  Spiral arms have been observed in more than a dozen protoplanetary disks, yet
the origin of nearly all systems is under debate. Multi-epoch monitoring of
spiral arm morphology offers a dynamical way in distinguishing two leading arm
formation mechanisms: companion-driven, and gravitational instability
induction, since these mechanisms predict distinct motion patterns. By
analyzing multi-epoch J-band observations of the SAO 206462 system using the
SPHERE instrument on the Very Large Telescope (VLT) in 2015 and 2016, we
measure the pattern motion for its two prominent spiral arms in polarized
light. On one hand, if both arms are comoving, they can be driven by a planet
at $86_{-13}^{+18}$ au on a circular orbit, with gravitational instability
motion ruled out. On the other hand, they can be driven by two planets at
$120_{-30}^{+30}$ au and $49_{-5}^{+6}$ au, offering a tentative evidence
(3.0$\sigma$) that the two spirals are moving independently. The independent
arm motion is possibly supported by our analysis of a re-reduction of archival
observations using the NICMOS instrument onboard the Hubble Space Telescope
(HST) in 1998 and 2005, yet artifacts including shadows can manifest spurious
arm motion in HST observations. We expect future re-observations to better
constrain the motion mechanism for the SAO 206462 spiral arms.
