# Does the thickness and diameter of a hollow earth give an affect to the overall gravity inside?

I looked at a few articles that talk about the hollow earth. All answers state that the gravity would be 0G no matter the position of an object inside.

Though what I'm not sure is that does the diameter of the space inside, as well as the thickness of the shell have any affect on the overall gravity inside the sphere?

• Do you literally mean a hollow earth , like scooping out a watermelon? There are several hypotheses under the same name on the Wikipedia page. – cutculus Jul 29 '17 at 20:43
• "All answers state that the gravity would be 0G". This is correct. Thickness or diameter don't matter. I don't understand what your question is. Can you edit to explain how your first paragraph doesn't answer the second. – James K Jul 29 '17 at 21:03

In principle you've answered your own question. Gravity inside a hollow sphere is always zero. More formally, the force exerted on any particle inside a hollow sphere, as a result of a uniform field that follows an inverse square law (gravity and electrostatic charge are two examples) is zero. This is the shell theorem.

Now the shell theorem applies to perfectly symmetrical hollow spheres. A hollow Earth need not be perfectly symmetrical. If the hollow Earth is oblate (like our own), or otherwise non-symmetrical (either on the outside or the inside), then the gravity field will not be uniform and not always zero everywhere. For this to be noticeable, though, the asymmetry would have to be rather large (unlike Earth's).

I have no idea how the math would work in that case, and also no idea whether that idea has ever been employed in fiction.

No. It does not matter what the shell is like, as long as it is spherically symmetrical. You can make the shell as big or as small as you want, and it will not affect conditions inside. Hyperphysics has a very detailed explanation of the math behind this.

Roughly speaking, if you will double the mass of the shell, you will double* gravity — and 2 * 0g = 0g giving you no effect for a perfect shell.

As you can see, Earth is not a perfect sphere. If your hollow Earth will be similarly imperfect, then increasing its mass will change effects of global imperfections, like having more highlands on one side. Especially if you will scale these imperfections with scaling size. For details, you will need to run some simulations.

* not exactly but imprecision doesn't really matter anyway.