We calculate the friction force between two semi-infinite solids in relative
parallel motion (velocity V), and separated by a vacuum gap of width d. The
friction force result from coupling via a fluctuating electromagnetic field,
and can be considered as the dissipative part of the van der Waals interaction.
We consider the dependence of the friction force on the temperature T, and
present a detailed discussion of the limiting cases of small and large V and
d.Comment: 15 pages, No figure