It has been heavily questioned by many: The mechanics of a Hollow Earth.

Here I would like to provide one explanation on how the inside of a Hollow Earth could work, given my current understanding of physics, and the huge limitations Hollow Earth comes with, such as the lack of a dynamo core.

Let’s first have a look at what happen to a human body on the surface of the globe.

We have 1 super important force acting on us right now: gravity. This is calculated by the above formula, where big M is the Earth’s mass, little m is our mass, and r is the distance to where gravity is acting from. r happens to be the radius of Earth here. Let’s throw electromagnetism out the window, mainly because the hollow Earth doesn’t have an explanation for that and, of course, electromagnetism isn’t really making a difference in our motion.

And yes, nerds, we are also throwing the weak and strong forces out too for the obvious reasons. I didn’t forget.

There’s also the contact force which is equal to gravity, otherwise we’d fall through the crust into the core. But all in all, these are the 2 forces that keep us on the ground, not floating, and not falling.

What happens in a hollow Earth?

Let’s look at the simplest case first: You’re smack bang in the centre:

No problem! Ideally these arrows should extend to the red ring which is the “centre” of the Hollow Earth, but Google Drawings is a pain!

There’s no contact force; you’re not touching anything, but it isn’t necessary here. In this case the force of gravity acts all around you in equal magnitude. The distance to the “centre” of the Earth (red ring) is the same in all directions, and the mass (big M) is uniformly distributed around you as well. The forces all cancel each other out, meaning there’s no net force on you and you just float.

That must be pretty lonely, but what about if you’re next to the inner surface?

Hmm, not as easy to see what’s going on!

The distance to the centre is no longer the same everywhere, but the mass M hasn’t changed, so you’re going to get pulled towards the part of the surface you’re closest to, right?

Not quite!

It is true that now the distance r is no longer the same in all directions, but neither is the mass. To see this, let’s take only two opposite directions:

I’ve drawn the dotted perpendicular chord to separate the sides of the Hollow Earth. The top mass is much closer to the person, but the mass itself is small. The bottom mass is (in general) further away from the person, but it is much greater than the top mass.

If we refer back to the formula, we find these 2 forces will be equal and opposite, so they’ll cancel out. If we extend this model to all directions, the same will apply and we’ll get the exact same result as if we’re in the very centre of the diagram like before.

If there’s no net force, we’ll just be floating around like before. There’s no force holding people to the inner surface!

If that’s the case, then how would subterraneans live in a Hollow Earth?

They wouldn’t. That’s the magic of ✨ＳｃＩｅＮｃＥ✨ 🌍