Explore a Peculiar Hollow Earth World: Gravity Possibilities

[Huggas]

Hello everyone!
I am an undergraduate getting three bachelors in Anthropology/Ecology/Neurosciences (just to show off of course), but sadly no true physics (I don't show off that much, I'm still humble).
I am working on a personal project, a tabletop role playing game, where my main struggle is its main premises.

My world is a hollow Earth type world (yes I read the rules of this forum, this is just a RPG, and it's fantasy, nothing linked with the conspiracy, it just makes it interesting), and all I would like to ask is one question.

So imagine, their known and livable world is in a sphere, from which:
1, their "ground" is infinite in depth as it spreads away forever.
2, their center of the universe is the center of their sphere.
3, their "space" is first "empty", but gets denser and denser as it gets closer to the center of the sphere.
(Thus there is ultimately sphere in the sphere, where the "ground" is infinite, and the "space" is finite and small)

My question here is about gravity, and I would like to see if there is any possibility of it:

As the mass (to be defined) of their dense ground pulls everything towards the ground, and the mass (to be defined) of the universe gets denser at a certain point,

Could there be, depending of the defined masses of ground and space,
1, a livable environment where it would still be that of a normal gravity like on earth,
2, a zone in between ground and space where there would be no real forces (and/or they both get cancelled),
3, a place where the force from the dense "space" center of the sphere pulls objects.
And otherwise, if we removed the assumption #3 (just an empty space, no densification), could possiblities 1&2 happen?

Thanks to everyone for their time and attention, I look forward to your answers!

P.S.: I just made a fast sketch of the concept, just to broadly give the idea as it's quite far stretched.

 

[trurle]

Basically, you need non-Newtonian gravity (falling with distance faster than reverse-square rule)
Also, this forum may be not optimal place for such posts.
alternatehistory.com
for example have more discussions on world-building.

 

[Rive]

...their dense ground pulls everything towards the ground...

That's not working this way. In this configuration as far as it is 'gravity' what is in effect there will be no gravity in the whole internal part at all.
Maybe you can give it a spin to create some artificial gravity: together with a real mass at the center it'll work ~ the way you described, but it'll has its own problems at the poles.

 

[Huggas]

That's not working this way. In this configuration as far as it is 'gravity' what is in effect there will be no gravity in the whole internal part at all.
Maybe you can give it a spin to create some artificial gravity: together with a real mass at the center it'll work ~ the way you described, but it'll has its own problems at the poles.

I see, thank you for your input.
Of course I am a neophyte in the domain of gravitational physics, and I thought that the formula for gravitational force depended on masses and radius.
Could you please explain me, briefly if possible, why there wouldn't be gravity inside the sphere, and whether having a dense center or an empty center would impact this.
Also, if "I give it a spin", where should the spin be, should the dense center be spinning, the whole sphere, or the ground, or everything altogether in different directions?
Lastly, if so, what are the problem you spoke about at the poles (the poles of the center or the whole sphere or both)?

To my first replier, how would non-Newtonian physics help this case, how this could happen?

Thank you so much!

 

[DaveC426913]

Could you please explain me, briefly if possible, why there wouldn't be gravity inside the sphere,

Newton's shell Theorem shows that any point inside a hollow sphere whose walls are of uniform density (or infinite), gravity cancels out to zero everywhere inside.

A1 is twice as far from P as A2, but its area is squared. When you plug this into the formula for gravity, it turns out gravity pulls exactly the same in very direction, no matter where you are inside the sphere.

If you put a mass at the centre, objects will be attracted to it, exactly as if you were not inside a shell.

Spinning would produce artificial gravity, just like a space station.*
Rotating artificial gravity only works if you are rotating with the body. This means, if you can stop yourself from rotating with the body (i.e. counter spin), you will float without falling. unfortunately, you will be under a constant wind from antispinward, which will ultimately push you toward the ground.

* but how could you spin something whose diameter is infinite?

 

[DaveC426913]

To my first replier, how would non-Newtonian physics help this case, how this could happen?

Magic.

This is already a fantasy - why does it need a rational explanation? Unless your players are actually going to solve the mystery of how the world works, they should be satisfied with it being hand waved.

 

[Rive]

Lastly, if so, what are the problem you spoke about at the poles (the poles of the center or the whole sphere or both)?

Look up the 'Ringworld', that more or less describes the proposed situation within limits of acceptable science and it'll spare you a lot of questions.

Regarding the poles: it is about a different construct, a 'Dyson sphere'. The weakness of a Dyson sphere is that artificial gravity is proportional to the distance from the axle of rotation so it does not work around the poles (and it has the wrong direction everywhere else but around the equator). So this pole part of the construct must be strong enough to withstand the gravity of the center object. This problem does not apply for a 'Ringworld'.

 

[Klystron]

Look up the 'Ringworld', that more or less describes the proposed situation within limits of acceptable science and it'll spare you a lot of questions.

No argument that 'Ringworld' series covers the physics quite well for fiction. Principle author Larry Niven created 'Known Space' over many stories and novels linked with common themes such as hominid species entering a third phase of life after consuming 'tree of life' roots; explained in the novel 'Protector'. Child => adult => protector. This is probably enough to enjoy 'Ringworld', though 'Protector' is a great novel in itself.

Since you study anthropology and ecology, you may enjoy John Varley's 'Gaea' trilogy. Unlike 'Ringworld', the torus (ring) is enclosed and is habitable from the rim to the hub due to several ingenious plot devices such as hollow spokes used to provide breathable atmosphere. Weather events are featured in the 3 'Gaea' novels to a greater extent than I recollect in 'Ringworld'. 'Titan', though still gigantic, is modest in size compared to 'Ringworld' permitting more interaction among the inhabitants without requiring teleportation devices.

Several authors featured hollow Earth scenarios, popular in late 19th, early 20th century literature; but Niven's science is vastly superior. Robert Heinlein visits hollow Earth myths among representative story ideas in novel 'Number of the Beast'. Umberto Eco explains the history and social significance of hollow Earth theories in novel 'Foucault's Pendulum' from a (skeptical) occult viewpoint. Enjoy!

 

[Huggas]

Dammit! It's not today I'll prove we live in a concave earth!.
Alright, thanks to everyone, especially for surviving my lack of knowledge in certain fields (non-Newtonian physics haha!). I listened to everyone and searched more on the subjects you gave me. I came to another but similar idea.

What if it's a normal planet, an oblate spheroid (bigger but less dense than Earth), but the known world of my population is only a "hole" in the planet.
Could this be a thing?

Imagine a 30M km2 hole (size of Africa) with 10km "walls" surrounding it... Tell me if I am accurate:
- A healthy gravity would occur in the hole, more than on the surface sure, but this could be adapted with the planet's size.
- A light source could go through most of it depending on the planet's rotation and distance.
- There could be a viable ecosystem inside it.
- There is no other physical phenomenon that I don't know of which would destroy my childish imagination.

If these four pass... What could have made this hole?
An asteroid or similar?
Tectonic plates subduction (or is there a more accurate phenomenon I do not know)?
Thor's right foot?

Thanks to everyone, one more time, I really enjoy it!

 

[Rive]

Could this be a thing?

Yes. Air pressure is your friend for this: if at the bottom of the hole you have ~ the same atmosphere and gravity as on Earth, then you will have ~ the same atmosphere at the edge that we have here at 10km high. Quite a fence I would say.

Thor's right foot?

I'm afraid that's the most 'realistic'

Sci-Fi has many branches but only the 'hard Sci-Fi' is which requires everything written to comply with the actual state of science. Despite being claimed so not many author can write real hard Sci-Fi and those who can are really an elite club. Maybe you should tune your expectations a bit and try to write 'just' something good instead of 'hard'? There are a lot of legitimate plot devices what can turn impossible to possible without turning the 'sci' part a joke - especially if the focus is not on physics. Instead of throwing your ideas to the meat grinder here, hm?

 

[some bloke]

I'm not 100% sure on this, but I think that if you were to have a central sphere which somehow exerted a force, additional to gravity, which only affected the atmosphere and not the people or structures, you could achieve the "effect" of gravity in the other direction in that it pushes the people against the outer surface, due to buoyancy.

Unrealistic example: The air is magnetic, the central "orb" is an immensely powerful magnet. The people have nothing magnetic in their bodies (realistic, as everything magnetic will have been dragged to the orb before life even started evolving). So the effect of buoyancy, being the comparative forces in a direction of the orb, will pull the air a disproportionate amount more than it pulls the people, so the people will "float" to the surface.

Will also allow you some poetic license to have flying creatures which have magnetic elements in them to "catch the gauss flux" and glide on it.

This would have some odd side-effects, like "gravity" being greater the "higher" from the "ground" you are (my what a lot of inverted commas!). IE buoyancy will be more the deeper into the air you go, the closer to the central orb.

To solve the origins of the "hole in the ground" idea, you can always just go for the mystery approach. Presumably, for no-one to have climbed out, the walls must be smooth. Perhaps a failed test-run of a planet killing laser, which only fired so far into the planet before blowing up it's own planet, countless millennia ago. Perhaps it was built by a long lost civilisation. Perhaps it is a prison. Perhaps the planet is artificial and the civilisation started from a lost craft which crashed there.