Yofed asked me to update my blog just as the same time as I decided to make a post about a podcast I heard. The podcast is beyondscience it is a podcast about stuff that are "beyond science" i.e things that science isn't advanced enought to explain or that are theories not yet reconized by the scientific community. They talk about things like antigravity, alien abduction angels and stuff of that nature, they use very articulate and informed speaker that put the subject in a quasi scientific way. One of their show was about a "scientific" theory called hollow earth.
Hollow Earth is as it name indicate the theory that the earth is not a quasi solid ball of matter but an hollow shell with life and stuff in the middle. It claims that the Earth cross is some 300 km deep and much more dense then what we think and that every planet bigger then 600 km radius is hollow. Up to now there is nothing in the theory that is really easilly disprovable (well aside from the observation of sound wave from earth quake that are consistant with an solid core earth...) since it would not change anything to any models of the solar system or gravity. Next this particular proponent of the hollow earth theory claims that the human race probably has evolved (along with thousand or millions of other species not seen on the surface). He said that the inside of the shell would experience a slightly lower gravity lower gravity (and that human are adapted to this slightly lower gravity explaining some of the "difficulty" of human on the surface) and this is where I call it bull.
An attractive force on a spherical shell as some interesting proprieties, no that this is in no way tied to the theory of a core filled earth, gravity or any other theory the hollow earth guy call false or misguided, it is simply doing the mathematic of an attractive force no matter the kind. Also this as been proven by experiment with hollow shells. To understand the proprieties let my try to explain them to you in non mathematical terms (advaced if yomath are kind hard to duplicate on the web). First off you probably agree that outside the shell the force pulls toward the center, if you put an object and draw arrow toward every point of the sphere no arrow are symetrical and the lenght of the arrow is the inverse of the pull it experience. Adding all those arrow together with get you a single arrow pointing toward the center. Up to now no problem, but if you look at the inside of the sphere now problem appears. If you try the same experience you see that there are arrow pointing in opposite direction. In fact you probably have at least two arrow that are exactly the same lenght but in exact opposite direction. The effect of the force of those two arrows on the object is exactly zero. If you take a small angle not directly below the object but having it appex at the object and project it on the other side you find that they cover a much larger area on the other side of the sphere. If you add the arrows of those two side (pulling toward the sphere close to object and the one pulling toward the the sphere far from the object) you find that the two resulting arrow are of the same lenght but opposing direction i.e their net effect on the object is zero. Now you will ask but what about the point directly bellow the object... well exactly the same thing happen the other side cancel this attraction exactly. The end result is simple: in the inside of an hollow shell producing an attractive force their is no net force. Meaning that inside an hollow earth the gravity is exactly 0. This phenomena explain why if you dug deep enough you would find that the earth gravity you felt would be lower: less and less matter would contribut to the gravity you fell.
Event if before the gravity argument I was pretty sure that the hollow earth theory could not work (the density of earth would have to be about 20 times larger then what is measured with radio wave) but I was willing to give it a chance. However considering that it is impossible for most if not all of the prediction of this theory (i.e. life and gravity inside the shell) to be true (without having to resort to any theory they do not find acceptable, simply by looking at how an attractive force work..) I have to say that this is an impossible situation. Hell even if the species living inside had somewhow find a way to build gravity generator it couldn't produce a gravity all over the sphere. In short the earth "could" be hollow but nothing could be living inside.