Editing 1366: Train

{{incomplete|First explain - please check or correct anything I may have done wrong.}}

This comic plays on the fact that a choice of a reference frame is arbitrary, due to the principles of Special and General Relativity. Rather than viewing this situation as a train causing itself to move relative to an immobile Earth, Randall provides the unconventional perspective of a train remaining fixed in space while causing the Earth itself (and all the stars in the sky) to rotate instead. In principle either perspective is equally valid. Due to General Relativity, the logic of the comic is correct even for accelerating trains; a train rider could not experimentally distinguish between their train remaining fixed while the universe accelerates, from the train accelerating as the universe remains fixed.

The title text takes this further by saying that elevators do the same thing by pulling the Earth up and down, rather than moving up and down by themselves.

These examples use the train and the elevator as fixed points to define relative travel. The more common method to define movement is to use the Earth's surface as fixed point, but other reference points could be the {{w|Earth's_rotation|Earth's center}}, the {{w|Earth's_orbit|Sun}}, predefined {{w|Fixed_star|"fixed" stars}} or the {{w|Galactic_year|center of our galaxy}}. Each of these would result in a completely different movement speed:

* The speed of the train (stationary on the equator) relative to the earth's center: 465 m/s

* The speed of the train (on earth) relative the center of our galaxy: 230 km/s

It should also be noted that the comic's logic only holds true for single, isolated trains; if all the trains on the planet were actually causing rotation, then their mutually exclusive directions would cause jams. The trains would, however, have a minute, immeasurable effect on the Earth's rotation (see [http://what-if.xkcd.com/41/ what-if? 41: Go West] and [[162: Angular Momentum|comic 162: Angular Momentum]]).

TRAIN:

''On the upper edge of a circle presumably representing Earth (in a very not-to-scale way), [[Cueball]] is in a train car, looking to the right and muttering "...almost...". The train tracks run between another Cueball standing at the 9:00 position, and [[Hairy]] standing at the 2:00 position. There's yet another Cueball standing at the 6:00 position, between some snow-capped mountains that come to his knees, and some low hills. There's a counterclockwise arrow in the middle of the circle, and motion lines indicate that everyone and everything on the planet is moving counterclockwise, except for the train, which is motionless.''

A machine that grabs the

Earth by metal rails and

rotates it until the part

you want is near you