Wangalade wrote: ↑Tue Apr 09, 2019 4:32 amyeah . . . . the hollow world screws with tectonic plates and continental drift, basically it wouldn't happen. continental drift is dependent on the heat and pressure derived from the layers of superheated liquids and solids. Specifically the Athenosphere(upper Mantle) is what causes the tectonic plates to shift and expand and contract; as it is, the hollow models created by thorf just don't have a thick enough shell to support the athenosphere, and even if it did, the density of the material would have to be drastically increased to make up for the hollow center. the physics and geology of the whole situation is the main reason I'm hesitant to have the hollow world in my version of Mystara. I've been thinking about just transplanting the general cultures of the hollow world to a corresponding place in davania or skothar(nithia to Arypt). I have been searching for any scientific/pseudo-science explanation that would support the hollow world AND maintain the presence of tectonic plates, and none I have seen are remotely plausible. The closest I have come is this jewish guy who rejects western/christian science and in the process throws out gravity and explains the effects of gravity with electro-magnetism and some other weird stuff. I don't think his work is really correct or logical, but he's the only person I've seen who works through the equations and everything in a rigourously scientific manner.Big Mac wrote: ↑Thu Apr 04, 2019 1:16 pm
Has anyone looked at the real-world geological processes that were happening back in the Jurrasic Earth and looked at how that sort of process might still work on a planet that is actually a hollow world?
Could the Known World still have continental drift and all that other stuff going on without land needing to go in and out of the Polar Openings?
Or could the idea of things being "moved into the Hollow World" happen literally sometimes?
If you don't have that going on, how else can the Skothar mountain ranges be explained?
So the bottom line, either the world is solid and has tectonic plates, ergo continental drift, or the world is hollow and has a static surface (therefore not being earth in the age of magic). Of course you could always say 'because magic!' and have both
This got me inspired to go deep researching the last couple days. As I stated above physics precludes a planetary body that is hollow from normally existing, and this is the reason I have not included the hollow world in my version of Mystara so far. There has always been certain things about it that have appealed to me though, I just really dislike the presentation of most of the offical hollow world products. To explain away the problems with magic and Immortals in complete disregard of physics is unacceptable to me.Thorf wrote: ↑Sat Apr 13, 2019 1:09 amObviously you have a point, in that by its very nature the Hollow World precludes the real life model of the Earth. However, Aaron Allston did come up with an explanation that at least explains gravity and volcanism, and could probably be extended to include plate tectonics: the magical material of the World Shield, which is basically just a way of compressing what should be the extremely thick core into a vastly thinner layer.Wangalade wrote: ↑Tue Apr 09, 2019 4:32 amSpecifically the Athenosphere(upper Mantle) is what causes the tectonic plates to shift and expand and contract; as it is, the hollow models created by thorf just don't have a thick enough shell to support the athenosphere, and even if it did, the density of the material would have to be drastically increased to make up for the hollow center.
Whether you accept this as an explanation of course is entirely up to you, but at the very least the explanation is there in official sources. And let's face it, the whole idea of the Hollow World is so entirely beyond the realms of science that there's only so far we can go in "explaining" it anyway.
One more thing: although the core is clearly not there, or rather has been hugely compressed into a very thin layer, the asthenosphere and the layers immediately below it do indeed have enough space. Checking this article at Wikipedia:
Official sources tend to give the crust as 1,200 miles, I believe. That's enough to provide Mystara with all the top layers (inside as well as out), and the top of the mantle.
My model currently has a depth of 3,414 miles, which is enough depth to provide the outer and inner worlds with the top layers plus the full mantle — everything but the core, really.
One last thing: it seems clear from Mystara's underground realms that its underworld is already vastly different from the real world's. Isn't this true of all fantasy that has people living well below the surface world? In reality, things get hotter and hotter, with more and more pressure as you go further down, do they not? Clearly this is not the case in D&D underworlds, which tend to be depicted as cold caves and caverns.
In conclusion, I'd argue that none of this really matters anyway. After all, whether or not the planet currently has plate tectonics, Mystara's mountains are clearly based on plate tectonics since they're inspired by a map of the Earth. Unsatisfactory as this may be as an in-world explanation, it's certainly rather useful to us cartographers in terms of terrain design.
So after doing some digging into the whys and hows of planet formation and speaking with my brother(currently attending university to become an engineer) for help with some equations I think I've come up with a solution for both the structural integrity of the hollow world and the reversal of gravity in the interior.
Let's start by talking about why hollow planets dont exist. Within Astrophysics there is a principle called Hydrostatic Equilibrium. Essentially, this is a balance between gravity and pressure. Gravity pushes all the mass of a planetary (or star) body inward toward a single point, while the substance of the body resists compression and builds up pressure pushing outward away from the center of gravity. When these two forces are equal, there is an equilibrium. If a body is not in equilibrium it will either expand or contract depending on whether the force exerted by gravity or pressure is greater. Hydrostatic Equilibrium is what causes all planets and Stars to be spherical in shape.
The reason a hollow world is unstable is because we have removed the material in the center exerting outward pressure, so now there is vacant space into which the rest of the planetary mass will collapse. Now, you may be asking why a solid object would change shape and collapse. The answer to this requires a different way of thinking about solids and liquids. Most of the mantle and core of the earth would normally be called a solid. But we're talking about millions of tons of material here constantly being acted upon by both pressure and gravity and heat(a significant portion of the heat in the Core comes from Radiocative materials). On a geological timescale the mantle and core behave viscously, basically the main body of the earth is a fluid. So when any significant hole is opened up inside the earth, the mass surrounding it would flow into the empty space until a new equilibrium was reached.
Now for the solutions.
I'm going to explain the physical structure of a viable hollow planet first. We understand at this point that a hollow earth can't exist because the interior of the earth is viscous. In order to support a cavity inside the center of the planet a solid non viscous material is required. If the mass of the earth didn't behave as a fluid, then Hydrostatic Equilibrium would be irrelevant because these principles only apply to fluids. The extremely easy solution is just to make the hollow world be a dead planet, meaning the core and mantle are completely cooled down and the intrior has solidified so it is no longer viscous. This would preclude the presence of continental drift though because the movement of tectonic plates is dependant on the presence of the asthenosphere which is itself dependant on the unique conditions of the upper mantle.
An alternative option which I prefer is to have an inner shell of solid non viscous material that would support all the viscous layers of the rest of the earth so that it wouldn't collapse in on itself. I think it's better explained through an image:
My next step with this is to determine what material the inner shell should be made of and how thick it needs to be. The shell must be able to act as a replacement of the pressure that would normally be in the center of the earth. There are a series of equations that I must iterate to determine exactly what type of material for the shell is best.
First I pick a substance and decide on how thick I want the shell to be. From there I calculate Hydrostatic Equilibrium
dP=-D(h)G(h)dh where P is pressure, D is density, G is gravity, h is height
Then I calculate stress exerted on the shell
Rs=P((R^3-r^3)/(r^3-R^3)) where Rs is radial stress, R is outerradius of the shell, r is inner radius of the shell
Cs=(P/2)(2(R^3+r^3)/(r^3-R^3)) where Cs is Circumferential stress
Equivalent Total Stress=Rs-Cs which must be less than the yield strength of the material comprising this inner shell. This will be a process of trial and error, picking a material doing calculations and figuring out what works out the best.
It's getting late and I have work in the morning. I will talk about the gravity of the inner world tomorrow and some ideas about Urt and Megaliths in general