away from foundational issues. Deep problems and anomalous observations remain unaddressed. New theories such as string theory seek to resolve these issues, but are presently untested. In this essay, I evaluate the foundational assumptions of modern physics and propose new physical principles. I reject the notion that spacetime is a manifold, the existence of static background structure in the universe, the symmetry interpretation of covariance, and a number of related assumptions. The central new principle I propose is the causal metric hypothesis, which characterizes the observed properties of the physical universe as manifestations of causal structure. More precisely, the classical causal metric hypothesis states that the metric properties of classical spacetime arise from a binary relation on a set, representing direct influences between pairs of events. Rafael Sorkin’s maxim, “order plus number equals geometry” is a special case. The quantum causal metric hypothesis states that the phases associated with directed paths in causal configuration space, under Feynman’s sum-over-histories approach to quantum theory, are determined by the causal structures of their constituent universes. The resulting approach to fundamental physics is called quantum causal theory.