  The van der Waals friction between two semi-infinite solids, and between a
small neutral particle and semi-infinite solid is reconsidered on the basis of
thermal quantum field theory in the Matsubara formulation. The calculation of
the friction to linear order in the sliding velocity is reduced to the finding
of the equilibrium Green functions. Thus this approach cab be extended for
bodsies with complex geometry. The friction calculated in this approach agrees
with the friction calculated using a dynamical modification of the Lifshitz
theory, which is based on the fluctuation-dissipation therem. We show that the
van der Waals fricxtion can be measured in non-contact friction experiment
using state-of-the art equipment.
