The floating Sigourney Weaver special effect is an actual physical effect, not an optical effect. The actress was put in a full body cast and attached to a post that was hidden in the curtains. According to the commentary, this effect came from director Ivan Reitman’s Broadway experience.
Does anyone know how they made that scene? Where is she lying?
Maybe the same way it was done in Ghostbusters?
There’s also levitation in Solaryis.