Andrews, Mark2015-11-092015-11-090002-9505http://hdl.handle.net/1885/16410We discuss four general features of force-free evolution: (1) The spatial spread of any packet changes with time in a very simple way. (2) Over sufficiently short periods of time (whose duration is related to the spread in momentum of the packet) the probability distribution moves but there is little change in shape. (3) After a sufficiently long period (related to the initial spatial spread) the packet settles into a simple form simply related to the momentum distribution in the packet. In this asymptotic regime, the shape of the probability distribution no longer changes except for its scale, which increases linearly with the time. (4) There is an infinite denumerable set of simple wave packets (the Hermite-Gauss packets) that do not change shape as they evolve.© 2007 American Association of Physics Teachers. Publisher permission to archive the version was granted via email on 19/11/2015. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in American Journal of Physics and may be found at https://doi.org/10.1119/1.29826282007-1210.1119/1.29826282015-12-08