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the setting premises are explained in this other question Survival of an Industrial Revolution city after being transported to a fantasy world

What I had in mind today is this: on Stitch theres an area (say less than 10 miles radius) where the underground is made of a very lightweight material; imagine a cone that starts from the surface and ends reaching the planet core that is made of this alien material (I imagine this material similar to cork wood). The surface area looks like a kinda normal temperate valley, but when you enter it you almost immediately start being in low gravity, almost like walking on the moon. Is there any way to make this premise scientifically based and not in any way be magic related?

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Claudio Bonifazi
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  • related: http://worldbuilding.stackexchange.com/questions/63296/why-would-an-antigravity-abyss-exist-in-a-deep-cavern – kingledion Mar 22 '17 at 12:15

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There's a problem here: unless gravity is very different in this world you're not going to feel much lighter. you don't just experience gravity from the things directly bellow you. You're also going to get pulled towards the rest of the planet. At best gravity would be a tiny tiny tiny bit different at the center of the area.

https://en.wikipedia.org/wiki/Shell_theorem

Take it to extremes: even if the entire world was hollow, if the total mass of the shell was similar was similar you would still experience a gravitational pull towards the center point as if the shell was not actually hollow.

Murphy
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Yes, but the result will be small.

To get a significant result, you have to make a very big and very deep - thousands of kms - hole in the Earth and fill it with the lighter material.

Here comes another problem: the more dense material will make a significant hidrostatic pressure to the lighter.

I didn't find a better image quickly, but around so:

enter image description here

Here the cellar is filled with air. If it would be filled by some lighter thing as earth, a similar hydrostatic pressure would also exist.

The deepest mines are around 4 km deep, maybe some tens of km would be possible with an extreme effort.

If some magical thing does it, maybe some hundreds of kms is highest limit. For example, some light nanostructure could be enough light and also hard.

It would reduce the gravity at most with some percents.

Gray Sheep
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  • So the lighter material would inevitably be crushed all around by the other planet matter,. I suppose I'll just scrap this one, thanks for the infos – Claudio Bonifazi Mar 22 '17 at 00:45
  • @ClaudioBonifazi Not surely crushed, but pushed out. Like if you blow shaving foam on water. The shaving foam is nearly so light as air, while the water is much denser -> the shaving foam is pushed out of the water. In such sizes, also the solid materials behave like fluids, even (for example) super-hard but super-light beryllium-oxide crystalls can't resist the pressure of hundreds of km of earth. – Gray Sheep Mar 22 '17 at 18:19
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If you replace an entire conical section of the planet and replace it with a lighter material, the first thing you need to understand that it will shift the centre of gravity. As a result, the distance will be greater from centre to designated surface reducing the gravity experienced. But this reduction will not be limited to the 10 mile radius. Instead it will be lowest at the centre of the area and gradually spread all around the world. Also gravity diametrically opposite will have highest gravity on the planet.

If you want only the 10 mile radius are to have lower gravity, you will need a material underneath that generates powerful magnetic field can make organic objects lighter due to diamagnetism. Or something that reduces curvature of space would also reduce gravity.

Deepon GhoseRoy
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Replace the area with some other fluid that is denser than normal air. This will increase buoyancy and create the illusion that things are much lighter than they are.

Christopher King
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The area is not longer on the planet, but has shifted to another dimension without gravity and THE PORTAL IS STILL OPEN.
Though I call it portal it is not necessarily a door but a spherical field, like the surface of a buble, that transport you to that dimension/plane.
Since the portal completely encompasses the zone you cannot step (sorry, float) out of it to the rest of the dimension without returning to earth and everything (light, air, animals, people) crosses freely through in and out of the field without realising they are leaving the planet or coming back to it.

ThreeLifes
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Rotating iron core creates a spherical magnatic field. Of course it gets weaker the further you get from the core. So the higher the mountain the weaker the field. Not in any significant amount of course. The only other way to have the gravity change so much in that way is magic.

Also wouldn't it catch fire if it extends to the very core, touching magma and the like?

Mormacil
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