Observation of the thermal Casimir force

TL;DR

This study experimentally observed the thermal Casimir force between gold plates at micrometer separations, confirming the Drude model's accuracy and clarifying the low-frequency dielectric response relevant for quantum fluctuation forces.

Contribution

First experimental measurement of the thermal Casimir force at micrometer separations, validating the Drude model over the plasma model for thermal fluctuation calculations.

Findings

Thermal Casimir force observed between gold plates from 0.7 to 7 micrometers.

Results agree with the Drude model including thermal effects.

The plasma model is inconsistent with the measurements.

Abstract

Quantum theory predicts the existence of the Casimir force between macroscopic bodies, due to the zero-point energy of electromagnetic field modes around them. This quantum fluctuation-induced force has been experimentally observed for metallic and semiconducting bodies, although the measurements to date have been unable to clearly settle the question of the correct low-frequency form of the dielectric constant dispersion (the Drude model or the plasma model) to be used for calculating the Casimir forces. At finite temperature a thermal Casimir force, due to thermal, rather than quantum, fluctuations of the electromagnetic field, has been theoretically predicted long ago. Here we report the experimental observation of the thermal Casimir force between two gold plates. We measured the attractive force between a flat and a spherical plate for separations between 0.7 $\mu$m and 7 $\mu$m. An electrostatic force caused by potential patches on the plates' surfaces is included in the analysis. The experimental results are in excellent agreement (reduced $\chi^2$ of 1.04) with the Casimir force calculated using the Drude model, including the T=300 K thermal force, which dominates over the quantum fluctuation-induced force at separations greater than 3 $\mu$m. The plasma model result is excluded in the measured separation range.