Ask the Experts; August 2003; Scientific American Magazine; by Staff Editor; 1 Page(s)

Mark Shegelski, professor of physics at the University of Northern British Columbia, offers this answer: Theoretically, yes. For this conjectural trip, let us ignore friction, the rotation of the earth and other complications. Just picture a hole or tunnel that enters the earth at one point, goes straight through the center and comes back to the surface at the opposite side of the planet. If we treat the mass distribution in the earth as uniform (for simplicity's sake), a person could fall into the tunnel and then return to the surface on the other side in a manner much like the motion of a pendulum. Assume that the person's journey began with an initial speed of zero kilometers an hour (he simply dropped into the hole). His speed would increase and reach a maximum at the center of the earth, then decrease until he reached the surface-at which point the speed would again fall to zero. The gravitational force exerted on the traveler would be proportional to his distance from the center of the earth: it is at a maximum at the surface and zero at the center. The total trip time would be about 42 minutes. If there were no friction, no energy would be lost, so our traveler could oscillate through the tunnel repeatedly.

This jaunt could not occur in the real world for a number of reasons. Among them: the implausibility of building a tunnel 12,756 kilometers long, displacing all the material in the tunnel's proposed path, and surviving the journey through a passageway that runs through the earth's molten outer core and inner core-where the temperature is about 6,000 degrees Celsius.