explain xkcd - User contributions [en]

1366: Train

2015-10-21T04:12:28Z

188.114.106.89: /* Explanation */

{{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==
This comic, which appeared the day before {{w|National Train Day}}, plays on the fact that a choice of a {{w|Inertial frame of reference|reference frame}} is arbitrary, leading to the {{w|Principle of relativity|"Principle of relativity"}} 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 — though in practice different trains often move in mutually-exclusive directions, thus each train would have to define its own frame of reference. There is a quotation, attributed to Einstein, that he once asked a ticket collector, "What time does Oxford stop at this train?"

Changing the reference frame into the inside of the train only means that you see the outside world in a different reference, since the train doesn't really move the Earth, it simply appears that way from the inside.

From the Newtonian perspective this choice of frame is valid, but results in unnecessarily complicated math; the equation of motion would include terms for centrifugal, Coriolis and other so-called "fictional forces" see [[123: Centrifugal Force]]. {{w|Isaac Newton|Newton}} supposes the existence of "inertial frames", in which these forces are zero, and the surface of the Earth approximates an inertial frame well. In General Relativity, the presence of mass in a system curves the {{w|spacetime}} around of it. The train-earth system could be modelled in general relativity, taking the train as fixed. However the resulting equations would be complex, and not amenable to an exact solution.

The title text expands on this to include elevators, which change a person's position relative to the center of the Earth. From a passenger's perspective, it would appear as though the Earth's position was instead being changed in space.

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 (1,674 km/h or 1,040 mph)
*The speed of the train (on earth) relative to the sun: 30 km/s (108,000 km/h or 67,000 mph)
*The speed of the train (on earth) relative the center of our galaxy: 220 km/s (828,000 km/h or 514,000 mph)

The train, as seen from an inertial frame, doesn't seem to rotate the earth, but it does in fact 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]]).

==Transcript==
:'''Train:'''
:[On the upper edge of a circle representing the Earth, Cueball is in a train car looking to his left.]
: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.]
:[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

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Physics]]

Talk:1581: Birthday

2015-09-25T04:56:29Z

188.114.106.89:

http://www.cnbc.com/2015/09/23/happy-birthday-song-now-in-public-domain.html [[User:Mwburden|mwburden]] ([[User talk:Mwburden|talk]]) 11:09, 23 September 2015 (UTC)

https://ia601904.us.archive.org/13/items/gov.uscourts.cacd.564772/gov.uscourts.cacd.564772.docket.html [[Special:Contributions/162.158.92.245|162.158.92.245]] 11:26, 23 September 2015 (UTC)

Why is there a <nowiki><code></nowiki> box around the transcript? {{User:17jiangz1/signature|12:51, 23 September 2015}}
:Because someone wrote the text with a space between each line instead of beginning each line with ":"
Like this
:Instead of like this --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 13:29, 23 September 2015 (UTC)

There are nine musical notes, not six.
[[Special:Contributions/173.245.50.92|173.245.50.92]] 13:40, 23 September 2015 (UTC)
:There are six musical notes (in pictures, two connected eighth notes are one note picture), not nine. [[Special:Contributions/188.114.106.89|188.114.106.89]] 04:56, 25 September 2015 (UTC)

"Some might claim" seems wishy-washy to me. Perhaps it would be better to say "calling the cops in such situations is neither socially appropriate nor beneficial: this is the source of the humor in this comment." [[User:Djbrasier|Djbrasier]] ([[User talk:Djbrasier|talk]]) 14:15, 23 September 2015 (UTC)

This appears to be one of the few comics with <em> both </em> Black Hat and White Hat. [[Special:Contributions/199.27.133.116|199.27.133.116]] 15:47, 23 September 2015 (UTC)

I don't have a sub to law360, nor do I wish to bother giving them my throwaway email. What was the basis of the ruling? Why is this public domain now? {{unsigned|NotLock}}
:It's not, text updated. [[Special:Contributions/108.162.250.161|108.162.250.161]] 06:39, 24 September 2015 (UTC)

The third line implies that "xkcd" should be pronounced as having two syllables. {{unsigned ip|108.162.250.163}}

:Clearly you have never heard "Happy Birthday" sung to someone with a really long name. It doesn't matter if it's two syllables, you just stretch it out. [[Special:Contributions/173.245.48.123|173.245.48.123]] 04:08, 24 September 2015 (UTC)

:The song can accommodate names of different lengths, consider the following https://youtu.be/vWs3035D69k?t=1m23s [[Special:Contributions/108.162.250.180|108.162.250.180]] 08:45, 24 September 2015 (UTC)

AFAIK The ruling only applies to the lyrics, they still have a valid copyright to the music, so if you sing the song you had better make sure it's to a suitable tune that is out of copyright! {{unsigned ip|141.101.99.80}}

:it was ruled that their copyright only covers a specific piano arrangement which is not the tune in use today, so sing your heart out: [[Special:Contributions/173.245.54.7|173.245.54.7]] 14:00, 24 September 2015 (UTC)

One in 366 xkcd fans turned to the page today and initially thought that Randall had used a code not dissimilar to what Google uses to change the banner to a birthday greeting on the users' birthday. I was one of those fans... :-) [[Special:Contributions/141.101.99.23|141.101.99.23]] 09:33, 24 September 2015 (UTC)
:You mean one in 365,25... ;-) Happy Birthday with yesterday! --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 11:37, 24 September 2015 (UTC)

On the cake most of the characters are separated by space and a dot, except beret guy and cueball who are separated by space and a heart, any thoughts? --Cris {{unsigned ip|108.162.241.195}}

:the explanation says that is Rob to the right of the heart, and that Cueball is in the top layer. Personally I can't tell the difference between those two stick figures in this cartoon. --Martin {{unsigned ip|108.162.249.183}}

:Heart in the middle of the cake! [[Special:Contributions/188.114.106.89|188.114.106.89]] 04:56, 25 September 2015 (UTC)

Talk:1537: Types

2015-06-13T03:35:44Z

188.114.106.89: Adding to the debate between theories about a language joke or a programming joke on line [10].

Relevant: WAT talk https://www.destroyallsoftware.com/talks/wat {{unsigned ip|‎108.162.254.108}}

Are (6) and (7) about completing sequences?

If the sequence was [1, 2, 3, ?] we would expect the ? to be a placeholder for 4. So [1, 2, 3]+2 is wrong := FALSE. But [1, 2, 3]+4 is correct := TRUE. {{unsigned ip|141.101.99.22}}

"+2 appears to be applying a unary + to the number 2" : or it adds the number of the line, 10, to 2 => 12. Also, the eleventh line, "2+2" may add 2 to all the following 2, explaining line 12. (that theory is from a friend of mine) [[User:Seipas|Seipas]] ([[User talk:Seipas|talk]]) 12:17, 12 June 2015 (UTC)
: Also, for the lines 6 and 7, the operation "[1,2,3]+x" may add x to the set [1,2,3] and return true if the operation succeeded or false if not. Adding 2 to the set [1,2,3] returns false because 2 is already in [1,2,3]. [[User:Seipas|Seipas]] ([[User talk:Seipas|talk]]) 12:23, 12 June 2015 (UTC)

Yellowish Blue: http://www.livescience.com/17948-red-green-blue-yellow-stunning-colors.html is NaN! {{unsigned ip|108.162.221.129}}

''"The ironic thing is that fractions with 2 in the nominator are not the kind of numbers that typically suffer from floating point impreciseness."''
- This is not technically correct. Should read "fractions with 'power of 2' in the '''de'''nominator. However, the 3/2 would cause precision errors. {{unsigned ip|108.162.221.129}}
: I don't know proper English wording for things, but 3/2=3*2^-1, so it would be represented exactly under IEEE-754 too. [[Special:Contributions/141.101.89.217|141.101.89.217]] 13:58, 12 June 2015 (UTC)

Is there more to this comic, a fixed set of rules that can tie all the examples together, or does each line make its own joke independently? [[Special:Contributions/108.162.219.5|108.162.219.5]] 12:54, 12 June 2015 (UTC)

;"normally"

<blockquote>This would make sense if it was <code>[] + 2</code></blockquote>

It really wouldn't. Javascript returns <code>"2"</code> (god knows why) and Python gives an error. Don't really feel like testing many other languages, but I also think it's not really a logical assumption to make at all. Can't think of a reason for <code>[] + 2</code> to return <code>[2]</code>... ever. It ''might'' make a little bit of sense in Randall's oddly typed language, but not in any sane one. --[[User:TotempaaltJ|TotempaaltJ]] ([[User talk:TotempaaltJ|talk]]) 12:35, 12 June 2015 (UTC)

: Javascript first converts <code>[]</code> (the empty array) to the empty string (using the rule "stringify each element and join with a comma"), then treats the operation as <code>"" + 2</code>, which results in conversion of the other operand to string and then concatenation. [[Special:Contributions/141.101.97.214|141.101.97.214]] 12:46, 12 June 2015 (UTC)

line 4: asci code of N + 2 = asci code of P [[User:SirKitKat|sirKitKat]] ([[User talk:SirKitKat|talk]]) 13:07, 12 June 2015 (UTC)

My favourite xkcd in a while. =8o) Of the list I got a good laugh out of numbers 8 and 13. [[User:Jarod997|Jarod997]] ([[User talk:Jarod997|talk]]) 13:11, 12 June 2015 (UTC)

I think a lot of this is his joke about programming languages loving the number 4. 2 + "2" = "4", [1,2,3] + 4 = true, 2+2 = DONE, and the range one all seem to support this. Also reminds me of this: http://xkcd.com/221/ {{unsigned ip|173.245.52.112}}

Why isn't yellowish blue just green? [[User:Djbrasier|Djbrasier]] ([[User talk:Djbrasier|talk]]) 16:18, 12 June 2015 (UTC)
:Because yellow and blue don't make green. [[Special:Contributions/108.162.237.158|108.162.237.158]] 23:33, 12 June 2015 (UTC)
::It does with my paint kit. Isn't that subtractive mixing. I feel like I've just traveled to a version of [[1268: Alternate Universe]], except I'm the only one here who went to kindergarten. What am I missing? [[User:Djbrasier|Djbrasier]] ([[User talk:Djbrasier|talk]]) 02:28, 13 June 2015 (UTC)
::The only {{w|color wheel}} I know has purple (not blue) opposite yellow and orange (not yellow) opposite blue. If that is incorrect, then wikipedia needs some serious editing. [[User:Djbrasier|Djbrasier]] ([[User talk:Djbrasier|talk]]) 02:31, 13 June 2015 (UTC)

line 4: I read NaP as Not a Problem. [[Special:Contributions/141.101.104.12|141.101.104.12]] 17:00, 12 June 2015 (UTC)
: So did I. [[User:Xynariz|Xynariz]] ([[User talk:Xynariz|talk]]) 23:12, 12 June 2015 (UTC)

Line 3 is missing its prompt. There does not appear to be any relevance to the joke, nor has anyone yet explained why it should be missing. Typo? [[Special:Contributions/108.162.221.183|108.162.221.183]] 17:10, 12 June 2015 (UTC)

Note that some programming languages avoid the problem of overloaded '+' operator between operands of vividly different types by using other symbols for string concatenation (be it "a"~"b" or "a"."b") and numerical addition. The real WTF is abusing '+' for string concatenation, which has very different properties from numerical addition, not being symmetrical for example: concat("aa", "bb") == "aabb", while concat("bb", "aa") == "bbaa" != "aabb". --[[User:JakubNarebski|JakubNarebski]] ([[User talk:JakubNarebski|talk]]) 17:38, 12 June 2015 (UTC)

Series of comics? I don't recall any others about Randall's new programming language... [[Special:Contributions/141.101.98.29|141.101.98.29]] 19:13, 12 June 2015 (UTC)

;+2
I think this is a japanese language joke. The + sign can also refer to the kanji 十, which is 10 in japanese. This would explain the result being twelve. 十二, or 10 2, is twelve in japanese. {{unsigned|Rafaeladson}}

I think number 5 is an escaped quote (two consecutive double quotes yields one double quote), a plus sign, and another escaped quote. The result is shown with an alternate form of escaped quotes (the apostrophe and double quote can both be used to show a string). NSIS scripting language uses this notation.[[Special:Contributions/108.162.221.180|108.162.221.180]] 20:19, 12 June 2015 (UTC)

Clearly this is what the xkcd phone's OS is written in (with some help from StackOverflow) {{unsigned ip|162.158.68.113}}

Great job at explaining the outputs. I clearly would have missed some interpretations without your insights. [[Special:Contributions/108.162.254.146|108.162.254.146]] 21:10, 12 June 2015 (UTC)

The joke on line [10] really doesn't seem to be a Chinese/Japanese language joke. We can see that the language interacts much more directly with line numbers from the inter-line joke between lines [11] and [14], where line [12] becomes [14] because the value of 2 has become 4. This is provable by observing that the line after [14] is [13], showing that the previous line really is still line [12], it simply displays as [14] because the value of 2 has changed. This absurdly direct interaction between the code and its line number makes the joke on line [10] make a lot more sense, as a Chinese/Japanese language joke here seems much too contrived and out-of-place considering the nature of the other jokes in the comic. Not to mention, if the joke on line [10] was really concerning the code's interaction with its line number, it would set up nicely for introducing the inter-line joke between lines [11] and [14].[[Special:Contributions/188.114.106.89|188.114.106.89]] 03:35, 13 June 2015 (UTC)