explain xkcd - User contributions [en]

2622: Angular Diameter Turnaround

2022-05-22T18:53:44Z

NightWolf1223: Undo revision 273650 by 108.162.246.124 (talk)

{{comic
| number = 2622
| date = May 20, 2022
| title = Angular Diameter Turnaround
| image = angular_diameter_turnaround.png
| titletext = Thank you to Katie Mack for teaching me about this effect, and to Janelle Shane for describing redshifts as 'like galaxies sinking into a pool of dilute blood,' which is how I'll see them from now on.
}}

==Explanation==
{{incomplete|Created by a Galaxy-branded phone with minimal writing experience- Please change this comment when editing this page. Do NOT delete this tag too soon.}}
This comic references multiple physics and maths concepts, including {{w|Angular diameter}}, {{w|Angular diameter distance}}, {{w|Redshift}}, and {{w|phones|mobile phones}}, although mobile phones are not a core science at this time{{Citation needed}}.

The comic shows the galaxies of the universe as Samsung Galaxy mobile phones, pairing the age we see them at from earth, the degree they are redshifted, and how much of the sky they take up, known as their angular diameter. The mobile phones that are closer and older have depleted batteries, whereas the batteries are full for those phones from which the light is still only beginning to reach us. This is how galaxies appear in the sky, if they were phones that had batteries lasting billions of years, light reaching us from deeper into the past as it comes from objects that are farther away. Phones at a low battery may be a reference to [[1373: Screenshot]], where Randall commented that it is hard to pay attention to any phone with a low battery as the need to charge it is so urgent.

An important takeaway from this comic is that the events that occurred at the very start of our universe are etched in our sky ''as if they are still happening now'', in a detailed faint timeline, that we are still learning more and more from. Using the mobile phone metaphor helps as, when the technology space was young and smaller there were {{w|mobile phones}}, such as the original {{w|iPhone}}, which one might still remember despite there being many more recent and better ones in a more crowded marketspace.

Randall's intent appears to be to highlight how just a few very distant galaxies occupy incredibly large proportions of the sky, and are seen as they were at a very young age. Mobile phones have this similarity, of massive presence, relatively early stages of new technology, and bringing information from far away.

The large galaxies can be seen dark red in the background, as if the unimaginably ancient child galactic bodies are looming forebodingly behind everything else. The title text refers to galaxies falling into a pool of dilute blood as if the void beyond were hell. This completely misses the point as to what red shift is, being an effect on the wavelength of light, rather than light being filtered through regions of infernal suffering or far galaxies being stained by the blood of the enemies of Android phones.

This physical concept has a lot of juxtaposition of things that usually contradict, and Randall has put energy into attempting to highlight that.

Katie Mack tweet: https://twitter.com/AstroKatie/status/1516548836709343238

{{w|Spacetime diagram}} possibly has reasonable visualisations of the kinds of relations bodies have when they are moving this far apart, including angular diameter distance. Simultaneity no longer exists at such distances. Distance is debated too, although that would be a different article.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:Angular Diameter Turnaround
:Illustrated using phones instead of galaxies
:(Brightness and redshift adjusted to keep phones visible)
:Things that are far away look smaller, but things that are ''really'' far away look ''bigger'', because when their light was emitted, the universe was small and they were close to us.

{{comic discussion}}
[[Category:Astronomy]]
[[Category:Math]]
[[Category:Physics]]
[[Category:Smartphones]]
[[Category:Comics with color]]
[[Category:Comics featuring real people]] 
[[Category:Puns]]

2014: JWST Delays

2022-05-21T01:51:47Z

NightWolf1223: Undo revision 272244 by Explain xkcd server admin (talk)

<noinclude>:''"2014", this comic's number, redirects here. For the comic named "2014", see [[1311: 2014]].''</noinclude>
{{comic
| number = 2014
| date = July 2, 2018
| title = JWST Delays
| image = jwst_delays.png
| titletext = Since delays should get less likely closer to the launch, most astronomers in 2018 believed the expansion of the schedule was slowing, but by early 2020 new measurements indicated that it was actually accelerating.
}}

==Explanation==

The {{w|James Webb Space Telescope}} (JWST) is a {{w|space telescope}} created to be the successor of the {{w|Hubble Space Telescope}}.

The telescope has been in development since 1996, but has been plagued by numerous delays and cost overruns. This comic was likely inspired by the most recent [https://nasa.gov/press-release/nasa-completes-webb-telescope-review-commits-to-launch-in-early-2021 delay announcement], which was posted on June 27, 2018. At that time, the JWST was scheduled to launch on March 30, 2021.

* In July 2020, this was pushed back further to October 31, 2021 due to the coronavirus pandemic.
* In June 2021, it was announced that the launch day is likely slip to at least mid-November 2021.
* On September 8, 2021, ESA announced that the official planned launch date is [https://www.esa.int/Science_Exploration/Space_Science/Webb/Targeted_launch_date_for_Webb_18_December_2021 December 18, 2021.]
* On November 22, 2021, NASA announced that the official planned launch date was delayed by four days to December 22, 2021, following a problem encountered when mating JWST to its payload adapter. This date was referenced in [[2550: Webb]].
* On December 15, 2021, NASA announced that the official planned launch date was delayed by two days to December 24, 2021, following a [https://www.space.com/james-webb-space-telescope-launch-delay-december-24 communications issue between JWST and the launch vehicle]
* On December 21, 2021, NASA announced that the official planned launch date was delayed by one day to December 25, 2021 due to [https://www.nasa.gov/feature/james-webb-space-telescope-launch-update adverse weather at the launch site]
* On December 25, 2021, the telescope was successfully launched, which Randall anticipated with this comic: [[2559: December 25th Launch]].

This comic portrays the launch delays and the new predicted launch years and the times at which those predictions were made. There have been so many delays in this project that you can plot a line of best fit with a surprisingly high degree of accuracy. Randall says optimistically that the line’s slope is less than one (there is less than one year of ''new'' delay per year of elapsed time), implying, of course, that if events continue without further intervention, it will eventually be built, with a predicted date of late 2026.

The title text alludes to the famous research over the {{w|Accelerating expansion of the universe|universe’s accelerating expansion}}. The expansion had been predicted to be slowing due to gravity from everything in the universe; instead, it was found to be accelerating since about 5 billion years ago. Here, Randall looks at the apparently ever-delaying schedule and observes that the delay per time does not decrease, although the date gets nearer (which should help to schedule the launch date, as research and unknown parameters are replaced with engineering and exact predictions and measurements). However, this delay inflation contradicts Randall's usage of a linear trendline. Given the {{w|COVID-19 pandemic}} brought some additional delays in 2020 and 2021, the "early 2020" date was perhaps unintentionally prescient.

The Wikipedia article linked above includes a {{w|James Webb Space Telescope#Cost and schedule issues|table}} which provides the data points for the chart:

{| class=wikitable
! width=35 | Year !! Planned<br />launch !! Time left<br />(years)
|-
| 1997 || 2007 || 10
|-
| 1998 || 2007 || 9
|-
| 1999 || 2007 to 2008 || 8-9
|-
| 2000 || 2009 || 9
|-
| 2002 || 2010 || 8
|-
| 2003 || 2011 || 8
|-
| 2005 || 2013 || 8
|-
| 2006 || 2014 || 8
|-
| 2008 || 2014 || 6
|-
| 2010 || 2015 to 2016 || 5-6
|-
| 2011 || 2018 || 7
|-
| 2013 || 2018 || 5
|-
| 2017 || 2019 || 2
|-
|2018 || 2020 || 2
|-
|2018 || 2021 || 3
|-
|2020 || 2021 || 1
|-
|2021 || 2021 || 0
|}

==Transcript==
:[Top caption, in the panel:]
:James Webb Space Telescope
:[Subtitle of top caption:]
:Launch Delays

:[There is a positive-quadrant only line graph. The x- axis is labeled 'Current Date' and the y axis is labeled 'Planned Launch Date'. The dates on both of the axes range from 1995 to 2030.]
:[In the graph are 15 points, starting at (1997,2007) and extending at a slope of a little less than one. The most recent one is labeled 'Now: 2021'.]
:[There are two lines on the graph: a red one and a dashed black one. The red one is a regression of the points on the graph. It has a slope of about ⅔. The black one is a line with a slope of one. They intersect at the point (2026,2026), marked by the label 'Late 2026'?]

:[Caption below the panel:]
:Look, at least the slope is less than one.

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Astronomy]]
[[Category:Telescopes]]
[[Category:Line graphs]]
[[Category:Extrapolation]]

1064: Front Door

2022-05-20T22:46:19Z

NightWolf1223: Undo revision 271704 by Explain xkcd server admin (talk)

{{comic
| number = 1064
| date = June 4, 2012
| title = Front Door
| image = front_door.png
| titletext = FYI: I'll be releasing a wolf into a randomly-chosen front yard sometime in the next 30 years. Now your fear is reasonable, and you don't need to feel embarrassed anymore. Problem solved!
}}

==Explanation==
This comic is showing in graphical form three different actions/feelings as [[Randall]] is approaching his front door at night. His "Fear That There's Something Behind Me" rises, so then his "Forward Speed" rises to get to his door faster. And then when he gets in the door and finds that in fact, there is nothing behind him, his "Embarrassment" rises. The y-axis of the chart is not labeled, so it is presumed to indicate amount, as in how embarrassed Randall is.

The interesting fact revealed is that Randall gets more concerned that someone is behind him when he walks up the steps to this door, and not while walking in the yard where there might be more hiding places. The worst fear is just before he walks up the steps to lock himself in. This is probably because now that he is almost home and safe, he now has to fumble with the key concentrating on getting the key in the lock, thus not being able to pay attention to what's coming up behind him.

This is of course an irrational thought, but it seems to happen to many people, and has certainly been used for suspense in many movies, whether it is "Did I hear a noise upstairs?" or "Is that an axe murderer hiding in that hedge?" or "There might be a [[:Category:Velociraptors|velociraptor]] lurking nearby". (However in all of these cases getting inside would probably not help you anyway just adding to the funny part of how the fear drops once inside. See [[87: Velociraptors]] for why getting inside probably also would not help against them.)

The title text attempts to alleviate the problem of the following embarrassment by introducing potential threat that could affect anybody. Since Randall mentioned that he will randomly release a wolf into a front yard sometime in the next thirty years, there will always be a fear that a wolf has been released onto your front yard. In that case, the fear one feels will be justified and not embarrassing, as it is possible that there is indeed a wolf released by Randall into their front yard. However, even assuming that Randall can find and release a wolf onto someone's front yard, the chance of this happening to you is minuscule.

==Transcript==
:[A graph with three colored lines. There is no scale on the Y-axis but the X-axis represents geographic location and has four labels. From the Y-axis to around the midpoint is "Yard", with an arrow from the word yard pointing left, at a point beyond the midpoint there is a tick labeled "Steps", at a point after that is another tick labeled "Door", and all points afterward are "Inside" with an arrow pointing right from the word inside, there are no tick for the first and last label. The three lines are all labeled with a small lines going from the label to the lines. The blue line begins to slowly increase from the start, with a slight dip further into the yard, and a steep increase right before the steps, maxing on the steps, and declining steeply at the door, bottoming out once inside. The gray line is at a steady medium height until it gets to the steps, at which point it shoots upwards, and then slowly declines once inside. The red line stays at 0 until just before the steps, where it begins to trend upwards, spikes at the door, and begins to slowly decline once inside. Above the curves to the left there is a black frame with the title of the graph:]
:Walking Back to My Front Door at Night:
:X-axis labels: Yard Steps Door Inside
:Blue line: Fear That There's Something Behind Me
:Gray line: Forward Speed
:Red line: Embarrassment

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Line graphs]]
[[Category:Animals]] 
[[Category:Psychology]]

303: Compiling

2022-05-20T22:45:28Z

NightWolf1223: Undo revision 271706 by Explain xkcd server admin (talk)

{{comic
| number = 303
| date = August 15, 2007
| title = Compiling
| image = compiling.png
| titletext = 'Are you stealing those LCDs?' 'Yeah, but I'm doing it while my code compiles.'
}}

==Explanation==
Computer programming involves writing instructions for a computer to follow, in a specific {{w|programming language}}, which is largely human readable and writable, at least to programmers who understand that language. However, for the computers to follow instructions, they need to be given {{w|machine code}} — the actual "language" that computers "speak" and one that ''can'' be written directly with the correct tools, but would be too tedious and error-prone for just about any practical modern project where alternatives exist, where anything more than a {{w|%22Hello,_World!%22_program|Hello World}} could be awkward to implement straight into machine-code.

Conversion from the more conveniently human-writable code into {{w|computer-executable files}} is performed by {{w|Assembly language#Assembler|assemblers}}, {{w|Interpreter_(computing)|interpreters}}, or {{w|compilers}}.

Programs can be written in {{w|Assembly language|assembly code}}, which is basically just a set of mnemonics that make machine code much easier for a human to remember and correctly parse; the human-written assembly code is then run through a simple assembler to convert it directly into machine code. Assembly coding is necessary whenever one is programming for a completely new architecture (one for which no other tools yet exist), and is still used in some other situations (as it allows the code to be optimized more closely for the system on which it is to run than is possible with other types of coding), but is still fairly tedious and error-prone, and assembly code needs to be completely rewritten if one wants to port it to a computer with a different architecture.

Interpreters (e.g. that for {{w|PHP}} for one example) generally read through the code, or script, each line at a time as and when required, and has to do a lot of work with various processing overheads and the risk of hitting an invalid instruction or mistake in syntax that it can't handle. It also requires that a relevant version of the interpreter exist on any machine that has to run the script and perhaps some additional knowledge by the end-user.

For widely distributed (and especially commercial) programs, some form of compilation will instead be used. Compiling may have just one computer system read through the man-written code and (barring errors) produces the equivalent stand-alone and direct machine-readable code, suitable for a given range of computers. This process might involve several passes to check for 'obvious' errors in the code, as well as converting some programming concepts that are easiest for humans to understand into equivalent concepts that may be far easier for the computer to work with.

As such, compiling takes a certain amount of time at the time of production. Normally, this takes a few seconds, but, depending on the size of the project and the power of the computer doing the compilation, the time required to compile a program may measure in minutes, or even hours. As of 2015, the {{w|Linux Kernel}} contains over 19 million lines of code, arguably a massive job for any compiler, but if done correctly, it saves time for all the people who will ultimately be using its output.

Thus, when Cueball is caught wasting time at work, he argues that such activities are not worse than any other possible ones, at this moment. If his job is writing code and compiling it, then there may be nothing else that he ''can'' do right now. He cannot usefully tweak the code before it finishes compiling and the expected result checked.

The title text takes this a step further. Cueball claims that ''all'' activities are equally benign while the code is compiling — and that includes committing illegal acts, such as stealing {{w|LCD}}s.

Nine years after this comic was released, [[Randall]] made a comic called [[1755: Old Days]] about how compiling worked in the old days. It was Cueball who asked. The next comic after that, [[1756: I'm With Her]], was released the Monday before the {{w|2016 United States presidential election}}. And in that comic, a Cueball with a sword on an office chair like in this comic is featured. Seems realistic that Randall had that politically loaded comic ready for some time, and when finding and deciding to use this old version of Cueball, he may have gotten inspired by the compiling theme to make Old Days.

==Transcript==
:The #1 Programmer Excuse for Legitimately Slacking Off: "My code's compiling."
:[Two programmers are sword-fighting on office chairs in a hallway. An unseen manager calls them back to work through an open office door.]
:Manager: Hey! Get back to work!
:Cueball: Compiling!
:Manager: Oh. Carry on.

==Trivia==
*A T-shirt based on this comic is available in the [https://store.xkcd.com/products/compiling xkcd store].
*This comic is available as a signed print in the [https://store.xkcd.com/products/signed-prints xkcd store].

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Programming]]
[[Category:Comics with xkcd store products]]

2004: Sun and Earth

2022-05-20T22:44:55Z

NightWolf1223: Undo revision 271707 by Explain xkcd server admin (talk)

{{comic
| number = 2004
| date = June 8, 2018
| title = Sun and Earth
| image = sun_and_earth.png
| titletext = But we don't need to worry about the boiling masses sandwiching the thin layer in which we live, since we're so fragile and short-lived that it's unlikely to kill us before something else does! Wait, why doesn't that sound reassuring?
}}

==Explanation==
This is one of a number of comics which describe everyday events in unusual terms, making them sound really weird. In this case, both the Sun and the Earth are "massive convective systems [blasting] huge plumes of heat", which contrasts sharply with the daily idea of the Sun being a ball in the sky and the Earth the thing under our feet.

{{w|Free convection}} is based on an difference in density. What is colder is typically denser, so gravity forces it downwards, displacing what is hotter (and less dense) upward (This should not be confused with {{w|forced convection}}, which uses fans or other devices that are not practical to build on the scale of a planet). In the sun, most of the energy to drive this process comes from nuclear fusion, specifically the fusion of hydrogen into helium. We cannot directly see inside of the earth,{{Citation needed}} but its core is known to be much hotter than its surface.

The magnitude of these systems gives you an idea of the size of the fluctuations you can expect. The sun is very massive, meaning the fluctuations in its convective or heat-dissipating behavior are very large. This is an instance of the {{w|Fluctuation-dissipation_theorem|Fluctuation-Dissipation theorem}}. These fluctuations take the form of a solar flare, as explained below. For a more thorough (but non-technical) explanation of the role of gravity and entropy in such systems, see [https://arxiv.org/abs/0907.0659 this].

The {{w|Sun}} produces great amounts of light and heat and blasts it towards us, which is why we can live on Earth. Since Ludwig Boltzmann pointed out the fact in 1875, people have been working on establishing exactly how such far from equilibrium systems as life might depend upon, or be formed by ([https://www.quantamagazine.org/first-support-for-a-physics-theory-of-life-20170726/ like this article]), such massive entropy gradients as between the sun and earth (or rather the sun and empty space). Main sequence stars like the sun transport energy by {{w|Radiation_zone|radiation}} and by {{w|Convection_zone|convective currents}} of {{w|Plasma (physics)|plasma}}, bringing the heat generated in the core of the sun to its surface. These quickly moving charged particles create a massive magnetic field, which occasionally gets concentrated into a {{w|solar prominence}} which can snap, causing a large amount of charged particles to get shot into space as a {{w|solar flare}}. If the Earth happens to be in the direction of the solar flare, we can notice all sorts of interesting and often damaging effects. Thankfully, there are lots of other directions for the sun to shoot solar flares, so they don't come by the Earth that often.

The {{w|Earth#Internal_structure|Earth's interior}} is also very hot. {{w|Mantle convection}} causes {{w|plate tectonics}} which is the main cause of {{w|Volcano|volcanic activity}} (next to {{w|Mantle_plume|mantle plumes}}), which essentially also consists of huge blasts of heat.

This could sound like a very bad scenario, but the title text reminds us that the real scenario we live in is far worse, as we are not likely to die from a Sun blast or volcano eruption. In doing this, he indirectly points out the hard truth about our lives: that they're limited and they're short, and it is far easier to die of because of other things. In this way Randall attempts to give the reader an existential crisis; he concludes that his statement did not help to reassure himself.

This comic was likely inspired by the recent eruptions of the {{w|Kīlauea}} and {{w|Volcán de Fuego}}. In contrast, solar activity is currently low, because the {{w|Sunspot#Solar_cycle|sunspot solar cycle}} is in the low end of the 11-year cycle.

==Transcript==
:[The entire comic is within a panel. At the bottom of the image a curved shape depicting a small part of the Earth's surface and labeled as "Earth" is shown. At the top a similar sized shape but opposite curve is labeled as "Sun". The surface of the Sun is seething while on Earth's surface a few plants, two birds, and Cueball together with Megan are visible.]

:[Two arrows pointing to the Sun and the Earth with a caption applying to both of them:]
:Massive convective systems which occasionally blast huge plumes of heat at us without warning through mechanisms we can't directly observe and don't really understand.

:Cueball: What a nice day!

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Science]]
[[Category:Volcanoes]]
[[Category:Astronomy]]

109: Spoiler Alert

2022-05-16T23:04:09Z

NightWolf1223: Undo revision 269705 by The author of xkcd (talk) rv/v

{{comic
| number = 109
| date = May 31, 2006
| title = Spoiler Alert
| image = spoiler_alert.png
| titletext = And then it turns out they're both Tyler Durden.
}}

==Explanation==

This comic refers to several unexpected plot twists from various Hollywood movies and combines them into one giant twist invented by [[Randall Munroe|Randall]]. A "{{w|spoiler (media)|spoiler}}" is a term used to describe information about the plot of any media that could spoil the media for someone who has not viewed it. The term "spoiler alert" has become popularized as a warning to potential readers used to precede such spoilers, particularly in online posting. It is also a phrase often used ironically or angrily to suggest that something someone has just said is a spoiler. It is also used jokingly to suggest that something just said (or is about to be said) was a spoiler so long ago that "everyone" should have heard of it by now (e.g. "Spoiler alert, Vader is Luke's father").

{{w|Severus Snape}} is a character from J.K. Rowling's ''{{w|Harry Potter}}'' series of books.

{{w|Trinity (The Matrix)|Trinity}} is a character from ''{{w|The Matrix (franchise)|The Matrix}}'' trilogy of movies.

Rosebud is from the 1941 film ''{{w|Citizen Kane}}''.

From the title text, {{w|Fight Club (novel)#Tyler Durden|Tyler Durden}} is a character from the {{w|Fight Club (novel)|novel}} and movie ''{{w|Fight Club}}''.

All four references share the common ground that they are all involved in significant events or ideas in their respective movies that have been often spoiled by careless viewers for those who have not yet seen the movies. Here, the relevant events are mashed together into one and spoiled in one go.

=== Spoilers ===
In this comic, Snape is depicted knocking Trinity off a high place with a wooden sled named Rosebud.

* Snape {{Spoiler|kills someone important, by knocking them off a building.}}

* Trinity {{Spoiler|dies}}

* Rosebud is {{Spoiler|a sled}}

* Tyler Durden, mentioned in the title text, is {{Spoiler|both characters}}

As mentioned above, this is an amalgamation of four spoilers from four different stories:

* In {{w|Harry Potter}}, Snape (a professor at the Hogwarts school) {{Spoiler|kills Dumbledore (the headmaster) at the top of the Astronomy Tower}} in the penultimate book of the series, "{{w|Harry Potter and the Half-Blood Prince}}" (in the final book, we learn that it was {{Spoiler|part of Dumbledore's plan}}).

* Trinity (the female protagonist in {{w|The Matrix (franchise)|''The Matrix'' series}}) {{Spoiler|is killed}} in the third film, ''{{w|The Matrix Revolutions}}''.

* The central mystery of the classic film ''{{w|Citizen Kane}}'' is the meaning of "Rosebud" (the title character's last word), which is revealed at the end of the film to be {{Spoiler|the name of his childhood sled}}.

* The title text refers to the film ''{{w|Fight Club}}'', at the end of which it is revealed that the character played by {{w|Edward Norton}} {{Spoiler|is actually Tyler Durden (the name}} {{w|Brad Pitt}}'s {{Spoiler|character goes by); and Pitt's character is really just a figment of the Norton character's imagination; and that the viewer has been watching from Norton's point of view, seeing Pitt doing things Norton did not want to admit to himself he was capable of. In other words, the two roles are one and the same character. Near the end of the movie, Edward Norton kills Tyler Durden (his split personality) at the top of a skyscraper}}.

==Transcript==
:[At the tope of the panel there is a large heading:]
:'''Spoiler Alert!'''

:[Below that Severus Snape with long black hair is smacking a trenchcoat-clad Trinity off the top of a building with a brown sled. Below them is the following caption:]
:Snape kills Trinity with Rosebud!

{{comic discussion}}

[[Category:Comics with color]]
[[Category:The Matrix]]
[[Category:Harry Potter]]
[[Category:Fiction]]
[[Category:Fight Club]]