Just as you can always count on Gilbert and Sullivan for a rousing finale, full of sound and fury, and signifying nothing, you can always count on Mr. Niven to come up with the most fascinating places.
The obvious answer is the city is a giant grav vehicle, but their are other ways to float a city. This planet has a standard atmosphere, this would be a subgiant that is just the right distance from its star so that its temperate atmospheric layer is at 1 bar equivalent to Earth, it gets warmer the further down you go and colder the further up you go. Another version has a thin atmosphere, this subgiant is closer to its star than the standard atmosphere subgiant, so you have to be at a higher altitude where the air is thinner to experience confortable temperatures. On another sort of gas subgiant, the planet is further away from the sun, so humans must decend lower where the air is thicker to experience comfortable temperatures. Since the sun is fairly close in this instance, a city can be floated by a solar heated hot air balloon. Another solution is to use a fusion reactor to heat the air in the balloon. Alternatively the city can be a giant flying wing powered by the above two methods. Remember that the gravity is only one half that of Earth, a fact that assists flying, you can build bigger airplanes because of this.
Not much more of a problem than flying a fat man on an airplane. If the technology is so frail that you worry about the weight of individual humans, its not ready to float a city. Anyway the fat man doesn't weigh as much as you think, there is only 1/2-G here. As for Police states, some are some aren't same as in space stations and on worlds with size 0 or 1. On this world, the populous is dependednt on technology to keep from falling, on a vacuum world, they need it to breath. If someone falls, there is time to rescue him. On Earth terminal velocity is 100 miles per hour, here since the gravity is less, but the atmosphere is just as thick, terminal velocity is more like 50 mph. the air also doesn't get thicker and warmer asquickly with decent as it does on Earth, so their is time for an air/raft to swoop down and catch him. There is no ground to hit, it is only that the air will get thicker and warmer eventually presure cooking him, but before that he falls slower as the air thickens up. Falling is not as serious an immediate danger as it is on Earth, but you do have to be rescued to survive.
I'm afraid that Jupiter's Atmosphere is a fairly radiation free environment. Sure Jupiter's Van Allen belts are the most lethal in the Solar system, but that's in space. Once you get past and underneath it, their is no danger from that source. What kills you are those charged particles collected from the Solar wind and accelerated and concentrated within Jupiter's magnetic field. The particles get dumped into Jupiter's atmosphere at the poles where it produces a gigantic Aurora Borealis causing a greenish glow to appear in the sky at those latitudes. My idea was to create a sort of gas giant with a standard breathable atmosphere. To do that, you need the gas giant to be smaller than any in our solar system so all the hydrogen and helium escapes, and you need the gas giant to occupy Earth's position in the Solar system. The planet's atmosphere is composed of the same components as Earth's. As you go down deeper, the concentration of carbon dioxide becomes greater and conditions become more Venus-like. water clouds fill the skies at all levels. The deeper you go the hotter it is, but the increased air pressure keeps the rain drops from boiling away. At a certain level where its hot and the sun appears red there is plantlife in the CO2 atmosphere. The plants are in the form of tiny green oxygen balloons. The oxygen, produced by the plants is lighter than the CO2, so it floats in green clouds way below but visible to the level of human habitation. It is these that make the atmosphere above breathable to humans. Below this level the planet is very dark. You can call these plants Areoplankton, they are very tollerant of high temperatures and pressures. Most of the native organisms live at this level. higher up, the air is clear and the sky is blue with a yellow Earth-like sun. The lifestyle on this planet is very "Jetsonian", if you know what I mean, in that cartoon you rarely ever get to see the ground. Here there is no ground. Deep down inside this planet is a liquid core made up of salt water. This water circulates producing a magnetic field. Inside this water core is a tiny rocky inner core make in part of crystalized carbon dioxide, carbon, and rock. All this is unreachable to the humans living in the upper atmosphere.
"Queerer". J.B.S. Haldane said that the universe was queerer than we can imagine. The quote's obviously morphed out of its original shape because of the additional meaning "queer" has acquired since he first said it. Well, you did say you were misquoting him
