  A tidal disruption event (TDE) occurs when a supermassive black hole rips
apart a passing star. Part of the stellar material falls toward the black hole,
forming an accretion disk that in some cases launches a relativistic jet. We
performed optical polarimetry observations of a TDE, AT 2020mot. We find a peak
linear polarization degree of $25\pm4$%, consistent with highly polarized
synchrotron radiation, as is typically observed from relativistic jets.
However, our radio observations, taken up to 8 months after the optical peak,
do not detect the corresponding radio emission expected from a relativistic
jet. We suggest that the linearly polarized optical emission instead arises
from shocks that occur during accretion disk formation, as the stream of
stellar material collides with itself.
