explain xkcd - User contributions [en]

3108: Laser Danger

2025-06-28T23:22:54Z

Sophon: /* Explanation */ Modify explanation of why a cat attack is not possible in real life

{{comic
| number = 3108
| date = June 27, 2025
| title = Laser Danger
| image = laser_danger_2x.png
| imagesize = 684x272px
| noexpand = true
| titletext = To combat the threat, many airlines are installing wing-mounted spray bottles.
}}

==Explanation==
{{incomplete|This page was created TO DISTRACT A GIANT KITTEN. Don't remove this notice too soon.}}
Shining a laser at a plane is a federal crime in the United States, and similarly proscribed in many other locations. A sufficiently powerful laser can disorient, distract and/or blind the pilot operating the aircraft. This can prove particularly dangerous to the safety of the aircraft and its occupants during takeoff and landing, when planes are likely to have altitudes and orientations particularly susceptible to laser interference, and are phases that are already hazardous periods of flight. This 'use' of lasers was previously discussed in [[3030: Lasering Incidents]].

Megan humorously claims that there's another reason why lasering a plane is illegal: to avoid provoking cats into leaping at them. Cats are known to [[729: Laser Pointer|chase and jump onto]] the dots created by laser pointers. Cats also prey on birds, with estimates of 1.3 to 3.7 billion birds killed each year. Of course, a cat would be unable to jump to the height of a flying plane{{Citation needed}}. If the cat were able to reach the plane, it would find itself hilariously outsized, though colliding with the plane midair could cause damage akin to a bird strike (e.g. shattered windshield or engine failure).

The title text alludes to the fact that cats do not like getting wet, and one of the methods people use to discourage cats from a place (or activity) that is unwanted is to use spray bottles to wet their fur. The spray bottle might also emit a hissing sound, which cats associate with other cats threatening them. The "wing mounted spray bottles" on aircraft could be a reference to fuel dump tubes, which spray out fuel to lighten the aircraft, commonly used before emergency landings (especially soon after take-off, when a nearly full load of fuel is now more trouble than it should have been). To combat against actual physical threats to aircraft in real life, El Al (The Israeli national airline) and government aircraft {{w|Air Force One|used by heads of state}} often have various {{w|Flare (countermeasure)|countermeasures}} installed.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

Megan: Shining laser pointers at planes is a federal crime. It's incredibly dangerous.

Cueball: Oh, because it can blind the pilot?

Megan: That's one reason...

[A plane is shown, with a green laser pointer aimed at it.]

[The laser disappears. A cat, approximately the same size as the plane, pounces the plane and sends it tumbling.]

{{comic discussion}}<noinclude>

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Aviation]]
[[Category:Cats]]
[[Category:Comics with color]]

Talk:3105: Interoperability

2025-06-22T01:15:31Z

Sophon:

Well, it is not interoperability that would be the potential problem, but potential interconnectedness. Some systems are just not meant to be connected to the rest of the network`21:48, 20 June 2025 (UTC)

There will be more interoperability concerns than the track gauge. Such as the up stop wheels. And side wheels. [[User:Henke37|Henke37]] ([[User talk:Henke37|talk]]) 21:55, 20 June 2025 (UTC)

> "''two standards: US rail systems predominantly have a 143.5cm gauge''" Why cite "US"? 4 ft 8+1⁄2 in was developed in England. The US got confused and Abe Lincoln is credited with ordering first 5' (won't work) then 4 ft 8+1⁄2 in (most US rails were so close to 4'8.5" that they mostly changed in a weekend). Rest of world used UK or US machines, or copied them, with the main exceptions fading away over a century. (Well, Australia was still jacking cars mid-route when I was young.)
https://en.wikipedia.org/wiki/Track_gauge#%22Standard%22_gauge_appears
The width of two horses' asses, a Roman chariot, is often cited as if pre-industrial mechanics standardized.
--[[User:PRR|PRR]] ([[User talk:PRR|talk]]) 05:39, 21 June 2025 (UTC)
:I 'cited' the US, for simplicity. Being British, yes (like many things) it was invented/established here, but Randall typically goes by US expectations of rail-gauge (actually 143.51cm, due to being still exactly US Customary Units-based, whilst the UK is sufficiently metricised). There'll be readers in [https://commons.wikimedia.org/wiki/File:World_RR_Gauge_Map.agr.png non-trivially large parts of the world] where the comic mismatches ''their'' local standards, and rather than go into the whole Stephenson-and-onwards thing (noting that Brunel had a point about wider being better for at least some reasons!), I'd just "americanize" it directly. It was a direct replacement for some "(in this instance)" insertions that sort of implied that track gauges ''could'' be different from those mentioned, but seemed neither to explain the worldwide variation nor account for pretty much all railway(/railroad) track in the US, especially 'subway' systems, definitely was this (give or take a tenth of a millimetre).
:Though I spent some time rephrasing things (for example, mentioning the US Customary thing, then removing it because – again — basing it on Randall's own directly cited value of exactly 1435mm (but in cm) seemed more in the spirit of things). Possibly I didn't neaten it down as much as I might, had I not tried to shoehorn those later-removed snippets in.
:Had also thought to provide links to the roller coaster gauge (also presumed to be "those RCs that Randall considers standard", but had a hard time tracking anything down. I actually found many places with a ''different'' quoted track-width. (e.g. 120cm, unless that was maybe outside-to-outside vs. 110cm inside-to-inside, with the perfectly valid possibility of the coaster-rails being each 50mm tubing, and thus actually ''is'' the same? More research needed!) Closest I could find was something about "woodie / <some germanic name I forget>" systems being 110cm, in a search-engine summary of a reddit article, with a partial URL given (as part of that SE-Summary, cut off with ellipses). But visiting that reddit, ''I couldn't find the original full text'', and I got a browser warning on trying to go to the base 'quoted' URI (long-expired and now cybersquatted by a dodgy page-redirection thing?), so eventually gave up on that and concentrated on other little changes/restructurings that I felt improved the article flow (rightly or wrongly).
:Anyway, that is a boiled down why-and-wherefore of how it ended up said the way it was said (probably been re-edited, by the time you read this). Far too much background material to add to the article, I think, or even try to shove in Trivia (except maybe the 4'8.5"==1435.1mm thing?), but might interest the true connoisseur of such thought processes and tangential information who reads this bit... ;) [[Special:Contributions/82.132.216.63|82.132.216.63]] 08:52, 21 June 2025 (UTC)
:...I tweaked it myself (on top of the intermediate edit that I won't argue with). Not sure I'd have used the word "predominantly" for 'only' 55% of global track, if writing it fresh this morning. Definitely predominant for the US, though. Considered "mostly", etc, of course... [[Special:Contributions/82.132.216.63|82.132.216.63]] 09:17, 21 June 2025 (UTC)

As a coaster and rail nerd, it's basically impossible to actually do this (and not just for the obvious safety problems). The engineering between a rail line and a coaster track (and the wheels that run on them) are completely different. Rails have an I-beam shaped cross section and are fixed to the ground, and trains use steel wheels that have flanges on the inner side keeping the cars from running off the track. Steel roller coaster tracks have a circular cross section and the cars have wheels that are (usually) polyurethane, with additional sets of wheels on the side and running under the track (upstop wheels) keeping them from flying off when pulling Gs. Even if you corrected for any track spacing and shape difference somehow, a roller coaster train would immediately slam its upstop wheels into the ground and stop if you tried to transfer it onto a regular grounded rail line. Likewise, since roller coasters don't have flanges, they often have support beams crossing the inner space between the rails, so a train trying to navigate a coaster track would slam its flanges into the support beams and either immediately derail or else start shearing the entire track apart, if it didn't already derail from the wheel flanges not having any grip on the circular coaster rails. [[User:Optimore|Optimore]] ([[User talk:Optimore|talk]]) 07:07, 21 June 2025 (UTC)
:Resolving such issues would be part of Phase 2. --[[Special:Contributions/81.96.108.67|81.96.108.67]] 07:32, 21 June 2025 (UTC)

Why does the explanation repeatedly and pointedly mention "subway systems" when the comic doesn't mention subways at all (but rather intercity train lines)? [[User:Sophon|Sophon]] ([[User talk:Sophon|talk]]) 01:15, 22 June 2025 (UTC)

Talk:3090: Sail Physics

2025-05-16T20:47:55Z

Sophon:

After the last step, the sailors would then need to ground the boat to avoid being pushed in a circle, wouldn't they? [[User:Sophon|Sophon]] ([[User talk:Sophon|talk]]) 20:47, 16 May 2025 (UTC)

3043: Muons

2025-01-27T21:11:59Z

Sophon: /* Explanation */

{{comic
| number = 3043
| date = January 27, 2025
| title = Muons
| image = muons_2x.png
| imagesize = 284x388px
| noexpand = true
| titletext = Update: I've been banned from the physics department for the way I pronounce "Doppler effect."
}}

==Explanation==
{{incomplete|Created by a BOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOT. Do NOT delete this tag too soon.}}

{{w|Time dilation}} is the concept from {{w|special relativity}} where faster moving objects feel like time is slowing down. Because the ‘regular speed’ muons are moving at a relatively normal speed, Cueball pronounces it properly, but because time slows down for the faster moving muons, Cueball adjusts this, and pronounces it much slower, as if he is being slowed down from talking about them.

The title text references the {{w|Doppler effect}}, the change in frequency of a wave when the observer is moving relative to the source. Extrapolating from the time dilation example, Cueball likely pronounces "Doppler effect" at a high or low pitch (or perhaps starting from a high and dropping to a low pitch like the sound of a passing ambulance siren). Evidently, the physics department does not appreciate Cueball's creativity and has fired him.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
[Cueball is standing in front of a whiteboard, which contains a diagram depicting a muon passing through the atmosphere, the equation for the Lorentz factor, and some illegible text. He's facing away from the whiteboard and holding a pointer that points towards the diagram.]

Cueball: Muons created in the upper atmosphere decay immediately, but fast moving muuuuuoooons are able to reach the surface due to their longer half-lives.

[Caption below comic]:

Physics tip: Remember to adjust your pronunciations to acccount for time dilation.

{{comic discussion}}

3043: Muons

2025-01-27T21:04:40Z

Sophon:

{{comic
| number = 3043
| date = January 27, 2025
| title = Muons
| image = muons_2x.png
| imagesize = 284x388px
| noexpand = true
| titletext = Update: I've been banned from the physics department for the way I pronounce "Doppler effect."
}}

==Explanation==
{{incomplete|Created by a BOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOT. Do NOT delete this tag too soon.}}

{{w|Time dilation}} is the concept from {{w|special relativity}} where faster moving objects feel like time is slowing down. Because the ‘regular speed’ muons are moving at a relatively normal speed, Cueball pronounces it properly, but because time slows down for the faster moving muons, Cueball adjusts this, and pronounces it much slower, as if he is being slowed down from talking about them.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
[Cueball is standing in front of a whiteboard, which contains a diagram depicting a muon passing through the atmosphere, the equation for the Lorentz factor, and some illegible text. He's facing away from the whiteboard and holding a pointer that points towards the diagram.]

Cueball: Muons created in the upper atmosphere decay immediately, but fast moving muuuuuoooons are able to reach the surface due to their longer half-lives.

[Caption below comic]:

Physics tip: Remember to adjust your pronunciations to acccount for time dilation.

{{comic discussion}}

Talk:3042: T. Rex Evolution

2025-01-24T18:45:34Z

Sophon:

too soon bro 😭😭😭😭 [[User:CalibansCreations|'''Caliban''']] ([[User talk:CalibansCreations|talk]]) 12:49, 24 January 2025 (UTC)

The next stage in T-Rex would be a massive lizard with incredible bite force and barely any limbs... an Alligator [[Special:Contributions/172.70.115.198|172.70.115.198]] 13:31, 24 January 2025 (UTC)
: Well, we do have large, legless animals with big mouths and teeth ... they're called 'snakes'. [[Special:Contributions/172.70.178.44|172.70.178.44]] 15:25, 24 January 2025 (UTC)
:two enter keys to form a new line [[User:CalibansCreations|'''Caliban''']] ([[User talk:CalibansCreations|talk]]) 13:33, 24 January 2025 (UTC)

So that explains the loch ness monster?
[[Special:Contributions/172.70.250.195|172.70.250.195]] 14:47, 24 January 2025 (UTC)
: Probably not. T. Rex lived in America. --[[User:Coconut Galaxy|Coconut Galaxy]] ([[User talk:Coconut Galaxy|talk]]) 14:50, 24 January 2025 (UTC)
::Since when has obvious contradictory facts gotten in the way of cryptids? [[Special:Contributions/108.162.238.104|108.162.238.104]] 15:04, 24 January 2025 (UTC)
::Not another CG account ''sigh'' '''[[User:42.book.addict|42.book.addict]][[User talk:42.book.addict|Talk to me!]]''' 16:02, 24 January 2025 (UTC)
:::alright, calm down, it's just the person(s?) who tried to start a conscript ARG on this wiki IIRC [[User:CalibansCreations|'''Caliban''']] ([[User talk:CalibansCreations|talk]]) 17:14, 24 January 2025 (UTC)
::::I guess you’re right. I should probably follow {{w|WP:GOODFAITH}} more when it comes to matters like this '''[[User:42.book.addict|42.book.addict]][[User talk:42.book.addict|Talk to me!]]''' 17:47, 24 January 2025 (UTC)

So this is where snakes come from! {{unsigned|Petercordia|17:01, 24 January 2025 (UTC)}}

Looks like this is how the {{w|tsuchinoko}} came to be... [[User:TheGoomba98|TheGoomba98]] ([[User talk:TheGoomba98|talk]]) 17:12, 24 January 2025 (UTC)

Do we have any figures on just how strong the bite force would be? [[Special:Contributions/172.68.22.223|172.68.22.223]] 17:15, 24 January 2025 (UTC)

It seems like many here say that the T-Rex would become a snake, but it would have to simultaneously evolve to be able to move via the scales on its stomach, so I don't think it's guaranteed [[User:Sophon|Sophon]] ([[User talk:Sophon|talk]]) 18:45, 24 January 2025 (UTC)

Talk:3029: Sun Avoidance

2024-12-26T05:22:11Z

Sophon:

My first time editing the BOT name. [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 01:39, 26 December 2024 (UTC)
:Maybe change it to BOT GETTING NOTHING BUT SUNBURN FOR CHRISTMAS? [[Special:Contributions/198.41.227.177|198.41.227.177]] 03:47, 26 December 2024 (UTC)

Nothing for Christmas? xkcd has fallen [[User:Pie Guy|Pie Guy]] ([[User talk:Pie Guy|talk]]) 02:26, 26 December 2024 (UTC)
:Comic 3000 didn't have anything special for it either. Lame! [[Special:Contributions/172.70.210.68|172.70.210.68]] 03:44, 26 December 2024 (UTC)
:Well, Hannukah is the feast of lights, and Christians say Jesus is The Light, so it kind of fits. Kind of. [[Special:Contributions/198.41.227.177|198.41.227.177]] 03:50, 26 December 2024 (UTC)
XKCD wishes you a merry NOTHING and a happy new NOTHING. Hope you get lots of NOTHING this NOTHING! Remember to spend lots of NOTHING with your NOTHING! [[Special:Contributions/172.70.211.233|172.70.211.233]] 03:34, 26 December 2024 (UTC)

"This would be difficult, since at Parker's aphelion (furthest distance from the Sun in its orbit) it's still only about 7 million km, 35 million km from Solar Orbiter's orbit (and the probes would be much further apart if they're not on the same side of the Sun at the time)." -- What does aligning the probes have to do with the title text? Isn't the nudge at the aphelion meant to lower the perihelion into the Sun? (and not have anything to do with the proximity of the two probes) --[[User:Sophon|Sophon]] ([[User talk:Sophon|talk]]) 05:22, 26 December 2024 (UTC)

Talk:3026: Linear Sort

2024-12-18T17:46:22Z

Sophon: add signature

First in linear time![[User:Mr. I|Mr. I]] ([[User talk:Mr. I|talk]]) 13:28, 18 December 2024 (UTC)

Due to the fact that O(nlog(n)) outgrows O(n), the Linear Sort is not actually linear. [[Special:Contributions/162.158.174.227|162.158.174.227]] 14:21, 18 December 2024 (UTC)
:If your sleep() function can handle negative arguments "correctly", then I guess it could work. [[Special:Contributions/162.158.91.91|162.158.91.91]] 16:27, 18 December 2024 (UTC)

That was fast... [[User:CalibansCreations|'''Caliban''']] ([[User talk:CalibansCreations|talk]]) 15:35, 18 December 2024 (UTC)

Do I even want to know what Randall's thinking nowadays? [[User:Definitely Bill Cipher|⯅A dream demon⯅]] ([[User talk:Definitely Bill Cipher|talk]]) 16:02, 18 December 2024 (UTC)

The title text would be more correct if Randall used e.g. Timsort instead of Mergesort. They both have the same worst-case complexity O(n*log(n)), but the former is linear if the list was already in order, so best-case complexity is O(n). Mergesort COULD also be implemented this way, but its standard version is never linear. [[User:Bebidek|Bebidek]] ([[User talk:Bebidek|talk]]) 16:35, 18 December 2024 (UTC)

According to my estimates extrapolated from timing the sorting of 10 million random numbers on my computer, the break-even point where the algorithm becomes worse than linear is beyond the expected heat death of the universe. I did neglect the question of where to store the input array. --[[Special:Contributions/162.158.154.35|162.158.154.35]] 16:37, 18 December 2024 (UTC)
:If the numbers being sorted are unique, each would need a fair number of bits to store. (Fair meaning that the time to do the comparison would be non-negligible.) If they aren't, you can just bucket-sort them in linear time. Since we're assuming absurdly large memory capacity. [[Special:Contributions/162.158.186.253|162.158.186.253]] 17:14, 18 December 2024 (UTC)

What system was the person writing the description using where Sleep(n) takes a parameter in whole seconds rather than the usual milliseconds? [[Special:Contributions/172.70.216.162|172.70.216.162]] 17:20, 18 December 2024 (UTC)

If I had a nickel for every time I saw an O(n) sorting algorithm using "sleep"… But this one is actually different. The one I usually see feeds the to-be-sorted value into the sleep function, so it schedules "10" to be printed in 10 seconds, then schedules "3" to be printed in 3 seconds, etc., which would theoretically be linear time, if the sleep function was magic. [[User:Fabian42|Fabian42]] ([[User talk:Fabian42|talk]]) 17:25, 18 December 2024 (UTC)

This comic also critiques/points out the pitfalls of measuring time complexity using Big-O notation, such as an algorithm or solution that runs in linear time still being too slow for its intended use case. [[User:Sophon|Sophon]] ([[User talk:Sophon|talk]]) 17:46, 18 December 2024 (UTC)

Talk:3026: Linear Sort

2024-12-18T17:46:02Z

Sophon:

2938: Local Group

2024-05-28T06:28:13Z

Sophon: /* Transcript */ describe position of galaxies

{{comic
| number = 2938
| date = May 27, 2024
| title = Local Group
| image = local_group_2x.png
| imagesize = 547x266px
| noexpand = true
| titletext = Cosmologists estimate the spaghetti strand to be about 200 septillion calories, though it could be higher depending on the nutritional value of dark matter.
}}

==Explanation==
{{incomplete|Created by a COSMIC SPAGHETTI BOWL - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
In {{w|Timeline_of_the_far_future|less than 5 billion years}} into the future, the {{w|Andromeda Galaxy}} is expected to {{w|Andromeda–Milky Way collision|collide}} with the {{w|Milky Way}}. This comic suggests this will happen when both galaxies get into a kissing distance after having slurped the same cosmic spaghetti strand.

This is a spoof of a famous scene in the movie ''{{w|Lady and the Tramp}}'', where the titular dogs are eating spaghetti in an Italian restaurant. They happen to be eating the same strand from opposite ends, so they end up meeting in the middle and kissing. This scene has been mentioned before in [[2612: Lightsabers]].

It's possible that this is also a reference to {{w|Flying_Spaghetti_Monster|Pastafarianism}}, the spoof religion which claims that the universe might have been created by a "flying spaghetti monster". If both galaxies are eating one of the deity's "noodly appendages" they may be guilty of blasphemy.

The title text figure of 200 septillion (2×1026) calories for the Milky Way-Andromeda noodle equates to approximately 2500 calories per foot of noodle. This value makes sense only if Randall is referring to the physics/chemistry "small" {{w|calorie}} (= 4.184 joules) rather than the dietary "large" calorie (= 4184 joules). Dietary calories would be the usual usage in a food-related context like this one, but Randall was trained as a physicist and apparently used the small calorie here. Having 2.5 dietary calories per foot is roughly correct for a noodle size between thin spaghetti and angel hair pasta.

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

:[Two galaxies, positioned on the left and right sides of the panel, are each eating a single noodle of spaghetti, originating from a bowl of spaghetti in between them.]
:[Left galaxy:] Sluuurp
:[Right galaxy:] Sluuurp

:[Caption below the panel:]
:Astronomers have determined that the Milky Way and Andromeda are currently slurping up the same strand of cosmic spaghetti, suggesting that in 5 billion years they will likely kiss.

{{comic discussion}}

[[Category:Astronomy]]
[[Category:Food]]
[[Category:Cosmology]]