Shellworld

[Zor]

Lets say that you have a spherical body the same mass as earth (if need be made out of some super strong sci-fi material). However instead of being a solid mass, it is hollow. In place of a core it has a spherical chamber. How would gravity function on such a body as it has no solid center?

Zor

[Hawkwings]

I believe the math works out such that at any point in the volume, treat the center of mass as a point mass at the center of the sphere, consisting of the equivalent mass of everything at r = your position or less. So if you're standing on the surface, r = the radius of the entire thing, and the force of gravity that you feel is Gm1m2/r^2. If you're inside the empty core, you would feel zero gravity, as there is no mass center-ward of you. If you're somewhere inside the volume, then only the mass that is "inside" the imaginary sphere that your radius defines pulls you in. All the mass outside of the imaginary sphere also pulls you, but it exactly cancels itself out.

[Simon_Jester]

On the surface, you'd never notice; gravity would still be the same as it would be if you were standing on a solid planet of mass equal to whatever the mass of the hollow planet was.

Now, you would have a good chance of detecting this sort of thing using very precise gravity measurements (which are done today) or seismic waves, and you'd need one hell of a strong chamber wall to support the pressure of the material surrounding the hollow chamber on all sides.

[Dave]

On the surface, you'd never notice; gravity would still be the same as it would be if you were standing on a solid planet of mass equal to whatever the mass of the hollow planet was.

But you couldn't stand on the inside surface, could you?

[Enigma]

Are you talking about a Dyson Sphere?

[Stark]

I think he's talking about that thread Rye made like a week ago ('what if the mantle etc of Earth disappeared'). The answers are all the same.

[Number Theoretic]

Not exactly. In Rye's case, mantle and core disappear (and with them their mass, which is a substancial part of Earth's mass), so we had a lot less mass than before. Here, as far as i understand it, we talk about a hollow shell with the same mass as the Earth, so the shell's material must be denser than the crust of the Earth.

If the interior of this hollow shell is filled with air, would one experience being pulled to the shell's surface because of the centrifugal force?

[Alerik the Fortunate]

It would be noticeable at the equator, but it wouldn't be that great of a pull, since it could only equal the change in weight you feel at the equator versus the pole (and how much heavier to people in high latitudes report feeling than people in the tropics?). For all intents and purposes, the interior would be a vast zero-g chamber.

[Simon_Jester]

On the surface, you'd never notice; gravity would still be the same as it would be if you were standing on a solid planet of mass equal to whatever the mass of the hollow planet was.

But you couldn't stand on the inside surface, could you?

On the inside surface you'd be in microgravity.

If the planetary surface isn't rotating, you couldn't stand, but you could move around with light tethers or fairly weak magnetic boots, like astronauts on a large space station might. See the shell theorem

If the planet is rotating, it's not rotating very fast (once per 24 hours, for an Earthlike planet), so the centrifugal force you perceive* pushing you away from the planet's axis is weak. Not really enough to simulate gravity.

*Insert Standard Physics Disclaimer, explained here