Classical and Quantum Causal Interventions

TL;DR

This paper explores the physical foundations of causal interventions in classical and quantum systems, revealing thermodynamic constraints that make perfect interventions physically impossible.

Contribution

It provides a physical perspective on interventions, showing they are limited by thermodynamics and challenging the feasibility of ideal interventions like Pearl's do-calculus.

Findings

Interventions are constrained by thermodynamics in open systems.

Perfect 'atomic' interventions are physically impossible in classical and quantum contexts.

The paper bridges causal modeling with physical principles of measurement and feedback.

Abstract

Characterising causal structure is an activity that is ubiquitous across the sciences. Causal models are representational devices that can be used as oracles for future interventions, to predict how values of some variables will change in response to interventions on others. Recent work has generalised concepts from this field to situations involving quantum systems, resulting in a new notion of quantum causal structure. A key concept in both the classical and quantum context is that of an intervention. Interventions are the controlled operations required to identify causal structure and ultimately the feature that endows causal models with empirical meaning. Although interventions are a crucial feature of both the classical and quantum causal modelling frameworks, to date there has been no discussion of their physical basis. In this paper, we consider interventions from a physical perspective and show that, in both the classical and quantum case, they are constrained by the thermodynamics of measurement and feedback in open systems. We demonstrate that the perfect "atomic" or "surgical" interventions characterised by Pearl's famous do-calculus are physically impossible, and this is the case for both classical and quantum systems.