It is commonly thought that thermal photons with transverse electric polarization cannot exist in a planar metallic cavity whose size $a$ is smaller than the thermal wavelength ${\lambda }_T$, due to the absence of modes with $\lambda <2a$. Computations based on a realistic model of the mirrors contradict this expectation and show that a micron-sized metallic cavity is filled with nonresonant radiation having transverse electric polarization, following a non-Planckian spectrum, whose average density at room temperature is orders of magnitudes larger than that of a blackbody. We show that the spectrum of this radiation can be measured by observing the transition rates between hyperfine ground-state sublevels $1S_{1/2}(F,m_F)\to 1S_{1/2}(F^{\prime },m_F^{\prime })$ of D atoms passing in the gap between the mirrors of a Au cavity. Such a measurement would also shed light on a puzzle in the field of dispersion forces, regarding the sign and magnitude of the thermal Casimir force. Recent experiments with Au surfaces led to contradictory results, whose interpretation is highly controversial.

• Received 26 June 2019

DOI:https://doi.org/10.1103/PhysRevA.100.032501