# Thread: A hugely massive rocky planet is hollow, how gravity would be at the inner surface?

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## A hugely massive rocky planet is hollow, how gravity would be at the inner surface?

Hello guys.

Thinking about gravity I imagined a rocky planet as big as a star, or even bigger, but despite being massive as a whole, it would be hollow.

So, at the outside surface, gravity would be as normal as we know it.

What about at the inner surface?

Gravity would push you against the inner surface, or would pull you to fall to the middle of the inner void?

Does the size of the inner void, comparatively to the rocky shell, makes any difference?

Thanks a lot, and sorry for the newbie question.

2. The net gravitational force anywhere inside a massive uniform spherical shell is zero, according to something called Newton's Shell Theorem (useful search term for further reading).
So you'd be in free fall anywhere inside your hollow planet (assuming the mass distribution was even), even right next to the inner surface.

Grant Hutchison

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Originally Posted by grant hutchison
The net gravitational force anywhere inside a massive uniform spherical shell is zero, according to something called Newton's Shell Theorem (useful search term for further reading).
So you'd be in free fall anywhere inside your hollow planet (assuming the mass distribution was even), even right next to the inner surface.

Grant Hutchison
Thank you very much Grant, I will try to understand the theorem.

4. As a quick addendum to Grant's post, I'd like to add that on that scale imperfections in the shell thickness might be important in terms of the spelunker's experience as the leap from the inner surface.

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I'm not well versed in math or physics, but I think a lot about gravity, and I think we have it wrong somehow.

If I saw a class about Newton's Shell Theorem in my time at school, I can't remember, I'm 53 now so that was a long time ago.

But it is mind bending to think that if you have a hollow sphere made of black hole matter, extremely big, but with a very thin wall, you would float inside of it and right outside, very close to you, gravity would crush you so easily.

Back to gravity, call me a cracked pot but I can't help myself but thinking it's all about space time density.

Yeah, I said it.
Last edited by Danilo Carrazzone; 2021-Aug-25 at 06:12 AM.

6. As another addendum, Newton’s shell theorem works at all scales, but you cannot build a hollow shell on Earth and float about inside. Because external masses still exert gravity forces, by Newton’s gravity or by Einstein’s model. That is why your example uses a massive planet sized hollow with, presumably, no nearby masses of significance.

7. A large body would not remain hollow in Real Life of course. I'm sure you know but I think it's worth pointing out. The gravity of the outer mass would still pull the material inward from the top/surface, and the underside would move accordingly.

8. Originally Posted by Noclevername
A large body would not remain hollow in Real Life of course. I'm sure you know but I think it's worth pointing out. The gravity of the outer mass would still pull the material inward from the top/surface, and the underside would move accordingly.
If this is a rock planet, the loads are all compression in the skin, rock is good in compression, the route to being hollow is hypothetical but the thing could hold up. (But i have not shown calculations, )

9. Originally Posted by Danilo Carrazzone
... Back to gravity, call me a cracked pot but I can't help myself but thinking it's all about space time density.
You are new here. I recommend strongly that you read the rules, particularly about advocating non-mainstream ideas in Q&A and other mainstream parts of this forum.

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Originally Posted by antoniseb
You are new here. I recommend strongly that you read the rules, particularly about advocating non-mainstream ideas in Q&A and other mainstream parts of this forum.

Oh, I'm very sorry, it will not be repeated, although I'm not advocating, just taking in consideration.

About the massive planet not be strong enough to hold itself against it's own gravity pull, it was just a theoretical model, I am trying to understand what gravity really is.

11. Originally Posted by profloater
If this is a rock planet, the loads are all compression in the skin, rock is good in compression, the route to being hollow is hypothetical but the thing could hold up. (But i have not shown calculations, )
Rock isn’t that good in compression. A moderate sized hollow asteroid might be possible, but the diameter of Earth? Or even worse, as described here something that is supposed to be the diameter of a star would collapse from self-gravity. You need compression strength beyond what normal matter is capable of, or some other method of support (for instance, as a general concept, a number of spinning rings, spinning fast enough to counter self gravity, each offset from others to appear roughly spherical when seen from a distance and that form a hollow interior).

12. Originally Posted by Van Rijn
Rock isn’t that good in compression. A moderate sized hollow asteroid might be possible, but the diameter of Earth? Or even worse, as described here something that is supposed to be the diameter of a star would collapse from self-gravity. You need compression strength beyond what normal matter is capable of, or some other method of support (for instance, as a general concept, a number of spinning rings, spinning fast enough to counter self gravity, each offset from others to appear roughly spherical when seen from a distance and that form a hollow interior).
I was toying with some numbers, maybe will continue if I have time. As diameter increases the risk of buckling increases to dominate the failure mode and buckling of a thin spherical shell would be complex. If this were to be an artificial huge hollow sphere, you could design anti buckling features, even in rock as a concrete material. Internal pressure in a gas would make a difference too. I am sure you are right about scale.

13. Originally Posted by profloater
If this is a rock planet, the loads are all compression in the skin, rock is good in compression, the route to being hollow is hypothetical but the thing could hold up. (But i have not shown calculations, )
Solid rock is good in compression. After you get to the point of soft mantle, it decidedly loses that property. The mass described in the OP fulfills that condition.

14. Originally Posted by Danilo Carrazzone
….
About the massive planet not be strong enough to hold itself against it's own gravity pull, it was just a theoretical model, I am trying to understand what gravity really is.
You and others! Newton helped engineers with an equation good enough to go to the moon, but it does not explain gravity. Einstein went a lot further as you know, and produced a model that has passed many tests. Mass distorts space time and spacetime dictates how masses accelerate. But as you might ponder, that may be a better cosmic model but it gets no further than Newton did in explaining the mechanism. The other small scale model, started from quantum mechanics does not explain gravity either, does not even include it, but has also been tested extensively. Currently it is even more complicated because cosmic observations suggest more mass, dark matter, and another force, dark energy, opposing what we think of as gravity. Newton’s observational model was sufficient to prove his shell theorem. Like an onion you can consider a solid as a nest of shells. It leads to the model that inside a mass the gravity profile is linear, not the square law, with zero gravity at the centre, (assuming homogeneity). But this also assumes no significant external masses. And then Einstein predicted gravitational waves from far away massive encounters, and indeed they have been detected.

15. Originally Posted by profloater
...Newton’s observational model was sufficient to prove his shell theorem....
As I recall, Newton proved his shell theorem geometrically in his Principia.

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Originally Posted by profloater
You and others! Newton helped engineers with an equation good enough to go to the moon, but it does not explain gravity. Einstein went a lot further as you know, and produced a model that has passed many tests. Mass distorts space time and spacetime dictates how masses accelerate. But as you might ponder, that may be a better cosmic model but it gets no further than Newton did in explaining the mechanism. The other small scale model, started from quantum mechanics does not explain gravity either, does not even include it, but has also been tested extensively. Currently it is even more complicated because cosmic observations suggest more mass, dark matter, and another force, dark energy, opposing what we think of as gravity. Newton’s observational model was sufficient to prove his shell theorem. Like an onion you can consider a solid as a nest of shells. It leads to the model that inside a mass the gravity profile is linear, not the square law, with zero gravity at the centre, (assuming homogeneity). But this also assumes no significant external masses. And then Einstein predicted gravitational waves from far away massive encounters, and indeed they have been detected.
I think there is still a lot to think and to talk about gravity.

17. Originally Posted by Cougar
As I recall, Newton proved his shell theorem geometrically in his Principia.
But I think he used the square law in doing so and found the linear relation as a corollary. The square law does come from geometry if you assume a force arises from a point mass. Maybe he did then infer orbital mechanics from his assumption rather than observing inclined planes and so on. The behaviour of a falling mass does invoke his Laws. Otherwise acceleration toward Earth would vary with mass. Interesting point.

18. There are concepts for large artificial hollow structures that might allow something of this kind to be constructed. The Edersphere for instance is a large metal balloon surrounding a volume of gas, suggested by Dani Eder as a possible habitable megastructure.
https://www.orionsarm.com/eg-article/48472f9d56859
An analysis of this concept here
http://gravitationalballoon.blogspot...c-gravity.html

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A smaller version is the Gravity Balloon, which uses rocks on the outer surface to counterbalance the gas pressure. A large version would be something like a hollow planet, with no gravity inside (not quite true, but close enough).
http://gravitationalballoon.blogspot...y-balloon.html

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The Supraself concept by Paul Birch is a vast and massive construction made from an entire galaxy, supported by orbiting particles inside mass accelerators. This object would bend space and time significantly; some details on this idea may be found in the last segment of this article by Birch;
https://www.orionsarm.com/fm_store/C...%20at%20...pdf

19. Originally Posted by Danilo Carrazzone
I think there is still a lot to think and to talk about gravity.
Yes, there certainly is...but this is the Space/Astronomy Questions and Answers subforum. This is where one gets mainstream answers to questions and those answers cannot be argued on an against-the-mainstream basis.