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Yuki Akuma
2011-06-13, 11:31 AM
So. I'm writing a campaign setting. Things are a little vague right now, but there's something I need some help on.

Basically, the world is a planet, with the same mean density as Earth - so it has a gravity of around 1g. However, instead of having a liquid iron core, it has a follow air-filled core. This means that it doesn't have a magnetic field - or at least a far weaker one.

I have a question about the physics of the core. If one were inside the core, near the 'surface', which way would gravity 'pull'? 'Up', towards the surface, or 'down', towards the center of the planet? And if they were at the center, would they be crushed or weightless?

And yes I am aware I can make gravity behave however the heck I want, but I'd like the planet to be physically possible before I add in magic to tweak things.

Kol Korran
2011-06-13, 11:37 AM
i am no expert in physics, but i know some people who did the 'hollow world' angle declared the gravitational center is actually at the crust/ mantle/ surrounding layer. so if you were on the inside, you would walk on the inner surface, being pulled to it. the guys were physics students, and they tried to explain how it might be possible somehow, but my "it's physics! run away!" mind shut it out.

i remember something about it being made from a different material, and something about velocity...

not much help, but i hope it's some help.
good luck to ya!

Tengu_temp
2011-06-13, 11:42 AM
If you're inside a hollow sphere, you're not affected by its gravity. Everything inside the core is weightless.

Also, a planet with an air-filled core is not really physically possible anyway.

Eldan
2011-06-13, 11:43 AM
I don't know about hollow planets, specifically, but rotating rings, at least, have their "gravity" pulling outwards, as on spaceships. Though that isn't from mass. Dyson spheres/Ringworlds too, from what I remember.

hamlet
2011-06-13, 11:46 AM
1) In "real world" physics, gravity is the center of mass, which would be the dead center of the world itself, not the crust. That means people on the inside would be eventually drawn to the center of the planet where they would collect unless they had a means of getting away from that.

2) It's not horribly breakable to assert that a quirk of local physics, perhaps induced by a strong magical field or some such, would cause center of gravity to be located in the crust itself, but in real life, a hollow world is not really practical or sustainable.

3) Hollow World is, in fact, a TSR D&D setting from back in the 80's. It's quite good if you can track down an inexpensive copy.

EDIT: As for spinning planets . . . the planet in question would already have to be quite substantial (since you want surface gravity of about 1G but with a hollow world) since you've removed all the mass from the middle of the planet, but even then, you'd have to get it spinning awefully fast (to the point of absurdity) to form an appreciable -2G (-1G to overcome actual gravity towards center of planet and -1G to create simulated +1G "sticking" people to the inside surface) and even then, get people far enough away from land and they'll start to float, just like on the outside.

valadil
2011-06-13, 11:47 AM
I misread that as a "Hello World" campaign setting and now I'm getting ideas for my unplayed Cryptonomicon inspired Technocracy game.

Mastikator
2011-06-13, 11:52 AM
i am no expert in physics, but i know some people who did the 'hollow world' angle declared the gravitational center is actually at the crust/ mantle/ surrounding layer. so if you were on the inside, you would walk on the inner surface, being pulled to it. the guys were physics students, and they tried to explain how it might be possible somehow, but my "it's physics! run away!" mind shut it out.

i remember something about it being made from a different material, and something about velocity...

not much help, but i hope it's some help.
good luck to ya!

Maybe I can help. When you're outside the planet you're pulled to every atom the planet is made of, it equals out so that the sum of the force points to the center of the planet.
However, when you're inside you are being pulled by every atom still, but now they're all around you, the crust that is above you pulls you UP, which makes the total force weaker, and as you get closer to the core of the planet the weaker the gravity gets until you're at the center, being pulled in all directions equally (which cancels it out to zero).


Anyway, from a physics perspective, a planet can't be hollow, it'd implode. The spin it would require to keep it from imploding would cause it to be ripped apart and made into a donut-like shape, but the atmosphere would be cast away, as with the oceans.
Might work for a massive ring of rocks, not so much a planet.
You're gonna have to add magic to explain a hollow planet, or hope that your players didn't pass physics.

Eldan
2011-06-13, 11:54 AM
The planet is made entirely out of Adamantium, Sovereign Glue and Riverrine. There. Problem solved.

danzibr
2011-06-13, 12:02 PM
Basically, the world is a planet, with the same mean density as Earth - so it has a gravity of around 1g. However, instead of having a liquid iron core, it has a follow air-filled core. This means that it doesn't have a magnetic field - or at least a far weaker one.

I have a question about the physics of the core. If one were inside the core, near the 'surface', which way would gravity 'pull'? 'Up', towards the surface, or 'down', towards the center of the planet? And if they were at the center, would they be crushed or weightless?

And yes I am aware I can make gravity behave however the heck I want, but I'd like the planet to be physically possible before I add in magic to wteak things.

Yeah, people basically already said it, but... I'll throw in my two cents. I just took a class last semester, Geometry of Black Holes (a mathematical physics class) and I asked my teacher something like this.

First, if it has the same density but less volume (by removing the inside), you're decreasing the gravity.

If you're dead center on the inside (and it's evenly distributed), you feel no pull of gravity.

If you're in the center but not dead center, you should fall to the closest part of the... surface.

Yeah, looks like Mastikator knows a lot more physics.

Pink
2011-06-13, 12:04 PM
This thread interests me. The idea I'm getting from it right now is that a game starts on the inside of the hollow, which is all the known world (need to think how things like weather and day and night work inside a sphere, as well as how things like maps might differ...this is a curious train of though I like), and monsters come from these rents and gaps in the ground which lead to the surface and such, and basically have this weird reverse underdark experience.

As I think of this, didn't ff13 have a hollow metal moon type planet thing that had civilization living on the inside of the crust, so to speak?

Giant Panda
2011-06-13, 12:16 PM
I can say that in real world physics, in a non-hollow planet like earth, gravity pulls down towards the core, at at the center gravity is zero, so any objects at the very center would be weightless. So if you were beneath the surface in the real-world, you'd still be pulled to the core. I'm pretty sure I can bring out the maths for this as well. Although I doubt anyone takes it that seriously.

You'll also need magic to explain the lack of a magnetic field. Without it, not only would the surface be uninhabitable (I assume, hence the hollow-core world), but I'm pretty sure the cosmic radiation would heat up the center of the planet a fair bit as well. So, I think even in the core, without a magnetic field your hollow planet is going to be pretty radioactive.

If you had a denser surface, so gravity was the same, but with a hollow core, gravity would still pull to the center of the planet, even if it was hollow. This is because the center of mass for a spherical object is in the very center, assuming the sphere is of uniform density. So you'd still always be pulled towards the center of mass, the core, and so theoretically would still be weightless at the center where gravity = 0.

Of course, you couldn't really get a hollow planet, as it would collapse on itself to form a smaller non-hollow planet. So the center-of-mass stuff is purely mathematical theory. In practice, it would probably not work like that, but since it can't occur, it's the best idea I have.

I hope some of this helped!

The Big Dice
2011-06-13, 12:20 PM
Forget science. A hollow world works because it's a world and it's hollow. Simple as.

And I'll second the 80s D&D Hollow World setting as being pretty good. In fact, I tracked down a copy in near mint condition on ebay for a price that compared well with the price it had on release. I wonder if there's a 3.X conversion for that...

hamlet
2011-06-13, 12:33 PM
Forget science. A hollow world works because it's a world and it's hollow. Simple as.

And I'll second the 80s D&D Hollow World setting as being pretty good. In fact, I tracked down a copy in near mint condition on ebay for a price that compared well with the price it had on release. I wonder if there's a 3.X conversion for that...

Not to the best of my (admittedly limited) knowledge. But there's absolutely no reason it wouldn't work very well.

Sir Swindle89
2011-06-13, 12:38 PM
If you had a denser surface, so gravity was the same, but with a hollow core, gravity would still pull to the center of the planet, even if it was hollow. This is because the center of mass for a spherical object is in the very center, assuming the sphere is of uniform density. So you'd still always be pulled towards the center of mass, the core, and so theoretically would still be weightless at the center where gravity = 0.


A sphere with a hollow center is in fact not a uniform density by definition.

Mastikator is pretty much correct. 0/outward g at the center, weak(-ish depending on variables like crust thickness) G-forces at the inner surface (possibly and probably counteracted by spin)

As for wether it is possoble for a planet to be hollow or not it's totally possible for a sphere to be hollow in the center what are you thinking? It might not be a natural phenomenon but the gods can do it just fine, or perhaps the crust is honeycombed with contiguous adamantine veins or some such.

I am less certain on the lack of a metal core thing. there wouldn't be northern lights and there wouldn't be compasses. from a cosmic persective the moon is much more important to earth than the iron core.

Giant Panda
2011-06-13, 12:49 PM
A sphere with a hollow center is in fact not a uniform density by definition.


Yeahhh, I made a boo-boo there. You get the picture, though.



I am less certain on the lack of a metal core thing. there wouldn't be northern lights and there wouldn't be compasses. From a cosmic persective the moon is much more important to earth than the iron core.

The thing about the metallic core is that it generates the magnetic field that prevents cosmic particles from making the surface sizzle. The compass is just a product of the magnetic field, and the Northern Lights are a release of energy from cosmic particles striking the magnetic field, since the field is strongest at the poles. The field is somewhat like an umbrella, and the cosmic particles are the rain. The particles are deflected from the "umbrella," making the planet habitable. Without it, the planet would be pretty much uniformly hot and radioactive, since rock absorbs infra-red and heats up.

I may have got that wrong, but that's my understanding.

But as mentioned by Big Dice, it's D&D, the physics doesn't have to be correct.

Yora
2011-06-13, 12:56 PM
If you're dead center on the inside (and it's evenly distributed), you feel no pull of gravity.

If you're in the center but not dead center, you should fall to the closest part of the... surface.
That's also my assumption. All atoms pull equally, with atoms closer to you pulling stronger, so you end up being pulled to the nearest surface. However, the actual gravitational pull would have to be quite high to be 1G higher than the pull by the opposite site. But if the hollow space is large enough, the pull from the opposite side should be unnoticable. (The ISS is just 300km from the surface of a 6,300 km radius sphere and experiences almost no gravity noticable by the crew.)

averagejoe
2011-06-13, 01:13 PM
1) In "real world" physics, gravity is the center of mass, which would be the dead center of the world itself, not the crust. That means people on the inside would be eventually drawn to the center of the planet where they would collect unless they had a means of getting away from that.

This is actually a convenient approximation. In the case that you're sufficiently far away from a mass, you can approximate it as a point source of gravity located at the center of mass, and for spherical or roughly-spherical objects this happens to coincide with the center of the sphere. If a hollow planet has mass M, you have mass m, and you are r away from the center of that sphere, you'd experience a gravitational force of GMm/r^2, or the same force as if all the M was packed into that one point at the center. (This works out because of the symmetries of spherical shapes. Some of the planet is closer to you than that point, but the same amount of planet is the same distance further from that point at the same time.) However, this is only when you're outside of the sphere. I'd suspect that even near the surface of the planet, you'd experience a different gravity than with a filled in one, and not just magnitude-wise, but that's just speculation.

A good way to think about gravity is, every bit of mass has a gravitational attraction. Basically every piece of matter is tied to you with an invisible rubber band (so to speak).* So on the inside of a hollow world gravity would be a bit different. The center of mass of the planet remains at the center, but you would no longer feel a gravitational force toward that center. This is because you'd be much closer to some of the planet's mass than the rest of it, and the assumptions that allow you to treat the planet's mass as collected at a point in the center no longer apply. Gravitational strength is dependent on two things: 1) the mass of the object, and 2) the inverse square of the distance from the object. This means that, on the inner surface of the sphere, given an Earth-like radius or greater, the net gravitational force should be toward the "shell" of the planet, or away from the center. (I'm assuming that the gravitational effect of the air inside the planet is negligible, a non-trivial assumption, but useful for the moment.) Now, the gravity would be much weaker than on the surface of the planet (how much depends on the density of the material the planet is made of) but there would be a gravitational force keeping you on the inner surface of a hollow world.

At the center, keeping the same assumption that the gasses inside the planet have a negligible gravitational attraction, the center would technically have no gravitational force, but in a very unstable way. Assuming you could find that point (and especially if it's an irregular or oblong object, like a human), any slight disruption in position would create a net gravitational force and they would, "Fall," slowly at first, but then faster, toward the inner surface of the planet.

Now, gasses do have a net gravitational attraction, and so I imagine the gasses inside the planet would grow more dense as you got closer to the center of the planet, probably making it breathable at certain "depths," or "altitudes," (depending on how you think about it.) This also makes the gravitational attraction on the inner surface somewhat weaker, since there's this big mass at the center of the planet, and it's possible at some point if you go, "High," enough off the inner surface, the collected gasses would be sufficient to pull an object toward the center of the planet and keep it thereabouts. But at this stage it doesn't really hurt anything if you decide one way or the other; either one is, "feasible," enough for good sci fi, at least. (Though in the negligible air option, the air inside the planet would probably be much thinner than the air outside.)

Hoo, I have no idea if any of that made sense.


Basically, the world is a planet, with the same mean density as Earth - so it has a gravity of around 1g.

Just to be clear, this probably means that the actual matter of the planet is made of some denser-than-known-possible substance or substances, probably so dense that it would even be hard to magically manipulate it. (Which helps, as gravity on the inner surface is stronger the denser the planet's material is.) I'd imagine that any sort of tunneling would be impossible; the only traversing points would be in natural fissures and the like, and even those would expose you to extreme heats and pressures.

Fun fact: diamonds are not actually a stable state of carbon at temperatures and pressures livable to humans. They destabilize (i.e. become non-diamond carbon) just very slowly. It would be similar for a planet made out of something superdense, I imagine. Eventually the planet would break apart, just very slowly over eons.


The thing about the metallic core is that it generates the magnetic field that prevents cosmic particles from making the surface sizzle. The compass is just a product of the magnetic field, and the Northern Lights are a release of energy from cosmic particles striking the magnetic field, since the field is strongest at the poles. The field is somewhat like an umbrella, and the cosmic particles are the rain. The particles are deflected from the "umbrella," making the planet habitable. Without it, the planet would be pretty much uniformly hot and radioactive, since rock absorbs infra-red and heats up.

I may have got that wrong, but that's my understanding.

But as mentioned by Big Dice, it's D&D, the physics doesn't have to be correct.

The field isn't strongest at the poles. Actually, in some sense, the field is weakest at the poles, and so that's where the charged particles tend to go (though putting it like this might be misleading. Suffice it to say, charge particles get, "Funneled," through the poles of the magnetic field.) The auroras are caused by charged particle collisions, which are denser at the poles and easier to see anyways because of the long nights.

But, yeah, shielding would be a problem on the outer surface of that planet. There would need to be some sort of shielding from solar winds, and a good magnetic field would be the best way I know to do it. Hmmmm, maybe a moon with a strong magnetic field, making for the odd period of fierce bombardment on the surface. Could be cool, at least.


Anyway, from a physics perspective, a planet can't be hollow, it'd implode. The spin it would require to keep it from imploding would cause it to be ripped apart and made into a donut-like shape, but the atmosphere would be cast away, as with the oceans.
Might work for a massive ring of rocks, not so much a planet.
You're gonna have to add magic to explain a hollow planet, or hope that your players didn't pass physics.

Actually, from a physics perspective, an air-filled planet would break up or collapse. An implosion happens when the external pressure of a system is greater than the internal pressure of a system. Implosion as an English word usually just means, "To collapse inward," or something similar, but it also has a well defined scientific meaning, essentially the opposite of an explosion. On a planetary/stellar scale, implosions (http://en.wikipedia.org/wiki/Gravitational_collapse) actually happen when it gets so dense that it collapses due to its own gravity.

*Actually every piece of matter is attached to every piece of matter on you in this metaphor. And gravity itself is limited by the speed of light. But let's keep this simple.

hamlet
2011-06-13, 01:18 PM
This is actually a convenient approximation. In the case that you're sufficiently far away from a mass, you can approximate it as a point source of gravity located at the center of mass, and for spherical or roughly-spherical objects this happens to coincide with the center of the sphere. If a hollow planet has mass M, you have mass m, and you are r away from the center of that sphere, you'd experience a gravitational force of GMm/r^2, or the same force as if all the M was packed into that one point at the center. (This works out because of the symmetries of spherical shapes. Some of the planet is closer to you than that point, but the same amount of planet is the same distance further from that point at the same time.) However, this is only when you're outside of the sphere. I'd suspect that even near the surface of the planet, you'd experience a different gravity than with a filled in one, and not just magnitude-wise, but that's just speculation.

A good way to think about gravity is, every bit of mass has a gravitational attraction. Basically every piece of matter is tied to you with an invisible rubber band (so to speak).* So on the inside of a hollow world gravity would be a bit different. The center of mass of the planet remains at the center, but you would no longer feel a gravitational force toward that center. This is because you'd be much closer to some of the planet's mass than the rest of it, and the assumptions that allow you to treat the planet's mass as collected at a point in the center no longer apply. Gravitational strength is dependent on two things: 1) the mass of the object, and 2) the inverse square of the distance from the object. This means that, on the inner surface of the sphere, given an Earth-like radius or greater, the net gravitational force should be toward the "shell" of the planet, or away from the center. (I'm assuming that the gravitational effect of the air inside the planet is negligible, a non-trivial assumption, but useful for the moment.) Now, the gravity would be much weaker than on the surface of the planet (how much depends on the density of the material the planet is made of) but there would be a gravitational force keeping you on the inner surface of a hollow world.

At the center, keeping the same assumption that the gasses inside the planet have a negligible gravitational attraction, the center would technically have no gravitational force, but in a very unstable way. Assuming you could find that point (and especially if it's an irregular or oblong object, like a human), any slight disruption in position would create a net gravitational force and they would, "Fall," slowly at first, but then faster, toward the inner surface of the planet.

Now, gasses do have a net gravitational attraction, and so I imagine the gasses inside the planet would grow more dense as you got closer to the center of the planet, probably making it breathable at certain "depths," or "altitudes," (depending on how you think about it.) This also makes the gravitational attraction on the inner surface somewhat weaker, since there's this big mass at the center of the planet, and it's possible at some point if you go, "High," enough off the inner surface, the collected gasses would be sufficient to pull an object toward the center of the planet and keep it thereabouts. But at this stage it doesn't really hurt anything if you decide one way or the other; either one is, "feasible," enough for good sci fi, at least. (Though in the negligible air option, the air inside the planet would probably be much thinner than the air outside.)

Hoo, I have no idea if any of that made sense.




Well, it was explained to me by an astronomer about 10 years ago as being the way you quoted me as saying. He may have been giving me an idiots' version, and he might have just been wrong. Don't know.

And yes, I do understand the fact that all the mass of an object is pulling at all times, but that the net force typically works out to center of mass. I believed that to hold true for no matter where you are on or in the sphere because even within our planet, going down deeper under the survace, the force of gravity is still net center. Of course, that might be explained by the mass within that lacks in our "hollow" world, but I just don't know. It's well beyond my Literature Major's understanding of physics.

averagejoe
2011-06-13, 01:25 PM
Well, it was explained to me by an astronomer about 10 years ago as being the way you quoted me as saying. He may have been giving me an idiots' version, and he might have just been wrong. Don't know.

And yes, I do understand the fact that all the mass of an object is pulling at all times, but that the net force typically works out to center of mass. I believed that to hold true for no matter where you are on or in the sphere because even within our planet, going down deeper under the survace, the force of gravity is still net center. Of course, that might be explained by the mass within that lacks in our "hollow" world, but I just don't know. It's well beyond my Literature Major's understanding of physics.

Yes, that's essentially correct. We would still get pulled toward the center of our planet because it's still massive toward the center, and, also significantly, more densely massive. The basic thing is, the net force works out to be at the center of mass, but only when you're far away from that object. It's not an assumption you can keep using if you're interacting with an object, or somehow, "Within," it. In those cases you have to start doing the longhand, "Each point goes to each point," addition (so one hopes you have some good math tricks!)

I wasn't explaining this all to you, understand, my response to your post was just related to a lot of things I want to talk about. Gravity is somewhat more subtle a concept than most people appreciate, and a good understanding of it is difficult. Hell, I'm not even sure that I have one.

Yuki Akuma
2011-06-13, 01:30 PM
Okay, all of you have been a huge help, thank you. :smallbiggrin:

I was planning on having the surface be rather inhospitable - but not so deadly that you'd die from all the radiation. Perhaps the local star doesn't produce as much energy as our sun.

And yes I'm aware that a hollow planet would never form naturally - it was definitely made by someone, probably a deity. I can't see why a planet with a 'hollow' core (it's not really hollow, it has air in it) would have to collapse in on itself - if it's made from strong enough materials it could be rigid enough, surely.

(Also the planet itself is likely much larger than Earth - I don't really like the idea of superdense un-diggable material, and I don't really want to hand wave everything away with 'it's magic'. :smalltongue: Although I'm probably going to have to wandwave some things. >.>)

The Big Dice
2011-06-13, 01:36 PM
Okay, all of you have been a huge help, thank you. :smallbiggrin:

I was planning on having the surface be rather inhospitable - but not so deadly that you'd die from all the radiation. Perhaps the local star doesn't produce as much energy as our sun.

And yes I'm aware that a hollow planet would never form naturally - it was definitely made by someone, probably a deity. I can't see why a planet with a 'hollow' core (it's not really hollow, it has air in it) would have to collapse in on itself - if it's made from strong enough materials it could be rigid enough, surely.
As a random aside, the D&D Hollow World has a pinpoint portal to the Elemental Plane of Fire at the centre, giving it a permanent source of light and heat. And there are also so-called floating continents, so the inhabitants can keep track of time. Some of the floating continents are even inhabited.

Finally, the poles are not closed. I'm not sure what the technical name for a hollow ball with holes at either end it, but that's what Mystara is shaped like. But in theory, it's possible to head off walking north and end up inside the world.

averagejoe
2011-06-13, 02:04 PM
Okay, all of you have been a huge help, thank you. :smallbiggrin:

I was planning on having the surface be rather inhospitable - but not so deadly that you'd die from all the radiation. Perhaps the local star doesn't produce as much energy as our sun.

And yes I'm aware that a hollow planet would never form naturally - it was definitely made by someone, probably a deity. I can't see why a planet with a 'hollow' core (it's not really hollow, it has air in it) would have to collapse in on itself - if it's made from strong enough materials it could be rigid enough, surely.

(Also the planet itself is likely much larger than Earth - I don't really like the idea of superdense un-diggable material, and I don't really want to hand wave everything away with 'it's magic'. :smalltongue: Although I'm probably going to have to wandwave some things. >.>)

Then you'd at least have to make the planet's substance thick compared to its radius, so that the inner surface would be smaller than the outer one. Or you could handwave it, but I'm here to answer science questions, dammnit! :smalltongue:

I dont' actually know the effect of sustained bombardment of charged particles. A lot of cancer, probably. It's not a matter of energy, unless the surface dwellers have short life spans, because constant radiation exposure over time has as many problems as short term small doses. The people living there would need some form of actual shielding.

Mikeavelli
2011-06-13, 02:24 PM
As a random aside, the D&D Hollow World has a pinpoint portal to the Elemental Plane of Fire at the centre, giving it a permanent source of light and heat. And there are also so-called floating continents, so the inhabitants can keep track of time. Some of the floating continents are even inhabited.

Finally, the poles are not closed. I'm not sure what the technical name for a hollow ball with holes at either end it, but that's what Mystara is shaped like. But in theory, it's possible to head off walking north and end up inside the world.

Ahh, Mystara. It even had an adventure where one of the Immortals of Entropy was helping his mortal followers to create a Close Gate spell with a range of "line of sight" so they could close the sun and plunge the whole world into darkness. It was great fun.

Honestly, Mystara had it right, physically impossible worlds are ever-so-much fun, but it's usually best to just go all out with magical explanations for everything that can't possibly work through real-world physics.

[hr]

Another great fictional hollow world was Pyran, featured in the Death's Gate novels. Basically, at the end of an apocalyptic magical war, the losing group of wizards decided to screw everyone over, and split their world into four different worlds with elemental themes (earth, wind, fire, water) each connected by a giant magical gate.

Pyran, world of fire, was a gigantic hollow sphere with ever-present sunlight and all the physical problems handwaved away by magic. Due to the incredible amount of energy provided by the sun at the center, the entire planet became a gigantic rain forest, with trees and plants growing so large no-one at the time the novels took place had ever even seen the surface of the planet, they just built their cities into the tops of those giant trees.

Douglas
2011-06-13, 02:56 PM
I see a lot of people here revealing that they have never taken a physics class that specifically addresses things like this, or forgot these details. The descriptions of the basis of calculation are generally correct, but most of you are stating a result that seems based mostly on intuition and is incorrect.

Assume a hollow spherical shell made of a material with uniform density. Yes, the hollow space means the sphere as a whole doesn't have uniform density, but that's beside the point. The relevant bit about density is that, in the section that has material, density is uniform. Further, assume that the shell has uniform thickness and radius.

Outside:
From outside such a shell, every individual atom is exerting its tiny gravitational pull on you and they all add up to the kind of gravity you're used to. Gravity pulls you straight down towards the center as if all of the mass were actually at the center point instead of in a giant shell. The bits near you pull proportionately more than the bits far away, but there's a lot more that's far away than near and it all averages out. In fact, if you work out the actual math of it all (requires some calculus), pretending that all the mass is at the center is an exact equivalency at all distances, not just a long distance approximation.

Inside:
From inside such a shell, every individual atom is still exerting its tiny gravitational pull on you, and they still add up, but now they're pulling in different directions. The bits near you are pulling you towards that section of the shell - and away from the center - while the bits far from you are pulling you across the shell and towards the center. The bits nearer pull stronger proportionately, but again there's a lot more that's far away and on the other side. These pulls cancel out to some extent. In fact, if you work out the actual math (again requires calculus), no matter where in the interior you are, all of the gravitational pull cancels out to exactly 0 at all points.

End result:
Assuming a sufficiently dense material and appropriate radius and thickness, people on the outer surface would experience gravity just as if the world were solid all the way through. If someone were to dig a hole in the ground and descend through the shell, gravity would gradually decrease as he descends, eventually dropping to exactly nothing when he breaks through to the interior. For a solid planet, the decrease is exactly linear all the way through - dig half way to the center, and gravity's at half strength. A hollow planet would have a different progression because the distance from the center and the amount of mass still interior to you aren't synchronized in the same way, but I don't know off-hand what it is.

averagejoe
2011-06-13, 03:59 PM
Outside:
From outside such a shell, every individual atom is exerting its tiny gravitational pull on you and they all add up to the kind of gravity you're used to. Gravity pulls you straight down towards the center as if all of the mass were actually at the center point instead of in a giant shell. The bits near you pull proportionately more than the bits far away, but there's a lot more that's far away than near and it all averages out. In fact, if you work out the actual math of it all (requires some calculus), pretending that all the mass is at the center is an exact equivalency at all distances, not just a long distance approximation.

Sorry if I was unclear; I was accounting for irregular shapes, not just spheres, in which case the answer is more complex than "inside" and "outside." I did note that a perfect sphere behaves this way.


Inside:
From inside such a shell, every individual atom is still exerting its tiny gravitational pull on you, and they still add up, but now they're pulling in different directions. The bits near you are pulling you towards that section of the shell - and away from the center - while the bits far from you are pulling you across the shell and towards the center. The bits nearer pull stronger proportionately, but again there's a lot more that's far away and on the other side. These pulls cancel out to some extent. In fact, if you work out the actual math (again requires calculus), no matter where in the interior you are, all of the gravitational pull cancels out to exactly 0 at all points.

You know what, now that I think about it I have actually performed this calculation. >.> Whoops.

Randel
2011-06-13, 05:13 PM
Well, I once read a "choose your own adventure" book that involved a wormhole and had a part where you travel into an alternate world with hollow planets. In this one there were big clay-like planets that surround tiny stars but the starts radiate a sort of anti-gravity. So all the little people on the surface were being pushed onto the ground by the star in the center. I'm sure you can come up with a weird gravity/anti-gravity ratio where the star radiates enough anti-gravity to keep the people on the surface while having enough normal gravity to keep the planetary shell in place.

Or it could be spinning and have centrifugal force provide the gravity (like a ringworld) but if its a sphere then only the equator would be 'down' while the rest of the planets surface would be sloped due to differences in the centrifugal force coming at an angle. In short everything would fall towards the equator and trying to climb towards the sides would result in gravity weakening somewhat. Also, if you create a hot air balloon then you could see the land spinning away from you and you could travel all over the world (as a result of the world traveling away from you). On our Earth, gravity keeps our atmosphere somewhat in place, if this other world used something other than gravity to keep the people on its surface then the atmosphere would likely act different due to the centrifugal force not having any effect on it.

Hopefully this hollow world is air-tight or else the air might get forced out of it somehow and the whole thing could collapse like a balloon... unless its frame is holding it together in which case its probably made of adamantine and was made by physical gods.

Autolykos
2011-06-13, 06:32 PM
Well, having large parts of the planet hollow leads to all those problems pointed out above (I second the "no gravity anywhere inside" group - did the math). However, having a planet without a liquid core, but riddled with caverns and tunnels all the way to the bottom, would be kinda cool - even better than making the whole thing hollow IMHO (still not very realistic, the pressure from the ground above it would crush the tunnels). You'd still have weaker gravity as you moved down (since anything further outside than you cancels itself out - follows directly from those calculations), but that could have other cool effects (like allowing for far larger creatures).

danzibr
2011-06-13, 09:18 PM
Inside:
From inside such a shell, every individual atom is still exerting its tiny gravitational pull on you, and they still add up, but now they're pulling in different directions. The bits near you are pulling you towards that section of the shell - and away from the center - while the bits far from you are pulling you across the shell and towards the center. The bits nearer pull stronger proportionately, but again there's a lot more that's far away and on the other side. These pulls cancel out to some extent. In fact, if you work out the actual math (again requires calculus), no matter where in the interior you are, all of the gravitational pull cancels out to exactly 0 at all points.

Ya know... yeah, my earlier post is wrong. This is totally what my teacher said. He's a mathematical physicist (I'm just a scrub with an MA in pure math).

On a totally unrelated note, did anyone else think of Hueco Mundo from Bleach?

hamlet
2011-06-14, 10:42 AM
Well, according to the wiki article on "Hollow Planet," this is said regarding the matter of gravity:


The best scientific argument against that of a hollow Earth or any hollow planet is gravity. Massive objects tend to clump together gravitationally, creating non-hollow spherical objects we call stars and planets. The solid sphere is the best way in which to minimize the gravitational potential energy of a physical object; having hollowness is unfavorable in the energetic sense. In addition, ordinary matter is not strong enough to support a hollow shape of planetary size against the force of gravity; a planet-sized hollow shell with the known, observed thickness of the Earth's crust, would not be able to achieve hydrostatic equilibrium with its own mass and would collapse.

Someone on the inside of a hollow Earth would not experience a significant outward pull and could not easily stand on the inner surface; rather, the theory of gravity implies that a person on the inside would be nearly weightless. This was first shown by Newton, whose shell theorem mathematically predicts a gravitational force (from the shell) of zero everywhere inside a spherically symmetric hollow shell of matter, regardless of the shell's thickness. A tiny gravitational force would arise from the fact that the Earth does not have a perfectly symmetrical spherical shape, as well as forces from other bodies such as the Moon. The centrifugal force from the Earth's rotation would pull a person (on the inner surface) outwards if the person was traveling at the same velocity as the Earth's interior and was in contact with the ground on the interior, but even the maximum centrifugal force at the equator is only 1/300 of ordinary Earth gravity.

The mass of the planet also indicates that the hollow Earth hypothesis is unfeasible. Should the Earth be largely hollow, its mass would be much lower and thus its gravity on the outer surface would be much lower than it is.

Serpentine
2011-06-14, 10:53 AM
I recommend looking for anything written by genuine Hollow Earthers, on how they explain everything.
Yes, there are genuine Hollow Earthers. And Flat Earthers.

Tengu_temp
2011-06-14, 11:01 AM
I recommend looking for anything written by genuine Hollow Earthers, on how they explain everything.
Yes, there are genuine Hollow Earthers. And Flat Earthers.

Something tells me such people don't have a very good grasp of physics in general, though. I've yet to see a legitimate scientist claim that Earth is flat or hollow.

randomhero00
2011-06-14, 11:06 AM
The way I'd see a hollow planet working is to have the center be a churning mass of plasma, with so much pressure on it that its like a nuclear reactor. It pushes outward, and gravity pushes inward. This would still allow tectonic plates and the like. And still have a natural center of gravity (in the center of the planet). Its kind of how Saturn (or is it Jupiter? I forget) works. Except its all gas.

PS and it would still allow for a magnetic field.

Pechvarry
2011-06-14, 11:28 AM
One of my great, nerdy regrets is having never taken a physics course. :( That said, some ideas I was thinking about:

Magnetosphere: Because the planet lacks a molten core, what happens if you give the planet a large, dense moon? Something the size of our moon but with an extremely high metal content and orbiting closer. Could something like this be sufficient to envelop the planet in a protective magnetic field? If it's orbit is closer, would it being the source of a strong dynamo effect make it better or worse for the planet? This introduces its own problems, of course. Namely, a planet that's always in danger of imploding/ripping itself apart really doesn't need the external effects of a close-by gravity field. If the planet can survive this moon, then you also have something to aid volcanism, which indicates a world with a pretty thick mantle before reaching the air core.

Gravity: I really think Magic is gonna be the best solution here. Perhaps the internal continents all sit on vast, magical "tectonic plates" which are networked devices which push out on the planet, keeping it from collapse. As a side effect, they provide local gravity.

Finally, because hollow world stuff always makes me think of it, Septerra Core (http://en.wikipedia.org/wiki/Septerra_Core) had an incredibly fascinating setting.

Basically, a planet that consists of a giant spine, with 7 shells of continents all attached to the spine at various points. Each layer spins autonomously via gravity, and the energy of its spin is caught by the spine, which fuels the massive generator/supercomputer core. I can point out problems like "it's not a perpetual motion device and would eventually fail" or "where are all the oceans to keep these denizens of the shells alive?" etc, but at the end of the day, it's just plain cool.

mathemagician
2011-06-14, 12:05 PM
Instead of having the whole thing be hollow, what if it consisted of concentric shells? They could be connected with pillars which are given religious meaning, perhaps, or serve as bridges between two massive cities, or a place where war could be conducted. Or they could be disconnected, and freely spinning, which gives an interesting "when the x aligns" hook. Perhaps the only way between shells is when two teleportation circles line up.

Secret / Abandoned / whatever tunnels lead from the outside of your shell, to the inside, which reveals another shell to reach. Many adventures could be had trying to reach the center, for discovery sake, or maybe there's someone trapped there, whatever.

Just some ideas.

Othniel Edden
2011-06-14, 02:34 PM
You know with all that air getting thicker and thicker as you got to the center of mass, some of it might condense. Yup, those living on the inner surface might look up and see a giant ocean in the center of their sky.:smallamused:

hamlet
2011-06-14, 02:40 PM
You know with all that air getting thicker and thicker as you got to the center of mass, some of it might condense. Yup, those living on the inner surface might look up and see a giant ocean in the center of their sky.:smallamused:

*takes notes*

Do please continue . . .

Othniel Edden
2011-06-14, 11:57 PM
*takes notes*

Do please continue . . .

I wish I had more, cause I have a bunch of neat images swirling in my head. Might have to start one of those collaborative worldbuilding projects, just so yet another evocative versions of a hollow earth reaches the masses.

Serpentine
2011-06-15, 03:44 AM
Something tells me such people don't have a very good grasp of physics in general, though. I've yet to see a legitimate scientist claim that Earth is flat or hollow.No, but they should have pseudo-scientific explanations good enough to pass a casual glance and provide the framework for a fantasy world.

Yuki Akuma
2011-06-15, 04:55 AM
No, but they should have pseudo-scientific explanations good enough to pass a casual glance and provide the framework for a fantasy world.

I don't want pseudo-science. I want actual science. :smallfrown:

I know this seems weird but I'd like for some parts of the world to make some sort of rational sense. >.>

The 'no gravity in the core' thing I can work with...


I wish I had more, cause I have a bunch of neat images swirling in my head. Might have to start one of those collaborative worldbuilding projects, just so yet another evocative versions of a hollow earth reaches the masses.

I could always make a thread in the homebrew forum with the stuff I've got so far. :smalltongue:

Ravens_cry
2011-06-15, 06:45 AM
The trouble is, unless you make the planet, at least the inner part of the 'rind' some super strong unobtainium, ala Ringworld (http://en.wikipedia.org/wiki/Ringworld), any hollow world would immediately collapse and make a much smaller, hotter, planet. Still, if it was strong and heavy enough, those on the surface, would feel gravity, I think, the same way, and the mass would also pull those on the inner surface to it as well. I think.

Douglas
2011-06-15, 06:54 AM
and the mass would also pull those on the inner surface to it as well. I think.
No, we covered that already - gravity cancels out to nothing on the inside, and not just at the exact center. Variations in terrain and localized clumps of denser material could cause deviations from that, but it would take something really big to have an easily noticeable effect. A mountain range on the order of the Himalayas filled with dense metal ore deposits would probably be enough, but the apparent gravity on the inside surface under it would still be quite weak.

Sir Swindle89
2011-06-15, 07:08 AM
since gravity cancels you could just put in a massive network of tunnels or better yet INVERSE TUNNELS! (big rock pillars that branch and reconnect to each other that the populace jump between in a state of near weightlessness)

paddyfool
2011-06-15, 07:27 AM
Idea: a greater deity/overgod did it. The following is partly inspired by the Hourglass of Zihaja game setting:

Deity X decided to create a hollow world as a safe refuge for his creation. It is bounded on all sides by a thick enough layer of adamantium to have the right density to keep "down" on the outer surface, and above that, the equivalent of a planet's upper crust, above which dwell its inhabitants (which are whatever species X decided to introduce or create). They have since created other deities through their worship, who are the only beings to be aware of X's existence (he tithes their power, and arbitrates their disputes). Aside from a separate demiplane for each deity, all extra-planar transport is blocked by X.
This is the simplest version. Possible complications, some mutually contradictory, which you may find fun would include the following:


The hollow world is inside a vast interstellar, or even intergalactic, spacecraft.
A second world lies on the outside of the sphere, and sometimes things break through.
This hollow world is one of a number of linked spheres, and there are passageways between them.
X has extra-planar or extra-world adversaries who keep trying to break in.
The hollow world was not created by just one god, but a whole pantheon from A-Z who are in hiding from something worse.
An artificial sun, three-fifths of its mass blocked by a super-metal barrier, turns at the centre of the world; tilting on a regular interval just so as to replicate seasons etc.
Said "sun" is in fact a near-weightless city populated by archons, inevitables, fire elementals or some other servants of X.

Ravens_cry
2011-06-15, 07:27 AM
No, we covered that already - gravity cancels out to nothing on the inside, and not just at the exact center. Variations in terrain and localized clumps of denser material could cause deviations from that, but it would take something really big to have an easily noticeable effect. A mountain range on the order of the Himalayas filled with dense metal ore deposits would probably be enough, but the apparent gravity on the inside surface under it would still be quite weak.
Why would the gravity cancel out to nothing? Mass attracts. With no mass on the inside, the very definition of hollow, there is no mass to pull in the opposite direction.
Assuming the world is so hollow that it is little thicker then the crust on earth and there is a layer strong enough to maintain it's shape and heavy enough that it provides similar gravity as a filled sphere world to the people on the outside, the people on the inside layer would also be attracted to the rind.
http://img857.imageshack.us/img857/594/hollow.gif

Sir Swindle89
2011-06-15, 07:42 AM
Why would the gravity cancel out to nothing? Mass attracts. With no mass on the inside, the very definition of hollow, there is no mass to pull in the opposite direction.
Assuming the world is so hollow that it is little thicker then the crust on earth and there is a layer strong enough to maintain it's shape and heavy enough that it provides similar gravity as a filled sphere world to the people on the outside, the people on the inside layer would also be attracted to the rind.
http://img857.imageshack.us/img857/594/hollow.gif

Thats the thing there is a small amount of mass nereby that is pulling you to the outside. Theres a metric **** ton of mass further away pulling you away from the edge (or rather to the opposite edge).

Not to say you couldn't change the gravitational equation for game purposes (g*m1*m1)/(d^2) should do it. (assuming a moon and sun can be hand waved)

hamlet
2011-06-15, 08:14 AM
Why would the gravity cancel out to nothing? Mass attracts. With no mass on the inside, the very definition of hollow, there is no mass to pull in the opposite direction.
Assuming the world is so hollow that it is little thicker then the crust on earth and there is a layer strong enough to maintain it's shape and heavy enough that it provides similar gravity as a filled sphere world to the people on the outside, the people on the inside layer would also be attracted to the rind.
http://img857.imageshack.us/img857/594/hollow.gif

Because science says so according to the Wiki. And the Wiki knows all.:smallwink:

Might seem counter intuitive, but apparantly, that's the way of things.

Douglas
2011-06-15, 08:24 AM
Why would the gravity cancel out to nothing? Mass attracts. With no mass on the inside, the very definition of hollow, there is no mass to pull in the opposite direction.
Assuming the world is so hollow that it is little thicker then the crust on earth and there is a layer strong enough to maintain it's shape and heavy enough that it provides similar gravity as a filled sphere world to the people on the outside, the people on the inside layer would also be attracted to the rind.
http://img857.imageshack.us/img857/594/hollow.gif
I bolded the part that's wrong. There is mass pulling in the opposite direction - the far part of the shell. It's a lot farther away and that reduces how much it's pulling, but there's also a lot more of it and that increases how much it's pulling. The two factors of greater distance and greater mass cancel out exactly no matter where you happen to be inside the shell, resulting in precisely enough force to cancel the attraction of the nearer mass. This result is called the Shell Theorem (http://en.wikipedia.org/wiki/Shell_theorem), it was originally proven by Isaac Newton, and it is now a common subject in high school physics courses. I learned it in high school myself, and I've done the math to verify it at least once.

Ravens_cry
2011-06-15, 02:24 PM
I bolded the part that's wrong. There is mass pulling in the opposite direction - the far part of the shell. It's a lot farther away and that reduces how much it's pulling, but there's also a lot more of it and that increases how much it's pulling. The two factors of greater distance and greater mass cancel out exactly no matter where you happen to be inside the shell, resulting in precisely enough force to cancel the attraction of the nearer mass. This result is called the Shell Theorem (http://en.wikipedia.org/wiki/Shell_theorem), it was originally proven by Isaac Newton, and it is now a common subject in high school physics courses. I learned it in high school myself, and I've done the math to verify it at least once.
After I posted that, I wondered about that ,but if you done the maths, well, far be it from me and my crude squiggles to try and correct maths.
Hmm, other idea. This isn't gravity, but it does most of the same jobs, but what if, like most planets, the planet was spinning? Could you not get an effect like a centrifuge for the inner surface?

a_humble_lich
2011-06-15, 02:48 PM
After I posted that, I wondered about that ,but if you done the maths, well, far be it from me and my crude squiggles to try and correct maths.
Hmm, other idea. This isn't gravity, but it does most of the same jobs, but what if, like most planets, the planet was spinning? Could you not get an effect like a centrifuge for the inner surface?

That is a really amazing thing how the math all works out. And it is very nonintuitive.

As for spinning... it would sort of work. You would have apparent gravity at the equator. Then as you move north (or south) gravity would decrease until it became zero at the pole. Also, it would be difficult to justify being able to walk on the outside since the centrifugal force would most likely greatly overwhelm the weak gravity generated by a hollow planet and toss you into space.

averagejoe
2011-06-15, 02:58 PM
After I posted that, I wondered about that ,but if you done the maths, well, far be it from me and my crude squiggles to try and correct maths.
Hmm, other idea. This isn't gravity, but it does most of the same jobs, but what if, like most planets, the planet was spinning? Could you not get an effect like a centrifuge for the inner surface?

The effect would be very slight, and mostly significant near the equator. Also note that the force would be in the same direction from pole to pole, so even if it was significant, "Down," would change in direction relative to the ground as you get nearer to the poles.

Othniel Edden
2011-06-15, 03:17 PM
I could always make a thread in the homebrew forum with the stuff I've got so far. :smalltongue:
Do it. I'd love to help, as much as possible, with the world building. Building the biologic for something like this is incredibly fun.:smallbiggrin:

Douglas
2011-06-15, 03:28 PM
The idea of centrifugal fake gravity is possible, but making it significant would raise some serious scale issues. For example, if the hollow planet had the same size and day length as Earth, the centrifuge effect would give it pseudo-gravity on the inside at the equator roughly equal to 1/300th of Earth's gravity. Away from the equator it would be weaker and also not perpendicular to the ground.

Making the day shorter or the planet larger would increase the apparent gravity, but getting it anywhere near the strength we're accustomed to on Earth would take a drastic difference.

Othniel Edden
2011-06-15, 03:31 PM
Does this idea require that the planet be earth's average density to work?

Ravens_cry
2011-06-15, 03:35 PM
That is a really amazing thing how the math all works out. And it is very nonintuitive.

Yeah, physics can be like that. Wait scratch that, it is like that.


As for spinning... it would sort of work. You would have apparent gravity at the equator. Then as you move north (or south) gravity would decrease until it became zero at the pole. Also, it would be difficult to justify being able to walk on the outside since the centrifugal force would most likely greatly overwhelm the weak gravity generated by a hollow planet and toss you into space.
Hence the unobtainium for the husk being both super strong, it would need to be to take the strain, and super dense, we are probably talking white dwarf levels at least, to provide more mass. If it is dense enough, it would provide enough mass so that for surface, gravity is approximately Earth normal, right?

The effect would be very slight, and mostly significant near the equator. Also note that the force would be in the same direction from pole to pole, so even if it was significant, "Down," would change in direction relative to the ground as you get nearer to the poles.
Slight, but more then nothing. A light gravity world, with all the effects that would entail, would be an interesting way to do things, plus the weirdness of gravity being noticeably different at the poles and the equator.
Hehe, this is why I love science fiction, thinking of ideas, even if technically impossible, and looking at the logical implications of them.:smallbiggrin:

averagejoe
2011-06-15, 03:38 PM
The idea of centrifugal fake gravity is possible, but making it significant would raise some serious scale issues. For example, if the hollow planet had the same size and day length as Earth, the centrifuge effect would give it pseudo-gravity on the inside at the equator roughly equal to 1/300th of Earth's gravity. Away from the equator it would be weaker and also not perpendicular to the ground.

Making the day shorter or the planet larger would increase the apparent gravity, but getting it anywhere near the strength we're accustomed to on Earth would take a drastic difference.

Well, he's already said that the planet has Earth gravity on the surface, so it probably already is larger.

Othniel Edden
2011-06-15, 04:06 PM
I think the next thing that needs to be figured out is the surface area of the inner and outer surfaces. How much mass are we displacing?

a_humble_lich
2011-06-15, 07:28 PM
So this caused be to break out the calculator.

If we assume and Earth sized planet, Earth normal gravity on the surface, a depth of 10 km, spinning so as to provide .75 g at the equator (which means the outside equator will only have an effective gravity of .25 g) we get the following numbers.

1.) The shell has a volume of (4/3 pi (r_out^3-r_in^3) of 5.1e18 m^3.
2.) This gives a density of 1170 g/cm^3. Huge but not white dwarf levels (for comparison gold has a density of 19.3 g/cm^3).
3.) In order to spin at that rate I calculate that it would have to spin around once every 1.6 hours.

This world has several immediate features. First the high density of the crust means that you could have many strange gravitational anomalies. If you have large mountains the mountain would have a significant gravitational field of their own. Mountains on the scale of a few kilometers would have a gravitational field of about .2 g.

Second, there would be a point in the crust where the centrifugal force and the gravitation force exactly cancel each other.

Third, The much higher rotation rate would mean things like the Coriolis force will become much more important.

P.S. Sadly, many catgirls had to die to make this post.:smallfrown:

Yuki Akuma
2011-06-15, 09:33 PM
The original idea was to just scoop out the molten core and leave the rest - so the 'shell' would be somewhere between three thousand and five thousand kilometers thick.

Of course then the world on the inside would be pretty tiny, so I guess I should consider scooping more out.

(Or just make the planet bigger. It doesn't matter at this stage. My ideas are very, very rough.)

paddyfool
2011-06-15, 10:27 PM
So a shell would have no gravitational pull on its contents normally. This leaves us with the following options:


Limited gravitational replication by rotation
Artificial gravity produced by magic
Artificial gravity produced by technology so advanced as to be indestinguishable from magic
A largely weightless environment.


The 4th option would be the most work, but could also be rather fun.

Fri
2011-06-15, 10:35 PM
Well, since we already realized that real hollow planet like what you explained wouldn't work, are there other 'close enough' alternative that might satisfy you? For example, an artificial planet with gravity generator. Or it's actually just massive underground cavern that runs all around the planet's crust, but the inhabitant thought that they live in a hollow world.

Randel
2011-06-15, 10:59 PM
There is a series "The Integral Trees" by Larry Niven which takes place in a zero gravity environment and there are people who live on giant tree-like plants that grow there. (wikipedia page here (http://en.wikipedia.org/wiki/The_Integral_Trees))

the environment takes place in a sort of nebula thing that surrounds a neutron star with a gas giant positioned just right where its gravity helps keep the atmosphere contained. There are various life forms that live in this environment including massive trees that spin so that their branches and leaves collect moisture in the air and root like things (that are in the same area) can collect the nutrients.

The thing is that this spinning action creates a centrifugal force so that people can live among the branches with a sort of 'down' direction to work with (although that 'down' is spinning into space). There are other plant masses out there giant tumbleweed bushes and stuff that people can live inside.


Anyway, what if your hollow planet was zero gravity but contained a breathable atmosphere for everyone and the rock was laced with hot glowing minerals or something to create "sunlight". The people inside the hollow planet would have no natural gravity and would get sunlight from all around (or maybe just specific spots on the outer shell, or one half is bright and the other dark for whatever reason so there can be night and day).

But inside this shell and the zero gravity environment there are various spinning rings, trees, rods, or just big globs of water that people can live on. The rings and stuff with centrifugal force on them could all have "gravity" environments for people to walk around on although some would appear to have higher gravity than others. While there are other environs that don't have any centrifugal forces keeping things organized. Those ones would have a more three-dimensional landscape for people to work in.

Or... the "planet" could basically be a giant transparent bubble or something, like a big balloon full of breathable air. Maybe a god somewhere thought that regular planets were boring (and expensive) so they took a huge mass of breathable air, water, and earth and enclosed it in a big transparent membrane. That membrane could be diamond, a wall of force, or whatever... it just keeps all the air and water inside. Then all the people inside just deal with the zero gravity environment and there are rings and stuff spinning inside for them to live on. Sunlight would come from the star and travel through the transparent shell.


So instead of trying to have one big giant world that everyone lives on... make a big area with breathable air and have a bunch of smaller "worlds" existing inside it. Well, they would be more like islands than anything else. Weird spinning gravity-having islands floating in a big sky instead of an ocean.

Amiel
2011-06-15, 11:05 PM
What if there's a rapidly-spinning invisible moon that acts as the core of this Hollow World; it compels gravity away from itself, but at the same time, gravity prevails on both sides of the crust.

You will need some form of magic to ensure this Hollow World is inhabitable.

Othniel Edden
2011-06-16, 04:19 AM
We could use a second planet to move the center of Gravity more towards one side of the crust, and create a near normal gravity basin.of course if it was hollow the world might tear in half, and we could have two bowls of worlds sharing atmosphere. Maybe it could even be the former egg in which a god was hatched.:smallbiggrin:

Eldan
2011-06-16, 05:28 AM
Hmm. I'm going to kill tons of catgirls here, but how about a system like a

Klemperer Rosette? (http://en.wikipedia.org/wiki/Klemperer_rosette)

http://upload.wikimedia.org/wikipedia/commons/a/a7/Niven_Rosette.svg

Half a dozen or so large moons orbiting a hollow shell. They would all pull outwards, but be in balance with each other. The entire thing would of course be horribly unstable.

Draconi Redfir
2011-06-16, 06:57 AM
Has anyone ever thought of the possibility that perhaps the surface world is made up of some super-reflective metal that simply redirects the majority of the harmful radiaition away? This could allow those liveing on the inside of the planet a safe haven, and provide heat through the ground to keep them alive and warm.

Eldan
2011-06-16, 06:58 AM
It would have to be some strange one-way reflecting material, I think. Otherwise, they would be cooked in there pretty soon.

Draconi Redfir
2011-06-16, 07:09 AM
Well the entirety of the crust doesent need to be made of this material, only the outside, So say it could be a thin layer the hyper-mirror, then an extreamely thick layer of rock or something.

Eldan
2011-06-16, 07:17 AM
Would still do the same: if it's reflective in both directions, the body can't radiate heat off into space and cooks itself. Heat would go into the ground. From there, it could either be reflected back by the shell, or back into the atmosphere.

Draconi Redfir
2011-06-16, 07:22 AM
Oh ok i see what you mean, if the back end of the metal doesent let the heat back into space, then no matter how little heat is getting inside the planet at a time, it will eventually build up and become an oven. that about it?

Eldan
2011-06-16, 07:30 AM
Well, if it's perfectly reflective, it will also reflect heat. No heat will get in, none will get out. However, if there is any source of heat inside, such as a living organism or a sun, it will eventually reach an arbitrarily high temperature.

Of course, if we do weirdly specific fantasy materials, make one that lets heat through, but not visible light or ionizing radiation.

paddyfool
2011-06-16, 07:35 AM
Another idea: an orb at the centre of the sphere which has a magical or supertech anti-gravity effect. It could double as an artificial sun, and other purposes.

The only oddity this would create would be that gravity might get higher as you go closer to the centre of the sphere, meaning that flying at any great height just wouldn't happen... but on the other hand, you could have sheltered chasms and things that are very low G, and other oddities, and it would fit with there being Something at the centre of the sphere which Somebody doesn't want disturbed, which is a fine plot point.

Eldan
2011-06-16, 07:42 AM
Hmm.

I'm running into problems here, as I have never heard a term for this, but:

The "sun" is made of a kind of non-matter that has negative mass, and therefore "radiates" antigravity. It came from outside the known universe (probably far realm?) and the entire planet was built around it as a kind of safety shell.

Sir Swindle89
2011-06-16, 08:04 AM
Would still do the same: if it's reflective in both directions, the body can't radiate heat off into space and cooks itself. Heat would go into the ground. From there, it could either be reflected back by the shell, or back into the atmosphere.

This is actually my area of expertise. A reflective surface only reflects radiation not conduction. So if for example the crust was made of a metal that oxidizes on contact with the external atmosphere into a highly reflective oxide (basically the opposite of what aluminum does) then heat from the local star would be mostly reflected. But the under layer would still absorb the heat produced on the inside and conduct it to the outside layer to be cooled.

Eldan
2011-06-16, 08:07 AM
Right. I do, of course, concur with someone who actually knows what he's talking about.

(My qualifications are a pretty good grade in Physics I in university and a lot of SciFi reading. :smallwink:)

The Big Dice
2011-06-16, 08:27 AM
This is all starting to sound very inhospitable. Not the kind of place you'd want to live or even go on holiday. It's also getting very vague and theoretical.

My advice, decide on a purpose. Who lives there? What do you want the world to be like? Why is it like this? Then figure out the justifications and explanations.

Sir Swindle89
2011-06-16, 08:37 AM
It's easy. Just change the gravitational constant of the universe.
I'm telling you the whole gravitation thing can be solved by changing the gravity law to have distance squared in the denominator.

-You would have internal surface gravity because the opposite side of the planet's gravity would become negligible.
-You wouldn't have to worry about structural stability as much because the planet wouldn't pull as hard on it's self.
-stellar level things like moons would still work because they change distance verry little.
-Going up 10 feet would be like going up 100 as far as the change in gravity is concerned but the gravitational difference when you're on an air plane is hardly noticable so it shouldn't be a problem(unless you want it to be, mushroom towers could be cool)
-clearly you're not really concerned with the math your self so it should go unnoticed by players other than making the setting work.

Also @Eldan i'm a Chem Engineering so student 90% of my classes are thermo. I'm not like a professor or somthing.

Yuki Akuma
2011-06-16, 09:35 AM
Okay yeah I'm pretty much going to have to handwave the fact that it doesn't collapse with "it's magic".

I like the "five giant moons" idea more than the "centrifugal force" idea. The "antigravity sun" idea has some nice potential, too.

I need to start an actual document for the setting one of these days. Right now all I have is "was made by a deific being of some sort", "monsters live on the surface and sometimes find their way into the more hospitibal 'bubble'" and "there is totally gravity somehow, but probably just around the equater".

What sort of societies could I have living in a hollow planet I wonder...

Draconi Redfir
2011-06-16, 09:35 AM
Hey if your gunna take the easy route you could always just say a wizard did it.

Pink
2011-06-16, 09:45 AM
Depending on how you do the light source, plants on the inside could be interesting. Depending how you do the crust, you could actually have a massive ancient plant where the leafy part is a hardy surface plant that benefits from the extreme heat, and has roots travelling to the inside, where it sorted sticks out of the ground like a thick wiry grass that could produce edible bulbs or tubers.

Sir Swindle89
2011-06-16, 10:07 AM
assuming they are humans the Lighting canbe very important. Too little and you get a Nostramo (http://warhammer40k.wikia.com/wiki/Nostramo) type situation where every one is miserable.

A general rule on societies (primitive ones/religions) is they tend to idolize any thing uncomprehensible, unattainable, or life giving.

Depending on the scale of the world they might not even know they are inside a sphere. Or that there could be an outside the sphere ifthey did realize the ground was concave.

It does seem terribly likely that the surface would be regaurded as the realm of the god, or afterlife, or boogy man.

Othniel Edden
2011-06-16, 12:36 PM
Okay yeah I'm pretty much going to have to handwave the fact that it doesn't collapse with "it's magic".

I like the "five giant moons" idea more than the "centrifugal force" idea. The "antigravity sun" idea has some nice potential, too.

I need to start an actual document for the setting one of these days. Right now all I have is "was made by a deific being of some sort", "monsters live on the surface and sometimes find their way into the more hospitibal 'bubble'" and "there is totally gravity somehow, but probably just around the equater".

What sort of societies could I have living in a hollow planet I wonder...
Now we've reached the worldbuilding portion. What species do you want in this world? What type of things do you envision with variant gravity habitats? How much water do we have within the sphere?

paddyfool
2011-06-16, 02:10 PM
So, going with some form of rotation or outside influence to cause a gravitational equator, and seeing where we can run with it...

Can I suggest low gravity dragons? Because, as flight of dragons tells us: "Wings of that size flapping at that rate could not possibly support a beast of your bulk!" However, for some given fraction of G, we'd have to assume that they could. Ergo: once it gets down to 0.2Gs (say), dragons can actually fly, and so perhaps could winged humanoids etc.

An optional extra would be if the immediate vicinity of the equator itself was too high in gravity for humans to comfortably inhabit (but not too high for fit humans to move around in - say, 2Gs), and was instead the domain of sturdier-than-human aliens (modelled on orcs and dwarves, perhaps?)

a_humble_lich
2011-06-16, 02:28 PM
I'm telling you the whole gravitation thing can be solved by changing the gravity law to have distance squared in the denominator.

-You would have internal surface gravity because the opposite side of the planet's gravity would become negligible.
-You wouldn't have to worry about structural stability as much because the planet wouldn't pull as hard on it's self.
-stellar level things like moons would still work because they change distance verry little.
-Going up 10 feet would be like going up 100 as far as the change in gravity is concerned but the gravitational difference when you're on an air plane is hardly noticable so it shouldn't be a problem(unless you want it to be, mushroom towers could be cool)
-clearly you're not really concerned with the math your self so it should go unnoticed by players other than making the setting work.

Also @Eldan i'm a Chem Engineering so student 90% of my classes are thermo. I'm not like a professor or somthing.

Well, I think this is a good solution, changing the gravitational law to be more short range would help. However, so people don't get confused I should point out that the gravitational force in the real world already has distance squared in the denominator. So what is wanted is to change it to distance cubed.

(I assume in your post you were talking about the gravitational potential which fall of like one over distance, and changing that to distance squared would work. In fact that would be exactly the same as changing the force to distance cubed, as the force is the derivative of the potential. I just didn't want people to be confused.)

Qwertystop
2011-06-16, 02:30 PM
Hmm. I'm going to kill tons of catgirls here, but how about a system like a

Klemperer Rosette? (http://en.wikipedia.org/wiki/Klemperer_rosette)

http://upload.wikimedia.org/wikipedia/commons/a/a7/Niven_Rosette.svg

Half a dozen or so large moons orbiting a hollow shell. They would all pull outwards, but be in balance with each other. The entire thing would of course be horribly unstable.

Do you read Niven? That'sexactly what the Puppeteers did when theyfled the core explosion.

Othniel Edden
2011-06-16, 03:15 PM
Do you read Niven? That'sexactly what the Puppeteers did when theyfled the core explosion.

Its mentioned in the linked article. :smalltongue:

Eldan
2011-06-16, 03:17 PM
Did you really think I'd know what a Klemperer Rosette was if I didn't read Niven? :smalltongue:

averagejoe
2011-06-16, 04:51 PM
Has anyone ever thought of the possibility that perhaps the surface world is made up of some super-reflective metal that simply redirects the majority of the harmful radiaition away? This could allow those liveing on the inside of the planet a safe haven, and provide heat through the ground to keep them alive and warm.

The problem is more for the people on the surface. Tons of rock and metal is, at the end of the day, fairly good radiation shielding, so long as it has any sensible thickness.


Okay yeah I'm pretty much going to have to handwave the fact that it doesn't collapse with "it's magic".

I like the "five giant moons" idea more than the "centrifugal force" idea. The "antigravity sun" idea has some nice potential, too.

I need to start an actual document for the setting one of these days. Right now all I have is "was made by a deific being of some sort", "monsters live on the surface and sometimes find their way into the more hospitibal 'bubble'" and "there is totally gravity somehow, but probably just around the equater".

What sort of societies could I have living in a hollow planet I wonder...

The thing is, having five giant moons wouldn't really work. Even ignoring the extreme instability of such a system, and the fact that tidal forces would be fairly extreme, it wouldn't provide any appreciable gravity in the direction you want. Orbiting objects are basically in freefall around each other.

Vemynal
2011-06-16, 07:14 PM
think about how fast the planet spins, then think about the ride at the fair that spins so fast you can stand on the walls, horizontal to the ground.

/physics

Qwertystop
2011-06-16, 07:26 PM
think about how fast the planet spins, then think about the ride at the fair that spins so fast you can stand on the walls, horizontal to the ground.

/physics

1: That's the Centrifugal Force option people were talking about.
2: That doesn't quite apply, because in that case, centrifugal force somewhat negates gravity, while in this case, it would replace gravity. Also, since this is a sloped surface, it would only be a floor at the equator. The farther you get from the equator, the more uphill it would be and the less gravity there would be.

Eldan
2011-06-17, 02:46 AM
Furthermore, as people have elaborated already, it would only be about 1/300 G.

Othniel Edden
2011-06-17, 10:30 AM
I think whats far more interesting are the potential creatures that could survive in these environments. Specialized survival methods are going to have to be used by creatures that remain on the surface, and those under the surface are going to have to be used to variant gravity. It also makes me wonder what type of magic system would form in this world, as the magic system would have to be used in these surroundings. Even if it was just DnD's you'd get interesting applications of spells, that you wouldn't get in other settings.(yay spider climb)

hamlet
2011-06-17, 10:45 AM
I think whats far more interesting are the potential creatures that could survive in these environments. Specialized survival methods are going to have to be used by creatures that remain on the surface, and those under the surface are going to have to be used to variant gravity. It also makes me wonder what type of magic system would form in this world, as the magic system would have to be used in these surroundings. Even if it was just DnD's you'd get interesting applications of spells, that you wouldn't get in other settings.(yay spider climb)

Imagine the plant life that would grow in such an environment, unhampered, effectively, by gravity. Huge floating communal colonies like non-aquatic algea fields. Trees miles high with tendril thin (comparitively) trunks. Etc.

Holocron Coder
2011-06-17, 11:55 AM
I like the idea of a hollow planet (reinforced by unobtainium) that uses extreme centrifugal force to simulate low (0.75G) gravity near the equator, with it decreasing towards the poles :smallsmile: It's giving me ideas :smallbiggrin:

Since the "poles" would have little or no gravity, that would be the realm of the dragons, who could only fly (and survive) in such low gravity.

However, people are able to get chunks of the unobtainium which can only be smelted in extreme heat (say, by the floating sun/core above), which is only easily reached at the poles, where you can effectively "fly" to the sun (of course, shielded by anti-fire/heat magic) in order to smelt some small amount of unobtainium.

It becomes this big trial to get a little bit smelted, with the danger of roasting alive, being eaten by dragons, being roasted alive BY dragons then eaten... :smallbiggrin:

And, just to add it, demons live on the surface, mutated beyond recognition by cosmic energy :smallcool: and occasionally break through to the inner world.

Also, enough unobtainium is eventually created to craft mining tools to get further unobtainium in bulk, which is eventually causing the collapse of sections of the world into the sun/core, opening the world up to the outside (yay apocalypse via mutated demons and cosmic energy) :smallcool:

Qwertystop
2011-06-17, 12:13 PM
Again, if the gravity was made by centrifugal force, then it would not just be lower gravity at the poles, it would be sloped. At the farthest points from the equator, the surface of the planed would be effectively a vertical wall, relative to the gravity. Down would be perpendicular to the surface at the equator, so getting farther would be an increasingly steeper mountain, eventually becoming a sloped wall.

Douglas
2011-06-17, 12:23 PM
Again, if the gravity was made by centrifugal force, then it would not just be lower gravity at the poles, it would be sloped. At the farthest points from the equator, the surface of the planed would be effectively a vertical wall, relative to the gravity. Down would be perpendicular to the surface at the equator, so getting farther would be an increasingly steeper mountain, eventually becoming a sloped wall.
One consequence of this, btw, would be all the water flowing to the equator to form a giant ring-shaped ocean covering the entire circumference of the equator. Water flows downhill, and "downhill" in this setup is mostly equivalent to "towards the equator".

Othniel Edden
2011-06-17, 12:31 PM
Which could give us Terraced Cliff side cities, where people's main diet would be fishing and farming floating plants. Perhaps natural caverns pervade these cliffs. Others would tend to live on the islands spanning the equator, or even underwater societies. At the steepest slopes we see winged humanoids, gliding about in the low gravity.

Holocron Coder
2011-06-17, 12:37 PM
Again, if the gravity was made by centrifugal force, then it would not just be lower gravity at the poles, it would be sloped. At the farthest points from the equator, the surface of the planed would be effectively a vertical wall, relative to the gravity. Down would be perpendicular to the surface at the equator, so getting farther would be an increasingly steeper mountain, eventually becoming a sloped wall.

That just ties into everything even more easily. You'd have to "climb" the surface to get to the point of zero gravity. Dragons would roost in the wall, etc, etc. :smallbiggrin:

Othniel Edden
2011-06-17, 04:58 PM
Yukia, this thread has inspired me, and a few other people, and I was wondering if I could use the data here to set up a Hollow World as a campaign setting over in the worldbuilders section? (Asking out of good taste)

Thesbian
2011-06-17, 06:56 PM
Wow, I really love the Centrifugal force idea the vertical-cliff towns that might result, along with the idea of the specialized creatures that would evolve.

I would definitely join in on the world builders thread if it were to happen.

hamlet
2011-06-18, 06:10 AM
Yukia, this thread has inspired me, and a few other people, and I was wondering if I could use the data here to set up a Hollow World as a campaign setting over in the worldbuilders section? (Asking out of good taste)

Just be sure to let us know where it is so that we can play too!

Riv Sonata
2011-06-18, 06:27 AM
I'm a little late to the game, and I don't know much about physics or anything else that might apply, really. As I read through this though, I did have one idea. Instead of a natural (or somewhat natural) planet, you could have the entire hollow structure be man-made, perhaps a space station, with a sci-fi or steampunk flavor perhaps. It could be something like Hollowtown (http://starwars.wikia.com/wiki/Hollowtown) from Star Wars Extended Universe.

Yuki Akuma
2011-06-18, 08:33 AM
Yukia, this thread has inspired me, and a few other people, and I was wondering if I could use the data here to set up a Hollow World as a campaign setting over in the worldbuilders section? (Asking out of good taste)

Go ahead. I'm not using it for anything at the moment.

(And yeah I'm starting to like the centrifugal force too.)

Radar
2011-06-18, 09:25 AM
This is actually my area of expertise. A reflective surface only reflects radiation not conduction. So if for example the crust was made of a metal that oxidizes on contact with the external atmosphere into a highly reflective oxide (basically the opposite of what aluminum does) then heat from the local star would be mostly reflected. But the under layer would still absorb the heat produced on the inside and conduct it to the outside layer to be cooled.
Thing is, the only method for cooling things in space is radiating the heat away. If the planet will have a very high albedo and a significant internal heat source, it will cook everyone on the inside. If one is only concerned with cosmic radiation, then a good kilometer of rock will stop anything. Suggested ring of heavy moons might be able to produce magnetic field needed to shield the surface.


I'm telling you the whole gravitation thing can be solved by changing the gravity law to have distance squared in the denominator.

-You would have internal surface gravity because the opposite side of the planet's gravity would become negligible.
-You wouldn't have to worry about structural stability as much because the planet wouldn't pull as hard on it's self.
-stellar level things like moons would still work because they change distance verry little.
-Going up 10 feet would be like going up 100 as far as the change in gravity is concerned but the gravitational difference when you're on an air plane is hardly noticable so it shouldn't be a problem(unless you want it to be, mushroom towers could be cool)
-clearly you're not really concerned with the math your self so it should go unnoticed by players other than making the setting work.

Also @Eldan i'm a Chem Engineering so student 90% of my classes are thermo. I'm not like a professor or somthing.
There are problems here. At stellar level everything would break apart, since with gravitational potential proportional to 1/r^2, there would be no stable orbits whatsover - systems would either collapse or each object would go it's own way.

Also: going up 10 feet wouldn't be like going up 100 feet - it can't be projected like this.

As for filling the inside with floating plants: depending on the ammount of those, there would be some clumping going on obviously. If you add the spinning surface and subsequently spinning internal atmosphere, one is most likely to find most floating things concentrated in a disc aligned with the equator. It will be a rather insignificant effect in short time-scales though - the presence of internal atmosphere with 1 bar pressure would induce a gravitational pull of about 0.002 N/kg near the surface (proportionally less closer to the center).
Air currents in such an enviroment might be really interesting.

Othniel Edden
2011-06-18, 02:36 PM
Just be sure to let us know where it is so that we can play too!

http://www.giantitp.com/forums/showthread.php?p=11235277#post11235277

Please help.