  Blackbody radiation around hot objects induces ac Stark shifts of the energy
levels of nearby atoms and molecules. These shifts are roughly proportional to
the fourth power of the temperature and induce a force decaying with the third
power of the distance from the object. We explicitly calculate the resulting
attractive blackbody optical dipole force for ground state hydrogen atoms.
Surprisingly, this force can surpass the repulsive radiation pressure and
actually pull the atoms against the radiation energy flow towards the surface
with a force stronger than gravity. We exemplify the dominance of the
"blackbody force" over gravity for hydrogen in a cloud of hot dust particles.
This overlooked force appears relevant in various astrophysical scenarios, in
particular, since analogous results hold for a wide class of other broadband
radiation sources.
