Open systems across scales

Abstract The view that our best current physics deals with effective systems has gained philosophical traction in the last two decades. A similar view about open systems has also been picking up steam in recent years. Yet little has been said about how the concepts of effective and open systems relate to each other despite their apparent kinship—both indeed seem at first sight to presuppose that the system in question is somehow incomplete. In this paper, I distinguish between two concepts of effectiveness and openness in quantum field theory, which provides a remarkably well-developed theoretical framework to make a first stab at the matter, and argue that on both counts, every realistic effective system in this context is also open. I conclude by highlighting how the discussion opens novel avenues for thinking of systems as open across scales.