  The merger of two dense stellar remnants including at least one neutron star
(NS) is predicted to produce gravitational waves (GWs) and short duration gamma
ray bursts (GRBs). In the process, neutron-rich material is ejected from the
system and heavy elements are synthesized by r-process nucleosynthesis. The
radioactive decay of these heavy elements produces additional transient
radiation termed "kilonova" or "macronova". We report the detection of linear
optical polarization P = (0.50 +/- 0.07)% at 1.46 days after detection of the
GWs from GW170817, a double neutron star merger associated with an optical
macronova counterpart and a short GRB. The optical emission from a macronova is
expected to be characterized by a blue, rapidly decaying, component and a red,
more slowly evolving, component due to material rich of heavy elements, the
lanthanides. The polarization measurement was made when the macronova was still
in its blue phase, during which there is an important contribution from a
lanthanide-free outflow. The low degree of polarization is consistent with
intrinsically unpolarized emission scattered by Galactic dust, suggesting a
symmetric geometry of the emitting region and low inclination of the merger
system. Stringent upper limits to the polarization degree from 2.45 - 9.48 days
post-burst are consistent with the lanthanides-rich macronova interpretation.
