Causality in Physics: From Galileo to Einstein, and Beyond

TL;DR

This paper surveys the historical and conceptual status of causality across classical mechanics, field theory, relativity, and quantum/statistical physics. It shows that causal language is often implicit or emergent rather than explicit in fundamental laws, with shifts from forces to variational principles, from action-at-a-distance to local fields, and from deterministic evolution to probabilistic amplitudes. It highlights how relativity encodes causal structure via light cones and the finite speed limit $c$, how quantum mechanics reshapes causality through amplitudes, entanglement, and multiple interpretations, and how macroscopic irreversibility introduces a thermodynamic arrow of time. The overall message is that causality is an effective, context-dependent notion arising from macroscopic conditions and the structure of physical laws, rather than a primitive ingredient of the theory, and it remains a working hypothesis in practice, not a guaranteed feature.

Abstract

Causality is one of the most fundamental -- and yet elusive -- concepts in physics. From its intuitive role in everyday experience to its formal and often implicit role in scientific theories, causality has challenged philosophers and physicists alike. In what follows, we take a brief historical and conceptual journey through classical and modern physics, tracing how causality has been treated, questioned, or protected in successive physical frameworks -- from Galilean mechanics to Newtonian dynamics, from Lagrangian and Hamiltonian formulations to special and general relativity, and finally to quantum mechanics and statistical physics. Our aim is to show how the notion of causality has repeatedly receded into the background of our most successful theories, even when it appears to be central to our everyday understanding of the world.