# How could the air be stopped from falling off of a flat world?

Imagine you had a flat earth scenario, in which the world is essentially a giant disc floating in space (I know that in reality any such world would be pulled into a sphere by gravity, but for the sake of the question assume that gravity just pulls "down" instead of towards the center of mass). Now direct your attention to the edge of the disc: without something to stop them, all of the air is just going to pour over it, leaving the planet incapable of supporting life.

What measures could be taken to prevent that from happening? One thing I've thought of is placing giant mountains along the entire rim, tall enough to hold in the whole atmosphere, but I'm interested in whether there are any other passive solutions that would work in the real world (ie, no magic or teleportation).

EDIT:

A recurring question seems to be "why wouldn't the Earth and everything on it be falling at an equal speed?" Let's hand-wave it away by saying that the Earth is already at the "bottom" of the universe, and so cannot fall any further. Things on the top, however, can still slide, jump, or fall off and then descend through space until they reach the "bottom" too. Arbitrary, yes, but I think it allows us to focus more on the question itself: how do we keep the atmosphere on top of the flat surface, besides adding huge mountains around the edges?

• Hm, if the air falls off, what stopps the earth from eroding sideways off the flat earth and falling off over time? What keeps water put? I feel like the mechanics of such a flat earth would be resistant to things spilling over the edge in the first place. Jul 31, 2016 at 19:19
• Why wouldn't everything be falling equally again? Jul 31, 2016 at 19:47
• @MartineVotvik Hmm, good questions. Let's assume for the sake of the argument that the earth itself is, at least at the edges, made of a solid enough material so that erosion is minimal. Meanwhile, I suppose that water and air are interchangeable in this question: Yes, for such a world to exist it would have to be resistant to things falling off, but HOW could we achieve that resistance?
– JNW
Jul 31, 2016 at 20:16
• @Durakken Also a great question. I suppose that I was assuming some intrinsic quality of the earth kept it stationary, while all of the loose objects on its surface were not. Perhaps the earth itself sits on some universal foundation, so it does not fall, but things would be quite capable of sliding off the the top of it.
– JNW
Jul 31, 2016 at 20:22
• The classic flat earth scenario isn't a disc floating in space... So are you asking about that or are you asking about a solution other than what is provided in every flat earth cosmology? Jul 31, 2016 at 20:43

The whole universe could be filled with air. That would also explain why the world isn't just in free-fall (and thus effectively gravitation-free): It is going at the limit speed where air resistance equals gravitational pull. Of course it would still not explain why the air doesn't fall as well.

Note that the effect for an inhabitant of the world would be that beyond the border of the world there's a strong upwards wind. Probably eddies would cause an outwards wind near the edge of the world.

Edit:

In response to your edit of the question:

If the universe has a bottom, and the flat earth rests on it, the filled-with-air universe gets even more plausible. Especially, there's no need for it to be completely filled with air; the air just sits on the bottom of the universe, and the flat earth is inside that universal atmosphere. For an observer on the flat earth, the only difference between that and the eternal-falling earth model is the absence of the off-earth wind (and the absence of needing to hand-wave away why the air is not falling as well; it's not falling because there's a bottom).

• Very creative! And makes a lot of sense. Not exactly along the lines that I was thinking, but definitely a solution within the bounds that I gave. Thanks for the answer.
– JNW
Aug 2, 2016 at 14:49
• This reminds me of a question I wrote about parachute cities... Nov 9, 2017 at 8:56

If your flat earth has a somewhat normal gravity, whether artificial or by having something like a super dense ring at the edge that balances the normal center-of-mass direction, the atmosphere would just be present on both sides of the disk and around the edges.

There would likely be intense storms at the edge because of clashing jet streams, so to the inhabitants, the other side would be invisible. If the bottom side is also inhabited, an occasional unlucky explorer might mysteriously end up on the top side, thrown there by a storm.

From your edit, I assume that the "gravity" of the Universe overpowers that of the earth itself, causing anything that is not at the "bottom" of the universe to fall down. That of course leaves the question of how much air is reaching the bottom or is already at the bottom, and what else is there.

If the "bottom" is like an infinite flat plain that only receives some pockets of matter when a flat planet or gas cloud reaches the bottom, the air falling off the earth would spread out so thin it would not be noticeable, essentially lost forever.

If on the other hand the amount of air having reached the bottom of the Universe is big enough to create measurable pressure, the air could be reclaimed in some way at the bottom of the flat earth. That might be true if the bottom receives new air and water in balance with the natural dispersion on the bottom. Of course, that would also mean the earth gets new air and water falling on top of it.

A kind of stone with thin channels might absorb the water (and air dissolved in it) through capillary action, though I'm not sure it would work against this universal gravity.

Or there are world trees growing roots down to the bottom, where they absorb water and air through osmotic pressure, after which they evaporate/expel the water and air from their leaves as part of their metabolism.

Neither of them would seem to balance the loss of atmosphere from the sides if there's not some barrier there, though they would make up for some loss.

Since you have already handwaved away some major physical laws, you could perhaps declare that air pressure isn't a thing in that universe. So the air is not pushing itself to the edges, it only falls off when it some bit of it is pushed over the edge.

Finally, if the flat earth is artificial, it could have either some force field (solving all issues by handwaving again) or an air recycling system where the air falling off the sides is captured some way down along the the edge, filtered (to extract any intrepid explorers and their balloons) and transported to a mountain or otherwise in the middle of the disk's surface. This would result in a constant wind from the center of the disk to the edges, making any air travel one-way and very risky.

The problem with reclaiming air in any way is "what is out there past the edge of the planet?", because no regular pump or suction device is going to compete with a vacuum out there. The closest I can imagine is a combination of strong ionizing radiation and shaped magnetic fields to force the now charged particles into openings in the side of the disk.

• Love the idea of somehow "reclaiming" air and water that has fallen off. If the earth was artificial, I wonder if there would be a mechanical way to suck up the air from the sides as it fell and send it back up through caves in the center quickly enough to have a stable cycle (via giant underground vacuums or something).
– JNW
Aug 2, 2016 at 15:02

Assuming your 'planet' orbits a star you will need your flat earth to rotate in someway to get a day/night cycle.

I can see several possible methods. Please don't shoot me on the science behind them, there are just theories.

1) The Frisbee. Your flat earth rotates around the central point. This will bring certain points closer and further away from the sun over the 'day/night's cycle. This scenario doesn't really give you full night.

There is also the orientation of your flat earth to take into account with the Frisbee. Is the whole surface facing the sun, or is the sun perpendicular to the flat earth. So the rays are coming side-on. The first would give you more an eternal day while the second would give you more of a night time(twilight) than face-on.

This rotational around a central point will also create different gravity fields the further away from the centre you get. Think of artificial gravity fields in rotating spacestations.

2) The Frisbee Wobble. Combined with the Frisbee method above, but with a wobble. This will allow the sun to be blocked by the raised edge of your flat earth.

3) The Coin Spin. Your flat earth is spinning on its edge. This spin will allow full day night cycles.

You can have your spin perpendicular to the sun (but that doesn't really give you your day/night cycle). I'm going to work with the spin axis in line with the orbit axis.

Rotation combined with atmosphere. So now you know how your flat earth spins. You can use 'simple' gravity and physics to work out how your atmosphere and water etc will stay on 'planet'.

With the Frisbee, the rotational effects should limit the atmosphere from flying off into space. Think of spinning a bucket of water around your head. The water stays inside even though it is horizontal. Hmm, this relies on a bottom to the bucket. So this would be your mountain range you mentioned.

The wobbly Frisbee may be more helpful. Here you would have the atmosphere spinning around, as it spins it extends further into space. However this time, the upward wobble of the flat earth rotation could increase the friction and create a 'temporary' barrier to the atmosphere. It then slides back onto the earth and moves towards the downward facing edge. By the time it gets there...It will be upwards facing again. This will likely make your central regions have more atmosphere than your outer edges. So the outer edges could have the same affects on plant and animal life that very high altitudes do.

The spinning coin has the chance to completely encapsulate your Flat earth planet in an atmosphere -much like it does for a round ball shaped planet. The atmosphere would be thicker on the opposite side to the direction of the spin. Ie, if your spin is left to right, the thicker atmosphere should always be on the right hand side of the coin.

Any water that fell over the edge of the flat earth would be caught back in the underlying section of the 'planet'. It wouldn't be lost. The same for erosion. Your flat earth could potentially erode along the edges but the rotation spin could move sediments from the one side to the other. You could have some countries growing smaller, while their far distant cousins on the other edge could be growing. Gives a new meaning to country stability! But erosion like that takes thousands of years so it would not be too noticeable.

This atmosphere could even cover the bottom edge, and with such a spin you could actually walk over the edge of your planet and walk 'upside down' on the underside. So you can use both sides of your flat earth (gives a whole new meaning to digging a hole to China!).

Regardless of which rotational device you choose, it would be very windy on your flat earth. Good luck working out the climate!

• A wobbly frisbee passing the atmosphere back and forth is a fascinating idea and one that had never occurred to me before. Huge +1
– JNW
Aug 2, 2016 at 15:07

Does gravity still push down if you are on the other side? Maybe, gravity will still pull things towards the ground, so that things that fall off of the disc will stay near it. As for the world eroding at the edges, it could be that if you get far enough from the edge, gravity shifts in direction to push you back towards the world.

I know that this does go beyond real world physics, but you did already establish that the direction of gravity can be changed, so it doesn't go beyond the physics of your world.

Maybe it doesn't matter if the air "falls off" as long as enough air is "falling on" at a suitable rate. Think of a waterfall filling a lake while at the same time the lake is draining into a river.

This could also be extended as an idea to explain various seasons on your world caused by the rate of falling air changing.

• So you're saying that more air could be falling to the surface of the planet from space to counteract the loss from the edges?
– JNW
Aug 2, 2016 at 14:53
• That is it exactly. Another way to think of it is a smoke machine on a theater stage. As long as the machine is on, smoke stays on the flat surface in spite of "falling off" the stage constantly Aug 3, 2016 at 15:39
• This world has the added benefit of not having to ever worry about air pollution - it all just falls away to be replaced by new air! Unless of course stuff falling off the world is eventually destined to fall back onto it, Portal-style... Aug 4, 2016 at 10:27

Sounds similar to: What would make scientists realize they were on a flat world?

In your classic flat earth model, everything revolves around the earth. The atmosphere stays in place because it's already as close as it's going to get to the earth, so there's nowhere for it to move to. You'll still need a ring of mountains around the edge to present dissipation/evaporation, but that's it. Think of it as a shallow bowl.

If this is a three dimensional earth (just very thin, but with tall three dimensional structures on one (only one?) of its surfaces, then I have no answer.

If, however, this is a two dimensional earth existing in three dimensional space/time, then the very nature of gravity in this scenario prevents the air from escaping off the "side" of the disk earth.

My argument is: Imagine a two dimensional rubber sheet as space. Two dimensional earth sits on this sheet and its mass distorts it through a third dimension, causing it to sag down forming a cylindrical depression. The depression represents the gravitational field. The field occurs at every point on the surface, which is hard to visualize, but that is not what we are interested in. We are interested in the effect at the edge, which is easy to visualize; any particle trying to exit the side of the earth encounters the steeply curved space/time sheet and must climb out to escape. Which means any item trying to escape the edge of the earth encounters a gravitational field that resists its escape. Or, to put it another way, escape off the edge encounters the same gravitational resistance as escape up (even though "up" doesn't mean a thing to the denizens of this world).

Or maybe its better to say, the vector of escape in such a world is off the edge, not "up".

As I said, however, this is a solution for a two dimensional world in three dimensional space time. Perhaps, however, this will suggest a solution to someone else...maybe using spacial distortion as part of the solution.

Two different answers spring to mind.

First, the world is the 'flat' of a petri dish. This is how we deal with providing 'flat worlds' for our experiments with bacteria - we supply walls to stop the agar from dripping off.

But that's not very interesting...

How about the flat earth not having edges at all? If you walk far enough south, you end up in the far North... If you walk far enough East, you end up in the far West... The world is flat, Jim, but not as we know it...