Gravity another question from a bus driver

[bigalzz]

if we accept that there is zero gravity at the center of the earth. then this would be true for stars and even black holes.. and if you move from the center, at some point there will be an "event horizon" (for want of a better term) where the gravity is half G. and further, if a cold body such as the moon has zero gravity at the center, it may very well have a cavity at the center??
i'm sure I'm wrong and i don't know what difference it would make but i'd like to find out for sure and why not. i have the burden of a curious but uneducated mind.

 

[Janus]

Even at the center of the Moon the pressure would be so great (about 50,000 atm), that no cavity could be supported.

 

[Drakkith]

As has been said in your other post, the net pull of gravity is zero at the center but not anywhere else. A 5 ton block on the surface of the Earth adds 5 tons of weight pressing down on the center of the Earth, as something must support this weight and the weight of the rest of the Earth as well.