Editing 1366: Train
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==Explanation== | ==Explanation== | ||
− | + | {{incomplete|How does it work in general relativity? A falling elevator would be valid but not the one which is moving in reality.}} | |
− | + | This comic plays on the fact that a choice of a {{w|Inertial frame of reference|reference frame}} is arbitrary, a {{w|Principle of relativity|basic principle}} in {{w|Albert Einstein}}'s theories of {{w|special relativity}} and {{w|general relativity}}. But at speeds much lower than the speed of light it also applies to the {{w|Classical mechanics|newtonian mechanics}}. | |
− | + | 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. | |
− | + | 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. | |
− | The | + | 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 to the sun: 29.78 km/s | ||
+ | * The speed of the train (on earth) relative the center of our galaxy: 220 km/s | ||
− | + | 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]]). | |
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==Transcript== | ==Transcript== | ||
− | + | :[On the upper edge of a circle representing the Earth in a wrong scale, Cueball is in a train car, looking to his left.] | |
− | :[On the upper edge of a circle representing the Earth, Cueball is in a train car looking to his left.] | ||
:Cueball:...almost... | :Cueball:...almost... | ||
:[The train tracks run between another person standing at the 2:00 position, and Hairy standing at the 9:30 position. There's yet another person standing at the 6:00 position, between some snow-capped mountains and some low hills.] | :[The train tracks run between another person standing at the 2:00 position, and Hairy standing at the 9:30 position. There's yet another person standing at the 6:00 position, between some snow-capped mountains 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.] | :[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 | + | :A machine that grabs the Earth by metal rails and rotates it until the part you want is near you |
{{comic discussion}} | {{comic discussion}} | ||
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[[Category:Comics featuring Cueball]] | [[Category:Comics featuring Cueball]] | ||
[[Category:Physics]] | [[Category:Physics]] | ||
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