  While it is generally assumed that several light beams propagate
independently in a refracting medium, the exception of laser beams may be
extended to usual time-incoherent light provided that conditions of
space-coherence are fulfilled. Very few molecules have convenient properties,
the simplest one being atomic hydrogen in 2S and 2P states (called H* here).
  The interaction increases the entropy of a set of beams without a permanent
excitation of H*, a loss of energy by a beam having a high Planck's temperature
producing a decrease of its frequency, and the thermal radiation getting
energy.
  Atomic hydrogen in its ground state is pumped to H* by Lyman alpha
absorptions, producing a redshift of the light. The combination of the Lyman
absorptions and the redshifts they produce, induce oscillations which generate
a spectrum in which the lines deduce from each other by relative frequency
shifts which are products of an integer by a constant z_b=0.062.
  These purely physical results may be applied in astrophysics, searching where
H* may appear. In particular, the computed spectra of the accreting neutron
stars, remarkably identical to the spectra of the quasars, may explain that
these stars seem never observed. The too high frequencies of the radio signals
from the Pioneer probes may result from a transfer of energy from the solar
light allowed by a cooling of the solar wind able to produce H*. A similar
transfer to the CMB may explain its anisotropy bound to the ecliptic.
