explain xkcd - User contributions [en]

3039: Human Altitude

2025-01-18T20:23:43Z

Prallax: noted the role of tall buildings

{{comic
| number = 3039
| date = January 17, 2025
| title = Human Altitude
| image = human_altitude_2x.png
| imagesize = 508x495px
| noexpand = true
| titletext = I wonder what surviving human held the record before balloons (excluding edge cases like jumping gaps on a mountain bridge). Probably it was someone falling from a cliff into snow or water, but maybe it involved something weird like a gunpowder explosion or volcano.
}}

==Explanation==
{{incomplete|Created after falling off a hot air balloon- Please change this comment when editing this page. Do NOT delete this tag too soon.}}
The comic purports to show the altitudes of humans over time, starting from a little after 1700. The conceit is that it indicates the ''single'' most altitudinous individual at any given time, so does not follow any particular person but would switch focus to whichever representative of humanity becomes "the highest up" (whether by rising above the previous leader, or by remaining high as the other loses their own elevation). There will necessarily be a degree of artistic interpretation and presumed trajectory of this particular marker, although the general trend of the line appears to be inspired by (some) actual factual realities. It uses a logarithmic vertical scale in order to indicate the finer details of 'low level' altitudes, yet fit the highest achievements onto the page. The measurements do not count altitude ''due'' to the ground beneath them, so a resident of {{w|Tibet}} or the {{w|Andes|high Peruvian Andes}} (for example) does not normally gain any particular advantage.

Prior to 1783, the {{w|Montgolfier brothers#Piloted flight, autumn_1783|first confirmed ascent}} of a human in a balloon, the line's high-points are indicated to be due to "various falls", i.e. a person who ''was'' on the top of a particularly high building/cliff/tree suddenly finding themselves (for an instant or two, at least) the person 'lucky' enough to be considered the furthest above the ground (it is at times like this that living at a higher absolute altitude ''might'' grant an 'advantage' to the individual who suddenly discovers their previously high standing-spot to no longer be as reliable as they thought). It also suggests that "catapult accidents", such as accidentally, or maybe [https://www.theguardian.com/uk/2005/nov/01/highereducation.students not so accidentally] being caught in a sling on a {{w|trebuchet}} when it is fired (indicated as "hilarious") may also contribute to the (momentary) gain in altitude. The limit to this period's ability to exist at altitude appears to be around 100 metres, which is perhaps mostly what a particular precipitous (and precarious) cliff-top might contribute to the situation.

Once {{w|balloon}} flights start, heights of up to 10km are attained. And though there were some {{w|List of ballooning accidents|dangers}} from this, as early aeronauts discovered, it might at least now be presumed that some of these peaks were attained by individuals who had previously marked a prior instantaneous altitude on the graph.

Shortly after the 1900s, {{w|airplanes}} dominate the graph. And the rise in utility of passenger aircraft (before World War 2; but especially afterwards, following a period where regular and extended high-altitude flight has been experienced by bomber pilots of various nations) ensures not only that there are people attaining greater and greater altitudes, but also that there are also always ''other'' people in the air, ensuring that the lesser 'maximum altitude' periods still have people a significant number of kilometres in the air.

Interestingly, the lower-limit, all the way up to the invention of the airplane, seems to stay at about two metres (around 1881, the lowest marked position seems to be only slightly above 1 metre), which might represent the possibility of there always being at least ''someone'' climbing up a ladder and/or jumping off of a hay-cart.

Once {{w|spaceflight}} becomes a thing (interestingly, marked around the late 1960s, though it actually started in April 1961), that greatly increases the upper spikes for the (implied) duration of the {{w|Orbital spaceflight|orbital flights}}.

The {{w|Apollo Program}} is then indicated by both label and a notable spike as (between {{w|Apollo 8}} in December 1968 and {{w|Apollo 17}} in December 1972), men from Earth were sent around the Moon and attained altitudes 'above the Earth' of approximately 400,000km in the process. Note that the disclaimer "(very approximate)" in the chart's title also applies here, as the graph shows less spikes than actual Moon orbitings or landings performed.

Since the end of the original Moon landings, the upper spikes settled down quite significantly back to 'only' generally low orbital distances, but the very latest era, marked "Space Station", seems to coincide with the current continuous inhabitation of space, which officially started in November 2000. Since that date, there has ''always'' been someone at approximately 400km altitude (give or take changes in the orbit, and of the terrain below), with occasionally some yet higher person(s) on certain missions (e.g. servicing the {{w|Hubble Space Telescope}}, May 2009 at 515km). The graph does not ''seem'' to show the blip created by {{w|Polaris Dawn}}'s 1,400 km 'new record' of September 2024, but this may be ''just'' off the right-hand edge of the graph.

Though the historical validity is sometimes argued, it is interesting to note that (as early as the 6th century CE), experiments with man-flying kites may have produced (semi-)brief spikes in the altitude record for the time. Gliders of the later era (starting roughly at the start of the 1800s) were probably eclipsed by the indicated balloons, but may have produced ''some'' of the spikes seen (above 10 metres but well below the multi-kilometre peaks), as occasional departures off the tops of hills were accomplished without quite so much ill-fortune, or at least without being ''entirely'' unintentional.

Also, workers and bell-ringers in medieval cathedrals, or attendants at the Lighthouse of Alexandria, would have been substantially above the "tens of meters" level. Moreover, the Eiffel tower has been open to visitors since its opening in 1899, which would have ensured some people to be at at least 276m, during the opening hours. This indicates that people standing on buildings and tall structures do not count for the purpose of the graph.

Tornadoes are another possible cause of high-altitude humans. There are multiple credible stories, [https://www.youtube.com/watch?v=pEPf6K-Y7GA| like this one], of people being lifted off the ground and surviving. In theory, they could have been lifted well over 100 meters and still survived.

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

:Height above Earth's surface of the highest-altitude human over time
:(very approximate)

:[A line graph is shown, with frequent spikes on the line. The y-axis is a logarithmic scale from 1 meter to 1,000,000 km. The x-axis shows years from about 1710 to 2025.]

:[Label between 1720s and 1780s, maximum height is roughly 100 meters:]
:Various falls and hilarious catapult accidents

:[Label with multiple arrows, from 1780s to 1910s, maximum height is roughly 10 km:]
:Balloon flights

:[Label with multiple arrows, from 1910s to 1960s, maximum height increases to roughly 100 km:]
:Airplane flights

:[Label with arrow, in the late 1960s, maximum height is roughly 500 km:]
:Spaceflight

:[Label with arrow, in the 1970s, maximum height is roughly 500,000 km:]
:Apollo Program

:[Label between 1990s and 2025, the average height after 2000 is roughly 500 km:]
:Space station

{{comic discussion}}

[[Category:Line graphs]]
[[Category:Timelines]]
[[Category:Space]]
[[Category:Aviation]]

1644: Stargazing

2025-01-08T10:51:30Z

Prallax: fixed inaccurate statements

{{comic
| number = 1644
| date = February 17, 2016
| title = Stargazing
| image = stargazing.png
| titletext = Some of you may be thinking, 'But wait, isn't the brightest star in our sky the Sun?' I think that's a great question and you should totally ask it. On the infinite tree of possible conversations spread out before us, I think that's definitely the most promising branch.
}}

==Explanation==
This is the first comic in the [[:Category:Stargazing|Stargazing]] series. It was followed by [[2017: Stargazing 2]] two and a half years later and [[2274: Stargazing 3]] four years later.

This comic opens on [[Megan]] as the host for a {{w|stargazing}} TV show, or simply a stargazing tour. She claims to be a doctor, although it is unclear what exactly she's a doctor in. Her remarks, however, may call her professionalism into question. (Originally the host was suspected to be a spoof on {{w|Brian Cox (physicist)|Brian Cox}}, see [[#Relevant TV-shows|below]], but at some later point [[Randall]] changed his official transcript thus making the host female rather than male as in the original version, see the [[#Trivia|trivia]] section below. Thus now the host is clearly Megan, which it could not have been originally when the host was described as a man by Randall).

Throughout the comic the host's tone and choice of words becomes increasingly unprofessional, referring to most of the stars as "shitty," personifying them based on different astronomical observations, and providing little useful information on the study of stars or how they work.

It seems that this is not an isolated issue as the host mentions that people keep asking her whether or not she is a real astronomer.

The host also continuously glosses over the arguably less exciting portions of a typical presentation on astronomy sharing only what she sees as "the good stuff." This penchant for only caring about something if it is interesting extends past astronomy as well as the host is too bored when reading the dictionary to look up the meaning of astronomer.

The comic derives much of its humor from the absurdity of the host's comments on various astronomical bodies. Although not technically incorrect, the way she presents the information is far from informative. (See details below on [[#The host's observations|the host's observations]]).

One of her observations regards the fact that {{w|Sirius}} is a {{w|binary star}}, a system where two stars orbit each other. So even though it is the brightest star as seen from Earth we only really see one of them, as the other is, to quote the host, "not even trying". Sirius A is "large" and "bright" {{w|main sequence}} white star, while Sirius B is a {{w|white dwarf}} with a little under half the mass, 0.49% the radius and only 0.22% the luminosity of Sirius A.

{{w|Andromeda Galaxy|Andromeda}} is the largest galaxy in our {{w|Local Group}}. It is 220,000 light years across and contains a trillion stars. Humans have difficulty conceptualizing distances of this scale. Suffice to say that it is very large.{{Citation needed}}

{{w|Betelgeuse}} is the 9th brightest star visible from earth. One of its prominent features is its visible redness. Within the next million years (or maybe only 100,000 years) it is expected to explode as a {{w|supernova}}, which will certainly be a spectacular sight. It could happen anytime now, and the host hopes it will be in her lifetime.

In the title text it is mentioned that the {{w|Sun}} is also a star and of course is much brighter than Sirius seen from Earth, and thus Sirius is technically not the brightest star in our sky (although it is in the night sky). The title text sarcastically encourages the audience to raise that obvious but irrelevant point (a standard joke when people mention bright stars) instead of asking a more interesting, informative, or fruitful question, when there are so many to ask regarding astronomy.

See also [[1371: Brightness]] and [[1342: Ancient Stars]]. Saying cool things about space to make people like you is mentioned in [[1746: Making Friends]].

===The host's observations===
Here is a list of the host's observations:
*Most {{w|Bright Star Catalogue|visible stars}} are still very faint, and just become background to the bright {{w|stars}} that form the named {{w|constellations}}.
**The host correctly states that they are just dots. (This is also true for the bright stars, but at least they are clearly distinguishable).
*{{w|Sirius}} is the {{w|Apparent magnitude|brightest}} star in our {{w|List of brightest stars|night sky}}. But it is not the brightest object in the night sky, as several of the planets, especially {{w|Venus}} and {{w|Jupiter}}, and of course the {{w|Moon}} are much brighter. It is also far from being one of the most {{w|Absolute magnitude|luminous stars}} in the {{w|Milky Way}}, but its proximity to Earth makes it the brightest in the night sky. For instance, {{w|Betelgeuse}}, mentioned later in the comic, is thousands of times more luminous than Sirius, but it is so farther away that it appears dimmer.
**The host thus names Sirius as the star in charge since it outshines all the others as seen from the {{w|Earth}}.
*Sirius is actually a star system consisting of two stars: a {{w|binary star}} system. But though Sirius A is twice the size of the {{w|Sun}} and much brighter, Sirius B is currently just a dim {{w|white dwarf}}, the remains of a much larger star that became a {{w|red giant}} before shedding its outer layers and collapsing into its current state around 120 million years ago. So now Sirius A completely outshines Sirius B, which actually is now a dead star with no further fusion inside its core.
**This is construed by the host as it is barely even trying, as it is now only radiating away the rest of the heat from the now-exposed core.
*{{w|Andromeda Galaxy|Andromeda}} is a {{w|spiral galaxy}}, like the Milky Way, and it is the largest galaxy in the {{w|Local Group}} where our own galaxy the Milky Way is the second largest. It is one of a few visible objects that are located outside the Milky Way. It is "only" 2.5 million light-years from the Sun and it is heading our way (or vice versa), and will {{w|Andromeda–Milky Way collision|collide with the Milky way}} in about 4 billion years (before the Sun goes into {{w|Sun#After_core_hydrogen_exhaustion|its red giant phase}}). Being 220,000 light years across and consisting of a trillion stars, it is somewhere between 1.2-2.2 times wider than the Milky Way and has 2.5-10 times as many stars. (The local group was also mentioned two comics ago, in [[1642: Gravitational Waves]], together with the much less well known third largest galaxy in the group the {{w|Triangulum Galaxy}}).
**It is therefore true when the host says that it is too big to try to understand, and thinking about it will make your head spin, so she suggests we do not think about it.
*{{w|Betelgeuse}} is a clearly visible (9th brightest) {{w|Red_supergiant|red supergiant}} {{w|Semiregular_variable_star|variable star}} located in the {{w|Orion (constellation)|constellation of Orion}}. It is one of the largest and most luminous observable stars (12th) and one of the few where it is clear that the light is not white. Most people can see that it is slightly red, whereas most other stars are so faint that they look white despite having different colors (when seeing Orion's two brightest stars, to remember which is which between Rigel and Betelgeuse, its diagonal opposite, just remember: Rigel is "R" like blue, and Betelgeuse is "B" like red). It is expected that Betelgeuse, being at a late stage of its {{w|Stellar_evolution|evolution}}, will go supernova within the next 100,000 years as a {{w|type II supernova}}. The exact time when it will become a {{w|Supernova}} is so uncertain that it could [http://earthsky.org/brightest-stars/betelgeuse-will-explode-someday#explode just as likely happen tomorrow] as in a 100,000 years. When it happens it will not be dangerous to anyone on Earth, but it will likely be visible even during the day, as it may even become as bright as the full Moon.
**When it does go nova, it will be a fantastic spectacle for everyone, but especially for anyone who likes the ''good stuff'' in space like the host, who cannot wait for the star to explode. Clearly she hopes it will be in her lifetime, and, although this is unlikely, there is a small chance that it might just happen.
*A {{w|meteor}} (also known as {{w|shooting star}}), is debris from space that rains down on Earth, and burns up in the atmosphere. This happens all the time, but you need to be either lucky, patient, or know the right time for one of the {{w|meteor showers}} to see one. Often they are visible for so short a time period, that it is difficult to share the experience with anyone, as it will be gone by the time they turn their head to look where you are pointing.
**The host becomes very excited when she spots such a meteor, especially because it is likely that her audience got to share the experience with her, as they were already looking in the same direction as her. But still she asks if they saw it, because it is so short lived.
*{{w|Outer space}} is the void that exists between {{w|Astronomical object|celestial bodies}}, including the Earth. There is by definition nothing there but {{w|vacuum}}, and the interesting part of space is thus not the space but the astronomical objects found out there.
**The host says that ''space is awesome'', which is a very un-astronomical comment, as explained above. Also her excitement for a simple shooting star is cause for the suspicion that is raised after her space comment.
*A {{w|dictionary}} is a reference resource for words. Unless otherwise specified (as a "Dictionary of ''<some theme>''"), it contains a significant proportion of ''all'' words in the language(s) it covers.
**The host considers the dictionary ''boring'', which suggests that she is not even familiar with how to zero in on a word and its definition. For a typical and expected English dictionary (unless she made further errors) the word "Astronomer" is actually quite near the start, but it still would have been a large number of words needing to be read past (e.g. "a ... aardvark ... abandon ... abbey ... abbreviation ... ability ...") if one tried to find the word by slavishly checking every entry on the way up to it. It adds to the suspicion that the host's actual expertise isn't in either space ''or'' any more normal and grounded matters but {{tvtropes|Cloudcuckoolander|somewhere else entirely}}.

===Relevant TV-shows===
The comic could be a reference to BBC's ''{{w|Stargazing Live}}'', which {{w|Brian Cox (physicist)|Brian Cox}} has appeared in since 2011. If drawn in xkcd style he would likely look like Megan. He has a PhD in high-energy {{w|particle physics}}, but not astronomy. The newest season of the show aired during January 2016 just a month before this comic's release. Brian Cox has also been the presenter of several other science programs, especially such as the ''{{w|Wonders of the Solar System}}'', ''{{w|Wonders of the Universe}}'' and ''{{w|Wonders of Life (TV series)|Wonders of Life}}''. Originally the host was described as male in the official transcript (see [[#Trivia|trivia]] below, making this seem more likely. For some reason Randall changed the host to female in the transcript later. Very strange, but for sure when he was male, it was obviously a Brian Cox spoof.

It could also be a reference to {{w|Jack Horkheimer}}'s PBS shows ''Star Hustler'' and ''{{w|Star Gazers}}''. Horkheimer, however, does not at all look like Megan, and he died 6 years ago. But he was not a doctor in astronomy, only getting into it when he started volunteering at the Miami Museum of Science's planetarium. He ended up writing shows for the planetarium and the PBS series developed from there. He rarely covered facts about the night sky that couldn't be found in any basic reference (possibly because the show was aimed at children and non-astronomy buffs), although he did get more in-depth about current astronomical events such as {{W|Comet Hale–Bopp}}.

==Transcript==
:[A thin panel with Megan as a TV-host is holding her hands up. She is drawn in white on a black background. Behind her is an audience drawn in faint gray lines consisting of Hairy (to the left) and two Cueball-like guys and Ponytail (seen in a rare full face position) to the right of the host. One of the Cueball-like guys is partly hidden behind the host.]
:Host: Welcome to stargazing, with your host, me.
:Host: I'm a doctor or whatever.

:[Same scene as before but in a broader panel, and the host is now holding only one hand up with a finger pointing up. The audience is the same four people, but now Hairy has moved further to the left in the panel to make room for a Megan-like woman also to the left of the host.]
:Host: I'm not gonna waste your time on the shitty stars.
:Host: Just the good stuff.
:Host: Honestly half of 'em just look like dots.

:[A frame-less drawing with a zoom out showing the group of six people in black silhouette on a white background. Part of the ground beneath them is shown as a black pool. The host is pointing up with one hand. The people have been rearranged, so left of the host is now a Cueball-like guy and a Megan-like woman, and to the right is the other Cueball-like guy, then Ponytail (seen from the side as usual) and Hairy. All are looking up following the host's directions.]
:Host: This is Sirius. It's the brightest star in our sky so it's in charge.
:Host: It's really two stars but one of them is barely even trying.
:Host: This is Andromeda, it's too big to think about, so let's not.

:[Zoom in of the host's upper body, again drawn in white on a black background. She is looking right gesturing with one arm raised, and the other still pointing up with a finger stretched out. Her audience is no longer shown.]
:Host: That red stars is Betelgeuse. It's gonna explode someday.
:Host: Can't happen soon enough, as far as I'm concerned. I-
:Host: ''Holy shit did you see that meteor!?!''
:Host: Space is ''awesome!''

:[Same scene as the previous panel, but the host has turned towards left looking at someone in the audience (not shown) who speaks off-screen. She has taken both her hands down for the first time.]
:Off-screen voice: Are you ''sure'' you're an astronomer?
:Host: People keep asking that, so I finally tried to look that word up in a dictionary, and ''wow'' is that book ever boring. No thank you.
:Off-screen voice: But-
:Host: ''Space!''

==Trivia==
*Randall changed the [http://www.explainxkcd.com/wiki/images/archive/4/48/20160221022727!stargazing.png original] posted version of the comic.
**The only thing that changed was in the third panel where '''''That's''' Andromeda'' was changed to the current version: '''''This is''' Andromeda''
*The official transcript [https://web.archive.org/web/20160913162302/http://xkcd.com/1646/info.0.json originally] used male pronouns for the TV host. It now (as of 2019) uses female pronouns for the host.
**The official transcripts seems to have been messed up on xkcd at the time being.
***The [http://xkcd.com/1644/info.0.json transcript for 1644] is thus at the moment a mix of that comics main info (top and bottom) which results in the correct title and title text, but the entire description in this transcript is describing the comic from two releases before no. [[1642]].
***This seems to be a general problem for comics released around that time...
***Thus the description of this comic, was first released when comic no. [[1646]] came out.
***This has not been corrected (4 years later at the time of writing)!
****So the official transcript for 1644 can be found [http://xkcd.com/1646/info.0.json here], together with the data for comic 1646.
**The current transcript is included here below:
::(Inverse color panel - white on black)
::[A television host in the foreground, speaking toward the reader. A group of other people are in the background behind them.]
::Host: Welcome to Stargazing, with your host, me. I'm a doctor or whatever.
::[She continues to talk.]
::Host: I'm not gonna waste your time on the shitty stars. Just the good stuff. Honestly half of 'em just look like dots.
::(Normal color panel - black on white)
::[A shot from far away of the host standing in the center of the group of people watching her, she points to the sky.]
::Host: This is Sirius. It's the brightest star in our sky so it's in charge. It's really two stars, but one of them is barely even trying. This is Andromeda. It's too big to think about, so let's not.
::(Inverse color panel)
::[Close-up on the host gesturing toward the sky behind her.]
::Host: That red star is Betelgeuse. It's gonna explode someday. Can't happen soon enough, as far as I'm concerned. I--
::HOLY SHIT DID YOU SEE THAT METEOR?!?!
::Space is awesome!
::[The host speaks to someone out of panel.]
::Other: Are you sure you're an astronomer?
::Host: People keep asking that, so I finally tried to look that word up in a dictionary, and wow is that book ever boring. No thank you.
::Other: But--
::Host: SPACE!
:*The original official transcript with male host is included here below (correcting a typo with a missing "s" in "stuff" and formatting to look like our normal transcripts):
::[A television host in the foreground, speaking toward the reader. A group of other people are in the background behind them.]
::Host: Welcome to Stargazing, with your host, me. I'm a doctor or whatever.
::[He continues to talk.]
::Host: I'm not gonna waste your time on the shitty stars. Just the good stuff. Honestly half of 'em just look like dots.
::[Normal color panel - black on white. A shot from far away of the host standing in the center of the group of people watching him, he points to the sky.]
::Host: This is Sirius. It's the brightest star in our sky so it's in charge. It's really two stars, but one of them is barely even trying. This is Andromeda. It's too big to think about, so let's not.
::[Inverse color panel. Close-up on the host gesturing toward the sky behind him.]
::Host: That red star is Betelgeuse. It's gonna explode someday. Can't happen soon enough, as far as I'm concerned. I-- ''HOLY SHIT DID YOU SEE THAT METEOR?!?!'' Space is ''awesome''!
::[The host speaks to someone out of panel.]
::Other: Are you ''sure'' you're an astronomer?
::Host: People keep asking that, so I finally tried to look that word up in a dictionary, and ''wow'' is that book ever boring. No ''thank'' you.
::Other: But--
::Host: ''SPACE!''

{{comic discussion}}

[[Category:Stargazing]]
[[Category:Comics sharing name|Stargazing]]
[[Category:Comics with inverted brightness]]
[[Category:Comics featuring Megan]]
[[Category:Comics featuring Hairy]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Multiple Cueballs]]
[[Category:Astronomy]]
[[Category:Space]]

3000: Experimental Astrophysics

2024-10-19T06:47:32Z

Prallax: /* Explanation */

{{comic
| number = 3000
| date = October 18, 2024
| title = Experimental Astrophysics
| image = experimental_astrophysics_2x.png
| imagesize = 317x490px
| noexpand = true
| titletext = Our experiment will be expensive, but we believe it will produce important spin-offs, especially if we manage to hit the sun from the right angle.
}}

==Explanation==
{{incomplete|Created by a SOLAR FLARE WITH ABNORMAL PULSAR READINGS - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
A {{w|Thorne–Żytkow object}} is an object theorized by Kip Thorne and Anna Żytkow, which is a red giant or red supergiant with a neutron star at its core. There have been no confirmed sightings, but HV2112 in the small Magellican cloud (SMC) and HV 11417 are both strong candidates.

Normally, astrophysical science is an observational discipline, in which the existing situation is understood from afar. It would not be unusual to seek funding for telescope time, or even an entirely new telescope, in order to make such investigations possible. The scene in the comic itself might be the logical conclusion of that kind of proposal/{{w|elevator pitch}}, complete with a diagram of how such an object may form, but the title of the comic (made explicit by the caption text) suggests that this is intended to be a 'practical' experiment to induce a TŻO into existence by actually sending a neutron star into our own Sun.

Cueball is complaining that no one is willing to fund this practical experiment to create the Thorne–Żytkow object, for direct observation. This is impractical, for several of reasons. First, you would need to find (or create!) a neutron star and bring it here, which requires amounts of energy, mass and precision that we have no current possibility of achieving. Secondly, having a neutron star anywhere near the Sun would most likely destroy the solar system as a whole.{{Citation needed}} Moreover, having a neutron star inside would immediately turn the Sun into a red giant, which normally takes billions of years and will [[1606|ruin the weather]]. The funding body, seemingly based on Earth, would probably find the end result would not generate a favorable {{w|environmental impact statement}}, given the almost inevitable destruction of their planet.

A spin-off usually means something additional that was not originally planned. The title text in this comic is a two-layer joke on the phrase "spin-off", meaning that hitting the Sun off-center (from the right angle) would likely have many unintended consequences, but also cause the Sun to start rotating abnormally, due to the added angular momentum, in the same way as hitting a ball on the side makes it spin. A similar idea (the engulfment of a planet or companion star) has been proposed to explain the apparently anomalous rotation of the red supergiant Betelgeuse.

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

:[Cueball is standing on a circular stage in front of a poster. The poster shows two stars, one much larger than the other. The smaller star has an arrow pointing from it to the center of the large star, where a similarly sized 'core' can be seen.]
:Cueball: A Thorne-Żytkow object is a hypothesized nested star — a red giant with a neutron star inside it.
:Cueball: So far, no TŻOs Have been definitively observed, but your grant could help us change that.

:[Caption below the panel:]
:We're struggling to get funding for our project to slingshot a neutron star into the Sun.

{{comic discussion}}

847: Stingray Nebula

2024-06-29T18:49:28Z

Prallax: Randall is possibly confusing the stingray nebula (not an explosion and not happened in 1987) with SN1987 (a supernova exploded in 1987)

{{comic
| number = 847
| date = January 14, 2011
| title = Stingray Nebula
| image = stingray_nebula.png
| titletext = Eärendil will patrol the walls of night only until the sun reaches red giant stage, engulfing the Morning Star on his brow. Light and high beauty are passing things as well.
}}

==Explanation==
The {{w|Stingray Nebula}}, in the constellation {{w|Ara (constellation)|Ara}}, is the youngest known planetary nebula in the Galaxy. It was formed by an {{w|asymptotic giant branch}} (AGB) B1 {{w|supergiant}}, which ejected its outer layers during the 1980s. It now appears as a {{w|white dwarf}} star in the center of an expanding gaseous nebula.

This comic went from a reassuring comic about stars at night giving hope in the darkness, but then as with most xkcd's, it took a turn. In this case, the twist is that because [[Cueball]]'s star he got attached to exploded into a nebula, we should only become attached to stars that aren't quite as volatile as the one that formed the Stingray Nebula.

The formation of a planetary nebula is not usually referred to as an explosion, since it is a quite slow process, which, in the case of the stingray nebula, took about 20 years (1970-1990). When Randal describes it as an explosion happening exactly in 1987, it is possible that he is confusing it with the famous supernova {{w|SN 1987A}}, the closest and brightest observed supernova since {{w|Kepler's supernova}} of 1604.

While talking about his star, Cueball shares with his friend a quote from the ''{{w|Lord of the Rings: The Return of the King}}'', the third installment in {{w|J. R. R. Tolkien|J.R.R. Tolkien's}} epic fantasy series. {{w|Samwise Gamgee|Sam}} and his friend {{w|Frodo Baggins|Frodo}} were forced to travel through {{w|Mordor}}, a land of fire and death, to destroy the malevolent {{w|One Ring}}. The journey's hardship took a psychological toll on each of them.

Cueball compares his struggles as a kid, and his friend's struggles in the present, to Sam and Frodo's arduous journey through Mordor. Both he and Sam were able to find beauty and solace in the glimmering of some distant light. By attaching their hopes, woes and feelings to this small point of hope, they both get a sense of perspective and comfort in the fact that there is beauty in a greater sense before them.

The title text ties into the Tolkien quote. {{w|Eärendil}}, with a {{w|Silmaril}} strapped to his ship, Vingilot, is the Tolkienian myth explanation for the planet {{w|Venus}}, which has historically been mistaken for a star due to its brightness, being known as the "morning star" or "evening star". Venus will eventually (in billions of years time) be engulfed by the sun's expansion into a red giant. The source of Sam's comfort is also temporary in the long term.

==Transcript==
:[Cueball and his Cueball-like friend to the left is shown as white silhouettes against a dark sky. They're sitting on top of a grassy hill also shown inverted as white.]
:Cueball: I know things are tough right now. When I was going through some difficult times as a kid, I would go up on the roof and look through my telescope.

:[Zoom in on the twos white heads and upper torso only.]
:Cueball: One day I found a tiny star in Ara that seemed friendly.
:Cueball: There were millions like it, but I decided that this one was mine.

:[Zoom in only on Cueball's head and torso.]
:Cueball: When things got bad, I'd go find that star, and think of my favorite Tolkien quote. It's from Sam's time in Mordor.

:[The next panel is diagonally downward to the right of the previous. The upper left corner overlaps. A single star is shown above the highest peak in a jagged chain of mountains with at least five other large peaks and several smaller ones. Above the star in the black sky is a long quote.]
:"There, peeping among the cloud-wrack above a dark tor high up in the mountains, Sam saw a white star twinkle for a while. The beauty of it smote his heart, as he looked up out of the forsaken land, and hope returned to him. For like a shaft, clear and cold, the thought pierced him that in the end the shadow was only a small and passing thing: There was light and high beauty forever beyond its reach."
::::—The Return of the King

:[Back to normal panels below the quote panel. Larger zoom than in panel two both showing the same of the two.]
:Friend: That's comforting!
:Cueball: It was rather undercut in 1987, when the light from my star's explosion reached Earth. The debris forms the Stingray Nebula.

:[Zooming further out even than the first panel, showing more of the grassy mound below the two.]
:Friend: There's probably a lesson there.
:Cueball: "Never trust an unstable asymptotic giant branch star. Stick with main sequences and dwarfs."
:Friend: I'll, uh, keep that in mind.

==Trivia==
*The title of this comic was subsequently referenced in the following [http://web.mit.edu/puzzle/www/2011/puzzles/katamari_damacy/unlikely_situations/ MIT Mystery Hunt puzzle].

{{comic discussion}}

[[Category:Comics with inverted brightness]]
[[Category:Comics featuring Cueball]]
[[Category:Multiple Cueballs]]
[[Category:LOTR]]
[[Category:Philosophy]]
[[Category:Space]]
[[Category:Astronomy]]
[[Category:Telescopes]]

2821: Path Minimization

2023-08-29T21:16:34Z

Prallax: moved section before paragraph about title text

{{comic
| number = 2821
| date = August 28, 2023
| title = Path Minimization
| image = path_minimization_2x.png
| imagesize = 562x559px
| noexpand = true
| titletext = Of course you get an ice cream cone for the swimmer too! You're not a monster.
}}

==Explanation==
{{incomplete|Created by WAITING AN HOUR BEFORE SWIMMING - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In this comic, it appears that Cueball, standing on shore, is observing a swimmer who is possibly (but not obviously) in distress. The comic illustrates five potential paths that can be taken to reach the swimmer, each with a different reason to make them viable, in the manner of demonstrating different optimal strategies that can be chosen.

The first path is a direct line from Cueball, straight to the swimmer, which allows for the minimum possible distance to be traveled, some on land and the remainder in the water.

The second path travels more obliquely from Cueball to the water and then at a sharper angle to the swimmer. This path would take the shortest amount of time, presuming that Cueball would move faster on land (covering more of the distance) and slower through the water (but less distance). The exact angles would depend on how much faster Cueball is on land than in the water. The relationship between speeds and angles is the same as that in {{w|Snell's law}} for light passing between two media.

The third path travels at a far more oblique angle to the water, such that the subsequent swimming path is entirely perpendicular to the shoreline, adding to the amount of time spent on land in order to minimize the time spent swimming. Depending on one's swimming ability versus running ability, this could be the safest path to take. It might also be more sensible to keep the target in clear sight for as long as possible, from the land, then aim exactly away from shore when both your head and theirs are barely at wave-height. But this is a completely different reason from the distance or time preferences.

The fourth path travels nearly parallel to the beach. In fact moving slightly ''away'' from the swimmer but towards an intermediate goal: an ice cream stand. After that, the path turns and aims straight towards the swimmer, as all the others eventually do (although it is not made clear at this point if Cueball will also be eating, or just carrying, an ice cream even whilst swimming).

The fifth and final path, barely visible directly above Cueball, is labeled as the path that ''maximizes'' time. This path, presumably, travels around the entire world, likely stopping for many, ''many'' rest breaks. It should be noted that, by the definition given, it is theoretically possible to stretch the maximum time taken out forever by simply walking away and never returning.

You could also fulfill the criteria of reaching the target in finite, but arbitrarily long, time by following a {{w|random walk}}(+swim) or even follow a {{w|space-filling curve}} carefully chosen to be the maximally finite scenario. Or you could simply choose any path, and stop for an arbitrarily long time, or travel at a speed approaching zero. In the comic, however, a requirement for simplicity of path may dictate the use of something close to the opposing {{w|great-circle distance}}, or a variation that has a maximal swim-time even without ''undue'' time-wasting detours, and assume equal speeds of travel on all routes.

The comic pokes fun at two famous physical/mathematical problems that are usually stated as happening on a beach. The first is the Lifeguard problem, used by Richard Feynman in his book ''QED'' to illustrate {{w|Fermat's principle}}, or principle of least time, which states that the path taken by a light ray between two given points is the path that can be traveled in the least time. This is closely related to {{w|Stationary-action principle}} for mechanical systems. In Feynman's words:

''"Finding the path of least time for light is like finding the path of least time for a lifeguard running and then swimming to rescue a drowning victim: the path of least distance has too much water in it; the path of least water has too much sand in it; the path of least time is a compromise between the two."'' - ''Richard Feynman, QED - The Strange Theory of Light and Matter (1988, Princeton University Press), Chapter 2.''

The second problem referenced in this comic is the [https://gametheory101.com/courses/game-theory-101/hotellings-game-and-the-median-voter-theorem Beach Vendor Problem], which is stated as follows. Suppose that on a long beach there are two ice cream vendors. Customers are uniformly distributed on the beach and each person will go get the ice cream at the closest vendor. Each vendor wants to maximize the number of customers that buy at their place. To minimize the customer's walking time, the optimal configuration would be to have one vendor at 1/4 of the beach length and the other at 3/4, but {{w|Hotelling's law}} predicts that the two shops will converge to the middle of the beach, in an attempt to steal as many customers as possible from the competition. This is a case of {{w|Nash equilibrium}} that is also related to the {{w|Median voter theorem}}. If the number of vendors is larger than 2, the problem may become [https://gametheory101.com/tag/hotellings-game/ considerably more complicated].

The title text adds the stipulation, to the ice-cream path, that you also carry an ice-cream to the target swimmer to 'justify' that choice of route. But how this squares with the reason to rendezvous with the swimmer, or the manner in which this would further complicate the swimming stage, goes unsaid. But it makes it clear that ''not'' doing this isn't considered socially permissible, whether or not he had stopped to eat an ice-cream of his own beforehand.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:Path that minimizes distance [A straight line from beach cueball to ocean cueball, bearing about 135]
:Path that minimizes time [A line from beach cueball to the waterline closer to horizontal, bearing about 120, then angling towards ocean cueball, bearing about 150]
:Path that minimizes swimming [A line from beach cueball to the waterline closest to ocean cueball, bearing about 115, then angling toward ocean cueball, bearing 180]
:Path that minimizes time until you get ice cream [A line from beach cueball to an ice cream stand manned by Ponytail, bearing about 90, then angling toward ocean cueball, bearing about 190]
:Path that maximizes time [A line from beach cueball away from the shore, bearing 0, fading and disappearing at the top of the panel, and reappearing at the bottom of the panel directly below ocean cueball]

{{comic discussion}}

[[Category:Food]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Multiple Cueballs]]
[[Category:Physics]]

Talk:2821: Path Minimization

2023-08-29T21:14:39Z

Prallax:

Judging from the angle of the arms, I do not believe that the swimmer is in distress. In fact I think the swimmer is just a future projection of Cueball, not a separate person.

:I agree that the swimmer does not appear to be in distress, although the title text suggests that it is probably a separate person.
:
:My personal interpretation is that the situation is "meeting a friend at the beach to get ice cream". Options are either a) meet the friend first and then swim back to get ice cream or b) get ice cream first and take it out to eat together in the water. [[Special:Contributions/172.70.86.54|172.70.86.54]] 04:02, 29 August 2023 (UTC)
:: Or the more sensible walk to the water's edge and wait for your friend to swim in to meet you... [[Special:Contributions/172.70.85.29|172.70.85.29]] 08:43, 29 August 2023 (UTC)
:I agree as well, I changed it to "possibly in distress". [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 04:43, 29 August 2023 (UTC)
::I changed it further. Still admits the possibility, but less possibly than "possibly" hints at. (It is, nonetheless, a (mostly!) typical "you ''need'' to get here, minimising <some form of time/distance/energy/endurance resource>, what is the optimal strategy" tbing that lifesaving courses might include to train you in such considerations. [[Special:Contributions/172.71.242.82|172.71.242.82]] 09:01, 29 August 2023 (UTC)

Technically, the path that minimizes swimming passed a nearby boat rental stand. (by the way, I didn't write the unsigned paragraph above this one) [[Special:Contributions/172.69.247.40|172.69.247.40]] 03:05, 29 August 2023 (UTC)

When I first saw the comic, I thought cueball was suspended on a tether in the air above the water, and the ice cream stand was floating. I didn't realize it was a beach until I read this explanation. [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 04:41, 29 August 2023 (UTC)

The ice cream path reminds me of the [[2407|bread-first search]] (similar punchline). [[Special:Contributions/172.71.182.158|172.71.182.158]] 06:55, 29 August 2023 (UTC)

Noting, in passing, that an almost "flosbury flop"-like path could have been shown, too. A gentle curve, bending to hit the shore somewhere between shortest-time and shortest-swim, that would allow maximum run-up to dive ''into'' the water at the right angle to then power straight towards the target with the original running impetus turned straight into initial swimming speed, with no sharp turns involved. (Also, an 'Aquaman' line, perhaps exactly complimentary to the land-favouring 'bent-leg' routes, minimising/reducing land in favour of water.) [[Special:Contributions/172.71.242.82|172.71.242.82]]

This was one of my first edits in this wiki, so I hope I didn't do anything wrong. I think this comic clearly references Feynman's Lifeguard problem (there is an almost identical diagram in his QED book) and the Ice-cream vendor problem. I wrote a couple of paragraphs explaining this, but they were promptly deleted by the next editor. Did I do something wrong? I think the explanation is incomplete without those references. [[User:Prallax|Prallax]] ([[User talk:Prallax|talk]]) 10:08, 29 August 2023 (UTC)
: Seeing the history, this is probably just an edit conflict, you should insert your contribution again. [[User:Cochonou|Cochonou]] ([[User talk:Cochonou|talk]]) 11:54, 29 August 2023 (UTC)
:: It looks like it was my edit that 'overwrit' Prallax's. Though I got no "edit conflict" notification (or I'd have at least tried to reintegrate my failed changes into the revised 'current prior' version, or at least revisit the diffs to reinstate the best of what I'd caused to vanish/undo). Thinking back, it ''might'' have been due to a server time-out which meant I resubmitted, fooling the back end about what version I was (thinking I was) actually adding to/revising... I'm pretty sure it occasionally does that, and probably more than I ever know about, if I don't have reason to check the page histories in detail...
:: Anyway, recovered the paragraphs and re-added them to the end (maybe should have been inserted before the traditionally final title-text-paragraph?) on the original editor's behalf, as they looked fairly good and I didn't see anything in them that I might want to change (not even markup).
:: So both apologies and congratulations to their author! They might later get tweaked/redone, b6 others, but as far as ('one-of-my-')first-time edits go I'd have personally been proud to have done as well, whatever my first (probably disasterous) edits might actually have been, oh so many years ago and lost in the mists of anonymous-IPdom... Keep it up, and welcome to the rabble! ;) [[Special:Contributions/172.70.91.8|172.70.91.8]] 12:45, 29 August 2023 (UTC)
::: Thank you for reverting and fixing this! I'm moving it before the paragraph about the title text as suggested. [[User:Prallax|Prallax]] ([[User talk:Prallax|talk]]) 21:14, 29 August 2023 (UTC)

Reminds me of https://xkcd.com/85/ [[User:Betamaj|Betamaj]] ([[User talk:Betamaj|talk]]) 12:06, 29 August 2023 (UTC)
:One of the first comments there mentions trying to reach a drowning swimmer. [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 14:14, 29 August 2023 (UTC)

I don't know if I agree with the Beach Vendor paragraph. There's only one vendor, and there's no reason for an ice cream stand on a beach to be a reference to the problem. (Ice cream stands on beaches are pretty common, at least near I live.) [[User:DownGoer|DownGoer]] ([[User talk:DownGoer|talk]]) 12:50, 29 August 2023 (UTC)

I question the first-paragraph comment about how the paths are "indicative of the path light takes between materials with different refractive indices". That applies to only 2 of the 5 paths. [[User:BunsenH|BunsenH]] ([[User talk:BunsenH|talk]]) 15:48, 29 August 2023 (UTC)
:Arguably just the one (extremely exotic materials aside) if you don't count the straight-there path (zero difference). But it is relevent to quote how the "optimal speed route" is equivalent to a refractive-path (for similar underlying mathematics) as an analogue, which is further down. [[Special:Contributions/172.70.90.232|172.70.90.232]] 18:20, 29 August 2023 (UTC)

Talk:2821: Path Minimization

2023-08-29T10:08:56Z

Prallax:

2821: Path Minimization

2023-08-29T07:51:42Z

Prallax: /* Explanation */

{{comic
| number = 2821
| date = August 28, 2023
| title = Path Minimization
| image = path_minimization_2x.png
| imagesize = 562x559px
| noexpand = true
| titletext = Of course you get an ice cream cone for the swimmer too! You're not a monster.
}}

==Explanation==
{{incomplete|Created by WAITING AN HOUR BEFORE SWIMMING - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In this comic, it appears that Cueball, standing on shore, is observing a swimmer who is possibly in distress. The comic illustrates 5 potential paths that can be taken to reach the swimmer, each with a different reason to make them viable.

The first path is a 45° angle away from Cueball, straight to the swimmer, which allows for the minimum possible distance to be traveled.

The second path travels at a 30° from Cueball to the water and then at a 20° angle to the swimmer. This path would take the shortest amount of time, since Cueball would cover more of the distance on land (moving faster) and less of the distance through water (moving slower). The exact angles would depend on how much faster Cueball is on land than in the water.

The third path travels at roughly a 23.2° angle to the water followed by a swimming path perpendicular to the shoreline, maximizing the amount of time spent on land and thus minimizing the time spent swimming. Depending on one's swimming ability versus running ability, this could be the safest path to take.

The fourth path travels horizontal, along the beach, technically moving slightly ''away'' from the swimmer, but towards an arguably more important goal: an ice cream stand in a straight line away from where Cueball is presently. After that detour, the path turns and aims towards the swimmer, as all the others do (although how one could eat ice cream whilst swimming to rescue someone, much less whilst carrying a second ice cream, is not explained).

The fifth and final path, barely visible directly above Cueball, is labeled as the path that ''maximizes'' time. This path, presumably, travels around the entire world, likely stopping for many, ''many'' rest breaks. It should be noted that, by the definition given, it is theoretically possible to stretch the maximum time taken out forever by simply walking away and never returning.

The comic pokes fun at two famous physical/mathematical problems that are usually stated as happening on a beach. The first is the Lifeguard problem, used by Richard Feynman in his book ''QED'' to illustrate {{w|Fermat's principle}}, or principle of least time, which states that the path taken by a light ray between two given points is the path that can be traveled in the least time. This is closely related to {{w|Stationary-action principle}} for mecanical systems. In Feynman words:

''"Finding the path of least time for light is like finding the path of least time for a lifeguard running and then swimming to rescue a drowning victim: the path of least distance has to much water in it; the path of least water has too much sand in it; the path of least time is a compromise between the two."'' - ''Richard Feynman, QED - The Strange Theory of Light and Matter (1988, Princeton University Press), Chapter 2.''

The second problem referenced in this comic is the [https://gametheory101.com/courses/game-theory-101/hotellings-game-and-the-median-voter-theorem Beach Vendor Problem], which is stated as follow. Suppose that on long beach there are two ice cream vendors. Customers are uniformly distributed on the beach and each person will go get the ice cream at the closest vendor. Each vendor wants to maximize the number of customers that buy at their place. To minimize the customer's walking time, the optimal configuration would be to have one vendor at 1/4 of the beach length and the other at 3/4, but {{w|Hotelling's law}} predicts that the two shops will converge to the middle of the beach, in an attempt to steal as much customers as possible from the competition. This is a case of {{w|Nash equilibrium}} that is also related to the {{w|Median voter theorem}}. If the number of vendors is larger than 2, the problem may become [https://gametheory101.com/tag/hotellings-game/ considerably more complicated].

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:Path that minimizes distance [A straight line from beach cueball to ocean cueball, bearing about 135]
:Path that minimizes time [A line from beach cueball to the waterline closer to horizontal, bearing about 120, then angling towards ocean cueball, bearing about 150]
:Path that minimizes swimming [A line from beach cueball to the waterline closest to ocean cueball, bearing about 115, then angling toward ocean cueball, bearing 180]
:Path that minimizes time until you get ice cream [A line from beach cueball to an ice cream stand manned by Ponytail, bearing about 90, then angling toward ocean cueball, bearing about 190]
:Path that maximizes time [A line from beach cueball away from the shore, bearing 0, fading and disappearing at the top of the panel, and reappearing at the bottom of the panel directly below ocean cueball]

{{comic discussion}}

[[Category:Food]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Multiple Cueballs]]

2821: Path Minimization

2023-08-29T07:47:38Z

Prallax: /* Explanation */

{{comic
| number = 2821
| date = August 28, 2023
| title = Path Minimization
| image = path_minimization_2x.png
| imagesize = 562x559px
| noexpand = true
| titletext = Of course you get an ice cream cone for the swimmer too! You're not a monster.
}}

==Explanation==
{{incomplete|Created by WAITING AN HOUR BEFORE SWIMMING - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In this comic, it appears that Cueball, standing on shore, is observing a swimmer who is possibly in distress. The comic illustrates 5 potential paths that can be taken to reach the swimmer, each with a different reason to make them viable.

The first path is a 45° angle away from Cueball, straight to the swimmer, which allows for the minimum possible distance to be traveled.

The second path travels at a 30° from Cueball to the water and then at a 20° angle to the swimmer. This path would take the shortest amount of time, since Cueball would cover more of the distance on land (moving faster) and less of the distance through water (moving slower). The exact angles would depend on how much faster Cueball is on land than in the water.

The third path travels at roughly a 23.2° angle to the water followed by a swimming path perpendicular to the shoreline, maximizing the amount of time spent on land and thus minimizing the time spent swimming. Depending on one's swimming ability versus running ability, this could be the safest path to take.

The fourth path travels horizontal, along the beach, technically moving slightly ''away'' from the swimmer, but towards an arguably more important goal: an ice cream stand in a straight line away from where Cueball is presently. After that detour, the path turns and aims towards the swimmer, as all the others do (although how one could eat ice cream whilst swimming to rescue someone, much less whilst carrying a second ice cream, is not explained).

The fifth and final path, barely visible directly above Cueball, is labeled as the path that ''maximizes'' time. This path, presumably, travels around the entire world, likely stopping for many, ''many'' rest breaks. It should be noted that, by the definition given, it is theoretically possible to stretch the maximum time taken out forever by simply walking away and never returning.

The comic pokes fun at two famous physical/mathematical problems that are usually stated as happening on a beach. The first is the Lifeguard problem, used by Richard Feynman in his book ''QED'' to illustrate {{w|Fermat's principle}}, or principle of least time, which states that the path taken by a light ray between two given points is the path that can be traveled in the least time. This is closely related to {{w|Stationary-action principle}} for mecanical systems. In Feynman words:

''"Finding the path of least time for light is like finding the path of least time for a lifeguard running and then swimming to rescue a drowning victim: the path of least distance has to much water in it; the path of least water has too much sand in it; the path of least time is a compromise between the two."'' - ''Richard Feynman, QED - The Strange Theory of Light and Matter (1988, Princeton University Press), Chapeter 2''

The second problem referenced in this comic is the [https://gametheory101.com/courses/game-theory-101/hotellings-game-and-the-median-voter-theorem Beach Vendor Problem], which is stated as follow. Suppose that on long beach there are two ice cream vendors. Customers are uniformly distributed on the beach and each person will go get the ice cream at the closest vendor. Each vendor wants to maximize the number of customers that buy at their place. To minimize the customer's walking time, the optimal configuration would be to have one vendor at 1/4 of the beach length and the other at 3/4, but {{w|Hotelling's law}} predicts that the two shops will converge to the middle of the beach, in an attempt to steal as much customers as possible from the competition. This is a case of {{w|Nash equilibrium}} that is also related to the {{w|Median voter theorem}}. If the number of vendors is larger than 2, the problem may become [https://gametheory101.com/tag/hotellings-game/ considerably more complicated].

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:Path that minimizes distance [A straight line from beach cueball to ocean cueball, bearing about 135]
:Path that minimizes time [A line from beach cueball to the waterline closer to horizontal, bearing about 120, then angling towards ocean cueball, bearing about 150]
:Path that minimizes swimming [A line from beach cueball to the waterline closest to ocean cueball, bearing about 115, then angling toward ocean cueball, bearing 180]
:Path that minimizes time until you get ice cream [A line from beach cueball to an ice cream stand manned by Ponytail, bearing about 90, then angling toward ocean cueball, bearing about 190]
:Path that maximizes time [A line from beach cueball away from the shore, bearing 0, fading and disappearing at the top of the panel, and reappearing at the bottom of the panel directly below ocean cueball]

{{comic discussion}}

[[Category:Food]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Multiple Cueballs]]

Talk:371: Compiler Complaint

2023-07-25T21:35:10Z

Prallax:

The test "whether build environment is sane" have nothing to do with libraries (those are tested later) or compilation at all (ditto). It seems to test ls and creating new files. Failing this test will surely means lot of problems, but certainly there are other possible sources of problems (some tested later), so not sure why it's labeled this way ... there is probably interresting history behind this.

The test goes like this (example taken from coreutils-5.94):
{ echo "$as_me:$LINENO: checking whether build environment is sane" >&5
echo $ECHO_N "checking whether build environment is sane... $ECHO_C" >&6; }
# Just in case
sleep 1
echo timestamp > conftest.file
# Do `set' in a subshell so we don't clobber the current shell's
# arguments. Must try -L first in case configure is actually a
# symlink; some systems play weird games with the mod time of symlinks
# (eg FreeBSD returns the mod time of the symlink's containing
# directory).
if ! (
set X `ls -Lt $srcdir/configure conftest.file 2> /dev/null`
if test "$*" = "X"; then
# -L didn't work.
set X `ls -t $srcdir/configure conftest.file`
fi
rm -f conftest.file
if test "$*" != "X $srcdir/configure conftest.file" \
&& test "$*" != "X conftest.file $srcdir/configure"; then

# If neither matched, then we have a broken ls. This can happen
# if, for instance, CONFIG_SHELL is bash and it inherits a
# broken ls alias from the environment. This has actually
# happened. Such a system could not be considered "sane".
{ { echo "$as_me:$LINENO: error: ls -t appears to fail. Make sure there is not a broken
alias in your environment" >&5
echo "$as_me: error: ls -t appears to fail. Make sure there is not a broken
alias in your environment" >&2;}
{ (exit 1); exit 1; }; }
fi

test "$2" = conftest.file
)
then
{ { echo "$as_me:$LINENO: error: newly created file is older than distributed files!
Check your system clock" >&5
echo "$as_me: error: newly created file is older than distributed files!
Check your system clock" >&2;}
{ (exit 1); exit 1; }; }
fi
{ echo "$as_me:$LINENO: result: yes" >&5
echo "${ECHO_T}yes" >&6; }
-- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 10:15, 17 September 2013 (UTC)

Made slightly more efficient. [[Special:Contributions/173.245.54.31|173.245.54.31]] 14:11, 22 March 2014 (UTC)

;segfaults
I do not think Randall is suggesting that segfaults are a compiler issue. The insane computer stopped the person before compiling, because this would be the only time that he could change the code to prevent possible segfaults during runtime.{{unsigned|Flewk}}
:You're totally right; unless it's an interpreter, or the compiler is running some code at compile-time, the compiler will almost never get a segfault itself due to a buggy program. I updated the explanation to clarify this. [[User:APerson|APerson]] ([[User talk:APerson|talk!]]) 06:17, 29 October 2018 (UTC)

;Reference in the title text
Could the title text be a reference to the ending of the movie "Identitiy" (2003)? SPOILER ALERT - At the end of the movie we see Timmy grinning and holding a spatula just before murdering Paris. And Timmy is very much insane. - END SPOILER [[User:Prallax|Prallax]] ([[User talk:Prallax|talk]]) 21:35, 25 July 2023 (UTC)

2764: Cosmological Nostalgia Content

2023-05-13T10:24:44Z

Prallax: expanded explanation of redshift

{{comic
| number = 2764
| date = April 17, 2023
| title = Cosmological Nostalgia Content
| image = cosmological_nostalgia_content_2x.png
| imagesize = 316x386px
| noexpand = true
| titletext = Later renouncing clickbait, Einstein called his inclusion of cosmological content in general relativity the biggest blunder of his career.
}}

==Explanation==
{{incomplete|1=Created by a z=90s KID - The entire start of this explanation with the made up word multimanteu that has nothing to do with this comic should be changes maybe just deleted! Needs more explanation of what the click bait in the title text refers to in the context of the Einstein blunder and why that belongs in this nostalgia comic. Do NOT delete this tag too soon.}}

Like many xkcds, this comic is a figurative {{w|Portmanteau|multimanteau}}, in this case a combination of "{{w|cosmological constant}}" (an astrophysics term related to {{w|dark energy}} and to the accelerated {{w|expansion of the universe}}) and "[https://movementstrategy.com/editorial/nostalgia-social-media/ nostalgia content]" (clickbait marketing aimed at a specific age group referencing pop culture from their youth). The [https://tvtropes.org canonical examples] of nostalgia content are "[https://knowyourmeme.com/memes/only-90s-kids--2 Only 90s Kids Remember...]" and "[https://knowyourmeme.com/memes/feel-old-yet Feel Old yet?]". Some people of relatively advanced years like to make comparisons to others in their age group of where events that they remember fit into history; e.g., "The first moon landing was closer to the end of World War I than to today."

In cosmology, z is the symbol for {{w|Redshift|redshift}}: photons traveling from an object that is moving away from the observer exhibit an increase in wavelength, resulting in its color shifting towards the red end of the spectrum, and the variable z quantifies the amount of shift. Due to the expansion of the universe, objects that are further away from us appear to be moving away from us at a faster velocity, resulting in higher redshift. As light has finite velocity, it takes a longer time for light from faraway objects to arrive at the observer[https://en.wikipedia.org/wiki/Speed_of_light#Spaceflight_and_astronomy]. So the light observed at the present must have been emitted by the faraway object further back in time. In this sense, after assuming a cosmological model, redshift and cosmic time can be put in a one-to-one relation and are often used interchangeably by astronomers. Redshift z = 10 would correspond to about 500 million years after the universe was formed, or almost 13 billion years ago[https://www.astro.ucla.edu/~wright/CosmoCalc.html]. Megan is stating that only people that were born at that time (z = 10) can remember when the first stars were still forming. At redshift z = 10 the matter filling the universe was mostly constituted of neutral hydrogen gas, referred by Megan as "cold" (it had a temperature of about 300 K) and "dark" (there were no light sources in the universe before the first stars formed. This epoch is indeed called the [https://en.wikipedia.org/wiki/Timeline_of_the_early_universe#Cosmic_Dark_Age cosmic dark ages]). The ultraviolet light emitted by the first stars started ionizing the hydrogen around them in expanding hot plasma bubbles. This process (called {{w|reionization}}) had probably already begun at z = 10, but was really completed only at about z = 6, when the intergalactic hydrogen was completely ionized, as it continues to be at the present day.

Megan also has a red tint (she is drawn in the color [https://www.color-hex.com/color/462424 #462424], a very dark red; here is a comparison of #462424 and black: &emsp; &emsp; ), indicating that she is one such z = 10 kid, because her image is red-shifted, in the literal sense. The irony is that even if she were actually born 13 billion years ago (which would be absurd{{citation needed}}), her image would not appear red-shifted to us now. She would only appear redshifted to a far away observer looking at the young Megan wandering about the primordal cold dark gas.

White Hat, meanwhile, is referencing the fact that the universe has {{w|Scale factor (cosmology)|three eras}}: radiation dominated, matter dominated, and dark energy dominated. As the universe expands, the density of radiation and matter decreases due to their dilution, causing the universe, which first started off being dominated by radiation, to then become dominated by matter, then by dark energy (which does not dilute as the universe expands). Dark energy dominated era, which is when "dark energy started accelerating the universe's expansion" started around 5 billion years ago while {{w|Evolution of bacteria|bacteria evolved around 3 billion years ago}}, meaning that they evolved closer to dark energy domination than to today.

The title and title text play with the similarity in sound between 'content' and 'constant', segueing between web(page) content and cosmological constant.

The title text refers to Einstein's inclusion of the {{w|Cosmological constant|cosmological constant}} to his theory of general relativity in order to attain a static model of the universe, which he later removed, reportedly referring to it as his "biggest blunder". Cosmological constant has, today, been generally accepted as a part of the current cosmological model, relating to the concept of dark energy. It is a pun on content and constant. The title text makes the claim that Einstein actually had cosmological content, which he used to clickbait, and he removed it to renounce clickbait.

==Transcript==
:[Megan (with a red tint) holds both her arms out and up while she is talking to Cueball to the left of her. To the right of Megan White Hat is holding both arms out and down while he is talking to Ponytail to the right of him.]
:Megan: Only z=10 kids remember watching the cool dark gas that suffused the universe being eaten away by expanding bubbles of plasma around the first stars!
:White Hat: Want to feel old? Bacteria evolved closer to when dark energy started accelerating the universe's expansion than to today!

:[Caption below the panel]:
:Cosmological Nostalgia Content

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Comics featuring White Hat]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring real people]] 
[[Category:Comics to make one feel old]]
[[Category:Comics with color]] 
[[Category:Cosmology]]
[[Category:Biology]] 
[[Category:Clickbait]]