A physical perspective on causality
Speaker: Erik Aurell, Professor, Royal Institute of Technology, Stockholm, Sweden.
Causes and effects are concepts that are easy to grasp in everyday life but difficult to combine with Physics on the fundamental level where interactions are always mutual. For instance, the Sun and the Planets attract each other reciprocally and only the much greater mass of the Sun makes it appear that the Planets orbit the Sun; if the masses were more equal the dynamics of the Solar System would be more like that of a multiple star. Russell in 1912 was the first to leverage this Newtonian view of the world to claim that there are in fact no such things as causes.
This poses a problem as ideas of cause and effect after all come very naturally in everyday life, and indeed rule much of it. Causality is furthermore nowadays a very active field of statistics and its uses have many societal and legal implications. It would therefore seem to be of interest to reconsider the question, and to ask if causality is really only, as Russell wrote, "...a relic of a bygone age, surviving, like the monarchy, only because it is erroneously supposed to do no harm."
I will argue that causes and effects have a place as simplified descriptions of physical systems driven out of equilibrium so that they only go one way. This restriction leaves out planetary systems and systems of modern fundamental Physics but encompasses a large fraction of the systems we encounter in everyday life, and indeed Life itself. The time reflection symmetry of Classical Mechanics is then broken by the presence of a heat bath and the inevitable increase of disorder in an environment which is not completely controlled. I will discuss parallels between causality and non-equilibrium physics. I will in particular discuss modern ways to compute non-equilibrium steady states, and if these can be used as a starting point for an alternative to causal analysis in the sense of Pearl.
B Russell "On the Notion of Cause" Proc Aristotelian Soc Vol 13 pp 1-26 (1912-13)
E Aurell, G Del Ferraro "Causal analysis, Correlation-Response and Dynamic cavity" Journal of Physics: Conference Series 699, 012002 (2016)