  Transient spiral waves of moderate amplitude cause substantial changes to the
angular momenta of many stars in a galaxy disk. Stars near to corotation are
affected most strongly: for a wave of ~20% overdensity, the rms change for
particles near to corotation is ~18% of their initial angular momenta. Yet
these changes produce no increase in random motion near corotation, while the
same wave causes mild heating near its Lindblad resonances. The direction of
radial migration depends both on initial location relative to corotation and on
azimuth. Streamlines of cold gas become very significantly distorted before
they cross, suggesting that gas is also mixed radially by this mechanism. Such
continuous churning of the disk has profound implications for abundance
gradients in galaxies. Furthermore, it is quite possible that the Sun has
migrated significantly from its radius of birth, consistent with the claims of
Wielen et al.
