  We scrutinize the light sail scenario of the first interstellar object (ISO)
1I/2017 U1 (`Oumuamua) by making comparisons between physical models and
observational constraints. These analyses can be generalized for future surveys
of `Oumuamua-like objects. The light sail goes through a drift in the
interstellar space due to the magnetic field and gas atoms, which poses
challenges to the navigation system. When the light sail enters the inner solar
system, the sideways radiation pressure leads to a considerable non-radial
displacement. The immensely high dimensional ratio and the tumbling motion
could cause a light curve with an extremely large amplitude and could even make
the light sail invisible from time to time. These observational features allow
us to examine the light sail scenario of interstellar objects.
  Our results show that the drift of the freely rotating light sail in the
interstellar medium is $\sim 100\,$au even if the travel distance is only 1 pc.
The probability is < 1.5\% for the expected brightness modulation of the light
sail to match with `Oumuamua's observed variation amplitude ($\sim$ 2.5 -- 3).
In addition, the probability is 0.4% for the tumbling light-sail to be visible
(brighter than V=27) in all 55 observations spread over two months after
discovery. Radiation pressure could cause a larger displacement that is normal
to the orbital plane for a lightsail than that for `Oumuamua. Also, the ratio
of anti-solar to sideways acceleration of `Oumuamua deviates from that of the
light sail by ~ 1.5{\sigma}. We suggest that `Oumuamua is unlikely a light
sail. The dynamics of an intruding light sail, if exist, has distinct
observational signatures, which can be quantitatively identified and analyzed
with our methods in future surveys.
