But for a patent to be granted, a competent practitioner in the field should be able to assemble the device based on the supplied information. that's the essence of a patent: the mechanism is revealed in exchange for rights over it. So if a patent has been granted for an antigravity machine, the examiner must have believed it would be possible to build one. Or at least to build whatever is described in the patent, which may not function perfectly as an antigravity machine ;-)
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
