In discussions, and novels, about deep space missions, we frequently read of the plan to spin the space ship to produce artificial gravity. This intuitively makes good sense. But would it work in a ship that is very, very, very far away from any stars or other matter? That is, if the ship is spinning, relative to what is it spinning? If the proximity to matter is irrelevant, then does the universe have some inherent sense of direction or "upness"?