# Effective thermodynamics for a marginal observer

## TL;DR

This paper develops an effective thermodynamic framework for observers who only measure marginal currents, demonstrating that fundamental principles like the fluctuation relation and second law still hold in such incomplete measurement scenarios.

## Contribution

It introduces a mathematical construction of hidden time reversal to derive thermodynamic relations from partial observations, extending the applicability of thermodynamics.

## Key findings

- Effective thermodynamics applies to marginal observers.
- Fluctuation relation holds under partial measurement.
- Second law remains valid with limited information.

## Abstract

Thermodynamics is usually formulated on the presumption that the observer has complete information about the system he/she deals with: no parasitic current, exact evaluation of the forces that drive the system. For example, the acclaimed Fluctuation Relation (FR), relating the probability of time-forward and time-reversed trajectories, assumes that the measurable transitions suffice to characterize the process as Markovian (in our case, a continuous-time jump process). However, most often the observer only measures a marginal current. We show that he/she will nonetheless produce an effective description that does not dispense with the fundamentals of thermodynamics, including the FR and the 2nd law. Our results stand on the mathematical construction of a hidden time reversal of the dynamics, and on the physical requirement that the observed current only accounts for a single transition in the configuration space of the system. We employ a simple abstract example to illustrate our results and to discuss the feasibility of generalizations.