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Lets just assume I want a Hollow earth, similar to movies like the Monsterverse, etc, but without the plant/animal life. what changes does earth's structure or physics require for that?

Around the core is preferred, but a hollow pocket works too

Clarifying the details

  • 100 million cubic meters, like triple the largest cave on earth, at least 10 miles tall
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  • $\begingroup$ @JBH not to mention that the pressure at mere 12 (twelve) kilometers deep is enough to make stone flow like water, so any cavity deeper than that would get filled with surrounding walls in a matter of years. And the fact that having a cavity in a planet would reduce its gravity, a cavity in the center would have local gravity as zero, and more. $\endgroup$
    – Vesper
    Commented Jun 6, 2023 at 5:51
  • $\begingroup$ Would it being filled with hot water be fine? $\endgroup$ Commented Jun 6, 2023 at 17:03

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A little adamantium and you're fine

Planets are not hollow because of gravity.

You can't mess with gravity without causing major problems (1).

Therefore you need a nearly infinitely strong material that can resist earth's gravity at earthlike size. Just as a hollow steel ball does not collapse from gravity, nor does your planet.

I believe Larry Niven had the main structure of the Ringworld made out of such a material. As in many things, he anticipated my great ideas and shamelessly stole them from the future, the cad.

(1) Maybe someone can figure out if it could be suspended just at one distance range in a semicoherent manner?.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Commented Jun 8, 2023 at 2:44
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Honestly this requires so much hadwavium, that you could probably make it more plausible by having an anti-singularity inside your planet. A "white hole" in the relativistic sense, that bends space-time outwards rather than inwards. You could then have the crust of your planet of arbitrary thickness, of any (including earth-like) material.

If you want to scify technobabble: A sphere of Dark fluid smashed at high speed into a still forming giant rocky planet, blowing some of it away but stopping at an approximate center of now expanding cloud of hot mass.

While some of the mass had an escape velocity, the remainder of the spinning cloud began to collapse towards there center where the sphere of negative mass was locked due to approximate uniformity of the surrounding mass. at certain distance the gravity of the cloud and the anti gravity of the white hole matched, forming a "hollow" sphere that contained the dark fluid in the center.

I used dark fluid, because of its theoretical association with negative mass, feel free to replace it with any other exotic material.

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unobtanium shell

As AncientGiantPottedPlant correctly observed, you need a shell form some strane material that prevents the hollow earth from collapsing and forming an ordinary planet. The material properties are impossible with normal matter, but as a writer you can call it "unobtanium" or something similar.

some kind of magic downforce

Let me give you a classic example of Physics 101: Calculate the gravitational pull inside a hollow sphere. The result will shock you: it's 0. Inside a hollow sphere there is no gravity that keeps you on the outside. To be even on the surface, you need some strange thing that in the center pushes away any matter against the inner surface of the sphere-shell.

Alteration: Cylinder World spinning!

To stay on the outside there is a physics solution that works within the confines of normal physics: a cylinder that is spinning along its axis can create the appearance of gravity. Note that it is actually the effective force from you changing the vector all the time, often summarized as the Centrifugal Force. Also, note that the edges of Cylinderworld do experience a gravitational effect to the plane that halves the cylinder, so only a distinct area around the central plane will be habitable at all with no gravitation.

The cylinder spinning also helps with stabilizing the structure. If you use a tube on the cylinder, you create a spinning torus, aka Ringworld

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  • $\begingroup$ Ringworld requires unobtanium to build. $\endgroup$
    – Mark
    Commented Jun 6, 2023 at 21:34
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    $\begingroup$ @Mark yes, but less of a strange unobtanium than hollow earth $\endgroup$
    – Trish
    Commented Jun 6, 2023 at 21:38
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Negative gravity

What you need is a negative gravity generator. Put one in the middle of your planet. it creates a negative gravity field that prevents the planet collapsing under its own weight. Of course, conservation of gravity requires the removed gravity to go somewhere. The result is that the planet appears heavier from outside. So people on the surface feel heavier, and the moon gets closer. Oh, and everyone inside is weightless. Don't forget the weightlessness.

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I Love Hollow Earth Theory

There is of course a rather extensive lore surrounding this, which I'll skip. The physics of such a form of planetary accretion require that matter partially "shield" space from being curved by other matter on the other side. Take our moon, for instance, just as an illustration.

The moon exerts a tidal force on the planet. Hollow earth physics would say that force is only being felt on the side of the planet closest to the moon, that the earth's matter is "shielding" its far side from that lunar gravity. And that example is just to illustrate. Perhaps at that distance there is some gravity affecting tides on the far side of the earth as well. We certainly don't want our theory to die that easily, lol.

More precisely, there just needs to be a diminishing return on how much matter can bend space. Up close and personal, they think 400 miles of dirt has curved space about as much as it will. When you accrete more interstellar dust, your nascent planet ball has no gravity at its center, so additional accretion is done as a shell.

The right computer model could have fun with this - calculating an enormous number of particles exerting gravity on each other, while partially shielding others by calculating a "shadow" that diminishes the effects inside it. Let that stew for a few billion years and see what you get. I don't think my ten year old Xeon is up for the job.

The biggest problem with this lovely theory is that all planets would have roughly the same surface gravity. Our adventures on Mars have killed all the fun!

Here are some images to illustrate:

enter image description here

enter image description here

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  • $\begingroup$ I would add that even under the standard conception of gravity, there's no gravity at the very center of our earth. So maybe matter could accrete to a shell form, even if that form is much thicker than hollow earthers propose. Thickness would be proportional to the size of the planet. In that way, you could explain different surface gravities around the solar system without giving up entirely on a very fun theory. $\endgroup$
    – Elkrat
    Commented Feb 15 at 18:04

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