The Hoopworld is right on the edge of the possible, but according to my research the science isn't pure fantasy.
A Hoopworld would be a structure that would place humans on the outside surface of a giant spinning torus, or donut. A Hoopworld could potentially have many different shapes, depending on the length of the revolution axis and the radius of the circle being revolved. Different-shaped tori would also affect gravity, the day-and-night cycle, and weather conditions.
The hoop would be spun along the axis of revolution to produce gravity and stabilization, and to provide night and day to different areas on the torus. Gravity would first depend on the amount of mass on the interior of the hoop, and would likely require an active support structure. Second, since the hoop would be spinning, centrifugal force would cause stronger gravity on the inner equator, since you’d be pulled toward the outside of the hoop in the direction of the ground. The outer equator would have less gravity, since you’d be pulled toward the outside of the hoop in the direction of the sky.
Tipping the torus toward or away from the sun would create seasons, affect weather, and could create a wide variety of climates over different parts of the torus’ surface.
And if you like this sort of artwork, please check out my new book project "Megastructures: The Visual Encyclopedia" here: