  The notion of fluctuation-induced forces is generalized to the cases where
the fluctuations have nonequilibrium origin. It is shown that a net force is
exerted on a single flat plate that restricts scale-free fluctuations of a
scalar field in a temperature gradient. This force tends to push the object to
the colder regions, which is a manifestation of thermophoresis or the Soret
effect. In the classic two-plate geometry, it is shown that the Casimir forces
exerted on the two plates differ from each other, and thus the Newton's third
law is violated.
