explain xkcd - User contributions [en]

1398: Snake Facts

2014-07-23T06:10:16Z

JB Gnome: changed second factoid interpretation

{{comic
| number = 1398
| date = July 23, 2014
| title = Snake Facts
| image = snake_facts.png
| titletext = Biologically speaking, what we call a 'snake' is actually a human digestive tract which has escaped from its host.
}}

==Explanation==
The comic lists a few 'factoids' about snakes, ranging from the mildly informative to the strictly tongue-in-cheek. The first factoid references the hypothesis that snake venom was an evolutionary development of saliva that, over time, gradually became more toxic as snakes with saliva that was able to subdue their prey possessed an evolutionary advantage. It then posits that the entire evolutionary branch that developed in venomous snakes began with a snake whose mutation gave him a mouth that was 'slightly more gross than usual'.

The second factoid references the 'longest snake in the world', citing a South American habitat. Given the habitat listed, it is possible the comic is referring to the Green Anaconda (Eunectes murinus), but the Green Anaconda, while an impressive creature and one of the longest snakes in the world, is generally not as long as the Reticulated Python (Python reticulatus) of Southeast Asia, which is generally recognized as the world's longest snake, though not its largest by mass. The factoid then states that the world's longest snake is 'believed to be over 60 years old'. As Anacondas generally do not live beyond 20 years in captivity, and likely less in the wild, the factoid appears to be in error once again. Its wording seems intended to imply that the snake in question is not a ''species'', but rather a ''single specimen''.

Additionally, the comic illustration accompanying the second factoid colors in a 'habitat range' on a map of South America that is snake-shaped, possibly implying that when it states 'The longest snake is found in Brazil, Chile, and Peru' that this snake is so long that it literally stretches from Brazil into Chile and Peru, and that the 'habitat' shaded on the map is, in fact, this mammoth snake's silhouette.

The final factoid is entirely tongue-in-cheek, beginning with a typical factoid trope in which a collection of related items are laid end-to-end, then the length is compared to something else in a way intended to surprise the reader that the collection of items is indeed, so extensive (literally and figuratively). For example, "If you laid all the veins and arteries in the human body end-to-end, they would stretch 60,000 miles". The third factoid takes the form of that trope and turns it into a joke, "If you laid all the bones in a snake end to end, you would have a snake." Obviously, you would not have an entire snake, literally, but you would have a skeleton that was recognizably that of a snake and could reasonably be referred to as 'a snake', albeit a skeletal one.

==Transcript==
{{incomplete transcript}}

{{comic discussion}}

1366: Train

2014-05-09T14:18:16Z

JB Gnome: Corrected misleading statement about accelerating trains being detectable.

{{comic
| number = 1366
| date = May 9, 2014
| title = Train
| image = train.png
| titletext = Trains rotate the Earth around various axes while elevators shift its position in space.
}}

==Explanation==
{{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|Sun}}, predefined {{w|Fixed_star|"fixed" stars}} or the {{w|Galactic_Center|center of our galaxy}}. Each of these would result in a completely different movement speed.

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/ 41: Go West] and [[162: Angular Momentum]]).

==Transcript==

TRAIN:

''A picture of a tiny planet (presumably meant to be Earth) with [[Cueball]] on the south, [[Hairy]] on the right, and [[Megan]] on the left. At the top is a train on tracks with another cueball in it.''

A machine that grabs the
Earth by metal rails and
rotates it until the part
you want is near you

{{comic discussion}}

Talk:1366: Train

2014-05-09T14:12:03Z

JB Gnome: correct for acceleration, due to general relativity

I just did an explanation from scratch for the first time, please could you tell me how I could improve it? Thanks :) [[User:Cheeselord99|Cheeselord99]] ([[User talk: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. [[Special:Contributions/108.162.221.5|108.162.221.5]] 07:11, 9 May 2014 (UTC)

:I created an account. The 108.162.221.5 ip address today is me, along with 108.162.221.53 today. [[User:Mikemk|Mikemk]] ([[User talk: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... [[Special:Contributions/141.101.89.211|141.101.89.211]] 10:05, 9 May 2014 (UTC)

:Explanation is good, but there are certainly related comics or maybe what-if ... I've found [http://what-if.xkcd.com/58/ Orbital Speed], but I think there were something mentioning how fast sun goes relatively to galaxy ... -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 10:14, 9 May 2014 (UTC)
::Found two related comics - any other? [[User:Condor70|Condor70]] ([[User talk: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

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). --[[User:RenniePet|RenniePet]] ([[User talk:RenniePet|talk]]) 13:24, 9 May 2014 (UTC)
: Second-last paragraph - my comment was written at the same time as another paragraph was added. --[[User:RenniePet|RenniePet]] ([[User talk: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).--[[User:Sturmonium|Sturmonium]] ([[User talk: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.--[[User:JB Gnome|JB Gnome]] ([[User talk:JB Gnome|talk]]) 14:12, 9 May 2014 (UTC)