Find out how we would be able to get anywhere in just under forty five minutes, if it weren't for that pesky molten core at the center of the earth.

Imagine getting anywhere on earth in under an hour, without expending any power. This is the idea behind the gravity train, a mode of transport that has been theoretical for almost four hundred years and will, unfortunately, remain theoretical for another four hundred.

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The concept of a gravity train was kicked around by no less a mind than Isaac Newton in the late sixteen hundreds. The concept behind it is simple. Tunnel between two spots on earth. Lay track. Put a train on the track at one end and let it fall into the hole. For the first half of the journey, it would just keep falling, being accelerated by the earth's gravity until it reached the midpoint of the journey. After that, although it wouldn't change direction, it would be climbing away from the center of the earth. Because the gravitational pull would now be coming from the opposite direction, the train would decelerate until it finally, gently, rolled out of the earth at its destination.

You don't have to be a geology major to see the problem here. It turns out that the earth, probably out of spite, is not filled with nice, solid bedrock. Instead it's got a red hot, gooey center that is distressingly unsuited for train travel. Even if the core of the earth were solid rock, the friction that the train sustained during the journey would deplete its energy enough that the last part would have to be heavily aided by the trains own power source. Since the train, during the last part of the journey, would probably have a very steep grade to climb, that power source would have to be really impressive.

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Obviously, the gravity train is a pipe dream. What separates it from the usual pipe dreams is one interesting fact: every ride on the gravity train would last the same amount of time. Whether you were riding the train for a few hundred miles, or from one side of the earth to the other, the ride would take forty-two minutes and twelve seconds. Although the trip from one side of the earth would be longer, the train would essentially be in free fall - Fun times!- for the first half of the journey. Any shorter journeys would be at a greater angle to the earth's gravitational pull, and thus make the train go slower. Any trip, any distance, it would take the exact same time.

Gravity: the great equalizer.