We report the polarized optical light curve of a gamma-ray burst afterglow obtained using the RoboPol instrument .
Observations began 655 seconds after the initial burst of gamma-rays from GRB 131030A , and continued uninterrupted for 2 hours .
The afterglow displayed a low , constant fractional linear polarization of p = ( 2.1 \pm 1.6 ) \% throughout , which is similar to the interstellar polarization measured on nearby stars .
The optical brightness decay is consistent with a forward-shock propagating in a medium of constant density , and the low polarization fraction indicates a disordered magnetic field in the shock front .
This supports the idea that the magnetic field is amplified by plasma instabilities on the shock front .
These plasma instabilities produce strong magnetic fields with random directions on scales much smaller than the total observable region of the shock , and the resulting randomly-oriented polarization vectors sum to produce a low net polarization over the total observable region of the shock .