De Sitter space and perpetuum mobile
  We give general arguments that any interacting non--conformal {\it classical}
field theory in de Sitter space leads to the possibility of constructing a
perpetuum mobile. The arguments are based on the observation that massive free
falling particles can radiate other massive particles on the classical level as
seen by the free falling observer. The intensity of the radiation process is
non-zero even for particles with any finite mass, i.e. with a wavelength which
is within the causal domain. Hence, we conclude that either de Sitter space can
not exist eternally or that one can build a perpetuum mobile.
