  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 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
non-resonant 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 black-body. We show that the spectrum of this
radiation can be measured by observing the transition rates between hyperfine
ground-state sub-levels $1S_{1/2}(F,m_F) \rightarrow 1S_{1/2}(F',m'_F)$ 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 much
controversial.
