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Here are some of the general characteristic regarding both FP and MR:

Flux Pinning

  • Requires a superconductor (usually type 2) that is cooled to extremely low temperatures
  • Requires a magnet whose magnetic fields will "activate" the flux pinning. The flux pinning consist of flux tubes which are small portions of a magnetic field piercing the superconductor and locking it in place.

Magnetic repulsion

  • Magnets that push away from one another rather than attract (caused by the magnets same poles facing one another, bearing the same charge) NOTE: Magnetic repulsion is used a lot with Maglev trains that practically "float" and can reach speeds of 500km/h. Hence this technology is already quite present in our modern day society.

It is also important to notice that both technologies work but require long tracks of magnets to keep the desired object floating/levitating.

Now if we imagine that cost is not an important factor and ressources are not finite, which technology would be more realistic to make a floating city come to life?

I also wonder how either of the technologies would affect the inhabitants living below. If flux pinning were to be used, then so would copious amounts of liquid nitrogen since that is what is currently used as the cooling agent in experiments for flux pinning. Would that amount of liquid nitrogen harm people on the ground, or could there be a way to keep the cold contained in lower compartments within the city itself and possibly vent it out in another manner?

I've also come across some designs of floating cities where magnets on the bottom of the city work to counteract the Earth's magnetic field. Furthermore, the lower half of the city spins, producing energy and working to keep the city balanced and stable while in the sky.

I'm by no means a expert in physics so a lot of this is quite new to me. However, I find the concept of a floating city utilizing magnetism so cool so I would like to stay within this field of technologies. Ultimately, if things just aren't feasible no matter what direction I go down, I would also be open to hearing about other options that could maintain a floating city.

Thanks!!

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    $\begingroup$ Neither technology is really feasible. Magnetic repulsion drops very fast with the distance, so to have your city float at any decent height at all, you'd need a large enough magnetic flux that it would start to interfere with biological processes. $\endgroup$
    – LSerni
    Apr 22 at 20:14
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    $\begingroup$ If your dystopia has half competent civil engineers? None.. floating cities are an engineering/maintenance nightmare... $\endgroup$
    – Questor
    Apr 22 at 21:32
  • $\begingroup$ @LSerni Not to mention the issues with ferromagnetic objects being anywhere in the vicinity. Including a significant fraction of the minerals in the nearby ground and in windblown dust. $\endgroup$ Apr 23 at 3:24
  • $\begingroup$ Feasible is boring. Requiring the city to meet real life requirements is boring. Expecting future science to be restrained by modern science is boring. Think about it: nobody in their right mind would actually build such a structure as it doesn't make sense from the perspectives of engineering, defense, ecology, or economy. But none of that should matter at all. Whether or not one makes sense over the other is irrelevant. Pick one and let us help you rationalize the choice - then write a good story for it and give us all the proverbial vulgar hand gesture. $\endgroup$
    – JBH
    Apr 23 at 3:29
  • $\begingroup$ Maglev trains and the technology associated with them are not at all common in modern society. Currently there is exactly 1 high speed maglev lines and 7 low speed maglev lines anywhere in the world. There is one low speed maglev line that was closed 2 years after opening and one high speed line under construction $\endgroup$
    – OT-64 SKOT
    Apr 23 at 6:03

3 Answers 3

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Frame Challenge: Don't Use Magnets

Even if you could levitate a city more than a couple of inches using magnets, the magnetic fields would be so intense that it would be like living inside of a giant MRI machine. You could not bring in any sort of metal or it would fall through you like a bullet, and electronics are a hard no, since the magnetic fields would be like an intense and constant EMP. Infact, as you pass about 7 Teslas even the human body begins to feel uncomfortable. In other words, all of the things that make a futuristic civilization won't work on a magnetically levitated city.

Helium is also a bad idea because the whole point of a city is to compress a lot of people and industries into a small space, but airships have such a terrible lift to volume ratio that you could not concentrate people into anything resembling an urban density.

Use the Casimir Effect Instead

Alcubierre warp drives are a theoretical kind of engine that would allow someone to accelerate a ship using warped spacetime, possibly even exceeding the speed of light. What Alcubierre and his peers discovered is that the amount of mass/energy required to propel a ship by bending a volume of spacetime is insanely high, but that you can create a nanoscopicly thin bubble of bent spacetime around an object to achieve the same effect for way less power. Alcubierre warp drives (in theory at least) have 3 notable characteristics that would make them ideal for city sized levitation.

  1. They have an inverse square-cube efficiency. This means that they are more power efficient the bigger they are; so, something big like a city would be more cost efficient to levitate than something smaller.
  2. They have no noticeable effect on anyone who is inside of the bubble; so, you can be sure that all of your advanced technology will still work inside of the city.
  3. They don't care how heavy thier load is; so, dense construction materials like concrete and steel will not be an issue. Alcubierre drives spend power to move a volume of space, not a weight of mass; so, you are not as constrained by building materials and personal belongings as you would be with any Newtonian levitation system.

So, if you assume your civilization can meet the power requirements to give a relatively small interstellar ship continuous acceleration using a warp bubble, then you can assume that they also have the technology to create a giant spherical structure that can maintain a modest 9.8m/s upward acceleration to counter gravity, and that said structure would be far more power efficient than a smaller object using the same technology. Ergo, if your setting has Warp Ships, then floating cities is a trivial application of the same technology. As of now, there is a known way to create these super thin distortions in space time using the Casimir Effect, so, even though we do not have any practical warp engines yet, the evidence says that this technology may very likely exist in the not to distant future.

Image Generated Using Dall-E

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Sorry, neither makes any sense, unless you're cool with your futuristic city floating an inch above the ground. Magnetism falls off rapidly with distance, so the distance between your city and the ground would have to be very small for this to work, and it would still require an enormous amount of energy.

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Frame challenge you could try (no guarantees you'd succeed): Tethered Aerostats!

Your 'city' is nothing more than giant collection of tethered balloons with modular living platforms hanging from a network of cables or framing below them. The problem is that in this city weight is everything. Only the lightest of construction materials would be used e.g duraluium, carbon fiber and timber veneers etc. Weight is at a premium so most inhabitants would live in sparsely furnished accommodation, sleep on the floor under blankets and own little more than they could carry. How rich or powerful you are? Decides the weight allotment your permitted. Water is also heavy so most people would only shower when it rains etc etc. The list goes on.

And as someone else pointed out above? It would still be a maintenance nightmare. So maybe the 'serfs' of your dystopian city spend their lives climbing out over the surface of aerostats (with minimal safety gear) inspecting, cleaning and patching any potential leaks.

Food and raw materials? That's another problem.

Maybe you could have separate floating (thermostatic) green houses but that would be hard to do. Maybe the city farms the earth in line of sight below it which increases with altitude (so potentially hundreds of square miles). Perhaps it uses it's height advantage to force the ground dwelling inhabitants below to feed them by threatening them with 'death from above' while also using the same threat to fend off would be raiders from their territory. Maybe it just sends out raiding teams to pillage other ground dwelling survivors some distance away for raw materials, slave labor and food. Perhaps it's all the above combined! That part is all up to you.

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