Split planet

I imagine the world like an atom, with the bits of land rotating about something in the center. You could put something interesting at the planet's core.

I'm also curious about what it would do to the ecology. Would one part be prone to spawning creatures of darkness and the other creatures of light? Would the areas change "polarities"? Would there be creatures that would have different forms to deal with the change from positive to negative and vice versa?

I imagine there is a way to traverse the parts of the planet besides through the atmosphere. Maybe instead of a traditional underdark, the undersides of the hemispheres are jagged and cavernous, leaving large natural openings in which magic spontaneously creates portals to link the halves together.

That's an interesting take. I haven't thought what it would do to the seasons. I like the idea of the gods splitting to rule different halves. I was thinking of having the PC's having no knowledge of this, come out of a cave, or over a mountain and having where the world should be, be replaced by floating islands and a large red glow in the distance (magma from the other half). Just thought that would be a cool moment. It also opens up some interesting game play ideas. Floating island pirate airships?

An odd thing occurs to me that could be interesting. I imagine this planet split apart, and the atmosphere now fills in all the areas between the different chunks of the planet, held together by gods of energy. So what that means is that there is less atmosphere for the once-surface of the planet (no new breathable air, but the breathable air has to cover more surface area now). So it's like a falling ocean level, leaving high elevation locations stranded without breathable air. Mountain-top fortresses and temples are now frigid and uninhabitable. Think of it like maybe 3,000 feet above sea level is equivalent to 10,000 or more. The small eroded mountain outside town would be like K-2, and everyone all over the planet has had to adapt to high-altitude living like people in the Andes. So you could look up in the too-clear sky to see these ruins at higher elevation, wondering who could have built them, since nothing can live there now - they might as well be on the surface of Mars.

My science brain just wants to dissect this idea like a wonderful little mind puzzle. Physics-wise, a lot probably wouldn't change. It's still the same amount of planet/mass in roughly the same area. Gravity will continue to cause those hunks of mass to mutually rotate around a shared center of mass (presumably the core). The orbital mechanics wouldn't really change all that much. On it's own, gravity would soon recombine any split planet chunks which implies that something somewhere is generating a force to countermand gravity. How the planet stays split could be an interesting question to ask.

The distribution of the atmosphere could be a pretty big problem (atmosphere's tending to be pretty thin to begin with). If that distribution is at all similar to Earth then I would expect the mostly likely scenario is that only place with a sufficiently dense atmosphere for habitation is probably going to be between the two halves of the planet. Outside the break, on what was once the surface, there wouldn't be enough air to really support conventional life. That could muck up the water cycle in some interesting ways too.

And if everything does move into the split (what a migration -that- would be!) imagine how that would change their daily life. The length of a "day" would depend on how close you were to the edge/surface of the planet and how the rotation lined up with sun. Although, if the split was along the equator I guess the opposite could be true and you could be lined up with the sun perpetually. In that instance "night" might be when shadows cast by the terrain happen to fall on a given area.

And there's a whole other side to this that hasn't come up yet. If the planet is split then it seems unlikely it maintains a rotating mantle (geologically speaking). Without a circulating, massive sphere of molten iron (and some other things) it won't have a natural electromagnetic field. That exposes the planet (and anyone living on it) to the full brunt/spectrum of the sun's radiation. That would almost certainly destroy any organic life on the surface short of a tardigrade (or terrasque). Organisms may migrate into the crack just to escape the harmful light on the surface. And any atmosphere that was left exposed to that radiation would eventually be eroded away (we're pretty sure that's what happened to Mars).

It occurs to me that it would probably take a few decades for the mantle to cool enough to walk on but that's probably something that could be hand-waved. The upside of moving your industry/craftsmen into the crack is that suddenly you would have access to all the heavier elements that tend to fall/shift down to the center of the planet. Super dense elements like osmium or iridium (real world adamantite) would suddenly become accessible. Whether or not craftsmen would be able to work with them might be another question.

It seems like walking between the two halves of the planet should have some weird effects gravity wise. All mass pulls on all other mass with equal intensity which diminishes with distance. So if we ignore all other factors and just employed a thought experiment where we crack the Earth in half and walked down the split... The mass probably wouldn't be perfectly distributed so it's likely one half would be a bit heavier than the other half. If the divide between them were small enough it's possible our imaginary trekker might "fall" towards the heavier half. But they would fall very slowly because they're still being pulled on by the lighter half. If they managed to walk all the way to the center (the core) then they're being tugged on by all the mass around them, which is roughly equidistant in every direction. So, I suspect, the closer something gets to the center of the planet, the lighter it would weigh. At the actual center it would probably become trapped at the exact center of all that mass (presumably where your prison is at). I wouldn't be surprised if the terrain reflects this, perhaps with a pair of "mountains" from each half that both peak in the prison.

In any case, you're free to use or not use as much physics as you like. I enjoy it as a thought experiment if only because I find it entertaining but you're obviously free to do as you like irregardless of physics.

Science-wise, sleep is an odd thing we're still sorting out. My understanding is that it seems to be linked to the function/health of the brain, particularly memory. Aside from the central nervous system the body doesn't really seem to need much rest/sleep, but the without it the CNS just falls apart. It seems unlikely to me that processes of adaptation could result in an organism that once needed sleep no longer needing it. That said: magic bridges all gaps.

Again, the simplest approach to the lack of an EM shield would be magic. Alternatively, if it is indeed a vacuum between the halves then it will take significantly longer for the mantle to cool. Instead of decades you'd be talking millennia. It's a little hard to wrap our heads around but a significant portion of the Earth's internal heat today is left over from it's young days as a molten ball of lava. The effect of this is that your mantle would still be molten, likely for thousands of years. Depending on the size of the gap, molten metal might flow between the halves, at least in glops and globbules. As long as the metal is molten and there is a temperature differential (which is more or less required by physics in this case) the metal will circulate and you'll have an EM field. It also presents a not inconsiderable challenge to anyone who might want to reach the prison. How would you go about walking from London to Moscow when 99% of the trip was made over magma and lava occasionally rained down from above or fell up from below? Toss in the aforementioned effects of gravity and it could get pretty weird.

So far as the night/day cycle you could get into some weird orbital physics such as a catywhompus rotation (looking at you Neptune). Assuming you're sticking with something Earth-like then not that much is likely to change. If some force is keeping air (and likely water) out of the divide then life on the surface of the planet would likely continue as before. The two halves would be separate though so they may develop along very different paths, sort of like how various species in Australia developed separately from their cousins in Africa.

So far as YouTube goes, my wife and I are fans of MinutePhysics along with the wider world of science channels. CrashCourse is perhaps the best but the various SciShows are excellent for staying up to date. Then there's a litany like Smarter Every Day and Veritasium that are a bit more eclectic but still pretty good.

That's what happens on the DMB forums.

Seems like a cool world.