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If there was a life-supporting planet that was cubical rather than spherical, would it be feasible for there to be a unique but similar civilisation on each 'Plane'?

I know that cube planets are impossible for a whole bunch of reasons involving centrifugal force, gravity and such, but in this case I want to try and justify it.

Planet Box was originally a Death Star-like megastructure created by an advanced alien race, shaped like a giant cube. The original alien race long since died out, but the megastructure remained. After a few billion years floating around in space and a lot of interstellar collisions, the megastructure fell into an orbit and became a planet in its own right. However, the original metal frame of the megastructure remains sturdy enough to support the planet in a vaguely-square shape, despite the orbital forces that try to flatten it into a sphere.

Planet Box orbits in a habitable zone around a sun, with a suitable gravity mass. It has rocks, water, and forests growing on each of its Faces. After a while, from the primordial soup, life and civilisation arise. The native Box People all live on a flat surface.

Of course, the Box People don't think that their world flat. Gravity, being what it is, is noticeably stronger at the centre of each Plane than at outskirts. Their world doesn't appear flat to them - because of the change in gravity with landscape, their perception is that it goes uphill. When you're on the edges, rain falls at an angle. To the native Box People, they would instead think that they are living in the bottom of a bowl.

See this post for where I got concept come from: https://gizmodo.com/how-gravity-would-be-different-if-the-world-were-a-cube-1492018223

All of the water and oceans pool in the very middle of the face. The edges of the Box World rise into unpassable six mountains that go higher than the atmosphere. On each Plane, there's a circle of livable terrain surrounding a huge ocean where life can survive.

This is my (very high-quality) sketch of how I imagine the geography of each Plane:

enter image description here

And, because Box World is a cube, there are 6 separate Planes. Life on each Plane is completely isolated - none of the native civilisations would have any idea about the other 5 sharing their planet. To them, the flat earth theory is just a crazy idea proposed by radical astronomers, when anyone sensible knows that the world is concaved. Absent of space travel, there would be no easy way for them to know otherwise.

So, two questions, both related to each other:

  • Would life on a cube-shaped planet be broadly feasible (i.e. allowing an acceptable dollop of handwavium)?

  • Would it ever be possible for a civilisation on one Plane to encounter life from another? No space travel allowed.

Even though the world is square and the corners rise higher than the atmosphere, would it be possible (with a great deal of difficulty) for any of the Box People to cross onto a different face? Could there conceivably be a channel between Planes that's low enough that someone might endure the high altitude and mountain climb across? Or could there be a deep enough tunnel that they end up crossing the edge underground to reach a different side?

Planet Box doesn't have to be strict cube - it could be a cube with rounded edges. It does, however, have to have six distinct faces.

Edit: This question is different from the linked previous one in the comments as it asks more specifically about travel and possible interaction between civilisations on different faces of the cube.

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    $\begingroup$ It's primarily gravity trying to make it round, along with minor details like erosion. $\endgroup$ – Separatrix Oct 23 at 13:29
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    $\begingroup$ FYI, lease note that while the rock will be a cube, both atmosphere and ocean will try to be a ball, or rather a dome over the center of each plane. With enough atmosphere, the edges might be within it, and hence be passable without breathing gear. Otherwise, you need a tunnel under the edge. And you kinda hope that atmopshere pressure is same on both planes that it connects. $\endgroup$ – Bald Bear Oct 23 at 15:05
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    $\begingroup$ You should pick up a copy of the Dodec duology by Dave Duncan (Children of Chaos and Mother of Lies), which features a dodecahedral world, which has many of the same issues at play as a cubic one. While the story is fantasy, with magic etc. Duncan paid a decent amount of attention to getting the physics of a planet with "faces" correct. The plot also deals with various groups crossing the edges (a phenomenally difficult, but not impossible task). $\endgroup$ – rsandler Oct 24 at 0:27
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    $\begingroup$ Just note that by definition of planet, this could NOT be called a planet (en.wikipedia.org/wiki/IAU_definition_of_planet) $\endgroup$ – Zizy Archer Oct 24 at 7:44

12 Answers 12

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would it be feasible for there to be a unique but similar civilisation on each 'Plane'?

Define "similar". Given how thoroughly you've cut off each face, there's no guarantee that inhabitants of different faces would have the same set of amino acids, let alone anything more complex in common. You'd need to set up some means for information (cultural or biological) to be shared between faces if you wanted them to be similar, or handwave some slightly dubious convergent evolution.

Their world doesn't appear flat to them - because of the change in gravity with landscape, their perception is that it goes uphill

The world will be visibly mostly flat from high points on the surface (perhaps via the first hot-air balloon flights). I'm pretty certain that surveying work will reveal the world to be flat. The oceans will curve slightly, of course, but not necessarily as much as they do on Earth so you might get much longer lines of sight. Long distance navigation is likely to be much easier as a result, but it will also be obvious that "down" is a slightly different direction as you move around on the ocean's surface.

As you ascend the rim mountain ranges, you might feel like you're climbing the edge of a bowl but you'll clearly be able to see the world is flat. You should also be able to get some obvious astronomical cues... if you lived in a bowl, the stars that are visible over the rim from one location will be different from those visible from a distant location.

Determining the actual shape of cubeworld without space travel will be hard, but if a correct theory of gravity arises the groundwork will be laid for the discovery of the shape.

Seismology might also be interesting, if there is any sort of tectonic or volcanic acivity, natural or otherwise. Seismic events on other faces, like asteroid impacts or nuclear weapon tests would be detectable, and could be established to be "beyond the edge". The exact seismological nature of the world might be too confusing and complex to be easily established

Also, did cubeworld pick up any moons? Those will be major telltales of the true size of the world, and observing them coming in an out of the shadow cast by the sun will also give big hints. Astronomers may also eventually be able to tell that other planets exist, and they have non-planar shapes. Those super-high-altitude mountains would be wonderful places to build telescopes...

Note that over the billions of years the cubeworld spent in flight, erosion, deposition and accretion of matter from other sources (like asteroids or comets) will tend to concentrate mass towards the centres of each plane, and pull it away from the mountain ranges, as gravity attempts to pull your weirdly-shaped object into a nice neat sphere. Your central oceans may in fact be more ring-like than circle-like. The underlying framework of the world might not be moveable in this way, but it might get laid bare at the summit ridgelines of the edge mountains. You might need other handwavium mechanisms to re-face your cube (which might also help with other things, like carbon cycling).

Would it ever be possible for a civilisation on one Plane to encounter life from another? No space travel allowed.

Space travel is a rather different thing, if you live on a world with atmosphere-piercing mountains. It is conceivable that you could build a habitable outpost up there with technology closer to the Victorian age than the space age. You certainly wouldn't need rocketry. Once you can make submarines, you have the basic skills to make something that can operate in uninhabitably thin atmosphere, too. Building a railway into the sky will be a challenging enterprise, but one a hell of a lot easier than actual space travel.

Would life on a cube-shaped planet be broadly feasible (i.e. allowing an acceptable dollop of handwavium)?

Could it evolve there? Might be tricky... cubeworld seems unlikely to have any kind of plate tectonics, and absent crust recycling you can end up with quite nutrient-poor areas which don't lend themselves to life (see Australia's vast internal deserts). Without some means to generate a magnetosphere you risk having your atmosphere be blown away by the solar wind, and harsh levels of surface radiation aren't life-friendly. Other fictional settings with constructed worlds have complex automated mechanisms for handling these things. This might be part of the handwavium you need.

Just because life arose on one face doesn't mean it would arise independently on another, depending on how common biogenesis is in your universe. You might need some other way of sharing organisms between faces, which might just be whatever event seeded your cubeworld with life in the first place, if it didn't evolve in-situ.

Could there conceivably be a channel between Planes that's low enough that someone might endure the high altitude and mountain climb across? Or could there be a deep enough tunnel that they end up crossing the edge underground to reach a different side?

You're the author. You get to decide the answers to those questions.

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    $\begingroup$ Many microbes are anaerobic. While complex life may not survive the trek across the edges, there should be plenty of microbes that can allow for a single biogenesis event for all 6 surfaces. $\endgroup$ – Nosajimiki - Reinstate Monica Oct 23 at 21:09
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    $\begingroup$ @Nosajimiki anaerobic is insufficient. Even an extremophile will find a UV-scoured dessicated rockpile unwelcoming. There's also the issue of mobility. A violent asteroid impact or volcanic eruption might, perhaps, be able to fling some spores over a mountain range, though there might be a dearth of nutrients on the other side... $\endgroup$ – Starfish Prime Oct 23 at 21:12
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    $\begingroup$ Extremophiles are exactly what you'll have. Because there is such a predictably smooth gradient of harsh conditions leading out the the edges of a plane, they will have millions of years and billions of generations to incrementally adapt as they slowly spread out to harsher and harsher conditions, and then millions more years to work thier way down the other side. $\endgroup$ – Nosajimiki - Reinstate Monica Oct 23 at 22:00
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    $\begingroup$ "Building a railway into the sky will be a challenging enterprise" History has shown us you can build a railway anywhere if you don't care about sacrificing the lives of enough people to build it! $\endgroup$ – Muuski Oct 23 at 22:00
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    $\begingroup$ As you ascend the rim mountain ranges, you might feel like you're climbing the edge of a bowl but you'll clearly be able to see the world is flat. You're technically correct but don't forget about vegetation. Trees will grow parallel to gravity so the trees on the outer edge will be slanted. To a human walking among the trees their environment will suggests that they are on an actual hill. Water will run "down" the hill. Even if you see the flat plane, it's not impossible for people to think their vision is being warped (e.g. lensing) as opposed to disbelieving their own equilibrioception. $\endgroup$ – Flater Oct 24 at 7:34
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They meet in the interior.

whirlpool source

This world is an ancient construct. Very durable, yes, but a construct of nonmagical materials nonetheless. The sturdy metal frame is holding up fine but the skin is a different matter - light, water and life are tough on all materials. Or a meteorite might make it down. The skin may develop a hole.

Exactly how this hole manifests depends on what is above and below. There might be substantial pressure differences and these might change over time, resulting in winds blowing in and out. Water might drain through.

I could imagine that the original construct had the life and atmosphere on the outside as a "green roof" of sorts - an ecological buffer for the underlying construct or an art project or both. Really the creators had stuff going on inside. Your intelligent life forms might venture down through the hole and realize what was inside. If it were humans they would no doubt start salvaging materials and tech and bringing that stuff topside.

Similar ventures occurring on the other planes of the cubes would give an opportunity for different civilizations to meet in the ancient alien underworlds of their planet.

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    $\begingroup$ A hole in the world where the water drains out is a bit of a civilisation ender, but it sounds like an awesome peri-apocalyptic setting. $\endgroup$ – Starfish Prime Oct 23 at 14:36
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    $\begingroup$ @StarfishPrime Not if it is just a lake! You would want to keep your ocean away from that hole. Maybe the plane centers have extra reinforcement. $\endgroup$ – Willk Oct 23 at 15:02
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    $\begingroup$ The picture, entertainingly, is of the "Glory Hole" at Monticello Dam. Thanks for that! $\endgroup$ – jdunlop Oct 23 at 20:36
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    $\begingroup$ @StarfishPrime All that water draining into the planet's interior has to go somewhere. It could be that it's being pumped out as water vapour by vents on those massive mountains, to continue the water cycle. The civilisations living on the cube wouldn't notice until they developed science to the point where they could calculate the expected amount of evaporation from their sea and noticed it didn't account for how much rain they get. $\endgroup$ – anaximander Oct 24 at 15:25
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    $\begingroup$ @anaximander the somewhere it has to go could be just some internal void. The volume of a cube is quite big compared to a thin layer of water on the outside, and the object was not necessarily intended to be a habitable world; the intro fluff certainly does not imply that. $\endgroup$ – Starfish Prime Oct 24 at 21:08
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The reason it's impossible in real life isn't about centrifugal force but totally about the gravity of the object itself. There aren't any materials strong enough to hold up the corners against the force of gravity. And we know how strong chemical bonds are, so it can't be some "ordinary undiscovered material". You need something almost magically strong, like scrith from Larry Niven's Ringworld. And then erosion will scrape off all the rock and leave this boring adamantine material exposed that you presumably can't drill into to build your railroad to the top of the corner.

Perhaps there could be a dynamic tectonic-like process that constantly maintains the cube shape over geologic timescales. Maybe the creators of this object set the process up for their own reasons. Then you wouldn't have the problem of all the dirt and impacting asteroids piling up in the center of each face as Starfish Prime describes. (Actually I see I am just restating Starfish Prime's proposal of "handwavium mechanisms to re-face your cube." So consider this an affirmation.)

I think the biggest challenge is that, at the radius you drew the ocean, the slope of the land relative to gravity will be pretty steep. Erosion will be pretty fast even with tectonic restoration.

I think there are lots of ways you can make this work. You can just make the cube be the one impossible thing in your story and make no attempt to explain it. All the fun would be in the consequences for life on the object, focusing more on the geometry of separation than on the physics. Or you can address all of the issues of erosion and hydrostatic equilibrium head on, and make that be part of the entertainment. There are other options too.

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  • $\begingroup$ "There aren't any materials strong enough to hold up the corners against the force of gravity." - how so? $\endgroup$ – Nathan Oct 25 at 11:07
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They would probably not even think that there is anything "beyond" the Wall Mountains.

With time, however, they would develop curiosity about what really is there. How does the world end?

So they start climbing, and constructing pressurised way stations that allow them to rise ever higher.

At a certain point they discover that the ground becomes brittle, fragile, and beneath the ground there is a surface of a very hard metal. For a while this reinforces the old religion that sees the World as the Cup of God. The world is actually the solid deposit on the inside of an enormous metal cup.

Then they climb again.

Two things can now happen.

One: they reach the border, and from there they see another world.

Two: the metal surface is not smooth but sometimes presents hollows, or protrusions - metal trees, pointing out like lances or flag antennas towards the stars; large blisters, long cylinders and other shapes pointing every which way. And sometimes, doors. Some so large and round that they seem dried mountain lakes, and some small enough that they are clearly recognizable as doors, except doors that lead into the ground.

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I call shenanigans on your initial premise: that they wouldn't figure out the planet was cubical until they had spaceflight. I'd predict they'd figure it out around 300-200 BC.

Why?

The greeks knew the world was round in 400-300 BC, but one of the biggest ways they knew was the fact that, on the same midday at different locations, one location held the sun directly overhead (its reflection could be seen at the bottom of wells) while another was always at an angle. Not only did the greeks know the earth was spherical, but this 'different angle' issue let Eratosthenes actually calculate the approximate radius of the earth in 240 BC.

Well, the people on Cube6 are going to have the same sorts of data points that are going to let them very quickly figure out that they aren't on a concave world like you're expecting:

  • The sun shines down at the exact same angle at all points on the cube face, disproving a concaved planet (similar to how on earth, it not being the same angle disproved a flat planar planet.)
  • Ships sailing off into the distance don't pull 'up' vertically (similar to how on earth, ships sail 'down' over the horizon, which was the first evidence the greeks had for a spherical planet, and why ships have a crow's nest.)
  • The exact same constellations are visible on all places on the cube face, disproving angle differences between locations (similar to how on earth, different constellations at different latitudes proved there was a curve.)

And, well, that's a bit of a problem for a species that's both curious and voraciously intelligent. Because if you plopped a renaissance society in that sort of situation, you can darned well bet they're going to use a lot of ingenuity to figure out an avenue of exploration.

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    $\begingroup$ They knew the world was flat around 300 BC. They wouldn't know it was cubical just then - for that, they would require some theory of gravity, which might be a bit strange for such a cube - it was easy enough to figure it out for a sphere because you are pulled towards its center, so it doesn't really change depending where you are. $\endgroup$ – Zizy Archer Oct 24 at 7:52
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    $\begingroup$ The theory of gravity by Aristotle came about before Eratosthenes' calculation of the earth's radius. There was also pretty sophisticated trigonometry at the time. If they were able to measure the earth's circumference by differences in shadow angle, it'd be the exact same math to measure the earth's facet dimensions by differences in gravity angle. Even if you disagree, keep in mind: the OP thought society wouldn't realize they were cube-bound until they had spaceflight. $\endgroup$ – Kevin Oct 24 at 12:56
  • $\begingroup$ Well your premise is really not correct. Every well on the face will see the sun at the exact same angle. It's flat. People in Leftopolis looking at a shadow will see the exact same shadow as people in Rightsburg on the same face, no matter where they are. Because it is not round. $\endgroup$ – Vogon Poet Oct 25 at 0:28
  • $\begingroup$ @VogonPoet - I'm not sure you understood what my premise was. My premise is that the same style of data points that convinced the greeks that we weren't on a plane, but the surface of a sphere - would be the same style of data points that would convince OP's denizens they weren't on a concave planet, but on a planar face of a cube. OP felt his denizens would be convinced they were on a concave planet until they achieved spaceflight. $\endgroup$ – Kevin Oct 25 at 0:38
  • $\begingroup$ OK but "proving" their world is flat does not allow them to discover what's over the edge. They still need to wait until space flight to meet anyone else. And you're right, I didn't get that OP was worried about their assuming a round world $\endgroup$ – Vogon Poet Oct 25 at 0:42
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It's not only possible that civilizations on each face would be different, even the atmospheres and biospheres would be different. You basically have 6 separate bowl-shaped planets. One could be a cold, dead vacuum while an adjacent face is a lush jungle, and a third is a Venus-like hell.

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Depending on the original purpose of the cube, there may have been plenty of organic compounds on the inside, originally. Those may very well have leaked out to the surfaces (either due to damage, exposing those compounds, or through deliberate design like exhaust vents or waste). That would give early life a common basis to grow from, which increases the chances that even separately evolved life would at least be "compatible". Although without significantly evolved creatures finding their way across the edges, life is going to be significantly different.

I agree with others about early life getting over the edges, but it would be more straight forward to just have access tunnels that run between edges, that have been exposed over the history of the cube. The tunnels could be rare, and given the vastness and inhospitably of the mountainous regions, they'd be hard to discover, but wildlife would likely discover it and spread as well.

Another likely possibility would be for asteroids to have clipped off some of the edges and peaks. Big sticky-outy things are likely to get hit! That could easily provide narrow passages between edges, if desired, which also could be difficult to find, but accessible once found.

It's unlikely you'd end up with anything human-like regardless, without some critical bit of cube-builder intervention or cause. But if near-human ancestors (for example, they had just evolved to the point where culture was important, primitive tools, very start of language) somehow found those passages between regions, their evolutionary histories would be radically different. Some would have died out, maybe only surviving for a few hundred generations. Some would have thrived, some might have not QUITE gotten over the cultural/evolutionary curve, and stayed as basically advanced apes.

As others have said, the environments could be quite different side to side (also depending on the axis of spin relative to the orbital plane! And also very dependent on exactly how/when organic transfer happened between the sides.

If there happens to be a moon, that makes it much more obvious that the planet is cubical, due to a possible shadow.

Also, there might be some interesting atmospheric affects going on with bending of sunlight or starlight due to the cube world. I'd probably need to draw some diagrams to figure that out.

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Short answer:

World-wide interaction at some point wouldn't be much different from what we have on Earth in my opinion

Long answer:

What's the size/density of it? Earth size/gravity? Atmosphere like ours? The edges of a cubic planet would feel like enormous mountains, possibly even higher than the Karman line (the atmosphere is a fluid, so it will effectively look like a dome from afar) so it is safe to assume that (specially animal) life would most likely evolve very differently in different faces. Possibly not even existing at all in some.

Now, theoretically, at some point a civilization would have tech capable of getting to the other faces. It will be to their people what the discovery of the Americas looked like, but certainly with very different life (if at all).

I think it would be very difficult that different species would evolve to this level at the same time on each face, so the first to get the tech would conquer the other ones and over time, you would have 6 "continents" politically controlled by different cultures of the same species. I mean, after a real long time.

At some point, you would have commercial inter-face travel with BFR-like rockets. The world would evolve into interplanetary travel much earlier than we did because of their necessity of building rockets just to travel say from the USA do England.

But overall, a civilization at the same civilization level as us would make it pretty Earth-like international interactions, just with more rockets and less planes

Edit: the thing with the atmosphere, sure it could be enough to cover all edges, but that would put the atmospheric pressure at extreme levels at sea level. Life could possibly thrive in such an environment, but that would still mean that the pressure at the edges would be so low that they will still need pressurized equipment to go from one face to another

And yeah, tunnels aren't an option. Rockets are much faster, safer and cheaper in this case

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With your naive model, what you would have is a spherical world with a cube-frame of adamant "walls" or "mountains" sticking out of it.

On each face, there would be a semi-hemisphere "chord" of a sphere pressed against the adamant. It would sort with heavier stuff (rock) mostly on the bottom, and water mostly on the top. Matter stuck to the adamnt "sticking out" parts would fall towards the semi-hemisphere and eventually erode down to being mostly round-ish.

If you want to avoid this, you have to mess with gravity. In two dimensions:

+---------------+
| * * * * * * * |
| *           * |
| *           * |
| *           * |
| *           * |
| *           * |
| *           * |
| * * * * * * * |
+---------------+

The adamant "box" is the outer lines. Suspended within it are a set of black holes, arranged not spherically but rather in a square (or in 3 dimensions, cube) shape.

This violates the assumptions that make gravity spherically symmetric.

To make things simple, we could even do this:

+---------------+
| *           * |
|               |
|               |
|               |
|               |
|               |
|               |
| *           * |
+---------------+

and place the massive singularities only at the corners.

The math to work out the gravitational potential along a "face" of the cube will now be tractable. For a first approximation do two:

 +--------------+
 | *          * |

we have the free parameters of the "depth" of the singularity and the distance between them.

Suppose they are at depth 1, and at the surface "right above them" they produce 1 unit of gravity, and they are 2 distance units apart.

Then at the + there is 70% gravity at a 45 degree angle. At the midpoint of the side, there is 1 g strait down.

Directly above the * there is 1 g down plus a 1/sqrt(3) component at a 2:1 gradient angle.

You get a "peanut" shaped gravitational field.

With careful singularity placement, you can make the sides of the cube far closer to gravitationally "flat". The corners will still protrude, and unless you are extremely careful will be mountains of exposed adamant. You might be able to keep the edges within the atmosphere; you would probably still expect exposed adamant here, as there lacks a process to refresh any material that "slides down".

Fun arrangements of singularities, like rings on two opposing faces, might actually make some parts of some of the edges become under water, and water able to flow from one face to another.

I won't posit what kind of crazy technology is holding singularities (or other crazy-dense yet small masses) up in that kind of strange lattice. To pull this off, the gravitational masses have to both be significant and close to the surface; as you get further away, everything looks like a sphere.

In the limit, if you had an even dense grid of heavy masses arranged over an infinite flat grid, you get a level gravitational field. Cutting such a field off with edges results in the edges being "uphill". Adding singularities near the edges makes them "more downhill", removing them makes them "more uphill".

If I was serious about this, I'd start writing software that uses the principle of superimposition to let me place point masses and see the planes of level gravitational potential.

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Storms and winds will toss bacteria, seeds and spores high enough to get over the ridges separating the faces, so the plant biology will be largely the same. Since the plants are the same, there will need to be some similarity between the animals, since they have to eat the plants.

The odd shape will lead to powerful jet streams connecting the atmosphere over each face. Powerful storms such as tornadic waterspouts will occasionally suck up fish and amphibian eggs from one face and deposit them on the others, meaning they will share some similar marine life.

Now add in the dynamics of warfare. Whoever has the high ground can fire down on people trying to storm their fortress. Or losers can retreat there until they can launch a counterattack. Nations will build ever higher forts, learn how to build oxygen masks, and steadily get closer to the top of the ridge, until one face is able to get to the top and climb over.

Eventually, they build pressurized monorails to travel between faces. They can attach the rails to punctures in the skin caused by meteor strikes.

Interesting article on the physics: https://gizmodo.com/how-gravity-would-be-different-if-the-world-were-a-cube-1492018223

Gravity lines in most places do not point straight toward the center, unlike earth. That would act to make the climb seem less steep at first if you try to climb toward the edges.

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    $\begingroup$ Storms won't do it. Assuming a roughly Earth-sized box, the edges will be mountains that are effectively 2000 km tall, sticking well above the atmosphere. $\endgroup$ – Mark Oct 23 at 22:53
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Your world cannot exist in the form you've described.

To avoid being round, it must have very low gravity: at most a couple of percent of Earth's, even if it's solid rock. This is too little to maintain an atmosphere, since thermodynamics will keep a good portion of gas molecules moving above escape velocity until they've all leached off into space (even without a solar wind helping the process along). With no atmosphere, there's no ambient pressure at the surface so you can't have liquid either; also nothing to protect against ionizing radiation. Nothing could live on the surface without an enclosed habitat.

Something like the subsurface oceans of Jupiter's moons could work. With the right engineering, perhaps you could even have an internal atmosphere and landforms; but to protect against external impacts over countless millions of years, the "hull" would have to be extremely thick and heavy, which might make a hollow interior structurally infeasible. At the very least, this would take a truly Herculean feat of engineering, especially for it to last on evolutionary timescales - and I imagine a society with access to those sorts of resources would have better uses for them.

Even then, complex multicellular life took billions of years to evolve on Earth and your space habitat might need far longer still: a much smaller environment means a much smaller number of "evolutionary experiments" taking place at a given time. Whatever you got at the end of it all certainly wouldn't resemble the land life on Earth, due to the far weaker gravity.

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For the physics of this world you should look at this question discussing how large a cube can be. It is not only feasible, it is almost certain that each face of the cube would evolve and develop uniquely. Unless you are really throwing all physics out the window, you won’t be able to cross the corners or edges - they will be at such a high altitude you would need a space suit to travel over an edge.

So, keep just a small bit of reality and the societies on each face will be completely isolated until they develop space travel.

Tunneling, again, would require drilling technology far more advanced than what we have today. The depth of those tunnels will create unsurvivable pressure. Again, assuming you want a little bit of reality in this.

Also assuming some reality-based physics, Boxworld is made of something much more dense and rigid than granite or stone. They will never be drilling through whatever is holding it in that shape.

The faces can discover each other when spaceflight is developed.

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