英語時差：大坳

Last time we imagined a tunnel through the center of the earth. If such a thing existed, you could step into one end, fall all the way to the other side, and step out. Gravity would cause you to pick up almost exactly as much speed on the way down as would be needed to get you back up the other side. Once you passed the center of the earth, your speed would gradually decrease back to zero just at the point where you stepped out.

The trick comes in the "almost" part. It couldn't really be the case that you'd pick up exactly as much speed as would be needed to reach the same spot on the other side. Some energy always finds a way to escape.

Think of swinging on a swing. If you just push off once and then sit still, the swing doesn't keep going back and forth forever. Gradually it slows down.

It's the same situation when you step into that hole. Down you go, and when you come out the other side, your feet will stop at slightly less than ground-level. If you miss your chance to grab onto something, you will fall though the entire planet again. Back at your starting place you rise slightly less than last time, and so on.

See the trouble? It would be a fast way to travel, but miss your step and you will wind up yo-yoing inside the tunnel, each time rising slightly less than the time before. Eventually you will stop moving altogether and be stuck in the middle -- with a long climb home.