This is heavily inspired by sunless skies in that sunless skies' world allows for the use of steam engines as space ship- equivalent (using steam jets as thrusters, while still able to maintain a boiler). They have floating islands, relatively low to zero gravity in between islands and of very directional components on these space ships like fireboxes and boilers. This implies to me that the "land" has intrinsic, directional gravity that's almost planar, rather than points like how the center of gravity is usually denoted. Ships would internally have this planar center of gravity as well. Further, it seems that they can breathe air while underway in the skies, somewhat like that seen in treasure planet, that solarpunk disney movie.

Disclaimer: I haven't actually bought nor played the game yet, I've only watched snippets of the playthrough. I plan on saving up to play it for myself, hence the very vague conjecture.


I want to build that into my own world in that there is a way to have a clear directionality of up and down while on land but zero gravity once in space, i.e. stepping off the border of the island, but maintaining a breathable atmosphere throughout space. This would mean that while gravity affects most things, the atmospheric gas that people breathe does not follow gravity or has its own intrinsic force that counteracts or cancels gravity. Though, this gas bit isn't a requirement.

TL;DR: Is there a way to form a gravitational system for the world that can:

  1. Support directional gravity on free floating items like islands and
  2. Support a homogenous or an effectively homogenous atmospheric pressure in zero gravity.

I'm not looking for hard science, just a system that makes sense and wouldn't fall into an endless fractal cycle of explaining the quirks of the explanations.

Things I've tried

I've toyed with the fact that gravity could be a dipole like magnetism but that'd mean people could be sucked to the bottom side of the island and stuck to their heads. Pity the poor fool whose gravity was bourne onto him sideways. Further, since gravity on the scales that we know of can effectively be considered a constant force, transferring that to an object whose scale isn't conducive to that provides the need for furious handwaving

Another one I've tried is that there is an intrinsic repellant force, but having gravity and antigravity forces would just subtract each other into a positive or negative gravity force so it wouldn't necessarily be explained well either. (I've given up on this one without much thought, is the disclaimer for this one)

Working solution- a little too complicated...

So far what has gotten me the closest that I've gotten is that there are three types of gravity: two distance based gravities and one directional gravity.

First distance based gravity is our normal gravity. This is somewhat weak compared to the other gravities.

The second distance gravity is the intrinsic "repellant" force** caused by another intrinsic property of mass that causes masses to want to push apart like similarly charged electrical point charges. In this world, it is not thermal energy that keeps the particles apart, although it does help, it is this secondary distance based gravity that does the heavy lifting. further, the secondary gravity follows a function whose parent function is something like $1- \log_a x$ rather than $1/x^2$ such that $F_{g_{total}}=F_{g_{primary}}-F_{g_{secondary}}$

**although it is found out that it both pulls and pushes, depending on exactly how far the thing is.

The third gravity would be based flux in that there is a 2 dimensional element to every single bit of matter and it can be thought of as having the area proportional to the mass of the thing it is associated with. This element will want to orient itself to have the maximum positive flux (opposite orientation would mean negative flux) of the tertiary gravitational field lines through itself. Most matter isn't really affected by this; this element has a weak, though perceptible, interaction with most matter (think magnetic polarization of metal, change orientation and it is alright but do it too much and it heats up), except for Gravitum which is considered nearly deadlocked with this 2d element of itself. It holds up most land and gravitum also is installed into the floorboards of ships so they would have proper direction of up and down. tertiary field lines are generated at the center of the universe by a massive ball of centrium, which has abnormally strong secondary and tertiary gravitational forces and a weak.

The universe's planets are generally flat islands orbiting the center of the universe, though the suns are still spherical due to its elements using first gravity to fuse and generate light.

This world establishes good directionality and (somewhat) the atmosphere but there's furious hand waving and I don't really know how to resolve most other problems with this universe in that technically people still can float off islands pretty easily, just spinning anything would cause it to heat up, etc.

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    $\begingroup$ The hard-science way to get this is to have the setting be a virtual world. As for why noone would want to get out of "The Matrix", it is set in a galaxy in the far future where all matter is computronium running simulations or being converted into energy for running digital reality. $\endgroup$ Feb 18, 2023 at 20:39
  • $\begingroup$ I kinda wanted to avoid the virtual world kind of thing. I intended to make a sort of Treasure planet sailing the high skies kinda deal but with drifting islands instead of planets. Kinda like a 3d sea. $\endgroup$
    – Harry Mu
    Feb 18, 2023 at 20:45
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    $\begingroup$ What about instead of flat planes, each island is like a ring that creates artificial gravity through centrifugal force, but if you step off you are in free fall in some larger space filled with air? Air pressure could be maintained if it was a finite space so there'd be nowhere for the air to go, though I'm not sure if it'd be possible to also have a star in that space without all the gas molecules falling into the sun in fairly short order. Alternately the air could be contained in some giant bubble-like megastructure. $\endgroup$
    – Hypnosifl
    Feb 19, 2023 at 22:11
  • $\begingroup$ Possible duplicate of worldbuilding.stackexchange.com/questions/102965/… $\endgroup$
    – wokopa
    Feb 19, 2023 at 22:57
  • $\begingroup$ @wokopa that's actually pretty close but I was looking to create a universe (mostly gravity system or other universal force system) with a universal "up and down" on top of making it possible to have roughly planar floating islands. $\endgroup$
    – Harry Mu
    Feb 20, 2023 at 2:52

4 Answers 4


Not in our universe

Let's do a classic thought experiment: there is no gravity. Every particle behaves like a ball on a billiard table. They hit, redirect and drift apart. Temperature is nothing but the speed of those balls. Without gravity and friction, a cloud of particles will ultimately drift apart more and more, until the cloud is far enough apart that the partial pressure of oxygen is no longer breathable.

Now, let's assume there is a sticktion force that makes any balls that meet stick together forever, but not have any effect on particles nearby. As a result, there will be a massive ball of a (solidified) matter and a slowly drifting apart cloud made from particles that didn't have an impact in their way.

Gravity is not our fictional stiction force in that collision doesn't stick particles together and that it has a distance effect, but if you have enough mass in the core, it's close enough: the attraction to the large body prevents that other particles escape. As long as your particles are slow enough to not escape the bubble (which might need a massive core) and fast enough to not spiral into the core, the cloud can exist in microgravity.

To have the bubble without gravity, a hard bubble shell is needed.

Due to matter and gravity being undirectional, directed gravity doesn't exist. It can't exist. Gravity exerted from Particle A on Particle B is always going towards the other particle - they attract each other.

There is no way to redefine gravity as a dipole without having the bubble either suddenly collapse as particles align and clump up or particles being ejected. This can be trivially tested by using a field of magnets: most will align and form a super magnet in the center, and a few unlucky ones are thrown off the table.

The interstellar sea

Let's for a moment forget about gravity and everything. Let's just define that interstellar space is breathable due to people getting a Handwavium Injection.

Now we define that each solar body creates a dipole Solar Horizon Field, that is neatly aligned: all suns align with their fields like magnets, creating an orderly directional field in a rough estimation. Interstellar objects align to this field and rotate on orbits that follow the strangely shaped surface that is spun up by the many stars - The Solar Sea. In the area of a single solar system, the Solar Sea can be estimated by the ecliptic surface over the equator of the star, though it's not a flat plane, it is distorted into a rippled wave...

Of course, this wouldn't work in our universe, but on the Stellar Sea, where you can breathe Handwavium, the suns all have an upside, and every island aligns to this Solar Horizon Field due to Narrativum. Ships use a device that keeps their upsides aligned with the interstellar field, and thus sailing "on" the Stellar Sea.

  • $\begingroup$ Yes I didn't quite intend to put it in our universe. I intended to rewrite some parts of physics itself in that world to make it happen. As for the billiard analogy, my secondary gravity has this thing in that at a certain range of distance, an antigravity force would temporarily exceed the normal gravity force and push them apart. They get close but push apart without contacting. Islands would be floating in this area, but they would also have their own gravity that their inhabitants stick to? $\endgroup$
    – Harry Mu
    Feb 18, 2023 at 20:49
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    $\begingroup$ @HarryMu as a thought experiment... try the Stellar Sea. The field is distinctly not gravity $\endgroup$
    – Trish
    Feb 18, 2023 at 20:59
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    $\begingroup$ This seems to solve my problem but I will leave the answered bit unchecked for a few days to allow ideas to flow in. $\endgroup$
    – Harry Mu
    Feb 18, 2023 at 23:09

Let me borrow from an answer of mine to this question

In your , your worlds swim through an actual Luminiferous Aether

In the good old days, long before humanity actually understood that space is remarkably empty, we though there was stuff that the worlds swam through. We called it a "aether," and believed it's what allowed light to travel between the sun and the Earth.

Today, the ability to simply declare such an aether exists in the Real Universe (or something close to it) is difficult because the motion of planets through their orbits and gravity would sweep the aether clear very quickly (on a cosmological scale).

But that's not how your universe works!

In your universe, the aether, let's call it "Dark Matter" (if only for the sake of poking fun at what could be a very realistic way of asserting such an aether in the Real World) is something you can't see... but it is something that can be warmed up... and since there's no where particularly for the heat to go (I'm about the throw thermodynamics out the window, but breaking windows is fun), it's available to warm your planets.

A rule like this does have consequences

If you set a rule that says the aether exists but basically doesn't interact with your planets (meaning it's not just dust being sucked down to the planet surface), that means it's being pushed aside (by the magnetosphere, dontchaknow) as the planet orbits. But that means you have a bow wave leading the planet in its orbit and a wake trailing the planet. That would have some fun consequences with how light is seen by people on your planet, especially at dawn and dusk.

It also means there could be the consequences of friction as your planet moves. In the Real World, friction would eventually slow the planet, causing it to spiral into the Sun. But in your , this doesn't happen. The sun's gravity, perhaps, impels the planet, keeping it moving and overcoming the friction. This means you could have some amazing aurora-like effects during dawn. And if your story includes space flight, avoiding the friction effects would be a whomping big deal.

Another benefit is that spacecraft could get rid of excess heat through convection rather than radiation. Ooooh, that would be a beni. Ridding a craft of heat in the Real World is a big deal.

Finally, allowing a solar system full of planets to swirl around in what can only be called a lovely soup means that the aether is also being stirred, resulting in a whirlpool or vortex around the sun. I'd like to imagine that the aether's natural state is to not do this, otherwise it would negate both of my previous two suggestions. But it does mean that celestial navigators must deal with currents, eddies, and other navigational hazards in three dimensions that are usually only seen by seafaring craft in (simplifying) two dimensions.

Your world can build on this same idea, your universe has an aether. It's an entirely breathable atmosphere that permeates throughout the universe. It isn't ignited by stars or other celestial bodies because the solar wind (and its equivalent for other celestial bodies) pushes the aether back. Thus, you can't breathe all the way up to the coronasphere of a sun. Why would you want to?

You'll need to invoke a number of other rules, like the idea that the aether doesn't cause gravity like "other matter" and doesn't carry heat like "other matter" (in other words, you sun can keep things warm on an island/asteroid/planet, but the further you get from the planet, the colder things get). But setting that aside. Boom.

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    $\begingroup$ This is actually an interesting thought. I was on the fence on whether or not I should add magic (that comes with its own universe creation rules) but I guess one ability an astronaut can learn would be aether breathing where aether gets converted to the human store of mana and then that is used in an ongoing magic spell to create and use oxygen. But that's already a tangent. Without the magic interpretation, aether being "another gas" probably isn't all that usefully breathable ( like huffing nitrogen) so some consideration needed. But combine this and Trish's stellar sea It looks pretty good $\endgroup$
    – Harry Mu
    Feb 19, 2023 at 3:18
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    $\begingroup$ @HarryMu Yeah. I was tempted to try and figure out some kind of molecular membrane that could constrain the atmosphere such that it was held together in zero-G, but frankly, that seems more magical than this, especially if it's to be punctured for travel. $\endgroup$
    – JBH
    Feb 19, 2023 at 7:13

I'm reminded of Niven's "The Integral Trees" (https://en.wikipedia.org/wiki/The_Integral_Trees) where the bulk of the action takes place in a gas torus around a star rather than on a planet's surface. As such, in most cases gravity is insignificant.

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    $\begingroup$ While I was looking for something similar, I was still hoping that "localized gravity" existed on islands floating around in space as well as a universal up or down. Integral trees would seem to only satisfy the breathable atmosphere in a general zero gravity situation. Thanks for the tip though! I shall investigate more. $\endgroup$
    – Harry Mu
    Feb 19, 2023 at 19:03
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    $\begingroup$ The problem with a relatively thin floating island is that whatever imparted the gravity would also be likely to impart an intolerable gradient. An alternative would be "Roger Dean style" islands: see e.g. the cover from en.wikipedia.org/wiki/Yessongs $\endgroup$ Feb 19, 2023 at 20:12

A shellworld[1][2][3] is any of several types of hypothetical megastructures:

The fourth type listed is:

Completely hollow shell worlds can also be created on a planetary or larger scale by contained gas alone, also called bubbleworlds or gravitational balloons, as long as the outward pressure from the contained gas balances the gravitational contraction of the entire structure, resulting in no net force on the shell. The scale is limited only by the mass of gas enclosed; the shell can be made of any mundane material. The shell can have an additional atmosphere on the outside.[5][6]


People or spacecraft could enter such a shell world w if the shell has airlocks.

The amount of gravity in such a shell world would depend on its volume and the density of gas and the total amount of gas.

A small enough enclosed bubble of gas could have microgravity.

This is not what is requested, but it is known to be scientifically possible.


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