GRAVITY EXPRESS

"When I was in high school", writes physicist, A. Zee*, "I read about how gravity might be exploited to create a new mode of travel. Suppose a tunnel could be dug straight down through the center of the earth to the other side. Look down into it. Scary! An apparently bottomless pit. We have bought our ticket and now we are invited in. We take a deep breath and we jump." "Whoosh! We fall straight down, just as Alice once fell into the rabbit hole. We go faster and faster. It is that falling dream come true! We go rushing past the center of the earth. The moment we go past the center, gravity starts to act as a brake. It tries to pull us back to the center. We start to slow down. Will our momentum carry us through? Yes! Provided there is no friction, we would just make it to the exit hole on the other side. We would also need help getting out of the hole, lest we fall right back in."

"In practice, there is friction, and we need a little boost to get us to the end. Of course, we would also need to be inside a capsule strong enough to withstand the searing heat at the center of the earth, not to mention the lightning speed we will get up to as we zoom past the center." "Since we know the strength of the gravitational force, we can easily figure out how long the trip would take. It turns out to take forty-two minutes! No airplane with current technology could possibly get you there that fast. Better yet, no fuel is needed for this ’gravity express.’ We just let gravity do its thing." "You are suspicious. What is the catch? There is no catch, aside from the expense and engineering difficulty of building a heatproof tunnel that would not collapse through the center of the earth. The physics is perfectly sensible. Gravity always is trying to pull us down; we will just let it."

*An Old Man’s Toy--Gravity at Work and Play in Einstein’s Universe, Macmillan Publishing Company, N.Y., 1989.