I am on an astrophysics blog site, and occasionally am asked to add a little extra physics expertise to the discussion. In one discussion a blogger asked about the possibility of having a hollow earth. Letís discuss a few things about the real earth first. Most of the heaviest elements that are found on the earth are concentrated toward the center of our planet, due to their higher density. These heavier elements also happen to be the ones that are radioactive. Most scientists today believe that the heat that keeps the center of the earth molten is generated by the nuclear decay of these heavy elements and probably a slow rate of nuclear fission too. So the center of the planet is quite hot and has a deadly level of ionizing radiation. It is actually a much more hostile environment than outer space. Even if we could open up an empty sphere, it would be a very uncomfortable place to sit.
The question I was asked to comment on in this blog thread was, what kind of gravitational forces would one feel in this hollow earth scenario? Imagine the sphere is relatively small, say eight feet in diameter. Donít forget your lead underwear and a high power air conditioner.
Reply to Butch Shadwell at b.shadwell@ieee.org (email), 904-223-4510 (fax), 904-223-4465 (v), 3308 Queen Palm Dr., Jacksonville, FL 32250-2328. (http://www.shadtechserv.com) The names of correct respondents may be mentioned in the solution column.
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My astrophysics blog was wondering about the hollow earth theory, and came up with this question, what kind of gravitational forces would one feel in this hollow earth scenario? Imagine the sphere is relatively small, say eight feet in diameter.
I didn't get any correct answers this month. Some folks assumed that if the earth is a homogeneous sphere and the void is a concentric sphere, that there would be a net zero gravity at the center. This is true as far as it goes, but I asked what forces a person would feel in this place. The net force due to gravity is only zero exactly at the center of the empty sphere in the middle. Also, a spherical homogeneous object centered at this point would have equal forces pulling in all directions. However, most people are neither spherical nor homogeneous. Parts of you that happened to be closer to the walls of this void would feel forces tugging at them and denser parts would have greater tugging than less dense parts. So if your center of mass was at the center of this void, your extremities would be pulled away in each direction. Before very long, you would end up lying against the wall. But I bet you already knew that.
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