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So I have seen the never ending clash between those who believe in a Spherical Earth and those who believe in a Flat Earth. It thus made me wonder what a Flat Earth would even be like. One of the things I wondered and want to find answers to is its gravity, specifically its nature.

For this question, I will give some context.

  1. The Flat Earth has the same overall mass as the Earth we know.
  2. The entire surface area of the Flat Earth is equivalent to the surface area of Earth at Ground Zero. Of course, due to mountains and cities, there might be some uneven ground, just like our Earth.
  3. The other end of the Flat Earth is just ice and rock. This will weigh the same as the mass of the rest of our Earth (i.e. Core and Mantle). Most of this is evenly spread out, just like the other side.

So, something like this picture below:

So, something like this.

With this in mind, what would gravity be like on this Flat Earth? It will certainly not be 1 g anymore, that is for sure. Would there even be gravity? Can the structure even hold?

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    $\begingroup$ Science based ? Impossible. There's even confusion between weight and mass. If it's the mass of a planet it'll be in hydrostatic equilibrium (round). I fear there's no science based anwser here. $\endgroup$
    – user78828
    Dec 9, 2020 at 13:19
  • $\begingroup$ I agree with a_donda, there's no way this can be possible. $\endgroup$
    – The Darke Lorde
    Dec 9, 2020 at 13:46
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    $\begingroup$ Sorry, mass, not weight. Always been mass. $\endgroup$
    – CYCLOPSCORE
    Dec 10, 2020 at 1:01
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    $\begingroup$ Not sure if I made this understood: this shape can not exist on any scientific base. That's what makes the answers piffle. A planetary mass will be (that's even part of the definition) drawn into a spherical shape from its own gravity. Without the "science based" tag you're free to go anywhere :-) $\endgroup$
    – user78828
    Dec 10, 2020 at 9:26
  • $\begingroup$ A flat Earth is not possible in any form with modern science and fundamentally violates principles found in relativity, classical gravitation, cosmology, dynamics and the planetary sciences (geology, etc.). It is a conspiracy theory because it requires all relevant data used in those fields to be falsified in a concerted effort to hide the truth. Of course this leads to the issue of extraordinary claims and all that. Flat Earth physics would need to be very different which is why adherents usually go to strange fringe hypotheses (electric universe, density dynamics, conscious universe...etc) $\endgroup$
    – user110866
    Dec 10, 2020 at 23:41

7 Answers 7

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what would gravity be like on this Flat Earth? It will certainly not be 1 g anymore, that is for sure. Would there even be gravity? Can the structure even hold?

If you start with a disc satisfying your specs, our science tells you that it will exert gravity. As a consequence of this gravity and the finite resistance of the materials it is made of, it will crumble into a spheroid and turn into the planet we know.

Yes, the rearrangement would liberate enough energy to mess up with water and life on the surface, so forget of the greenery for a while.

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    $\begingroup$ Imagine a molten hellhole, thats what it will look like $\endgroup$
    – Sync
    Dec 9, 2020 at 13:19
  • $\begingroup$ also, i wonder if certain parts of the disk will be broken off and maybe create our moon... $\endgroup$
    – Sync
    Dec 9, 2020 at 13:20
  • $\begingroup$ @Sync That would require an external force. Everything on the flat earth would be pulling things together. $\endgroup$
    – Mary
    Dec 9, 2020 at 13:45
  • $\begingroup$ true, but the irregular structure of the disk may result in something like what happens when you crack a cookie in half $\endgroup$
    – Sync
    Dec 9, 2020 at 13:46
  • $\begingroup$ @Sync except the quote on quote "crumbs" would just be pulled in and added to the rest of the planet $\endgroup$
    – Firestryke
    Dec 9, 2020 at 14:33
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If you had a disc shaped earth that somehow doesn't collapse into a ball, it would still exert gravity towards its center. As a consequence, the direction of the force you feel would tilt as you move towards the rim, and it would feel like the inside of a bowl. There's a visualization of that on the vsauce youtube channel.

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  • $\begingroup$ That kinda sounds like Minecraft UHC. $\endgroup$
    – CYCLOPSCORE
    Dec 10, 2020 at 1:00
  • $\begingroup$ Not the inside of a bowl, the outside of a bowl. The rim would be the peak of a mountain range thousands of miles high (yes, running right out of the atmosphere), and the slope would depend on how thick and how massive the disk is. $\endgroup$
    – Zeiss Ikon
    Sep 22 at 19:15
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2 years 9 months, 5 existing answers, and no one has actually answered the very simple question that was actually asked.

Ignore how such a structure is made stable. If you actually flattened out the Earth like this, what would the gravity be like?

At the center of the disk and near the surface--the "North Pole"--the gravitational environment is well-approximated as that of an infinite plane. An infinite plane produces a uniform gravitational field above it whose strength is directly proportional to the areal density of mass below. So, we just take the area of the Earth and divide by the mass--we don't even need to bother figuring out the exact dimenions of the resulting cylinder, and then multiply by $2\pi G$, and we get an answer of $~4.91m/s^2$--so, about half a g. Which makes sense, because, if you flatten out the Earth like that, you are moving most of the mass farther away from the North Pole than where it started, so the force should get weaker.

Now, this plane is of course not actually infinite, so as you move a significant distance away from the North Pole, corrections from the edge conditions will become significant--high above the North Pole, gravity will eventually start to decrease even further, and as you move along the surface away from the North Pole gravity will both decrease in strength and start tilting so it feels like you are walking uphill. That's what will force the structure to collapse back into a sphere, unless you arrange for a counter-force. Turns out just spinning the disk is not sufficient--the inward gravitational force changes according to a complicated curve that is not matched by the linear increase in centrifugal force. You can imagine supermaterials, like Larry Niven's scrith, that rigidly hold the structure in the flat disk shape against gravity, but if you accept a slight modification, you can get a stable structure by spinning the disk and letting it thin towards the edges--so, it's actually a highly oblate spheroid with an extremely short day, in which case gravity always remains perpendicular to the ground, ends up slightly higher than the uniform plane estimate at the North Pole, and smoothly decreases to whatever lower limit you want based on the chosen spin rate--down to zero, where the equator is effectively in orbit--as you get towards the edge.

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    $\begingroup$ In other words, you've got Hal Clement's Mesklin, but with < 1 G at the poles, instead of ~ 3 G at the equator... I've seen this calculated, day length runs to something like 24 minutes and equatorial gravity is tiny (meaning you lose all the atmosphere to space in geologically short time). $\endgroup$
    – Zeiss Ikon
    Sep 22 at 19:19
  • $\begingroup$ @ZeissIkon Atmosphere loss depends on escape velocity, not surface gravity, and that doesn't change nearly as much, so while atmosphere loss would be accelerated, it may still be held enough to make the planet habitable. $\endgroup$
    – Logan R. Kearsley
    Sep 22 at 23:22
  • $\begingroup$ One of the questions being asked was "would the structure hold". No, it wouldn't, so the rest is somewhat academic. $\endgroup$
    – jdunlop
    Sep 23 at 0:56
  • $\begingroup$ @jdunlop That was only one question, and not the first. And the scenario is underspecified to give a definitive answer to it. So I find it rather embarrassing that no one else bothered to give the very simple straightforward answer to the primary question, regardless of whether or not it is purely academic. $\endgroup$
    – Logan R. Kearsley
    Sep 23 at 2:19
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    $\begingroup$ @jdunlop The spinning-disk planet is gravitationally stable in terms of the rocky body -- it's just an extreme case of what you can see through a telescope if you look at Jupiter (12 hour rotation) or Saturn (10 hour rotation). My only objection was atmosphere loss. $\endgroup$
    – Zeiss Ikon
    Sep 25 at 11:07
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One option circulating in the worldbuilding community of flat earthers is to create gravity not by mass but by acceleration. The world is constantly accelerated upwards with an acceleration of 9.81 m/s². What causes that acceleration? Why does it only affect Earth and the rest of the solar system but not the things on Earth? Where is the solar system going? That is left as an exercise to the reader.

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Normally, a flat disk of a planet will crumble and collapse into a ball due to physics and the laws of gravity. There will probably be none of the greenery you see on the original image. But if you could override physics to allow for exceptions such as those of flat planets without them collapsing, you could have both gravity drawing everything into the center and spin gravity. Those two could possibly balance each other out, but small irregularities may allow for things like wind. Concerning the sun and the moon: I’m not certain. The sun would have to be like a lamp which only casts light onto one surface of the planet at a time, and if you want oceanic tides, you would need a “magical” moon which would have to project the pulling force into the right places.

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If you're going with an Earth-sized disc, the problem is, the gravity will still go towards the center of mass, so unless there's such thing as a technology that allows gravity to go in one direction for the inhabited side, the atmosphere and oceans will pool up around the center. And that's assuming that the structure is stable in the first place.

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In order for a flat earth to be possible, you would have to rewrite physics. If you want your world to be plausible, I suggest the following laws, replacing gravity.

Law of down: Everything is pulled downward in a force equal to it's mass.

This still leaves the same issues, for example, how would air stay on the planet? What causes wind? I suggest you have a barrier that keeps it in, visible or otherwise.

Alternatively, ignoring the physics of things collapsing, you could have regular old gravity, still drawing everything to the center, but you could also have a spin gravity, that puts things outward. I'm no physicist, but I am sure you could do it. Both forces would be exact enough to balance exactly, perhaps with some irregularities in behavior. This may allow wind, but I am uncertain. The sun would have to be a 'lamp' of sorts, only shedding light on a certain part of the planet, while it spins, different parts would be covered. Maybe if the surface upon which the planet was on was infinitely durable (Which is impossible in a finite universe.) then this would not collapse, and still operate under the same physics. You could still see pretty much any mountain in this world, and no way to explain tides, unless you also have a moon 'lamp' that projects pulling forces in the right spots. The 2 'lamps' and the indestructible platform are the only magical things here now!

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