  We study the van der Waals friction between two flat metal surfaces in
relative motion. For good conductors we find that normal relative motion gives
a much larger friction than for parallel relative motion. The friction may
increase by many order of magnitude when the surfaces are covered by
adsorbates, or can support low-frequency surface plasmons. In this case the
friction is determined by resonant photon tunneling between adsorbate
vibrational modes, or surface plasmon modes.
