The evolution of quantum packets in the presence of an impenetrable boundar
y is studied. Due to an effective 'quantum repulsive force', the mean value
of the momentum in the direction perpendicular to the boundary gradually c
hanges in time at the expense of the energy of quantum fluctuations, and th
e packet becomes significantly narrower than it would be in the free space
under the same initial conditions. For zero initial mean transverse velocit
y, the asymptotical value of the transverse momentum depends only on the in
itial form of the packet, being independent of its initial position (provid
ed this position is much greater than the initial transverse width of the p
acket). As a consequence, narrow beams of ultracold particles (e.g., atoms
with velocities about 1 m/s and the initial transverse uncertainty in posit
ion about 0.01 mu m) directed along an impenetrable surface will be deflect
ed to observable angles due to the quantum (wave) nature of particles. The
possibilities of the experimental verification of the effect are discussed.
The conclusion on the inelasticity of collisions between ultracold particl
es due to the transformations of the shapes of their wave functions is made
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