Gravity Is the Same All the Way Through
Alex Klotz, a doctoral student at the time and now a postdoctoral researcher at MIT, doesn’t buy that answer. “I remember just looking at a graph of what the inside of the Earth is like in terms of density and trying to figure out how that would affect the time it takes to fall,” he says.
The crust, where we live, is much less dense than the mantle below, which is less dense than the core. When he considered Earth’s true composition, and the varying accelerations this composition created, Klotz found the train’s trip takes just 38 minutes. But, more interestingly, he could also get this new result by ditching density and making his own simplification: Gravity stays constant the whole trip. It works because gravity doesn’t change drastically until the train gets halfway to the core. At that point, the contraption is moving so fast that the seconds spent near the center hardly matter. So by this reasoning, Klotz says, the gravity train needs an updated schedule.