  We experimentally demonstrate optical dipole trapping of a cloud of cold
atoms by means of a dynamically coupled mode of a high-finesse cavity. We show
that the trap requires a collective action of the atoms, i.e. a single atom
would not be trapped under the same laser drive conditions. The atoms pull the
frequency of the mode closer to resonance, thereby allowing the necessary light
intensity for trapping into the cavity. The back-action of the atoms on the
trapping light mode is also manifested by the non-exponential collapse of the
trap.
