Hollow planets

Larry: "If your pillow is smaller than a bed I'd be inclined to agree that stuff mixes."

My pillow is indeed smaller then a bed. Certainly not the size of a planet as I was making a comparison with a planet core, where gravity is very low.
(If there are materials that wouldn't melt or burn in the earth's core bring them on.


Phil: "Not even hydrogen can exist as a gas at that temperature and pressure."

I suspect that too, but I don't now the pressure and temperature inside the earth, nor the way hydrogen reacts to that.

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So, on one side we have a process that takes heavy elements towards the centre using gravity. It's less effective the deeper you get.
On the other side we have thermal diffusion, convection currents and tidal effects to mix the stuff up, more or less randomly, but likely it gets more effective with higher temperature.
The two sides will find a balance, like Phil described very well.

Maybe there is such a balance where there is a heavy core, then a very low-density region and then the mantle, but i doubt it. Whatever may be, the words 'hollow planet' seem to stir up all the wrong associations.

The data from "S"&"P" waves might be overly massaged by modelers to get the commonly accepted view of the interior of Earth. The rest of the reasoning here is guess work. The only data is seismic waves and it seems to me that data can be read in any way one likes.

Please note correction to my last post, above.

Phil, your argument could be strong if you can show that a solid sphere reacts differently then a hollow sphere to centrifugal and gravitational forces.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Michiel</i>
<br />Phil, your argument could be strong if you can show that a solid sphere reacts differently then a hollow sphere to centrifugal and gravitational forces.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

It is possible for a planet-size hollow shell to exist for a time---after some immensly powerful intelligent beings constructed it. However, the logistics of such a project are mind boggling, and such an artificial construct would be highly unstable.

So long as the hollow sphere is intact and uniform, I believe gravity and centrifugal forces would act the same as for a solid sphere. But what happens when a meteor punches a hole in the hollow shell? The loose fragments would gravitate gently toward the new center of gravity of the damaged shell, which would be slightly off center away from the hole. So a solid sphere would begin to grow inside the hollow shell. Each time another chunk breaks off at the edge of the hole, it would add to the mass of the solid sphere inside---thus increasing the imballance. Eventually the last chunk of shell would join the solid ball inside.

The simplest argument against a hollow sphere is the fact that it would have far more gravitational energy than a solid ball. The collapse of the shell into a solid ball would convert that gravitational potential to heat, making the solid ball very hot. Reversing that process would require a tremendous energy input.

Every step along the way, angular momentum would be conserved; so the new solid sphere would spin faster than the original shell. Conversely, converting a solid ball into a hollow shell would decrease the angular velocity. But all we need to know about centrifugal force is that there is far too little of it make the center hollow.

If you could magically speed up the Earth's rotation indefinitely, the poles would move closer together and the equator would bulge more. Since the mass is not homogenious, it would eventually form two or more pieces which would move farther apart until they seperate into a main planet and one or more moons. Increasing the angular momentum of the system further would cause the pieces to orbit farther apart in a plane---not in a sphere.

I've wasted far too much time debunking this idiotic theory. It is not worthy the attention it is getting. So don't hold your breath waiting for me post to this thread again.

Phil: "So long as the hollow sphere is intact and uniform, I believe gravity and centrifugal forces would act the same as for a solid sphere."

In that case we're back to square one (or sphere one for that matter).

I've always been curious about the inner working of our planet. Unfortunatly there is little data to go on, so until someone comes up with a realizable experiment it's like Jim says: we can do only some educated guesses. (And post them in a geology topic...)