  In 1964, Refsdal hypothesized that a supernova whose light traversed multiple
paths around a strong gravitational lens could be used to measure the rate of
cosmic expansion. We report the discovery of such a system. In Hubble Space
Telescope imaging, we have found four images of a single supernova forming an
Einstein cross configuration around a redshift z=0.54 elliptical galaxy in the
MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates
multiple images of the z=1.49 spiral supernova host galaxy, and a future
appearance of the supernova elsewhere in the cluster field is expected. The
magnifications and staggered arrivals of the supernova images probe the cosmic
expansion rate, as well as the distribution of matter in the galaxy and cluster
lenses.
