  The spectrum of zero-point radiation is relativistically invariant and its
spectral density function is therefore inversely proportional to the cubes of
its wavelengths. For its energy to be finite, there must exist a minimum
wavelength, $q_\lambda$. The measurements of the apparent attraction between
two uncharged conductor plates, placed in a vacuum at a temperature close to
absolute zero, made by Sparnaay in 1958 allow us to deduce that the energy flow
of the zero-point radiation which comes of or into an area $(q_\lambda)^2$,
corresponds with the emission of one photon of wavelength $q_\lambda$ per
$q_\tau$ $(q_\tau=q_\lambda/c)$, plus one photon of wavelength $2q_\lambda$ per
$2^3q_\tau$, etc., up to one photon of wavelength $nq_\lambda$ per $n^3q_\tau$.
This energy flow is enormous, but Sparnaay's experiments implied only photons
whose wavelengths were greater than $5\times10^{-5}$ cm, and zero-point
radiation may include only photons with wavelengths greater than $xq_\lambda$,
being $x$ an integer, perhaps very great.
