"Hollow World" campaign setting help

[Yuki Akuma]

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]

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]

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]

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]

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]

I misread that as a "Hello World" campaign setting and now I'm getting ideas for my unplayed Cryptonomicon inspired Technocracy game.

[Mastikator]

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]

The planet is made entirely out of Adamantium, Sovereign Glue and Riverrine. There. Problem solved.

[danzibr]

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]

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]

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]

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]

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]

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]

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]

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]

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 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]

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]

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]

Okay, all of you have been a huge help, thank you.

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'. Although I'm probably going to have to wandwave some things. >.>)

[The Big Dice]

Okay, all of you have been a huge help, thank you.

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]

Okay, all of you have been a huge help, thank you.

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'. 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!

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]

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]

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]

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]

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]

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]

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]

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]

I recommend looking for anything written by genuine Hollow Earthers, on how they explain everything.
Yes, there are genuine Hollow Earthers. And Flat Earthers.