  The measurement of pion source radii in Pb-Pb collisions with the ALICE
experiment at LHC is presented. By the analysis of Bose-Einstein enhancement in
the correlation function of identical pions information on the size and
evolution of the source region in space-time can be obtained. The source radii
observed at LHC are larger than at RHIC, approximately scaling with the cube
root of the particle multiplicity. A decrease of source radii with increasing
pair momentum, indicating a collective expansion of the medium, was observed at
RHIC and SPS and is also expected at LHC energies. The ALICE data confirm this
and provide strict constraints on dynamical models describing the medium
expansion. The results suggest that the created fireball has a higher
temperature and a longer lifetime as compared to lower energies.
