Part of the show UFOs, Mars and Space Science
Jeff via email asked:
You talked in the past about dropping an object down a tunnel that's been drilled right through the earth. I've been in all night debates about this. What would happen if a person jumped into the hole? Also, what would happen when you got to the middle? Would gravity be different, because the world is all around you, rather than you being on top of it. Also, would the air pressure in the tube be so great that you'd be squashed flat?
The first thing we need to think about is gravity itself. When you're stood on the surface of the earth, the gravity that's affecting you is from the whole mass of the earth beneath your feet. As you move towards the centre of the earth, the amount of mass beneath your feet is decreasing. So when you get to the centre, the force of gravity exerting on you would be zero. But by this stage you would have reached a velocity, so you would pass through the centre and move through the other side. However, then gravity will start working on you again, and you will be pulled back down towards the centre. Therefore, you will oscillate. If you assume that there is air all around you, there would be air resistance, which would cause you to lose some energy along the way. This would cause you to eventually come to a stand still. As for the air pressure, if you look at a deep sea diver, for every ten metres they go down under the sea, they get the equivalent of the whole earth's atmosphere on top of them again. The reason that this isn't a problem for them is because it's pressing on them in all directions. It's both inside and outside, so it all equalises out. So I don't think that you would be squashed flat. You would probably get the bends though, because you'd be flying out of the hole so fast.