  `Oumuamua, the first detected interstellar visitor to the solar system,
exhibits non-gravitational acceleration in its trajectory. Ruling out other
means of propulsion, such as the evaporation of material via a cometary tail,
it has been argued that radiation pressure is responsible for this
acceleration. From this, the mass of the object must be approximately 40
tonnes, and given its dimensions, `Oumuamua must have a thickness of ~1 mm if
of a similar rock/iron composition as the Earth. This raises the much
publicised possibility that `Oumuamua is artificial in origin, sent
intentionally across interstellar space by an alien civilisation, This
conclusion, however, relies upon the common misapprehension that light (solar)
sails can accelerate to a considerable fraction of the speed of light,
permitting rapid interstellar travel. We show that such speeds are unattainable
for conceptual man-made sails and that, based upon its observed parameters,
`Oumuamua would require half a billion years just to travel to our solar system
from its closest likely system of origin. These cosmological time-scales make
it very unlikely that this is a probe sent by an alien civilisation.
