  Accelerating non-spreading wavepackets in nonrelativistic free particle
system, with probability distribution having an Airy function profile, were
discovered by Berry and Balazs (1979), and have been subsequently realised in
several optical experiments. It is shown that these wavepackets are actually
Perelomov coherent states. It is found that the Galilean invariance of the
Schrodinger equation plays a key role in making these states unique and giving
rise to their unusual propagation properties.
