  We discuss the transport of a tracer particle through the Bose Einstein
condensate of a Bose gas. The particle interacts with the atoms in the Bose gas
through two-body interactions. In the limiting regime where the particle is
very heavy and the Bose gas is very dense, but very weakly interacting
("mean-field limit"), the dynamics of this system corresponds to classical
Hamiltonian dynamics. We show that, in this limit, the particle is decelerated
by emission of gapless modes into the condensate (Cerenkov radiation). For an
ideal gas, the particle eventually comes to rest. In an interacting Bose gas,
the particle is decelerated until its speed equals the propagation speed of the
Goldstone modes of the condensate. This is a model of "Hamiltonian friction".
It is also of interest in connection with the phenomenon of "decoherence" in
quantum mechanics. It is based on work we have carried out in collaboration
with D Egli, IM Sigal and A Soffer.
