  Spiral arms, if they are massive, exert gravitational torques that transport
angular momentum radially within galactic disks. These torques depend not on
the pattern speed or permanence of the arms but only on the nonaxisymmetric
mass distribution. Hence the torques can be measured from photometry. We
demonstrate this using $gri$ CCD data for M100 (NGC 4321). Since we find
consistency among the three bands, we believe that dust and young stars in the
arms do not seriously bias our results. If the present epoch is representative,
the timescale for redistribution of angular momentum in M100 is $5-10$ Gyr, the
main uncertainty being the mass-to-light ratio of the disk.
