1366: Train

Explain xkcd: It's 'cause you're dumb.
Revision as of 15:46, 9 May 2014 by (talk) (Transcript: The description lacked major details, such as the fact that train-Cueball has dialogue.)
Jump to: navigation, search
Trains rotate the Earth around various axes while elevators shift its position in space.
Title text: Trains rotate the Earth around various axes while elevators shift its position in space.


Ambox notice.png This explanation may be incomplete or incorrect: First explain - please check or correct anything I may have done wrong.
If you can address this issue, please edit the page! Thanks.

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 Earth's center, the Sun, predefined "fixed" stars or the 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 earths center: 465 m/s
  • The speed of the train (on earth) relative to the sun: 30 km/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 41: Go West and 162: Angular Momentum).



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

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


I just did an explanation from scratch for the first time, please could you tell me how I could improve it? Thanks :) Cheeselord99 (talk) 07:02, 9 May 2014 (UTC)

Am I the only one who gets Inflation when going to xkcd.com (without the www)? This comic shows at www.xkcd.com and m.xkcd.com however. 07:11, 9 May 2014 (UTC)

I created an account. The ip address today is me, along with today. Mikemk (talk) 07:16, 9 May 2014 (UTC)
I thought today's comic was late. http://www.xkcd.com/1366/ kept on displaying "Web-page not available" (browser thing, not server-thing), then I checked here. So. Oh, http://www.xkcd.com/ also... Hmmm... That's not right. Oh, "Ping request could not find host www.xkcd.com. Please check the name and try again." DNS errors? Only those trying via cached details get anything? Things are not working for xkcd.com or m.xkcd.com either. So, DNS poisoning or human error of some kind? Not the place to discuss this, I know, sorry... 10:05, 9 May 2014 (UTC)
Explanation is good, but there are certainly related comics or maybe what-if ... I've found Orbital Speed, but I think there were something mentioning how fast sun goes relatively to galaxy ... -- Hkmaly (talk) 10:14, 9 May 2014 (UTC)
Found two related comics - any other? Condor70 (talk) 11:33, 9 May 2014 (UTC)

It sounds like the dark matter engine in Futurama:http://futurama.wikia.com/wiki/Dark_matter_engine (talk) (please sign your comments with ~~~~)

That's just what I was thinking! -- 17:53, 12 May 2014 (UTC)

I think the last paragraph, considering the situation from the point of view of multiple trains, is not relevant. The whole concept of what makes this idea funny and interesting is that you MUST view the situation from the point of view of a single train (or elevator). --RenniePet (talk) 13:24, 9 May 2014 (UTC)

Second-last paragraph - my comment was written at the same time as another paragraph was added. --RenniePet (talk) 13:26, 9 May 2014 (UTC)

I do not understand what the last paragraph is suggesting as it seems to violate the 3rd Newtonian law of motion.

The last paragraph is not correct, the Earth would also experience an acceleration (albeit a small one).--Sturmonium (talk) 13:54, 9 May 2014 (UTC)

This line: "The logic of the comic also fails when taking acceleration into account. Whether the train or earth is moving can be determined by which one experiences a force due to acceleration or deacceleration when the train starts." is incorrect, according to the principle of General Relativity. You cannot experimentally distinguish between your own acceleration against a fixed universe, and your position remaining fixed against an accelerating universe. This applies for rotation as well; if you fix the reference frame of the train rider, the acceleration of the universe creates gravity waves that cause any rider on the train to experience what feels like an acceleration. Therefore, the logic of the comic is indeed correct, even for accelerating trains. I will correct this edit.--JB Gnome (talk) 14:12, 9 May 2014 (UTC)

But the comic doesn't say that the train accelerates the universe: rather, it just accelerates the Earth. Does that make a difference? 14:34, 9 May 2014 (UTC)

Does anyone have an idea where "train guy" is heading? He's saying "almost", like he's almost there but wasn't sure if there was something more. Maybe he's timing when he needs to jump off the train? 14:58, 9 May 2014 (UTC)Pat

I see this comic as a nice ab absurdo for the many people who think the sun rotates around us, and further to those who claim the earth has 6 thousand years etc... 18:12, 9 May 2014 (UTC)

I don't have a citation, but some traditional Polynesian navigation works using this view. Their "maps" are made of a grid of bush materials where intersections are stars or islands (possibly with a pebble tied on to represent the island). They consider the map and the earth to be connected, and you don't move along the map - the map moves. So you don't go to another island, you bring it to you. At night you move the stars to the right place, and during the day you paddle the sea and land so they are in the right place and direction.--DivePeak (talk) 21:15, 9 May 2014 (UTC)

A train rotating the Earth is NOT physically equivalent to a train traversing the Earth. It would be true for a flat Earth, but rotation is absolute, see Newton's bucket argument. [1] --Gleyshon

I am only a layperson, but as i read it the explanation confuses coordinate systems for the description of motion with physical explanation. The reasons why the train cannot be seen as accelerating the earth/universe are so numerous (even ignoring the other objects moving in different ways) it's just for subjective fun that i'll point out a few: The rest mass of the earth is not relative, so neither is the force needed to accelerate it -- force which the train cannot create by a factor of kajillions (roughly ;^). And if it did, the area would turn into plasma. Sticking to the comic, there isn't enough friction/rigidity/etc for the wheels/rails to "grab the earth". (Notice these are local observations unaffected by any relativity.) If somehow the earth could be accelerated in this manner, the rest of the universe wouldn't go along with it. And if it did, astronomical objects wouldn't change apparent position until their light reached us. Finally, rotation of massive bodies creates Frame-dragging (the twisting of spacetime) which is locally observable. So: the point of the comic isn't that the view presented is counter-intuitively valid (with awkward math); rather the comic is funny because of how badly this view fails. But i don't think i'm the person to rewrite the explanation... Noobgeek (talk) 16:59, 15 May 2014 (UTC)

That physics are easy to understand:
  • At the train you just see the Earth is moving.
  • On Earth surface you just see that the train is moving.
Just get into the train without any knowledge about the size or mass of the Earth, and you just see the surface beyond you is moving. I will try to enhance the explain on this matter. --Dgbrt (talk) 21:39, 15 May 2014 (UTC)
Hi there! You're right that you can often exclude information and be left with various "relativities". But (to give only one argument) the comic explicitly describes the entire planet being grabbed and rotated to a new orientation. Mountains and all :^) Noobgeek (talk) 15:07, 16 May 2014 (UTC)

It's fun to play with the physics, but the explanation is just wrong re the comic expressing a valid perspective in physics, most directly for the reason supplied by the title text: Multiple trains and elevators are moving relative to each other, so their mode of operation can't be to stay still and shift the earth. (The point of the title text might be to make the false-physics joke extra clear.) I'm going to rewrite the explanation if no one else does -- but this will be my first big edit, so before i do, to be a polite noob, i'm requesting objections. Noobgeek (talk) 19:58, 23 May 2014 (UTC)

hey cool, i live in oxford Beanie talk 11:24, 1 July 2021 (UTC)