Observation of the thermal Casimir force

Quantum theory predicts the existence of the Casimir force between macroscopic bodies, a force arising from the zero-point energy of electromagnetic field modes around them. A thermal Casimir force, due to thermal rather than quantum fluctuations of the electromagnetic field at finite temperature, was 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. Previous measurements of the quantum-fluctuation-induced force have been unable to clearly settle the question of whether the correct low-frequency form of the dielectric constant dispersion for calculating Casimir forces is the Drude model or the plasma model. Our 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.