explain xkcd - User contributions [en]

1489: Fundamental Forces

2015-02-20T05:45:55Z

Homunq: Explanation and begin transcript. Written on tablet.

{{comic
| number = 1489
| date = February 20, 2015
| title = Fundamental Forces
| image = fundamental_forces.png
| titletext = "Of these four forces, there's one we don't really understand." "Is it the weak force or the strong--" "It's gravity."
}}

==Explanation==
Cueball is acting here as someone teaching physics at a basic level, perhaps a high school science teacher. As is typical for someone in that position, he understands the general idea of the four fundamental forces, but his understanding gets progressively more sketchy about the details. The off-panel audience, probably a student or class, is interested, but quickly begins to realize Cueball's lack of understanding. Instead of acknowledging the problem directly, Cueball simply blusters onwards.

The title text refers to the fact that it is gravity, which appears to be the simplest and easiest to understand of the four forces when first introduced, which turns out to be the hardest to reconcile with a coherent (quantum) understanding of all four forces together.

==Transcript==
Frame 1:

Cueball: There are four fundamental forces between particles. (1) Gravity, which obeys the inverse square law, F_gravity=G(m_1 m_2/d^2)
Off panel audience: OK...

Frame 2:
Cueball: (2) Electromagnetism
Off panel audience:
Frame 3:
Cueball: (2) Electromagnetism
Off panel audience:
Frame 4:
Cueball: (2) Electromagnetism
Off panel audience:

{{comic discussion}}

1301: File Extensions

2013-12-09T12:45:12Z

Homunq:

{{comic
| number = 1301
| date = December 9, 2013
| title = File Extensions
| image = file_extensions.png
| titletext = I have never been lied to by data in a .txt file which has been hand-aligned.
}}

==Explanation==
{{incomplete|Needs more detail}}

The form of presentation can say a lot about the content. {{w|File extension}}s are a quick way of checking the type of a file, and this comic comments on how file extensions can tell us a few things things about the contents.

* {{w|.tex}} is for {{w|TeX}} and {{w|LaTeX}} source files; it's often used by academics. .tex means serious business.
* {{w|.pdf}} is a document format by Adobe, frequently used for publication — a final product, a polished work.
* {{w|.csv}} is just a bunch of data delimited by commas, probably computer-generated and containing raw data (from, say, a scientific experiment).
* {{w|.txt}} is a plain text file; it's usually programmers who use these (e.g. README files).
* {{w|.svg}} is a vector graphics format used a lot for diagrams, such as on Wikipedia.
* {{w|.xls}} and {{w|.xlsx}} are Excel spreadsheets. Mostly these contain a mix of raw data (as with .csv) and calculations using that data. Excel is part of the Microsoft Office bundle.
* {{w|.doc}} is a rich-text document format, used by Word as part of the Microsoft Office bundle. Any clown with Windows could easily make one of these, which is probably why Randall doesn't trust it much.
* {{w|.png}} is a bitmap image format designed for the Internet. It is enjoying wide popularity for providing crisp, full-color images with lossless (invisible) compression.
* .ppt refers to a Microsoft Office Powerpoint file.
* {{w|.jpg}} or {{w|jpeg}} is a bitmap image format, excellent for storing photos, but not too good for most other things. This file format is prone to annoying compression artifacts; storing numerical or textual information in a JPEG file is typically a bad idea. Digital cameras use .jpg, so .jpeg means that the photo has been edited with a photo manipulation software or that the file doesn't contain a photo at all. Therefore, you cant't trust the content of a .jpeg file.
* {{w|.gif}} is a bitmap image format capable of short animations. It was once ''the'' Internet image file format until PNG gradually replaced it for many good reasons. It made a comeback in recent years, mostly for silly clips of cats falling into boxes. Since its the only common format for animated images, it's also the preferred format for blinking website ads that tell that you're the 100,000,000 visitor and have won a prize ([[570]] makes fun on this).

==Transcript==
:Trustworthiness of Information by File Extension
:[A bar graph charting this. No units or figures are given, but for ease of comprehension this transcript will arbitrarily designate the highest score as "+100"; subsequent scores are estimates based on the size of their bars.]
::.tex: +100
::.pdf: +89
::.csv: +85
::.txt: +67
::.svg: +65
::.xls/.xlsx: +49
::.doc: +21
::.png: +15
::.ppt: +14
::.jpg: +3
::.jpeg: -8
::.gif: -36

{{comic discussion}}
[[Category:Charts]]

1287: Puzzle

2013-11-06T08:28:57Z

Homunq: /* Explanation */

{{comic
| number = 1287
| date = November 6, 2013
| title = Puzzle
| image = puzzle.png
| titletext = Prediction for Carlsen v. Anand: ... 25. Qb8+ Nxb8 26. Rd8# f6 27. "... dude." Qf5 28. "The game is over, dude." Qxg5 29. Rxe8 0-1 30. "Dude, your move can't be '0-1'. Don't write that down." [Black flips board]
}}

==Explanation==
{{incomplete}}

The game of {{w|Go (game)|go}} (also called Weiqi or Baduk) is usually played on the 19x19 intersections of a grid, but sometimes a faster, simpler version is played on the 9x9 intersections of a grid (which thus has 8x8 squares, as a chessboard, though they are not colored in an alternating pattern). In the comic, white has chess figures and plays against black, which uses go stones.

White also, confusingly, has an extra (third) bishop.

{{w|Magnus Carlsen}} and {{w|Viswanathan Anand}} are professional chess players. The text is in the format of a game transcript, but black continues to make moves after white wins the game (checkmate is denoted by #). White eventually responds by taking black's king; black's "move" is to declare that he (black) won. The rest is clear.

==Transcript==
:[A game board with 8x8 white squares and black borders, like a goboard or an all white chessboard, there are white chess pieces on the bottom and five black go pieces on the edges in the center of the board.]
:White to continue insisting this is a chessboard

{{comic discussion}}
[[Category:Chess]]

Talk:1287: Puzzle

2013-11-06T08:28:05Z

Homunq:

Black thinks he's playing Go and white thinks he's playing chess. Although a 7 x 7 board is a bit small for go, it is not unusual for a beginner to play on such a board {{unsigned|hax}}

It is a 9x9 go board! (usually used for learning, as its smaller, less strategic, and quicker to finish game, whereas regular go is played on 19x19 intersections). Olivier.

::You beat me to it. "Less strategic" also means "more tactical". In my experience, 9x9 boards are rare (mostly, people would just use part of a 19x19 board), but when they do exist, they have 4 handicap intersections marked with dots. [[User:Homunq|Homunq]] ([[User talk:Homunq|talk]]) 08:28, 6 November 2013 (UTC)

1287: Puzzle

2013-11-06T08:19:08Z

Homunq: /* Explanation */

{{comic
| number = 1287
| date = November 6, 2013
| title = Puzzle
| image = puzzle.png
| titletext = Prediction for Carlsen v. Anand: ... 25. Qb8+ Nxb8 26. Rd8# f6 27. "... dude." Qf5 28. "The game is over, dude." Qxg5 29. Rxe8 0-1 30. "Dude, your move can't be '0-1'. Don't write that down." [Black flips board]
}}

==Explanation==
{{incomplete}}

The game of [http://en.wikipedia.org/wiki/Go (game)|go] (also called Weixi or Baduk) is usually played on the 19x19 intersections of a grid, but sometimes a faster, simpler version is played on the 9x9 intersections of a grid (which thus has 8x8 squares, as a chessboard, though they are not colored in an alternating pattern). In the comic, white has chess figures and plays against black, which uses go stones.

White also, confusingly, has an extra (third) bishop.

[http://en.wikipedia.org/wiki/Magnus_Carlsen|Magnus Carlsen] and [http://en.wikipedia.org/wiki/Viswanathan_Anand|Viswanathan Anand] are professional chess players. The text is in the format of a game transcript, but black continues to make moves after white wins the game (checkmate is denoted by #). White eventually responds by taking black's king; black's "move" is to declare that he (black) won. The rest is clear.

==Transcript==
{{incomplete transcript}}

{{comic discussion}}
[[Category:Chess]]

1287: Puzzle

2013-11-06T08:18:09Z

Homunq: /* Explanation */

{{comic
| number = 1287
| date = November 6, 2013
| title = Puzzle
| image = puzzle.png
| titletext = Prediction for Carlsen v. Anand: ... 25. Qb8+ Nxb8 26. Rd8# f6 27. "... dude." Qf5 28. "The game is over, dude." Qxg5 29. Rxe8 0-1 30. "Dude, your move can't be '0-1'. Don't write that down." [Black flips board]
}}

==Explanation==
{{incomplete}}

The game of [http://en.wikipedia.org/wiki/Go (game)|go] (also called Weixi or Baduk) is usually played on the 19x19 intersections of a grid, but sometimes played on the 9x9 intersections of a grid (which thus has 8x8 squares, as a chessboard, though they are not colored in an alternating pattern). In the comic, white has chess figures and plays against black, which uses go stones.

White also, confusingly, has an extra (third) bishop.

[http://en.wikipedia.org/wiki/Magnus_Carlsen|Magnus Carlsen] and [http://en.wikipedia.org/wiki/Viswanathan_Anand|Viswanathan Anand] are professional chess players. The text is in the format of a game transcript, but black continues to make moves after white wins the game (checkmate is denoted by #). White eventually responds by taking black's king; black's "move" is to declare that he (black) won. The rest is clear.

==Transcript==
{{incomplete transcript}}

{{comic discussion}}
[[Category:Chess]]

Talk:1276: Angular Size

2013-10-12T14:11:55Z

Homunq:

What is the meaning of "football field" in panel #2? --[[User:Kevang|Kevang]] ([[User talk:Kevang|talk]]) 04:50, 11 October 2013 (UTC)
:I was wondering the same thing. Probably misplaced text. [[User:Irino.|Irino.]] ([[User talk:Irino.|talk]]) 05:49, 11 October 2013 (UTC)
::It does seem to be misplaced, but if that's the only glitch, this is the only panel without a unique reference object. "20 football pitches long" isn't all that easy to grasp. [[User:Jameslucas|jameslucas]] ([[User talk:Jameslucas|" "]] / [[Special:Contributions/Jameslucas|+]]) 09:09, 11 October 2013 (UTC)
:::The image is fixed by Randall. I did an update here.--[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 11:28, 11 October 2013 (UTC)
::::Well, that's a letdown. I'm surprised Randall didn't use Heathrow. [[User:Jameslucas|jameslucas]] ([[User talk:Jameslucas|" "]] / [[Special:Contributions/Jameslucas|+]]) 13:42, 11 October 2013 (UTC)

I haven't done any lookups or maths to check these, but give the size of these as "stars" in the sky, everything from panel 2 onwards seems to me to be an order of magnitude or two too large. [[User:Markhurd|Mark Hurd]] ([[User talk:Markhurd|talk]]) 05:17, 11 October 2013 (UTC)
: Not really. You see the stars and planets as points because their angular size is lower than your eyes' resolution. They have measurable (or, in case of really distant or small objects, computable) angular sizes. For stars etc. these angular sizes are really small - but Earth is quite big, so if you cut a portion of a sphere the radius of Earth corresponding to these small solid angles, you get sizable areas. I haven't checked Randall's math, but I'd rather believe his results. If it is non-intuitive for you consider the Sun and Moon example - when observed by naked eye, the Moon looks for you as being the size of a dime held up in your hand - and yet it's shadow during an eclipse covers quite an area of Earth's surface. It is true that sizes of some of these "footprints" are quite surprising compared to other ones. [[Special:Contributions/89.174.214.74|89.174.214.74]] 08:55, 11 October 2013 (UTC)
:: Definitely surprising. I'll put faith in Randall doing his math correctly, but still needed to check on a couple of these because they did elicit a "What? No. Really? Can't be." reaction. Using the formula described in the Explanation above, for Venus I get 12742 km (Earth radius) * 12104 km (Venus diameter) / 38000000 (shortest distance to Venus) = 2.03 km.
:: Hard to picture that something that is such a small dot in the sky is actually directly over such a large patch of ground. But there you are.
[[Special:Contributions/67.51.59.66|67.51.59.66]] 17:11, 11 October 2013 (UTC)
::: The question that sprung to my mind was, ''which'' distance is he using for the planets and asteroids, since those vary hugely depending on where objects are relative to each other along their orbits. Is he going with closest approach, maybe? Or the distance that we happen to be at just this instant? --[[User:Rmharman|Rmharman]] ([[User talk:Rmharman|talk]]) 21:42, 11 October 2013 (UTC)

Does someone know how to use LaTeX formulas? And if so, can they translate my formula into something more pleasing to the eye? [[User:Irino.|Irino.]] ([[User talk:Irino.|talk]]) 05:49, 11 October 2013 (UTC)

According to the wikipedia page, the M25 is 117 miles long. That sounds more like "37 miles across" to me. [[User:Kaa-ching|Kaa-ching]] ([[User talk:Kaa-ching|talk]]) 08:46, 11 October 2013 (UTC)
: it originally stated 15 miles, someone has fixed it now. Thanks! [[User:Kaa-ching|Kaa-ching]] ([[User talk:Kaa-ching|talk]]) 11:35, 11 October 2013 (UTC)

Neither the sun or moon, nor Messier 25 (declination -19°) can ever culminate in the zenith over London. :-( Admittedly, Townsville, Australia would be sort of overwhelmed by M25. --[[Special:Contributions/129.13.72.198|129.13.72.198]] 11:27, 11 October 2013 (UTC)
: M25 is a reference to the highway that surrounds London, not the Messier object, which is probably nowhere near the angular size of the moon. [[Special:Contributions/65.129.214.100|65.129.214.100]] 15:17, 11 October 2013 (UTC)
Does anyone know why the exoplanet "HD 189733 b" is labled as "Permadeath" ? Same question for the other weird names in the same pannel (the "tilde on keyboard" one) ? [[User:Jahvascriptmaniac|Jahvascriptmaniac]] ([[User talk:Jahvascriptmaniac|talk]]) 11:32, 11 October 2013 (UTC)
: A reference to [[1253|Exoplanet Names]]. [[User:Squornshellous Beta|Squornshellous Beta]] ([[User talk:Squornshellous Beta|talk]]) 12:08, 11 October 2013 (UTC)

If you were looking from the center of the earth, as the situation suggests, wouldn't the M25 be reversed, east-to-west, as you look at the sun and the moon?--[[Special:Contributions/76.105.133.220|76.105.133.220]] 16:09, 11 October 2013 (UTC)
: I visualize it as looking down on Earth, with the "shadow" of the celestial object on top of the M25/soccer field/laptop/etc. [[Special:Contributions/67.51.59.66|67.51.59.66]] 17:02, 11 October 2013 (UTC)

Has anyone noticed that Voyager 1 and 2 look like viruses? Kind of funny considering they're next to E. Coli...
[[User:StelarCF|StelarCF]] ([[User talk:StelarCF|talk]]) 17:54, 11 October 2013 (UTC)

: Yeah, I thought that too. It's a happy thought. Why, you ask? Well, with vastly diminished (or - in the course of time - zero) output from RTG power sources, they're like weakened (or inactivated) viruses - that we've sent out to the rest of the Universe, to any other intelligent lifeforms that may find them. What does that remind you of?

: VACCINES! We've vaccinated aliens to the human condition :D To the Earthly condition even.

: [PS - that's a happy thought because I choose to interpret from a cross-contamination standpoint. Which, in this case, allows them to observe us in our own locale, and establish our intrinsic nature - before a two-way interaction with us, in 'shared space' :P, and observing us through the medium of those interactions.] [[Special:Contributions/220.224.246.97|220.224.246.97]] 18:46, 11 October 2013 (UTC)

Does "Explain XKCD" installation of MediaWiki has [http://www.mathjax.org/ MathJax] [plugin] installed for writing mathematics formulas? --[[User:JakubNarebski|JakubNarebski]] ([[User talk:JakubNarebski|talk]]) 08:15, 12 October 2013 (UTC)

Too bad he didn't do the Pleiades. I mean, instead of using the Vatican, he could have used something geeky: Bletchley Park or something (though that's probably not big enough). [[User:Homunq|Homunq]] ([[User talk:Homunq|talk]]) 14:11, 12 October 2013 (UTC)

1276: Angular Size

2013-10-12T14:02:09Z

Homunq: /* Explanation */

{{comic
| number = 1276
| date = October 11, 2013
| title = Angular Size
| image = angular size.png
| titletext = If the celestial sphere were mapped to the Earth's surface, astronomy would get a LOT easier; you'd just need a magnifying glass.
}}

==Explanation==
{{incomplete}}
This comic is a comparison of the {{w|angular diameter|angular diameters}} (or apparent diameter) of various of celestial objects at the surface of the earth relative to a vertex at the center of the Earth as diagrammed in the opening panel. The objects' scales are compared to actual objects on earth for comparison. Each size given is for the object at its closest approach to earth.

London's {{w|M25 motorway}} is around 60 km (35 miles) across, a {{w|soccer field}} is about 100 meters long, a {{w|Table tennis table|ping pong table}} is 274 centimeters long, a {{w|laptop}} is about 35 centimeters across, the {{w|tilde}} symbol on a keyboard is about 5 millimeters long, and a cell of ''{{w|Escherichia coli|E. coli}}'' is about 2 micrometers long.

A simple {{w|Intercept theorem|formula}} can be used to find the size on earth of a celestial object when the size of and distance to the object is known. This is done by taking the radius of the earth, multiplying by the diameter of the object, and dividing by the distance to the object from the center of the earth.

The space objects referenced in the panels are:

* The {{w|Sun}} and the {{w|Moon}} have approximately the same size (around 0.5 degrees of arc) when seen from the Earth.

* {{w|Mercury (planet)|Mercury}}, {{w|Venus}}, {{w|Mars}}, {{w|Jupiter}}, {{w|Saturn}}, {{w|Uranus}}, and {{w|Neptune}} are the other planets of the {{w|Solar System}}.

* {{w|Io (moon)|Io}}, {{w|Europa (moon)|Europa}}, {{w|Ganymede (moon)|Ganymede}}, and {{w|Callisto (moon)|Callisto}} are the main moons of Jupiter; {{w|Titan (moon)|Titan}} is the largest moon of Saturn; and {{w|Triton (moon)|Triton}} is the largest moon of Neptune. {{w|Ceres (dwarf planet)|Ceres}} and {{w|Pluto}} are {{w|dwarf planet}}s.

* {{w|Phobos (moon)|Phobos}} and {{w|Deimos (moon)|Deimos}} are the moons of Mars. {{w|Eris (dwarf planet)|Eris}} is another dwarf planet. {{w|R Doradus}} and {{w|Betelgeuse}} are giant stars, respectively around 180 and 640 light-years away. R Doradus is the star with the largest apparent diameter (other than the sun, of course).

* {{w|4942 Munroe}} is an asteroid [http://blog.xkcd.com/2013/09/30/asteroid-4942-munroe/ named] after xkcd author [[Randall]] Munroe. {{w|Proxima Centauri}}, {{W|Alpha Centauri|Alpha Centauri AB}}, {{w|Barnard's star}} and {{w|Sirius}} are nearby stars (all within 10 light-years from the Sun).

* {{w|HD 189733 b}}, {{w|Gliese 581 b}}, {{w|Gliese 667 Cc}}, {{w|82 G. Eridani#Planetary_system|HD 20794 c}}, {{w|Tau Ceti#Planets|Tau Ceti c}}, and {{w|KOI-1686.01}} are {{w|extrasolar planet}}s; the parenthetical names are references to the comic [[1253: Exoplanet Names]]. However, some of the planets' parenthetical names do not match the table in the previous comic. For example, HD 20794 c is called "Legoland" rather than "Moonchild" in [[1253: Exoplanet Names]].

* {{w|Voyager 1}} and {{w|Voyager 2|2}} are space probes launched in 1977, and currently around 125 and 100 {{w|astronomical unit}}s away, respectively.

The alt text, about astronomy being easier if the celestial sphere were mapped to the earth, should come as no surprise; essentially, it's equivalent to asking "what if the earth were a giant telescope." That is, although there's no actual optical setup which would enable such a mapping, an earth so mapped would be functioning as a giant telescope.

==Transcript==
{{incomplete transcript}}

{{comic discussion}}
[[Category:Math]]
[[Category:Astronomy]]