About thermometers and temperature

TL;DR

This paper explores mechanical models of thermometers capable of accurately measuring temperature in complex systems with long thermalization times, long-range interactions, or negative temperatures, revealing key design principles for such thermometers.

Contribution

It introduces a novel class of mechanical thermometer models suitable for non-standard thermodynamic systems, emphasizing their design and effectiveness through numerical analysis.

Findings

Thermometers can measure temperature in systems with long thermalization times.

Long-range systems' temperature readings match microcanonical temperature.

Bounded energy thermometers are necessary for negative temperature measurement.

Abstract

We discuss a class of mechanical models of thermometers and their minimal requirements to determine the temperature for systems out of the common scope of thermometry. In particular we consider: 1) anharmonic chains with long time of thermalization, such as the Fermi-Pasta-Ulam (FPU) model; 2) systems with long-range interactions where the equivalence of ensembles does not always hold; 3) systems featuring absolute negative temperatures. We show that for all the three classes of systems a mechanical thermometer model can be designed: a temporal average of a suitable mechanical observable of the thermometer is sufficient to get an estimate of the system's temperature. Several interesting lessons are learnt from our numerical study: 1) the long thermalization times in FPU-like systems do not affect the thermometer, which is not coupled to normal modes but to a group of microscopic degrees of freedom; 2) a thermometer coupled to a long-range system measures its microcanonical temperature, even at values of the total energy where its canonical temperature would be very different; 3) a thermometer to read absolute negative temperatures must have a bounded total energy (as the system), otherwise it heavily perturbs the system changing the sign of its temperature. Our study shows that in order to work in a correct way also in "non standard" cases, the proper model of thermometer must have a special functional form, e.g. the kinetic part cannot be quadratic.