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Writing about suspended cities within a planet's atmosphere! The story itself focuses on the air forces that fight for each of the hundreds of floating city states living in the sky, but that's here nor there. I would prefer the suspended land masses to be naturally occurring (predating technology).

I've come up with some options for how it would work, but I could use some help with looking in depth at each possible explanation for how the cities fly.

  1. Asteroid Lodestones - this is my favorite theory, as it has naturally occurring floating-land that has been around for millennia in this world. The theory is that over time, space rock crashes into a planet as it does with any planet. But these asteroids are naturally magnetized to be repulsed from the planet's surface. Therefore, the asteroids enter the atmosphere and get hung in the sky before impacting. Or after impacting. Either way, this allows for floating land without technological help.

  2. Magically powerful atmosphere - Piggybacking off theory number one, if the asteroids weren't NATURALLY magnetized to repel from the planet's surface, could the space rock by magnetized as it passed through a planet's atmosphere? Could, theoretically, the magnetic waves be that powerful in the upper layers of an atmosphere? And could something be magnetized in such a violent, intense way?

  3. Electromagnetism (maglev) - I have a rough understanding of this. Electric currents are needed to produce a magnetic charge, which keeps things afloat? Do the rocks themselves have to carry the current? If not, is there a separate magnet which has to conduct electricity, such as a tower or surface-based giant magnet? Similar to theory two, could an asteroid by electromagnetically charged when entering an atmosphere?

If you guys could give me feedback on these, any tweaks to these theories, and any other ideas about floating land masses I would be very grateful!

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    $\begingroup$ Arthur C. Clarke didn't bother to to explain how his utopian floating cities worked. He just narrated how bored his protagonist was with perfection while staring over the parapet at another city nearby. I think your answer, one way or another, is simply 'magic'...which can work any way you want it to work. $\endgroup$ – user535733 Oct 23 '18 at 1:49
  • $\begingroup$ I may indeed go that route if I have to, but I'd rather not. It would be really advantageous to the story if the atmosphere could play a role in the suspended cities. I want to create a mystical, bermuda triangle like mythos around the atmosphere. With that said, I need to more fully understand how the high layers of the atmospheres work. Do they hold powerful magnetic charges? Would the charges/polarity they give be related to the planet's magnetic charge/polarity? Would it even be possible for an atmosphere to magnetize a rock to be repulsed by the surface? $\endgroup$ – StarHustler Oct 23 '18 at 1:56
  • $\begingroup$ @StarHustler There won't be any thing real world that will work, because if it did, we would see it in real life. You can use any of the ideas you want even if they aren't scientifically possible, they just need to be believable. Just take the time and look back at what you are saying. Does the audience want to hear about a detailed explanation of why it works? Does the reason why they float really matter, does it have some plot around it or is it driving the story? You can brush it off by saying, The remnants of the asteroids remained floating across the sky, without having to dive into it. $\endgroup$ – Shadowzee Oct 23 '18 at 2:50
  • $\begingroup$ @user535733 As someone reasonably familiar with Arthur C Clarke's fiction (and non-fiction too), I don't recognize any story of his with utopian flying cities. Could you please give its title? $\endgroup$ – a4android Oct 23 '18 at 4:02
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    $\begingroup$ @user535733 - I agree to the idea that in fiction things should just "work." After all we want fiction to actually have surreal elements beyond what is known as rather "boring" and already widely explored reality. --- You want floating isles on which cities are built? Just do them. There can be mysticism, technology or natural phenomenon hidden beyond the comprehension of the characters themselves (from the view you can witness the world in the first place). Just be consistent in what you do! $\endgroup$ – Battle Oct 23 '18 at 11:57
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Magnetic field of a planet will not hold a city up. Even if it is strong enough, it can hold cities only at the two magnetic poles. Such city will need lots of magnets strategically placed and correctly oriented in its foundations, so it is unlikely to arise naturally.

You can have city in any spot if you have similar magnets placed in the ground below it. You cannot have entire planet magnetized. You really should read up on magnets, their north and south poles, and magnetic field lines.

You cannot magnetize atmosphere, and magnetic field that is off-planet and strong enough to hold up a city will make all iron fly into the sky.

If you want cities to be mobile, use dense atmosphere and balloons. Zeppelins were pretty robust when in the air, most of the crashes happened during landings.

If you want natural, have plants that grow air bubbles filled with methane or helium (harvested from underground sources). I know kelp has such bubbles (but underwater). Or you can natural gases making foam out of lava. Once foam solidifies, and if solid labs is separated from it, it will float in the air.

Edit: in response to OP's comments You really needs to learn more magnetic field and how has a direction.

Magnetic repulsion can only happen between two magnets, and only if they are lined up just right. Moreover, if magnets are simple shapes like balls, they will push each other away from that perfect alignment. You can try it with any two small magnets, and it will work like that with magnetic field of a planet.

To get stable leviation with magnets, you need special shape of lower magnets (or multiple magnets) E.g. this: http://web.mit.edu/viz/levitron/Physics.html Or more recent superconducting hovering skateboard (which can only hover over magnetic carpet).

Levitating above a magnetic pole will require very special arrangement of permanent magnets in the floating city, since planet is a ball magnet, not a ring like in levitron above.

Magnetic field of a planet cannot magnetize an asteroid to achieve the right shape of magnetic field. Moreover, such induced magnetic field is short-lived, and levitating above the pole will quickly re-magnetize the asteroid to be attracted to the pole.

You can have ancient/alien race construct the floating city with strong permanent magnets. Or you can just hand-wave some rocks with negative gravity (like in Spielberg's Avatar movie).

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  • $\begingroup$ But I think even if the cities are built in the magnetic poles, the polar attraction will still be small that small magnets will not float on poles. $\endgroup$ – Mr.J Oct 23 '18 at 2:42
  • $\begingroup$ @Me.I I said field needs to be strong enough. Eg planet's core rotates real fast. $\endgroup$ – Bald Bear Oct 23 '18 at 2:46
  • $\begingroup$ Even if it is strong enough, it can hold cities only at the two magnetic poles. But still, the planet's core is far from the planets crust, so even if the core rotates real fast, I still think the magnetism is small, maybe a big core? then thin mantle and crust? (if based on earth like category) Thank you for the time. $\endgroup$ – Mr.J Oct 23 '18 at 2:52
  • $\begingroup$ So magnetic repulsion can only occur at the poles? Would it not be possible to have asteroids made of material that is naturally magnetised to be repulsed from the planet's magnetic charge? $\endgroup$ – StarHustler Oct 23 '18 at 3:04
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    $\begingroup$ @StarHustler It is not ruled out that there is a solid core in gas giants like Jupiter, however, heat and pressure will render it inaccessible. $\endgroup$ – Otto Abnormalverbraucher Oct 23 '18 at 12:06
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My first bit of advice would be to drop the idea of magnets or anything magnetic as being the mechanism. To have it occur "naturally" would involve too many contortions of planetary mechanics and using maglev would be impractical as well - the inverse square law would mean that having an altitude of more than a few centimetres would require so much energy as to be ludicrous.

What I propose instead would be to have the lower regions of the atmosphere be a "soup" of very dense gases and your cities based on asteroids made up of a materiel that is less dense that, as a result floats on the denser atmospheric layers beneath.

This way you could still have:

  • naturally occurring landmasses that "fly"
  • a planetary surface beneath (you'd need a pretty pressure-resistant craft to reach it mind you - but not much beyond a submarine)
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  • $\begingroup$ Seems like magic may be my best route! Such great answers, guys! $\endgroup$ – StarHustler Oct 23 '18 at 13:20
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Kite city.

kite city https://www.artstation.com/artwork/nY1bO

Your city does not float. It soars. Your world is one with phenomenal updrafts and thermals. Huge, fragile cities kite above the surface. The city kite pilots work in shifts and their job is crucial - watch the clouds and the weather and keep the city afly. They have spinnakers and accessory kites they can deploy, and the great kite cities can fold in surprising ways - the citizenry react to these movements without breaking stride. Large cities can have defensive capabilities like dreadnoughts. Smaller cities converge on large vehicles. There is a spectrum from large to small, all kiting their way as the air takes them.

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Not a great physics person, but I am taking astro and enjoying it, so here goes (consult with someone else who knows physics).

From our friend science (please correct me, but this is angular momentum no?) http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html

Anyway, basically what this means is that the orbit time is equal to 4π^2 (a constant) over another constant times the mass of the orbiter (city) + the orbitee (planet) times the distance between them cubed.

With all that out of the way, I feel like it would be possible if you just had the cities orbiting really low? I guess that means they're going slowly, but you'd just have to make sure that they aren't hitting escape velocity enter image description here where M is the same M as the mass of the planet mentioned above, G is the same as G above, and R is the radius of the planet (I believe).

Here are some of the problems you'll have to deal with if you take this route:

  • you'll need to have a very thin atmosphere (because the city will need to keep moving at the speed it is going and the atmosphere will probably cause lots of resistance/heat (I'd imagine)
  • you could try to make the city and the planet have the same rotational speed as the orbit, so that basically from the ground it looks like the city is always directly ahead and the same part of the city always faces the moon (look up synchronous rotation, it's why we always see the same side of the moon).
  • if the cities speed up, they will need to be careful because they could hit the exit velocity and get ejected into space (which is kind of a fun thing to have to worry about in a story).

Anyway, some of the science in this post is probably wrong, but I enjoyed writing it and you might be able to pull some ideas out.

Oh, one more thing. You could try doing it space elevator style (esp. if what I recall is correct). You could launch something beyond orbit, but have it tethered to the planet (the big problem on earth is we don't have the material to do it, but that's easy to fix) and then basically it just swings with the earth's rotation. I'd do more research into this if that's what you're thinking of.

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Realistic Gravity Manipulation

So, there is a certain level of handwavium element to the execution of this idea, but the physics are kind of solid. Gravitoelectromagnetism. Doesn't have anything to do with electricity or magnets at all, but scientists are kinda jerks like that. Anyways, it was discovered a few years ago by Gravity Probe B that large rotational masses actually cause an effect called frame dragging, whereby space and time are kind of drug along and slightly twisted with the large rotational mass. It was discovered (at least on paper) that two of these rotational masses rotating in different directions could possibly cause some repulsive effects somewhat similar in nature to the effects of electrical charges or magnetic fields repelling each other, except with space, time, and gravity instead. So basically, in certain theoretical scenarios you can get gravitic fields to repel each other kinda like how magnets or electrical fields do. Also time and space has something to do with it. Its weird. I suck at math too. And physics.

Thing is, we're talking about two planetary masses, and the rotational rate to achieve cancellation has to be nearly relativistic right? So it's not exactly like we can just take two planets and spin them at a high percentage of the speed of light in opposite directions right? Well... kinda.

Take some extremely dense matter from a white dwarf and contain it in some sort of ring. Now you have something as massive as a planet, but just really really dense. Take two of these rings and accelerate the white dwarf matter inside of each ring in opposite directions to a percentage of the speed of light like a really big, really energy hungry particle accelerator.

Viola, opposing frame dragging effects begin. Now here is where things get really cool (Assuming the math is right). Anything sent through the center of these rings will be accelerated without experiencing any G forces. Its basically being pulled and pushed along at the same rate by opposing torsional fields of space and time and thus is in effect, no longer experiencing the effects of gravity. As long as the object sat in the sweet spot between these two cancelling fields it would in essence, just kind of sit there suspended. So you got one of these rings of hyper dense matter being accelerated at high speed below the city, and one somehow above it (in a geostationary orbit maybe?) and the city sits in the sweet spot between the two where gravity is no longer effecting it.

Problems!

The math is wrong: Somebody might have miscalculated some variables, or may have misinterpreted the findings and the entire theory turns out to be bunk. Frame dragging effects and their exact effects have yet to be accurately modeled and studied, it is believed that doing so would be incredibly difficult and until this is done all of this is still not considered to be confirmed science. The theory is relatively new and not very well studied yet.

Even if Not, Still Waving Hands Pretty Hard: Collecting mass that dense, containing it, then accelerating to relativistic speeds, then keeping it that way indefinitely requires a lot of energy. I illustrate the amount required vs what they are using it for in our terms, its like building a nuclear reactor just to run a single coffee maker. At minimum our race would have to have a Kardeshev 2 level ability to access and manipulate energy. (IE can harness 100% of the energy output of a star or its equivalent.) This level of technology is so unbelievably out of our reach that talking about specifically how they do it with our current level of development is basically waving our hands and grunting "magic." This phenomenon will most certainly never be found in nature or achieved by a pre-technological society. If you want to go that route you are just going to have to utilize the old "its just magic" explanation.

enter image description here

People Dwelling in the City Don't Have Gravity Either Pretty Self explanatory. The object in between these opposing fields do not experience gravity. Period. Nothing in the city is being effected by any gravitational fields outside of the area of nullification. At this point, you might as well just build an orbital city. I mean, maybe your Kardeshev 2 level society just really likes showing off and builds something impractical just because they can, Its been done before. Its just not a very practical way to do things. Maybe if we are really handwaving here the same fantastically advanced people somehow managed to make the field only effect the city, or found a way to counteract it within the confines of the city? In which case you are basically just saying "meh, its science." Then you just end up utilizing the same trope as the magic thing earlier.

enter image description here

Summary

Anti-gravity and gravity manipulation may very well be possible, at-least on a theoretical level, but not naturally. The idea that something naturally occurring or predating technological sources could achieve this with magnetic fields or something is bunk. The precisely balanced system of high energy effects required to make something like this happen simply doesn't occur naturally. So I have a proposal:

The great old ones trope. A Kardeshev 2 level society built all these floating cities a long, long time ago as a bunch of resorts or fancy ultra-technological space mansions or something. They all died, or ascended, or got bored and left or something a long long time ago too. Now a bunch of comparatively backwards people descended from their estate servant staff who got left behind live on these floating cities and islands with zero clue as to their origins or original purpose. They've just always been there and as such nobody considers that to be terribly odd or worth looking into. (Also the last time somebody got curios and went poking around they accidentally turned the gravity back on and everybody died so nobody does that anymore.)

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You'll be needing a bullshit explanation for this so lets get a bullshit explanation. Keeping even a few tons on the air is going to take nuclear bombs worth of energy daily, building the entire city out of magnets isnt going to give enough lift for your purposes.

So heres the bullshit I came up with: somebody, perhaps your own people or perhaps aliens who moved on beyond our reality, created matter with negative mass for all manner of purposes such as space travel. To make it easier to handle they gave the material attraction to itself similar to how water attracts itself, as negative mass would push itself away otherwise. Much of the waste coalesced and a bunch soaked into the ground on your planet. This reduced the mass of all the material it soaked into and make it into a lower than air mass, allowing it to float.

Advantage is that anything that soaked the negative mass can float without any special circumstances in the atmosphere or ground needed to remain airborne. And the negative mass could be used in some aircraft for example.

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