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I have a world I want to put together but I ran into a problem.

Description

This world is a cube infinite in all directions, exactly half of it is full of earth and half is space (where we see normal space stuff happening besides a place that makes the special "Suns" required for this world but I'll get to that in a second). The world has a fairly strong magnetic field for the purpose of supporting the "Suns" that light this world. The "Suns" are big, flat, flaming disks that (due to their composition) flare up and down every few hours to create a rather ugly day/night cycle. The last thing I know: life (in this world) is brought about from this magic, gold, sparkly dust. The more you have, the smarter you are.

Problem

Gravity. Of course I could hand-wave this but I would like to make it at least a little realistic. From what I have now the infinite mass would end up collapsing in on itself and killing everything. Is there a way around this? I was thinking something along the lines of massive, complex cave systems or low density soil. Would any of these ideas work or is my world doomed to self destruction?

If you need any clarification on any thing, let me know and I will be mor than happy to tell you what I can.

EDIT: oops forgot something important. Unlike the world in This question: Gravity on a Minecraftian world? I want the world to go all the way down to the end of infinity. Gives me a little more space to work with.

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    $\begingroup$ Can I ask...why infinite? This line " I want the world to go all the way down to the end of infinity" makes me think you consider infinity as a really big number (slightly smaller than infinity + 1) and not the concept that it is. Honestly, by extending something to infinity (which is effectively infinite hand-waving) you are so far out of believable that a three headed cow made of mystical green matter as the source of gravity in your world is a usable explanation. $\endgroup$
    – Twelfth
    Commented Apr 28, 2015 at 21:19
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    $\begingroup$ @Twelfth I thought "a little more space to work with" was sarcasm... $\endgroup$
    – 2012rcampion
    Commented Apr 28, 2015 at 21:27
  • $\begingroup$ I was reading an article on how pi is an infinite non repeating decimal and how if that is true it must contain all possible number combinations this if decoded properly it would contain everything from a copy of War and Peace to a perfect simulation of our world. I thought it would be cool to translate that into a world where everything was infinite and non repeating. The implications are quite interesting don't you think? $\endgroup$
    – unknown
    Commented Apr 28, 2015 at 21:34
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    $\begingroup$ @2012rcampion my saying "end of infinity" was not implying that infinity has an end but that the earth takes up all remaining space in its half. It was meant as an idiom. $\endgroup$
    – unknown
    Commented Apr 29, 2015 at 6:15
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    $\begingroup$ What does infinity offer you that really really really big doesn't accomplish? $\endgroup$
    – Lilienthal
    Commented Apr 29, 2015 at 10:51

7 Answers 7

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With this sort of cosmology, you can't get orbits. Plus the gravitational attraction between objects on a human scale is too small to notice.

With that in mind I'd just get rid of the notion that gravity is attraction between mass. Instead, just define a uniform gravitational field across all of space, pointing perpendicular to the infinite plane of the ground surface.

Mass is affected by this universe's gravity, but (unlike our universe) the reverse is not true: mass-energy distribution has no influence on gravity.

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  • $\begingroup$ So you are saying that a set state of constant gravity is the way to go? I.e. All gravity is set at 1G and that cannot change? $\endgroup$
    – unknown
    Commented Apr 28, 2015 at 21:19
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    $\begingroup$ Yeah, exactly like that. It will introduce some inconsistencies (general relativity no longer works the same, and angular momentum is no longer conserved), but unless your characters are physicists, those shouldn't matter to the story. $\endgroup$
    – 2012rcampion
    Commented Apr 28, 2015 at 21:22
  • $\begingroup$ If all of the celestial bodies are all feeling say one G of constant gravity, why is keeping them away up? $\endgroup$
    – unknown
    Commented Apr 28, 2015 at 21:23
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    $\begingroup$ @unknown "The world has a fairly strong magnetic field for the purpose of supporting the 'Suns'." Same thing could apply to planets. Although I'd just say they're made of the classical "celestial matter" that is massless and moves in perfect circles. $\endgroup$
    – 2012rcampion
    Commented Apr 28, 2015 at 21:29
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    $\begingroup$ You could also use this (although I wouldn't normally recommend this site, I think its perfect in your case). $\endgroup$
    – Christopher King
    Commented Oct 16, 2015 at 19:59
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It sounds like what you want / need is astonishingly (but not precisely) like that of the Unicorn Jelly web comic.

A Khex class cosmos is defined as being those that contain infinite planes of matter in some fashion. The cosmos of Widlan, here, is a prime example of one kind of khex class cosmos.

Description of Widlan
Description of Widlan

and

Khex class Cosmos
Khex class Universes

Widlan appears as an infinite, flat plane, with an apparently infinite atmosphere above it, as though the entire universe was half earth, and half air. In actuality, Widlan is more complex, being dimensionally wrapped so that it really consists of two planes. One of these is a plane of 'earth' that has only one side...if one were to dig down far enough, one would eventually come up somewhere else on the vast plane. The second plane is one of energy, 'fire' if you will, that extends far above, and parallel to, the plane of 'earth'. The inside of the Earth plane contains a layer of matter that is at Absolute Zero, and between the deathly cold of the middle of the earth plane, and the deadly fire of the Sunwall above, lays a temperate surface.

The Sunwall provides the energy source and the Absolute Zero layer provides the energy sink. Its cosmologists might be working on a solution for how the power gets recycled from one side to the other.

More importantly, it provides a relatively thin layer of matter so you don't end up with infinite gravity.

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    $\begingroup$ While this did not turn out to be the solution I choose I want to give your answer an upvote just for being so bizarrely awesome. $\endgroup$
    – unknown
    Commented Apr 29, 2015 at 6:32
  • $\begingroup$ One aspect that I really love about this site, is that I learn so much when I answer other people's questions. It gives me lots of great & new ideas! $\endgroup$
    – Jim2B
    Commented Feb 12, 2018 at 20:34
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As an aside, the entire concept requires so much handwaving that I'm not sure gravity is even your biggest obstacle, but I'll still try to answer it.

Do you have to fill an entire half of it with earth to get what you want?

Imagine instead a plane of earth bracketed by space on either side. The width should be apprxoximately the same as the diameter of our planet. It won't collapse on itself because there's no center point to collapse to, being infinite.

You'd then have two habitable sides, which doesn't matter since you have effectively the same total surface area either way to play with.

One note: Due to some interesting math, it's not possible to escape the gravity from an infinite wall. The acceleration is constant, regardless of distance - it's not like a sphere where gravity decreases the further you get away.

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  • $\begingroup$ If the density of the world-plane is approximately uniform, then gravity will decrease linearly as you tunnel through, reaching zero in the middle (just like the sperical Earth). I suspect that this world would have an infinite-sheet version of the Earth's geologic zones (core, mantle, and crust). Currents through the sheet-core could generate the magnetic fields you're looking for... $\endgroup$
    – 2012rcampion
    Commented Apr 28, 2015 at 21:25
  • $\begingroup$ If you wanted to accurately lift the conclusions and sources from my answer, you should note that the "width", or depth, should be the radius of the Earth, not the diameter :) $\endgroup$
    – Samuel
    Commented Apr 28, 2015 at 23:42
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Another solution would be that there isn't any gravity in this universe, but the ground plane is constantly accelerating upwards through infinite space. This should be indistinguishable from gravity.

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    $\begingroup$ Ah, the "Flat earther" solution, I like it. $\endgroup$
    – Deolater
    Commented Jun 11, 2015 at 17:56
  • $\begingroup$ I expanded on that same idea. pressure. $\endgroup$
    – JDługosz
    Commented Jun 12, 2015 at 7:50
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Simple: think about the entire block being accelerated like an elevator. The gravity experienced is the same constant throughout, not related to the mass.

So, have the universe have a constant vector of acceleration that affects certain kinds of fundamental materials. That's why all the rocks are in one half: they fall that way.

It does not explain the lack of rising pressure as you go deeper. Maybe pressure doesn't matter because material doesn't get crushed, and its strength increases with the pressure it is under, so a piller will scale the force vs strength no matter how tall. So in general, pressure is not something that can be measured (except by mixing materials).

It does allow the suns and other celetrial objects to have different rules: they're made of different fundamental materials.

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Well, one easy solution would be to make gravitation a ranged force. Make gravitons massive, but not too massive. Say it has a range of 1020 m. Any single particle would be attracted by the ground in a 1020 m radius hemisphere (a volume of about 1060 m3, compared to Earth's volume of around 1021 m3. Since there's no Iron core, it'll be a lot less dense, so you should be good to go). The sun will take a bit of work though.

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Short Version

You can have an infinite universe like this as long as all it did was exist.

Long Version

Assuming you have no edges, you should be able to arrange the mass of such a universe in a way where a series of gravitational nodes exist and are arranged harmonically such that they are tidally locked into a grid that doesn't move. It's like the grid Carbon forms in a Diamond except that the grid is tied together by gravitational forces between nodes instead of chemical bonds. That locked lattice grid is gravitationally stable-ish and would mean that any given part of the whole experiences only the gravity of it's local node. There are certain things this universe simply cannot have, an edge; the system is truly infinite in all dimensions or it falls in on itself. Net mass migration of any sort between nodes; if you change the node mass by any measurable amount the universe gets asymmetric and falls in on itself. Any massive celestial object that isn't a node; it would distort the lattice and the whole universe would fall in on itself. Any relative movement of the nodes beyond a certain very very low threshold or, you guessed it, the universe falls in on itself. Basically you could have it but you couldn't build it because it's inherently unstable until completed and do note that it's not amazingly stable once you do complete it.

Please note: I'm drawing on the idea of the Collapsium lattices from Wil McCarthy's work; I'm not altogether sure they would work in reality, the math does but I'm not sure about the physical reality.

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