The space isolating barrier is only locked relative to the magician's imagination. Those people are still on a planet with a day-night cycle and presumably seasons. It's a different universe, so who knows, but we might assume their star system is still a part of a galaxy. So, they are moving nonetheless, very fast, through space. Unless it's an extremely different universe where the planet is totally static, the sun orbits the planet, and so forth, but that seems like a stretch. The pondering here is locking it relative to the planet's surface, but not above that level (in scale). But why is that so? Why can't he lock it relative to the ship. So, it can't be moved on the ship's deck, but it will move with the ship.