https://www.explainxkcd.com/wiki/api.php?action=feedcontributions&user=162.158.106.216&feedformat=atomexplain xkcd - User contributions [en]2024-03-29T15:30:34ZUser contributionsMediaWiki 1.30.0https://www.explainxkcd.com/wiki/index.php?title=1406:_Universal_Converter_Box&diff=1769311406: Universal Converter Box2019-07-21T06:30:27Z<p>162.158.106.216: /* Left side */ DVI is in fact sort of compatible with VGA.</p>
<hr />
<div>{{comic<br />
| number = 1406<br />
| date = August 11, 2014<br />
| title = Universal Converter Box<br />
| image = universal_converter_box.png<br />
| titletext = Comes with a 50-lb sack of gender changers, and also an add-on device with a voltage selector and a zillion circular center pin DC adapter tips so you can power any of those devices from the 90s.<br />
}}<br />
<br />
==Explanation==<br />
Converter boxes are used to connect two or more devices together which otherwise couldn't be, due to differently shaped plugs, different voltages, or different protocols of communication.<br />
<br />
Converter boxes or converter cables are commonly found for several of the plugs at the top of the list - such as from USB to micro-USB. As this is supposed to be a Universal Converter Box, there are many connections.<br />
<br />
The humour from this comic comes from the sheer number of [[927: Standards|different standards]] that all claim to be the universal way to connect two devices, in their target market, as well as the progressively ridiculous conversions that this box is capable of doing, for example, converting audio from a 1/8&nbsp;inch / 3.5&nbsp;mm headphone jack, into a variety of fuel suitable for running your car.<br />
<br />
A connector is capable of making a connection to another connector only if the connectors are of the same style and the opposite gender ("male" connector is plug, "female" connector is socket), except for rare "genderless" connectors, such as the token ring mentioned above. Gender changers are devices with two connectors of the same gender. The "circular center pin DC adapter tips" in the title text are barrel jack power plugs. There are a large number of these style connectors, and many of these devices look the same, leading to frustration.<br />
<br />
===Different connectors===<br />
The plugs are numbered from top to bottom and incremented for every wire that comes directly out of the converter box.<br />
<br />
====Left side====<br />
#{{w|VGA connector|VGA}} (Video Graphics Array): This a video connector (standard is blue) that connects computers and monitors or projectors. It has fifteen pins in a D-shell. It's still one of the common type of video connectors.<br />
#{{w|Digital Visual Interface|DVI}} (Digital Visual Interface): This a video connector (standard is white) that uses a D-shell with flat pins. DVI is only partially compatible with VGA ports, itself being designed as a "universal" connection standard via its sub-types of DVI-D (digital-only), DVI-I (digital+analog), and the incredibly-rare DVI-A (analog-only).<br />
#{{w|HDMI}} (High Definition Multimedia Interface): This is an audio video connector that supports high definition video and audio.<br />
#{{w|Thunderbolt (interface)|Thunderbolt}}: Thunderbolt can transfer both video signals to a monitor, audio signals to speakers, and send and receive data at the same time, over the same port.<br />
#{{w|IEEE 1394|Firewire}} (IEEE 1394): A bidirectional data transfer connector, similar to USB, Firewire is used for networking computers, and connecting audio/video equipment to computers.<br />
#{{w|Component video|Component}} and {{w|RCA connector|RCA}}: Both component video and RCA are ways of transmitting video and audio signals. RCA is the name of the connector type. RCA uses one plugs per audio channel (e.g. left and right channels). RCA (composite) uses one plug for video where component uses three: Y (luma), Pb (Blue - Y), Pr (Red - Y).<br />
#{{w|Phone connector (audio)|1/8" audio/video}} (3.5&nbsp;mm phone connector): Best known as a headphone plug, but also used for other audio equipment and for some video equipment.<br />
#{{w|Parallel port}}: A port that used to be used to connect printers to PCs.<br />
#{{w|S-Video (analog video standard)|S-video}}: A video with the video signal split in Y (luma) and C (chroma).<br />
#{{w|In-flight entertainment#History|Airline pneumatic tube audio}}: The seat would contain the loudspeaker, and the headphone connected to this unit with a pneumatic tube to conduct the sound.<br />
#{{w|PS/2 port|PS/2}}, PS/3 and PS/4: The PS/2 connector was used for mouse and keyboard connections in older computers; it has been superseded by USB. There are no PS/3 or PS/4 connectors. This is a play on the {{w|PlayStation}} line of video game consoles, which have recently seen their second, third, and fourth generations abbreviated to PS2, PS3, and PS4.<br />
#{{w|NEMA connector|120V AC}}: This style of plug is used for domestic power outlets in the US, Canada, Mexico, and some other parts of the Americas. The pin marked "removable" is the ground pin. Not every device requires a ground pin, and some older power sockets do not have a hole for it.<br />
#{{w|Floppy disk|Floppy}}, {{w|Parallel ATA|IDE}}, {{w|Hard disk drive|2.5"}}, {{w|SCSI connector|SCSI}}: These are {{w|Insulation-displacement connector|IDC connectors}} for connecting to media drives to processors using different numbers of pins, and hence different widths of {{w|Ribbon cable|cable}}. Despite this similarity, real plugs would not work with break-away parts as the pinout has no similarities and the connectors are keyed differently.<br />
<br />
====Right side====<br />
#{{w|USB#Connectors and plugs|USB}}: Also known as USB-A. USBs are used for connecting various devices to computers, each other, and to power supplies and chargers. The USB standard has multiple connectors. Some of the others are below.<br />
#USB (weird other end): Also known as USB-B.<br />
#mini-USB/micro USB: Alternate smaller connections for USB communication.<br />
#macro USB: A joke about a larger version of USB.<br />
#{{w|F connector}}: A type of coaxial plug used for various television signals and for cable modems.<br />
#{{w|Optical fiber connector|Fiber}}: Optical fiber cables are used for various data transmission purposes and are often connected to devices with only a connector on the device, and none on the cable.<br />
#{{w|Registered jack#RJ11, RJ14, RJ25 wiring|RJ11}}/{{w|Ethernet over twisted pair|Ethernet}}: Ethernet connections, which use a {{w|TIA/EIA-568|TIA/EIA-568 connector}} (often mistakenly called RJ45 because of its visual similarity), are the most common fixed wire connection for computer networking. The RJ11 connector is used for land-line telephones.<br />
#{{w|Token ring}}: The token ring was a late-80s competitor to Ethernet for fixed-wire network connections. Its connectors were large and boxy, but were unique in that they were genderless.<br />
#{{w|MagSafe}}: Magnetically-attached power connectors used on Apple devices. The original MagSafe (introduced in 2006) was later replaced by MagSafe 2 (introduced in 2012); both come in "L" and "T" shapes as shown here for MagSafe and MagSafe 2, respectively, but are incompatible. MagSafe 3 and 4 do not actually exist yet (and probably never will, now Apple is using {{w|USB-C}} to charge its laptops). The MagSafe 3 charger appears to resemble the Apple Watch charger, interestingly. Also, the MagSafe 4 "connector" appears to be broken; this is likely a joke about the {{w|MagSafe#Defects|poor quality}} of the original MagSafe 1 cables.<br />
#{{w|Bluetooth#Communication and connection|Bluetooth dongle}}: A USB device that allows the converter to connect via the {{w|Bluetooth}} wireless networking standard to accessories like phones and computers for audio, general purpose file transfer, mouse and keyboard interaction and a wide variety of other uses.<br />
#{{w|SCART}}: An audio/video connector mostly used in Europe; it replaced other connectors like component video, but has itself been superseded by HDMI.<br />
#{{w|Tin can telephone|String}}: For connecting to a "tin can telephone", an analogue device for transmitting sound through a physical connection rather than electronically or via radio waves. Probably also a reference to {{w|CAN bus}}.<br />
#{{w|Fuel dispenser#Nozzles|Fuel nozzle}}, with a switch to choose between different {{w|octane rating}}s and {{w|diesel fuel}}: Dispensers for fossil fuels used to power internal combustion engines. Presumably, this would be the gasoline/petrol tip [see trivia].<br />
<br />
===Trivia===<br />
For some interfaces, such as USB, the female side is standard to the device while the male side is standard to the cable. For other interfaces, such as the RS-232 serial port, the conventions vary or there is no convention.<br />
<br />
The "universal" connector here doesn't support the proper RS-232, with the closest surrogate available being RJ-11. The other nearest analog would be the parallel port, available in Centronix and D-25-pin connectors.<br />
<br />
The SCSI connectors have been available as the "internal" connectors (see the "break-away" above) of 2 different widths, Centronix, 2 widths of the mini-D connectors with the easily bendable pins, 3 widths of the more reliable pin-less mini-connectors, and high-speed serial.<br />
<br />
Not only is there gender and connector type, but there are also different standards on what data/power is connected on each pin of the connector. Building a working connection often involved getting 3 or 4 adapters connected in a sequence to produce the right connector, gender and pin-out.<br />
<br />
Barrel jack power plugs were developed in the 1980s. The "barrel" has an inner diameter an outer diameter, and different style pins.<br />
<br />
A D-shell is a trapezoidal metal skirt that protects the pins, prevents the connector from being plugged in the wrong way, and makes the physical connection more secure.<br />
<br />
A VGA was developed in 1987, and with new versions being developed since then.<br />
<br />
DVI can be configured to support multiple modes such as DVI-D (digital only), DVI-A (analog only), or DVI-I (digital and analog).<br />
<br />
HDMI has slowly been replacing DVI and VGA ports on newer devices due to the simplicity and the smaller footprint and overall dimensions.<br />
<br />
Thunderbolt is far faster than almost any connector on the market for transferring data. However, the limited adoption by manufacturers, the higher costs of the hardware, and the security concerns inherent to the interface have limited the adoption by consumers.<br />
<br />
Because Firewire is designed to allow {{w|backplane}} access and {{w|direct memory access}} (DMA) to devices, there are additional conversion and security issues with it.<br />
<br />
The phone connector diameter of 1/8" is only an approximation using {{w|Imperial units}}. The standard actually specifies a size in the {{w|Metric system}} of 3.5&nbsp;mm. The video plug has 3 contacts (Tip, Ring and Sleeve) and the audio has 4 contacts (Tip, Ring, Ring and Sleeve).<br />
<br />
While no longer common in homes or offices, parallel connections are still used in some {{w|embedded system}}s.<br />
<br />
Airline pneumatic tube audio was used by in-flight entertainment systems manufactured from 1963 until 1979.<br />
<br />
Note that while AC adapters are necessary—and widely available—to suit sockets in other countries, this "universal" converter does not feature any other AC power plugs, but this could be accommodated using adapters.<br />
<br />
{{w|Cheater plug}}s exist to connect a NEMA grounding-type plug (three prongs) to a NEMA non-grounding receptacle (two slots), but the use of such an adapter can be hazardous if the grounding tab is not connected to electrical ground. A safer alternative is to replace the outlet with a {{w|Residual-current device|Ground Fault Circuit Interrupter (GFCI)}} breaker outlet.<br />
<br />
The computer media drive connectors are unlike the motherboard-powering connectors from the Power Supply Unit of a PC, which may involve multiple additional 4, 6 and 8-pin 'breakout' supply cables that have this feature and specially 'keyed' pin-sheaths as well to allow forward/backward compatibility between various versions of PSU and motherboard that could be used (and power-hungry GPUs of various kinds, as well).<br />
<br />
Note that some embedded systems such as cash registers actually do use larger USB connectors to include 12V and/or 24V power connections. These are not, however, called "macro-USB", and are not as large.<br />
<br />
Other countries often use RJ11-ended cables with locally-specific adapter-ends, e.g. the BS 6312 in Britain. Broadband microfilters may make use of this difference by splitting a relevant telephone plug standard into the local non-RJ11 style of telephone plug for an "audio-only" pass-through socket and an RJ11 for the router/modem to be cabled up to for the abstracted "data-only" signal — making an adapter for this will be nearly impossible.<br />
<br />
There are two common systems for showing octane numbers on fuel pumps; the numbers shown (87, 91, 93) most closely map to {{w|Octane rating#Anti-Knock Index (AKI) or (R+M)/2|Anti-Knock Index}} values which is used for the North American market and a number of other countries, the other system used in the rest of the world is Research Octane Number. In the AKI system; 87 octane (91 RON) is regular US, 91 octane (95 RON) is regular European, 93 octane (98 RON) is premium European, and in US both 91 and 93 are considered premium/super depending on the regulations of a particular state. Some states, such as California, forbid the sale of the gasoline above 91 octane. Only very rarely could both 91 and 93 be found at the same gas station. The typical line-up is "regular" (87), "plus" (89), and "premium"/"super" (depending on the state and on the fuel brand, 91, 92 or 93 octane). A standard diesel nozzle (24mm) is slightly larger diameter than a standard petrol nozzle (21mm) so you cannot tank diesel into a petrol car but if this nozzle has the petrol nozzle diameter you are still able to tank with it into some diesel cars. Some manufacturers such as Volkswagen fit a misfueling guard and fuel filler neck cap or have redesigned the fuel filler to prevent a petrol nozzle being used in a diesel car.<br />
<br />
Since the release of this comic, Apple has created a magnetic charging cable for its Apple Watch, which functions in the same manner as the current MagSafe 1 & 2 by using a magnet to connect to the device. This new charger looks identical to the fictional MagSafe 3 in the comic.<br />
<br />
==Transcript==<br />
:[Universal converter box with wires to connectors:]<br />
:VGA<br />
:DVI<br />
:HDMI<br />
:Thunderbolt<br />
:Firewire<br />
:Component<br />
:[sharing connectors with Component:]<br />
:RCA<br />
:1/8" Audio<br />
:1/8" Video<br />
:Parallel Port<br />
:S-Video<br />
:Airline Pneumatic Tube Audio<br />
:PS/2/3/4<br />
:120V AC<br />
::[pointing to ground pin:]<br />
::Removable<br />
:Floppy/IDE/2.5"/SCSI<br />
::[pointing to sections in IDC connector:]<br />
::Break here<br />
:USB<br />
:USB (weird other end)<br />
:Mini-USB<br />
:Micro USB<br />
:Macro USB<br />
:F Connector<br />
:Fiber<br />
:RJ11<br />
:Ethernet<br />
:Token Ring<br />
:MagSafe<br />
:MagSafe 2<br />
:MagSafe 3<br />
:MagSafe 4<br />
:Bluetooth Dongle<br />
:SCART<br />
:String (fits most cans)<br />
:[Fuel nozzle with selector for:]<br />
:87/91/93/Diesel<br />
<br />
{{comic discussion}}<br />
[[Category:Computers]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=Talk:2169:_Predictive_Models&diff=176032Talk:2169: Predictive Models2019-07-01T20:05:05Z<p>162.158.106.216: signing</p>
<hr />
<div><!--Please sign your posts with ~~~~ and don't delete this text. New comments should be added at the bottom.--><br />
<br />
If you click on the comic, it opens a page with error 404. Looking at the URL, it says "At the July 28th meeting", which I assume is the prediction result to the title text suggesting that they will be 1 month late. [[Special:Contributions/162.158.106.174|162.158.106.174]] 17:13, 28 June 2019 (UTC)<br />
<br />
: Fixsed it, my years of mediawiki knowledge have finally come to use. [[User:Iggyvolz|Iggyvolz]] ([[User talk:Iggyvolz|talk]])<br />
<br />
In the HTML tag for the link (the <a> tag surrounding the comic image) after the link it says "cancel the meeting! our cover is blown" [[User:Everlastingwonder|Everlastingwonder]] ([[User talk:Everlastingwonder|talk]]) 17:21, 28 June 2019 (UTC)<br />
<br />
In the [https://m.xkcd.com/2169/ mobile version], you can read «See also: [AT THE JULY 28TH MEETING][tab] "Cancel the meeting! Our cover is blown."» It leads to a 404, like the other examples in the comments here. [[Special:Contributions/172.69.44.136|172.69.44.136]] 17:31, 28 June 2019 (UTC)<br />
<br />
This looks a whole lot like Gmail's [https://ai.googleblog.com/2018/05/smart-compose-using-neural-networks-to.html Smart Compose] [[Special:Contributions/172.68.206.76|172.68.206.76]]<br />
<br />
Today GMail actually predicted the beginning of my mail correctly. I typed literally zero characters and it already knew how to continue. In the future, we won't even have to upload our brains to a computer, a backup will already be available there automatically. [[User:Fabian42|Fabian42]] ([[User talk:Fabian42|talk]]) 21:32, 28 June 2019 (UTC)<br />
<br />
Not a backup, a simulation. [[Special:Contributions/108.162.219.184|108.162.219.184]] 04:46, 29 June 2019 (UTC)<br />
<br />
: If you can't tell the difference, does it matter? [[Special:Contributions/173.245.48.147|173.245.48.147]] 17:04, 1 July 2019 (UTC)<br />
<br />
On my Mac the title text only shows "WE WILL ARREST THE REVOLUTION MEMBERS" while on my iPad (where you long press to see title texts) long pressing only shows the link. Weird. Also someone remind me to check the link again on July 28. [[User:Herobrine|Herobrine]] ([[User talk:Herobrine|talk]]) 13:10, 29 June 2019 (UTC)<br />
<br />
: On my Ubuntu system, both Firefox and Chrome display "WE WILL ARREST THE REVOLUTION MEMBERS" as the title text and "<nowiki>https://xkcd.com/[AT THE JULY 28TH MEETING][tab]</nowiki>" as the link target, which is also what's in the HTML source. Additionally, the HTML source is malformed, with quotes inside quotes in the href attribute. - [[User:Linneris|Linneris]] ([[User talk:Linneris|talk]]) 14:37, 29 June 2019 (UTC)<br />
::Malformed. Precisely! I think there was a glitch while the comic was uploaded, which used the title text as a link in addition to as the title text. It didn't include the last part due to the quotes. It will be either fixed or legitimate, or at least make the href a little nicer. [[User:Jacky720|That's right, Jacky720 just signed this]] ([[User talk:Jacky720|talk]] | [[Special:Contributions/Jacky720|contribs]]) 21:24, 29 June 2019 (UTC)<br />
:::Actually... Looking at the comic again (for the first time on my PC), I would like to rethink that. I think this is Randall's method of demonstrating the [tab]; clicking and looking at the URL. [EDIT] Man, the more I think, the weirder it gets. Maybe it's about how sometimes you can find the information on the client side in the code where it should be hidden? I don't know anymore. [[User:Jacky720|That's right, Jacky720 just signed this]] ([[User talk:Jacky720|talk]] | [[Special:Contributions/Jacky720|contribs]]) 21:27, 29 June 2019 (UTC)<br />
::When you look at the source of [https://xkcd.com/2169/%5BAT%20THE%20JULY%2028TH%20MEETING%5D%5Btab%5D that 404 page], you can see six HTML comments with the content ''a padding to disable MSIE and Chrome friendly error page''. This is to prevent MSIE and Chrome from displaying "helpful" proprietary error pages. If you change the link in the slightest, you will also get a 404 page, but without these comments. I assume that either this was a glitch (intended or unintended) and this particular 404 page was modified so that everyone can see that the authors are aware of it, *or* it's a hint pointing to somewhere else. A rabbit hole maybe? I would like the latter to be true, but I haven't found anything.--[[Special:Contributions/162.158.90.168|162.158.90.168]] 22:42, 29 June 2019 (UTC)<br />
:::Not for me. I see the same tiny Nginx 404 page with the same HTML source as any other 404 page due to invalid link on xkcd.com. - [[User:Linneris|Linneris]] ([[User talk:Linneris|talk]]) 07:14, 30 June 2019 (UTC)<br />
<br />
This reminds me of that time where via data analytics on things like shopping habits, [https://www.nytimes.com/2012/02/19/magazine/shopping-habits.html Target figured out that a teen girl was pregnant before her father did]. [[User:Ahiijny|Ahiijny]] ([[User talk:Ahiijny|talk]]) 06:42, 30 June 2019 (UTC)<br />
<br />
I tried this on google, and got "we will arrest chamisa" and "the meeting will be in room 27" and "our next meeting will be at 3 p.m. on wednesday". Any more? [[Special:Contributions/162.158.59.214|162.158.59.214]] 19:16, 30 June 2019 (UTC)<br />
<br />
I decided to see what a more sophisticated predictive model would do, so I plugged it into Talk to Transformer. The output: "Long live the revolution. Our next meeting will be at 10 a.m. on December 14 at the Cressey Building, 1636 S. Second St. Please invite your friends, family, and coworkers! For those interested in donating to the cause, please contact:" I'm legitimately impressed. [[User:Arcorann|Arcorann]] ([[User talk:Arcorann|talk]]) 01:03, 1 July 2019 (UTC)<br />
<br />
Thinking about predictive text, in combination with the advice on the futility of making people change their passwords frequently, perhaps systems which require people to change their passwords could be more helpful by observing the pattern the user is using, and suggesting what the next password should be. [https://www.troyhunt.com/passwords-evolved-authentication-guidance-for-the-modern-era/ Passwords Evolved: Authentication Guidance for the Modern Era] [[Special:Contributions/162.158.106.216|162.158.106.216]] 20:05, 1 July 2019 (UTC)</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=Talk:2169:_Predictive_Models&diff=176031Talk:2169: Predictive Models2019-07-01T20:04:28Z<p>162.158.106.216: Systems that do password expiration should offer suggestions for the next password the person is likely to generate</p>
<hr />
<div><!--Please sign your posts with ~~~~ and don't delete this text. New comments should be added at the bottom.--><br />
<br />
If you click on the comic, it opens a page with error 404. Looking at the URL, it says "At the July 28th meeting", which I assume is the prediction result to the title text suggesting that they will be 1 month late. [[Special:Contributions/162.158.106.174|162.158.106.174]] 17:13, 28 June 2019 (UTC)<br />
<br />
: Fixsed it, my years of mediawiki knowledge have finally come to use. [[User:Iggyvolz|Iggyvolz]] ([[User talk:Iggyvolz|talk]])<br />
<br />
In the HTML tag for the link (the <a> tag surrounding the comic image) after the link it says "cancel the meeting! our cover is blown" [[User:Everlastingwonder|Everlastingwonder]] ([[User talk:Everlastingwonder|talk]]) 17:21, 28 June 2019 (UTC)<br />
<br />
In the [https://m.xkcd.com/2169/ mobile version], you can read «See also: [AT THE JULY 28TH MEETING][tab] "Cancel the meeting! Our cover is blown."» It leads to a 404, like the other examples in the comments here. [[Special:Contributions/172.69.44.136|172.69.44.136]] 17:31, 28 June 2019 (UTC)<br />
<br />
This looks a whole lot like Gmail's [https://ai.googleblog.com/2018/05/smart-compose-using-neural-networks-to.html Smart Compose] [[Special:Contributions/172.68.206.76|172.68.206.76]]<br />
<br />
Today GMail actually predicted the beginning of my mail correctly. I typed literally zero characters and it already knew how to continue. In the future, we won't even have to upload our brains to a computer, a backup will already be available there automatically. [[User:Fabian42|Fabian42]] ([[User talk:Fabian42|talk]]) 21:32, 28 June 2019 (UTC)<br />
<br />
Not a backup, a simulation. [[Special:Contributions/108.162.219.184|108.162.219.184]] 04:46, 29 June 2019 (UTC)<br />
<br />
: If you can't tell the difference, does it matter? [[Special:Contributions/173.245.48.147|173.245.48.147]] 17:04, 1 July 2019 (UTC)<br />
<br />
On my Mac the title text only shows "WE WILL ARREST THE REVOLUTION MEMBERS" while on my iPad (where you long press to see title texts) long pressing only shows the link. Weird. Also someone remind me to check the link again on July 28. [[User:Herobrine|Herobrine]] ([[User talk:Herobrine|talk]]) 13:10, 29 June 2019 (UTC)<br />
<br />
: On my Ubuntu system, both Firefox and Chrome display "WE WILL ARREST THE REVOLUTION MEMBERS" as the title text and "<nowiki>https://xkcd.com/[AT THE JULY 28TH MEETING][tab]</nowiki>" as the link target, which is also what's in the HTML source. Additionally, the HTML source is malformed, with quotes inside quotes in the href attribute. - [[User:Linneris|Linneris]] ([[User talk:Linneris|talk]]) 14:37, 29 June 2019 (UTC)<br />
::Malformed. Precisely! I think there was a glitch while the comic was uploaded, which used the title text as a link in addition to as the title text. It didn't include the last part due to the quotes. It will be either fixed or legitimate, or at least make the href a little nicer. [[User:Jacky720|That's right, Jacky720 just signed this]] ([[User talk:Jacky720|talk]] | [[Special:Contributions/Jacky720|contribs]]) 21:24, 29 June 2019 (UTC)<br />
:::Actually... Looking at the comic again (for the first time on my PC), I would like to rethink that. I think this is Randall's method of demonstrating the [tab]; clicking and looking at the URL. [EDIT] Man, the more I think, the weirder it gets. Maybe it's about how sometimes you can find the information on the client side in the code where it should be hidden? I don't know anymore. [[User:Jacky720|That's right, Jacky720 just signed this]] ([[User talk:Jacky720|talk]] | [[Special:Contributions/Jacky720|contribs]]) 21:27, 29 June 2019 (UTC)<br />
::When you look at the source of [https://xkcd.com/2169/%5BAT%20THE%20JULY%2028TH%20MEETING%5D%5Btab%5D that 404 page], you can see six HTML comments with the content ''a padding to disable MSIE and Chrome friendly error page''. This is to prevent MSIE and Chrome from displaying "helpful" proprietary error pages. If you change the link in the slightest, you will also get a 404 page, but without these comments. I assume that either this was a glitch (intended or unintended) and this particular 404 page was modified so that everyone can see that the authors are aware of it, *or* it's a hint pointing to somewhere else. A rabbit hole maybe? I would like the latter to be true, but I haven't found anything.--[[Special:Contributions/162.158.90.168|162.158.90.168]] 22:42, 29 June 2019 (UTC)<br />
:::Not for me. I see the same tiny Nginx 404 page with the same HTML source as any other 404 page due to invalid link on xkcd.com. - [[User:Linneris|Linneris]] ([[User talk:Linneris|talk]]) 07:14, 30 June 2019 (UTC)<br />
<br />
This reminds me of that time where via data analytics on things like shopping habits, [https://www.nytimes.com/2012/02/19/magazine/shopping-habits.html Target figured out that a teen girl was pregnant before her father did]. [[User:Ahiijny|Ahiijny]] ([[User talk:Ahiijny|talk]]) 06:42, 30 June 2019 (UTC)<br />
<br />
I tried this on google, and got "we will arrest chamisa" and "the meeting will be in room 27" and "our next meeting will be at 3 p.m. on wednesday". Any more? [[Special:Contributions/162.158.59.214|162.158.59.214]] 19:16, 30 June 2019 (UTC)<br />
<br />
I decided to see what a more sophisticated predictive model would do, so I plugged it into Talk to Transformer. The output: "Long live the revolution. Our next meeting will be at 10 a.m. on December 14 at the Cressey Building, 1636 S. Second St. Please invite your friends, family, and coworkers! For those interested in donating to the cause, please contact:" I'm legitimately impressed. [[User:Arcorann|Arcorann]] ([[User talk:Arcorann|talk]]) 01:03, 1 July 2019 (UTC)<br />
<br />
Thinking about predictive text, in combination with the advice on the futility of making people change their passwords frequently, perhaps systems which require people to change their passwords could be more helpful by observing the pattern the user is using, and suggesting what the next password should be. [https://www.troyhunt.com/passwords-evolved-authentication-guidance-for-the-modern-era/ Passwords Evolved: Authentication Guidance for the Modern Era]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=2169:_Predictive_Models&diff=1760282169: Predictive Models2019-07-01T19:57:39Z<p>162.158.106.216: /* Explanation */ another example of predicting user input - password expiration policies</p>
<hr />
<div>{{comic<br />
| number = 2169<br />
| date = June 28, 2019<br />
| title = Predictive Models<br />
| image = predictive_models.png<br />
| titletext = WE WILL ARREST THE REVOLUTION MEMBERS [AT THE JULY 28TH MEETING][tab] "Cancel the meeting! Our cover is blown."<br />
}}<br />
<br />
*When the image is clicked the "Not Available" xkcd post opens up: [https://xkcd.com/%5BAT%2520THE%2520JULY%252028TH%2520MEETING%5D%5Btab%5D <nowiki>https://xkcd.com/[AT%20THE%20JULY%2028TH%20MEETING][tab]</nowiki>]<br />
==Explanation==<br />
{{incomplete|Created by a PREDICTIVE MODEL THAT WILL BE FIRST AGAINST THE WALL WHEN THE REVOLUTION COMES. Do NOT delete this tag too soon.}}<br />
<br />
{{w|Predictive text}} is a feature on many systems where as you type the system automatically suggests likely words or phrases to follow what you have written to that point. For instance, if you type "I'm heading" the system may suggest "home" or "back" as likely words to follow. Predictive systems usually use prior input to generate their predictions, so if you frequently type "Totally amazing!" the system will suggest "amazing!" every time you type "totally" even if you actually want to type "totally true" sometimes.<br />
<br />
In the comic, [[Cueball]] is using predictive text to uncover a plot against his organization/government, but instead of using only his personal input, the system is using input from ''all'' users. By typing in an obscure phrase related to revolution and a meeting, he gets the predictive text algorithm to display where and when the next supposedly secret meeting will be held based on other users input. This works because it is unlikely that anyone else other than revolutionaries would be typing this phrase, thus the only data the algorithm has to predict from is the actual message from the revolutionaries on their next meeting. The caption of the comic is pointing out that systems which use prior input for predictive purposes in this way can end up leaking information that might otherwise be considered private. (However, this method may produce outdated information. On June 29, 2019, typing in Google "Long live the revolution. Our next meeting will be at" gave the predicted completion "long live the revolution. our next meeting will be at comic con 2018", which would not be useful information to anyone looking for revolutionaries, because Comic-Con 2018 was already over.)<br />
<br />
As humanity adapts to a digital world, people are finding that their digital communications provide the illusion of confidentiality, with damaging results when the information leaks out. Real-life examples include a [https://www.bbc.co.uk/news/health-36391957 2016 British trainee doctor strike], where a technically-secure WhatsApp group leaked information to the press.<br />
<br />
The title text shows the revolutionaries using the same technique. By typing in "We will arrest the revolution members" they are hoping that the algorithm will suggest the time and date of their planned arrest, since no one other than the authorities would be typing in that phrase. Pressing the key [tab] to autocomplete that text produces "WE WILL ARREST THE REVOLUTION MEMBERS [AT THE JULY 28TH MEETING]", and the revolutionaries then say "Cancel the meeting! Our cover is blown." The revolutionaries have apparently made the serious mistake of holding secret meetings on regular, predictable dates (such as the 28th day of each month, the last date guaranteed to exist in any month of the Gregorian Calendar), and the authorities have successfully figured this out, either through the predictive-text attack or by other means.<br />
<br />
Both examples assume that the revolutionaries and the authorities would be talking about very secret information in the clear on a network accessible to their adversaries. In the real world people engaged in sensitive activities would communicate via code, encryption, or both, or would do so through what they believe to be secure channels. There is still the danger of secret information leaking via non-secret channels, however. <br />
<br />
{{w|Side-channel attack|Side-channel attacks}} use information gained from the implementation of a system to deduce supposedly protected information. A famous example occurred in World War II. The Germans kept tank production figures a secret, but they gave items like engine blocks sequential serial numbers. The Allies wanted to know exact tank production figures, so they solved the {{w|German tank problem}} by using statistical methods to analyze the distribution of these numbers on captured vehicles. They were able to predict tank production figures extremely accurately, to the point they predicted 270 tanks in a month when 276 were actually built. Thus the secret information on tank production leaked.<br />
<br />
Some systems require frequent password change, in an effort to limit danger from a password being discovered. However, people respond by chosing passwords in patterns, so it is easy to predict what subsequent passwords will be, given old ones. Thus defeating the purpose of requiring frequent changes.[https://www.troyhunt.com/passwords-evolved-authentication-guidance-for-the-modern-era/ Passwords Evolved: Authentication Guidance for the Modern Era]<br />
<br />
Although the comic title is "Predictive Models", the term {{w|Predictive modelling}} usually refers to computer programs that try to predict outcomes from data aggregation, such as reviewing health records to identify people most at risk from certain diseases based on weight, prior injuries, etc., before testing directly for the diseases themselves. This is similar to but not precisely like the example in the comic, since predictive text is using direct input to predict further input, while predictive modelling is using related input (such as make and model of a car along with driver acceleration patterns) to predict a different output (such as likelihood of a crash). Both predictive text and predictive modelling could leak information as the comic suggests, however. <br />
<br />
Predictive text and the possibility to leak unintended information has been parodied on xkcd before in [[1068: Swiftkey]].<br />
<br />
==Transcript==<br />
:[Cueball is sitting in an office chair at a desk typing on a laptop. Above him is the text he writes along with what the predictive text tool suggests, the latter in grey text. The TAB at the end is in a small frame.]<br />
:Cueball typing: Long live the revolution. Our next meeting will be at<span style="color:gray">| the docks at midnight on June 28 [tab]</span><br />
:Cueball: ''Aha, found them!''<br />
<br />
:[Caption below the panel:]<br />
:When you train predictive models on input from your users, it can leak information in unexpected ways.<br />
<br />
==Trivia==<br />
*Clicking on the comic takes you to this page: [https://xkcd.com/%5BAT%2520THE%2520JULY%252028TH%2520MEETING%5D%5Btab%5D <nowiki>https://xkcd.com/[AT%20THE%20JULY%2028TH%20MEETING][tab]</nowiki>], which as of this moment only shows "404 Not Found". <br />
**The anchor actually contains invalid HTML <nowiki><a href=" [AT THE JULY 28TH MEETING][tab] "Cancel the meeting! Our cover is blown.""></nowiki>. This would suggest that [[Randall]] didn't intend this behaviour.<br />
**It is also possible that [[Randall]] may add what he intends to add at a later date, most likely July 28, the date mentioned in the title text. In this case the page will likely remain this way until then.<br />
*Some browsers, only show the first part of the title text "WE WILL ARREST THE REVOLUTION MEMBERS." For example FireFox version 66 Windows does this, evidently some versions of Firefox and chrome do likewise on GNU/Linux.<br />
<br />
{{comic discussion}}<br />
<br />
[[Category:Comics featuring Cueball]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=2169:_Predictive_Models&diff=1760272169: Predictive Models2019-07-01T19:49:20Z<p>162.158.106.216: /* Explanation */ link to Side-channel attack, split long paragraph</p>
<hr />
<div>{{comic<br />
| number = 2169<br />
| date = June 28, 2019<br />
| title = Predictive Models<br />
| image = predictive_models.png<br />
| titletext = WE WILL ARREST THE REVOLUTION MEMBERS [AT THE JULY 28TH MEETING][tab] "Cancel the meeting! Our cover is blown."<br />
}}<br />
<br />
*When the image is clicked the "Not Available" xkcd post opens up: [https://xkcd.com/%5BAT%2520THE%2520JULY%252028TH%2520MEETING%5D%5Btab%5D <nowiki>https://xkcd.com/[AT%20THE%20JULY%2028TH%20MEETING][tab]</nowiki>]<br />
==Explanation==<br />
{{incomplete|Created by a PREDICTIVE MODEL THAT WILL BE FIRST AGAINST THE WALL WHEN THE REVOLUTION COMES. Do NOT delete this tag too soon.}}<br />
<br />
{{w|Predictive text}} is a feature on many systems where as you type the system automatically suggests likely words or phrases to follow what you have written to that point. For instance, if you type "I'm heading" the system may suggest "home" or "back" as likely words to follow. Predictive systems usually use prior input to generate their predictions, so if you frequently type "Totally amazing!" the system will suggest "amazing!" every time you type "totally" even if you actually want to type "totally true" sometimes.<br />
<br />
In the comic, [[Cueball]] is using predictive text to uncover a plot against his organization/government, but instead of using only his personal input, the system is using input from ''all'' users. By typing in an obscure phrase related to revolution and a meeting, he gets the predictive text algorithm to display where and when the next supposedly secret meeting will be held based on other users input. This works because it is unlikely that anyone else other than revolutionaries would be typing this phrase, thus the only data the algorithm has to predict from is the actual message from the revolutionaries on their next meeting. The caption of the comic is pointing out that systems which use prior input for predictive purposes in this way can end up leaking information that might otherwise be considered private. (However, this method may produce outdated information. On June 29, 2019, typing in Google "Long live the revolution. Our next meeting will be at" gave the predicted completion "long live the revolution. our next meeting will be at comic con 2018", which would not be useful information to anyone looking for revolutionaries, because Comic-Con 2018 was already over.)<br />
<br />
As humanity adapts to a digital world, people are finding that their digital communications provide the illusion of confidentiality, with damaging results when the information leaks out. Real-life examples include a [https://www.bbc.co.uk/news/health-36391957 2016 British trainee doctor strike], where a technically-secure WhatsApp group leaked information to the press.<br />
<br />
The title text shows the revolutionaries using the same technique. By typing in "We will arrest the revolution members" they are hoping that the algorithm will suggest the time and date of their planned arrest, since no one other than the authorities would be typing in that phrase. Pressing the key [tab] to autocomplete that text produces "WE WILL ARREST THE REVOLUTION MEMBERS [AT THE JULY 28TH MEETING]", and the revolutionaries then say "Cancel the meeting! Our cover is blown." The revolutionaries have apparently made the serious mistake of holding secret meetings on regular, predictable dates (such as the 28th day of each month, the last date guaranteed to exist in any month of the Gregorian Calendar), and the authorities have successfully figured this out, either through the predictive-text attack or by other means.<br />
<br />
Both examples assume that the revolutionaries and the authorities would be talking about very secret information in the clear on a network accessible to their adversaries. In the real world people engaged in sensitive activities would communicate via code, encryption, or both, or would do so through what they believe to be secure channels. There is still the danger of secret information leaking via non-secret channels, however. <br />
<br />
{{w|Side-channel attack|Side-channel attacks}} use information gained from the implementation of a system to deduce supposedly protected information. A famous example occurred in World War II. The Germans kept tank production figures a secret, but they gave items like engine blocks sequential serial numbers. The Allies wanted to know exact tank production figures, so they solved the {{w|German tank problem}} by using statistical methods to analyze the distribution of these numbers on captured vehicles. They were able to predict tank production figures extremely accurately, to the point they predicted 270 tanks in a month when 276 were actually built. Thus the secret information on tank production leaked.<br />
<br />
Although the comic title is "Predictive Models", the term {{w|Predictive modelling}} usually refers to computer programs that try to predict outcomes from data aggregation, such as reviewing health records to identify people most at risk from certain diseases based on weight, prior injuries, etc., before testing directly for the diseases themselves. This is similar to but not precisely like the example in the comic, since predictive text is using direct input to predict further input, while predictive modelling is using related input (such as make and model of a car along with driver acceleration patterns) to predict a different output (such as likelihood of a crash). Both predictive text and predictive modelling could leak information as the comic suggests, however. <br />
<br />
Predictive text and the possibility to leak unintended information has been parodied on xkcd before in [[1068: Swiftkey]].<br />
<br />
==Transcript==<br />
:[Cueball is sitting in an office chair at a desk typing on a laptop. Above him is the text he writes along with what the predictive text tool suggests, the latter in grey text. The TAB at the end is in a small frame.]<br />
:Cueball typing: Long live the revolution. Our next meeting will be at<span style="color:gray">| the docks at midnight on June 28 [tab]</span><br />
:Cueball: ''Aha, found them!''<br />
<br />
:[Caption below the panel:]<br />
:When you train predictive models on input from your users, it can leak information in unexpected ways.<br />
<br />
==Trivia==<br />
*Clicking on the comic takes you to this page: [https://xkcd.com/%5BAT%2520THE%2520JULY%252028TH%2520MEETING%5D%5Btab%5D <nowiki>https://xkcd.com/[AT%20THE%20JULY%2028TH%20MEETING][tab]</nowiki>], which as of this moment only shows "404 Not Found". <br />
**The anchor actually contains invalid HTML <nowiki><a href=" [AT THE JULY 28TH MEETING][tab] "Cancel the meeting! Our cover is blown.""></nowiki>. This would suggest that [[Randall]] didn't intend this behaviour.<br />
**It is also possible that [[Randall]] may add what he intends to add at a later date, most likely July 28, the date mentioned in the title text. In this case the page will likely remain this way until then.<br />
*Some browsers, only show the first part of the title text "WE WILL ARREST THE REVOLUTION MEMBERS." For example FireFox version 66 Windows does this, evidently some versions of Firefox and chrome do likewise on GNU/Linux.<br />
<br />
{{comic discussion}}<br />
<br />
[[Category:Comics featuring Cueball]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=1559:_Driving&diff=1614811559: Driving2018-08-19T19:16:26Z<p>162.158.106.216: Mentioned Continental states while excluding Alaska</p>
<hr />
<div>{{comic<br />
| number = 1559<br />
| date = August 3, 2015<br />
| title = Driving<br />
| image = driving.png<br />
| titletext = Sadly, it probably won't even have enough gas to make it to the first border crossing.}}<br />
<br />
==Explanation==<br />
A {{w|self-driving car}} is a car that requires no human interaction to navigate streets to a destination. Thus, when [[Black Hat]] places a rock that weighs "as much as a small adult" into the car's seat, he begins the process of fooling the car into thinking it has an occupant when it does not. His purpose in doing so appears to be to send the car to {{w|Anchorage}}, {{w|Alaska}}, which is presumably quite far from where Black Hat and [[Cueball]] are standing, thus taking the car far away from its owner with relatively little effort on the part of Black Hat. This is yet another evil prank from xkcd's resident [[classhole]].<br />
<br />
The title text references the fact that driving to Alaska from the {{w|contiguous 48 states|contiguous lower 48 states}} requires two border crossings, once into {{w|Canada}} from the mainland, and once from Canada into Alaska. The car apparently begins some distance from the Canadian border, since it will likely run out of gas before reaching Canada. Title text expresses regret about this probable failure; perhaps Randall was looking forward to the encounter between the border guards and the vehicle's "occupant." However, even if the car does not get to Anchorage, Black Hat will have created a serious problem for its owner who will have to report the car as stolen.<br />
<br />
Shortly after this comic appeared Tesla released footage of a robotic charger that can [http://www.bbc.co.uk/news/technology-33794085 connect itself to a Tesla automatically]. If this kind of technology becomes common then a self-driving electric car might be able to make a transcontinental journey without human intervention.<br />
<br />
At the time of the release of this comic there were no places where self-driving cars were for sale to individuals. However, several corporate owned test cars are frequently seen on public road (such as those operated by Google among others). {{w|Google_driverless_car|Nevada, Florida, California and Michigan}} were the first states to allow the testing of self-driving cars on public roads, and this legality is quickly spreading to many other states, as well as several countries in Europe. Alternatively, Randall might be setting this comic in an idyllic near future, wherein you could drive all over the country and in Canada with these cars!<br />
<br />
It could have been [[Beret Guy|Beret Guy's]] car - see [[1493: Meeting]]. Self-driving cars is a [[:Category:Self-driving cars|recurring topic]] on xkcd.<br />
<br />
==Transcript==<br />
:[Black Hat, carrying a rock, is walking toward Cueball, leaving a trail of sand.]<br />
:Black Hat: Would you guess this weighs as much as a small adult?<br />
:Cueball: What?<br />
:Cueball: Uh, probably.<br />
<br />
:[Black Hat walks past Cueball who turns to look after him.]<br />
:Black Hat: Great!<br />
<br />
:[Black Hat has walked out of the frame. Cueball is looking in the direction he left in. Several noises and voices are coming from off-panel.]<br />
:Off-panel: <nowiki>*</nowiki>Thump*<br />
:Car voice (off-panel): ''Please fasten your seatbelt.''<br />
:Off-panel: <nowiki>*</nowiki>click*<br />
:Black Hat (off-panel): Take me to Anchorage, Alaska.<br />
:Car voice (off-panel): ''Navigating''<br />
:Off-panel: <nowiki>*</nowiki>slam*<br />
<br />
:[Black Hat walks back in the panel towards Cueball.]<br />
:Car driving off:''Vrrrrrrrrrrrr<small>rrrr<small>rrrrr<small>rr</small></small></small>''<br />
:Black Hat: I love self-driving cars.<br />
:Cueball: ...Whose car was that?<br />
:Black Hat: Dunno, but they shouldn't have left it running.<br />
<br />
{{comic discussion}}<br />
<br />
[[Category:Comics featuring Black Hat]]<br />
[[Category:Comics featuring Cueball]]<br />
[[Category:Self-driving cars]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=2034:_Equations&diff=1614542034: Equations2018-08-19T00:03:01Z<p>162.158.106.216: Equation to encompass all of a given discipline</p>
<hr />
<div>{{comic<br />
| number = 2034<br />
| date = August 17, 2018<br />
| title = Equations<br />
| image = equations.png<br />
| titletext = All electromagnetic equations: The same as all fluid dynamics equations, but with the 8 and 23 replaced with the permittivity and permeability of free space, respectively.<br />
}}<br />
<br />
This comic gives a set of equations supposedly from different areas of science in mathematics, physics, and chemistry. To anyone not familiar with the field in question they look pretty similar to what you might find in research papers or on the relevant Wikipedia pages. To someone who knows even a little about the topic, they are clearly very wrong and only seem even worse the more you look at them. In many disciplines, the mathematical description of a large area is summed up in a small number of equations, such as Maxwell's equations for electromagnetism. In similar fashion, the equations here purport to encompass the whole of their given field.<br />
<br />
==Simplified Explanations==<br />
{{incomplete|Created by a mere human. Do NOT delete this tag too soon.}}<br />
<br />
;All chemistry equations<br />
This shows a parody of the common example chemistry equation of burning Methane and Oxygen (with added heat), to form water and carbon dioxide. However in this form "HEAT" is an actual molecule, rather than simply indicating the presence of heat to start the reaction. Thus the equation is modified to incorporate the fictional "HEAT" into the reaction.<br />
<br />
TODO: other simplified explanations.<br />
<br />
<br />
==Technical Explanations==<br />
{{incomplete|Created by an EQUATION. Do NOT delete this tag too soon.}}<br />
<br />
<br />
<br />
;All kinematics equations<br />
:<math>E = K_0t + \frac{1}{2}\rho vt^2</math><br />
{{w|Kinematics}} describes the motion of objects without considering mass or forces.<br />
<br />
This equation here literally states: "Energy equals a constant <math>K_0</math> multiplied by time, plus half of density multiplied by speed multiplied by time squared". <br />
<br />
The first term here is hard to interpret: it could be correct if <math>K_0</math> is a constant power applied to the system, but this symbol would more normally be used to denote an initial energy, in which case multiplying by <math>t</math> would be wrong. Alternatively, the term is similar to <math>k_B T</math> (sometimes written as ''kT''), a term that often appears in {{w|Statistical_mechanics|statistical mechanics}} equations, where ''k<sub>B</sub>'' (or ''k'') is {{w|Boltzmann_constant|the Boltzmann constant}}, and ''T'' is the {{w|Thermodynamic_temperature|absolute temperature}}. In this latter case, the term would have units of energy, consistent with the left side of the equation.<br />
<br />
The second term looks similar to the kinetic energy term <math> \frac{1}{2}\rho v^2 </math> in [http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html the Bernoulli equation] for fluids (or, more properly, the kinetic energy ''density'' in the fluid). <br />
<br />
The whole equation appears to be a play on the kinematics formula: <math>s = ut + \frac{1}{2}\ at^2</math>, where distance travelled (''s'') by a constantly accelerating object is determined by initial velocity (''u''), time (''t''), and acceleration (''a'')<br />
<br />
Kinematics is often one of the first topics covered in an introductory physics course, both at the high school and freshman college levels. As such, mixing in material from more advanced topics like statistical mechanics and the Bernoulli equation, even if done correctly, would be very confusing for a typical student learning kinematics.<br />
<br />
;All number theory equations<br />
:<math>K_n = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-e^{\pi-\infty})</math><br />
{{w|Number theory}} is a branch of mathematics primarily studying the properties of integers.<br />
<br />
Taken literally the equation says: "The nth K-number is equal to: the sum for all i from 0 to infinity, the sum for all pi from 0 to infinity; subtract pi from n, and multiply it with i minus e to the power of pi minus infinity". A twofold misconception can be seen here. The first is the reassignment of pi as a variable instead of the constant (3.14...). This might be a jab at how in number theory letters and numbers are used interchangeably, but where some letters are all of a sudden fixed constants. The second misconception is the use of infinity in the latter part of the formula. Naively this would signify that (with the reassigned pi values) the part in the power would range from minus infinity to zero. However, infinity is not a number and cannot be used as one without using a limit construct.<br />
<br />
;All fluid dynamic equations<br />
:<math>\frac{\partial}{\partial t}\nabla\cdot \rho = \frac{8}{23}<br />
\int\!\!\!\!\!\!\!\!\!\;\;\bigcirc\!\!\!\!\!\!\!\!\!\;\;\int<br />
\rho\,ds\,dt\cdot \rho\frac{\partial}{\partial\nabla}<br />
</math><br />
{{w|Fluid dynamics}} describes the movement of non-solid material. In particular for gases, the density <math>\rho</math> is often the most interesting quantity (for liquids, this is often just constant). A unique feature of fluid-dynamic equations is the presence of {{w|Advection|advection terms}}, which take the form of often strange-looking spatial derivatives. This equation turns this up to a new level by differentiating with respect to a differential operator <math>\nabla</math>, which does not make any sense at all. Also it has a contour integral which seems reminiscent to a closed-circle process like in a piston engine, but this does not really fit in the context (differential description of a gas), and it has a pair of {{w|Magic number (programming)|unexplained numbers}} <math>8</math> and <math>23</math>, probably alluding to the {{w|Heat capacity ratio|specific heat ratio}} which is often written out as the fraction <math>\tfrac{7}{5}</math>, whereas most other physics equations [[899: Number Line|avoid including any plain numbers higher than 4]].<br />
<br />
The title text stating that the electromagnetism equation is the same as the fluid dynamics equation, but with the arbitrary 8 and 23 replaced with the permittivity and permeability of free space is likely because electromagnetism equations often have relations to fluid dynamics, and because those two constants appear in the vast majority of electromagnetism equations.<br />
<br />
;All quantum mechanic equations<br />
:<math>|\psi_{x,y}\rangle = A(\psi) A(|x\rangle \otimes |y\rangle)</math><br />
{{w|Quantum mechanics}} is a fundamental theory in physics which describes nature at scales of atoms and below. It typically uses the {{w|Bra–ket notation|bra–ket notation}} in its formulae.<br />
<br />
This equation takes a state psi in the dimensions of x and y and equates it to an operator A performed on psi multiplied by the same operator performed on the tensor product of x and y. Since the state psi is already the tensor product of the states x and y, this is equivalent to performing the same unknown operator twice on psi, and unless this operator is its own inverse such as a bit-flip or Hermitian operator, this equation is therefore incorrect.<br />
<br />
;All chemistry equations<br />
:<math>\mathrm{CH}_4 + \mathrm{OH} + \mathrm{HEAT} \rightarrow \mathrm{H}_2\mathrm{O} + \mathrm{CH}_2 + \mathrm{H}_2 \mathrm{EAT}</math><br />
A {{w|Chemical equation|chemical equation}} represents a chemical reaction as a formula, with the reactant entities on the left-hand side, and the product entities on the right-hand side. The number of each element on the left side must match those on the right side. The energy produced or absorbed in this process is not included in that formula.<br />
<br />
This is a modification of the combustion of methane. The correct form is often taught and a good example problem but obviously there are more chemistry problems.<math>\mathrm{HEAT}</math> is normally shorthand for {{w|activation energy}}, but in Randall's version it's jokingly used as a chemical ingredient and becomes <math>\mathrm{H}_2\mathrm{EAT}</math>, taking the hydrogen atom freed by the combustion equation shown. The proper methane combustion equation would be: <math>\mathrm{CH}_4 + 2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2</math><br />
<br />
;All quantum gravity equations<br />
:<math>\mathrm{SU}(2)\mathrm{U}(1) \times \mathrm{SU}(\mathrm{U}(2))</math><br />
This is more similar to expressions which appear in {{w|Grand_Unified_Theory|Grand Unified Theory}} (GUT) than general quantum gravity. Unlike some of the other equations, this one has no interpretation which could make it mathematically correct. This is similar to the notations used to describe the symmetry group of a particular phenomena in terms of mathematical {{w|Lie_Group|Lie Groups}}. A real example would be the Standard Model of particle physics which has symmetry according to <math>\rm{SU(3)\times SU(2) \times U(1)}</math>. Here, <math>\rm{SU}</math> and <math>\rm{U}</math> denote the special unitary and unitary groups respectively with the numbers indicating the dimension of the group. Loosely, the three terms correspond to the symmetries of the strong force, weak force and electromagnetism although the exact correspondence is muddied by symmetry breaking and the Higgs mechanism.<br />
<br />
Of course, an expression missing an "=" sign, is difficult to interpret as an "equation", because equations normally express an "equality" of some kind. Nobody knows whether Randal refers to a horse, zebra, donkey or other equine here. <br />
<br />
Randall's version clearly involves some similar groups although without the <math>\times</math> symbol it is hard to work out what might be happening. A term like <math>\rm{SU(U(2))}</math> has no current interpretation in mathematics, if anyone thinks otherwise and possibly has a solution to the quantum gravity problem they should probably get in touch with someone about that.<br />
<br />
;All gauge theory equations<br />
:[[File:All gauge theory equations.png]]<br />
In physics, a {{w|Gauge theory|gauge theory}} is a type of field theory which is invariant to local transformations. The term gauge refers to any specific mathematical formalism to regulate redundant degrees of freedom.<br />
<br />
This equation looks broadly similar to the sorts of things which appear in gauge theory such as the equations which define {{w|Yang–Mills_theory#Quantization|Yang-Mills Theory}}. By the time physics has got this far in, people have normally run out of regular symbols making a lot of the equations look very daunting. The actual equations in this field rarely go far beyond the Greek alphabet though and no-one has yet to try putting hats on brackets. The appearance of many sub- and superscripts is normal (this links to the group theory origins of these equations) and for the layperson it can be impossible to determine which additions are labels on the symbols and which are indices for an {{w|Einstein_notation|Einstein Sum}}.<br />
<br />
The left-hand side <math>S_g</math> is the symbol for some {{w|Action_(physics)|action}}, in Yang-Mills theory this is actually used for a so-called "ghost action". On the right-hand side we have a large number of terms, most of which are hard to interpret without knowing Randall's thought processes (this is why real research papers should all label their equations thoroughly). The <math>\frac{1}{2\bar{\varepsilon}}</math> looks like a constant of proportionality which often appears in gauge theories. The factor of <math>i = \sqrt{-1}</math> is not unusual as many of these equations use complex numbers. The <math>\eth</math> symbol looks similar to a <math>\partial</math> partial derivative symbol especially as the {{w|Dirac_equation#Covariant_form_and_relativistic_invariance|Dirac Equation}} uses a slashed version as a convenient shorthand. <br />
<br />
The rest of the equation cannot be mathematically correct as the choice of indices used does not match that on the left-hand side (which has none). In particle physics subscripts (or superscripts) of greek letters (usually <math>\mu</math> or <math>\nu</math>) indicate terms which transform nicely under Lorentz transformations (special relativity). Roman indices from the beginning of the alphabet relate to various gauge transformation propetries, the triple index seen on <math>p^{abc}_v</math> would likely come from some <math>\rm{SU(3)}</math> transformation (related to the strong nuclear force). Since <math>S_g</math> has none of these (and is thus a scalar which remains constant under these operations), we would need the right-hand side to behave in the same way. Most of the indices which appear are unpaired and so will not result in a scalar making the equation very wrong. For those not familiar with this type of equation, this is similar to the mistake of messing up units, for instance setting a distance equal to a mass.<br />
<br />
;All cosmology equations<br />
:<math>H(t) + \Omega + G \cdot \Lambda \, \dots \begin{cases} \dots > 0 & \text{(Hubble model)} \\ \dots = 0 & \text{(Flat sphere model)} \\ \dots < 0 & \text{(Bright dark matter model)} \end{cases}<br />
</math><br />
This is a parody of equations defining the {{w|Hubble's_law#Derivation_of_the_Hubble_parameter|Hubble Parameter}} <math>H(t)</math> although it looks like Randall has become bored and not bothered to finish his equation. Such equations usually have several <math>\Omega</math> terms representing the contributions of different substances to the energy-density of the Universe (matter, radiation, dark energy etc.). In this context <math>G</math> could be Newton's constant and <math>\Lambda</math> is the cosmological constant (energy density of empty space) although seeing them appear multiplied and on the same footing as <math>H</math> is unusual (the dot is entirely unnecessary). Choosing to make <math>H</math> a function of time <math>t</math> and not of redshift <math>z</math> is also unusual.<br />
<br />
The second section looks like the inequalities used to show how the equation varies with the shape of the Universe, based on the value of the curvature parameter <math>\Omega_k</math>. A value of 0 indicates a flat Universe (this is more or less what we observe) while a positive /negative value indicates an open /closed curved Universe. Randall's choice of labels further makes fun of the field as both a flat sphere and bright dark matter are oxymoronic terms which would involve some rather strange model universes.<br />
<br />
;All truly deep physics equations<br />
:[[File:All truly deep physics equations.png]]<br />
<math>\hat H</math> is the Hamiltonian operator, which when applied to a system returns the total energy. In this context, U would usually be the potential energy. However, there is also a subscript 0 and a diacritic marking indicating some other variable. Much of physics is based on Lagrangian and Hamiltonian mechanics. The Lagrangian is defined as <math>\hat L = \hat K - \hat U </math> with K being the kinetic energy and U the potential. Hamiltonian mechanics uses the equation <math>\hat H = \hat K + \hat U </math>. The Hamiltonian must be conserved so taking the time derivative and setting it equal to zero is a powerful tool. The "principle of least action" allows most modern physics to be derived by setting the time derivative of the Lagrangian to zero.<br />
<br />
==Transcript==<br />
:[Nine equations are listed, three in the top row and two in each of the next three rows. Below each equation there are labels:]<br />
<br />
:E = K<sub>0</sub>t + 1/2 &rho;vt<sup>2</sup><br />
:All kinematics equations<br />
<br />
:K<sub>n</sub> = &sum;<sub>i=0</sub><sup>&infin;</sup>&sum;<sub>&pi;=0</sub><sup>&infin;</sup>(n-&pi;)(i-e<sup>&pi;-&infin;</sup>)<br />
:All number theory equations<br />
<br />
:&#x2202;/&#x2202;t &nabla; &sdot; &rho; = 8/23 (&#x222F; &rho; ds dt &sdot; &rho; &#x2202;/&#x2202;&nabla;)<br />
:All fluid dynamics equations<br />
<br />
:|&psi;<sub>x,y</sub>&#x232a; = A(&psi;) A(|x&#x232a;&#x2297; |y&#x232a;)<br />
:All quantum mechanics equations<br />
<br />
:CH<sub>4</sub> + OH + HEAT &rarr; H<sub>2</sub>O + CH<sub>2</sub> + H<sub>2</sub>EAT<br />
:All chemistry equations<br />
<br />
:SU(2)U(1) &times; SU(U(2))<br />
:All quantum gravity equations<br />
<br />
:S<sub>g</sub> = (-1)/(2&epsilon;&#x0304;) i &eth; (&#x302; &xi;<sub>0</sub> +&#x030a; p<sub>&epsilon;</sub> &rho;<sub>v</sub><sup>abc</sup> &eta;<sub>0</sub> )&#x302; f&#x0335;<sub>a</sub><sup>0</sup> &lambda;(&#x0292;&#x0306;) &psi;(0<sub>a</sub>)<br />
:All gauge theory equations<br />
<br />
:[There is a brace linking the three cases together.]<br />
:H(t) + &Omega; + G&sdot;&Lambda; ... <br />
:... > 0 (Hubble model)<br />
:... = 0 (Flat sphere model)<br />
:... < 0 (Bright dark matter model)<br />
:All cosmology equations<br />
<br />
:&#x0124; - u&#x0327;<sub>0</sub> = 0<br />
:All truly deep physics equations<br />
<br />
{{comic discussion}}<br />
<br />
[[Category:Science]]<br />
[[Category:Physics]]<br />
[[Category:Math]]<br />
[[Category:Chemistry]]<br />
[[Category:Astronomy]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=2034:_Equations&diff=1614322034: Equations2018-08-18T01:02:35Z<p>162.158.106.216: Clarified that it is possible to have variable density in liquids due to a temperature gradient.</p>
<hr />
<div>{{comic<br />
| number = 2034<br />
| date = August 17, 2018<br />
| title = Equations<br />
| image = equations.png<br />
| titletext = All electromagnetic equations: The same as all fluid dynamics equations, but with the 8 and 23 replaced with the permittivity and permeability of free space, respectively.<br />
}}<br />
<br />
This comic gives a set of equations supposedly from different areas of science in mathematics, physics, and chemistry. To anyone not familiar with the field in question they look pretty similar to what you might find in research papers or on the relevant Wikipedia pages. To someone who knows even a little about the topic, they are clearly very wrong and only seem even worse the more you look at them.<br />
<br />
==Simplified Explanations==<br />
{{incomplete|Created by a mere human. Do NOT delete this tag too soon.}}<br />
<br />
;All chemistry equations<br />
This shows a parody of the common example chemistry equation of burning Methane and Oxygen (with added heat), to form water and carbon dioxide. However in this form "HEAT" is an actual molecule, rather than simply indicating the presence of heat to start the reaction. Thus the equation is modified to incorporate the fictional "HEAT" into the reaction.<br />
<br />
TODO: other simplified explanations.<br />
<br />
<br />
==Technical Explanations==<br />
{{incomplete|Created by an EQUATION. Do NOT delete this tag too soon.}}<br />
<br />
<br />
<br />
;All kinematics equations<br />
:<math>E = K_0t + \frac{1}{2}\rho vt^2</math><br />
{{w|Kinematics}} describes the motion of objects without considering mass or forces.<br />
<br />
This equation here literally states: "Energy equals a constant <math>K_0</math> multiplied by time, plus half of density multiplied by speed multiplied by time squared". The first term here is hard to interpret: it could be correct if <math>K_0</math> is a constant power applied to the system, but this symbol would more normally be used to denote an initial energy, in which case so multiplying by <math>t</math> would be wrong. The second term looks similar to the kinetic energy term <math>\frac{1}{2}\rho v^2</math> in [http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html the Bernoulli equation] for fluids (or, more properly, the kinetic energy ''density'' in the fluid). The whole equation appears to be a play on the forumula: <math>s = ut + \frac{1}{2}\ at^2</math>, where distance travelled (''s'') by a constantly accelerating object is determined by initial velocity (''u''), time (''t''), and acceleration (''a'')<br />
<br />
;All number theory equations<br />
:<math>K_n = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-e^{\pi-\infty})</math><br />
{{w|Number theory}} is a branch of mathematics primarily to the study the properties of integers.<br />
<br />
Taken literally the equation says: "The nth K-number is equal to: for all i in 0 to infinity, for all pi in 0 to infinity; subtract pi from n, and multiply it with i minus e to the power of pi minus infinity". A twofold misconception can be seen here. The first is the reassignment of pi as a variable instead of the constant (3.14...). This might be a jab at how in number theory letters and numbers are used interchangeably, but where some letters are all of a sudden fixed constants. The second misconception is the use of infinity in the latter part of the formula. Naively this would signify that (with the reassigned pi values) the part in the power would range from minus infinity to zero. However, infinity is not a number and cannot be used as one without using a limit construct.<br />
<br />
;All fluid dynamic equations<br />
:<math>\frac{\partial}{\partial t}\nabla\cdot \rho = \frac{8}{23}<br />
\int\!\!\!\!\!\!\!\!\!\;\;\bigcirc\!\!\!\!\!\!\!\!\!\;\;\int<br />
\rho\,ds\,dt\cdot \rho\frac{\partial}{\partial\nabla}<br />
</math><br />
{{w|Fluid dynamics}} describes the movement of non-solid material. In particular for gases, the density <math>\rho</math> is often the most interesting quantity (for liquids, this is often just constant). A unique feature of fluid-dynamic equations is the presence of {{w|Advection|advection terms}}, which take the form of often strange-looking spatial derivatives. This equation turns this up to a new level by differentiating with respect to a differential operator <math>\nabla</math>, which does not make any sense at all. Also it has a contour integral which seems reminiscent to a closed-circle process like in a piston engine, but this does not really fit in the context (differential description of a gas), and it has a pair of {{w|Magic number (programming)|unexplained numbers}} <math>8</math> and <math>23</math>, probably alluding to the {{w|Heat capacity ratio|specific heat ratio}} which is often written out as the fraction <math>\tfrac{7}{5}</math>, whereas most other physics equations [[899: Number Line|avoid including any plain numbers higher than 4]].<br />
<br />
The title text stating that the electromagnetism equation is the same as the fluid dynamics equation, but with the arbitrary 8 and 23 replaced with the permittivity and permeability of free space is likely because electromagnetism equations often have relations to fluid dynamics, and because those two constants appear in the vast majority of electromagnetism equations.<br />
<br />
;All quantum mechanic equations<br />
:<math>|\psi_{x,y}\rangle = A(\psi) A(|x\rangle \otimes |y\rangle)</math><br />
{{w|Quantum mechanics}} is a fundamental theory in physics which describes the nature at scales of atoms and below. It typically uses the {{w|Bra–ket notation|bra–ket notation}} in its formulae.<br />
<br />
This equation takes a state psi in the dimensions of x and y and equates it to an operator A performed on psi multiplied by the same operator performed on the tensor product of x and y. Seeing as the state psi is already the tensor product of the states x and y, this is equivalent to performing the same unknown operator twice on psi, and unless this operator is its own inverse such as a bit-flip or Hermitian operator, this equation is therefore incorrect.<br />
<br />
;All chemistry equations<br />
:<math>\mathrm{CH}_4 + \mathrm{OH} + \mathrm{HEAT} \rightarrow \mathrm{H}_2\mathrm{O} + \mathrm{CH}_2 + \mathrm{H}_2 \mathrm{EAT}</math><br />
A {{w|Chemical equation|chemical equation}} is the symbolic representation of a chemical reaction in the form of symbols and formulae, wherein the reactant entities are given on the left-hand side and the product entities on the right-hand side. The number of each element on the left side must match them on the right side, the equation is balanced. The energy produced or absorbed in this process is not included in that formula.<br />
<br />
This here is a modification of the combustion of methane. The correct form is often taught and a good example problem but obviously there are more chemistry problems.<math>\mathrm{HEAT}</math> is normally shorthand for {{w|activation energy}}, but in Randall's version it's jokingly used as a chemical ingredient and becomes <math>\mathrm{H}_2\mathrm{EAT}</math>, taking the hydrogen atom freed by the combustion equation shown. To deliver the punchline while maintaining proper stoichiometry, <math>\mathrm{OH}</math> (which should be <math>\mathrm{OH}^-</math>, since the oxygen keeps a free electron when it combines with a single hydrogen) is shown instead of <math>\mathrm{O}_2</math>. The proper methane combustion equation would be: <math>\mathrm{CH}_4 + 2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2</math><br />
<br />
;All quantum gravity equations<br />
:<math>\mathrm{SU}(2)\mathrm{U}(1) \times \mathrm{SU}(\mathrm{U}(2))</math><br />
This is more similar to expressions which appear in {{w|Grand_Unified_Theory|Grand Unified Theory}} (GUT) than general quantum gravity. Unlike some of the other equations, this one has no interpretation which could make it mathematically correct. This is similar to the notations used to describe the symmetry group of a particular phenomena in terms of mathematical {{w|Lie_Group|Lie Groups}}. A real example would be the Standard Model of particle physics which has symmetry according to <math>\rm{SU(3)\times SU(2) \times U(1)}</math>. Here, <math>\rm{SU}</math> and <math>\rm{U}</math> denote the special unitary and unitary groups respectively with the numbers indicating the dimension of the group. Loosely, the three terms correspond to the symmetries of the strong force, weak force and electromagnetism although the exact correspondence is muddied by symmetry breaking and the Higgs mechanism.<br />
<br />
Of course, an expression missing an "=" sign, is difficult to interpret as an "equation", because equations normally express an "equality" of some kind. Nobody knows whether Randal refers to a horse here (equidae) <br />
<br />
Randall's version clearly involves some similar groups although without the <math>\times</math> symbol it is hard to work out what might be happening. A term like <math>\rm{SU(U(2))}</math> has no current interpretation in mathematics, if anyone thinks otherwise and possibly has a solution to the quantum gravity problem they should probably get in touch with someone about that.<br />
<br />
;All gauge theory equations<br />
:[[File:All gauge theory equations.png]]<br />
In physics, a {{w|Gauge theory|gauge theory}} is a type of field theory which is invariant to local transformations. The term gauge refers to any specific mathematical formalism to regulate redundant degrees of freedom.<br />
<br />
This equation looks broadly similar to the sorts of things which appear in gauge theory such as the equations which define {{w|Yang–Mills_theory#Quantization|Yang-Mills Theory}}. By the time physics has got this far in, people have normally run out of regular symbols making a lot of the equations look very daunting. The actual equations in this field rarely go far beyond the Greek alphabet though and no-one has yet to try putting hats on brackets. The appearance of many sub- and superscripts is normal (this links to the group theory origins of these equations) and for the layperson it can be impossible to determine which additions are labels on the symbols and which are indices for an {{w|Einstein_notation|Einstein Sum}}.<br />
<br />
The left-hand side <math>S_g</math> is the symbol for some {{w|Action_(physics)|action}}, in Yang-Mills theory this is actually used for a so-called "ghost action". On the right-hand side we have a large number of terms, most of which are hard to interpret without knowing Randall's thought processes (this is why real research papers should all label their equations thoroughly). The <math>\frac{1}{2\bar{\varepsilon}}</math> looks like a constant of proportionality which often appears in gauge theories. The factor of <math>i = \sqrt{-1}</math> is not unusual as many of these equations use complex numbers. The <math>\eth</math> symbol looks similar to a <math>\partial</math> partial derivative symbol especially as the {{w|Dirac_equation#Covariant_form_and_relativistic_invariance|Dirac Equation}} uses a slashed version as a convenient shorthand. <br />
<br />
The rest of the equation cannot be mathematically correct as the choice of indices used does not match that on the left-hand side (which has none). In particle physics subscripts (or superscripts) of greek letters (usually <math>\mu</math> or <math>\nu</math>) indicate terms which transform nicely under Lorentz transformations (special relativity). Roman indices from the beginning of the alphabet relate to various gauge transformation propetries, the triple index seen on <math>p^{abc}_v</math> would likely come from some <math>\rm{SU(3)}</math> transformation (related to the strong nuclear force). Since <math>S_g</math> has none of these (and is thus a scalar which remains constant under these operations), we would need the right-hand side to behave in the same way. Most of the indices which appear are unpaired and so will not result in a scalar making the equation very wrong. For those not familiar with this type of equation, it is a similar mistake messing up units and setting a distance equal to a mass.<br />
<br />
;All cosmology equations<br />
:<math>H(t) + \Omega + G \cdot \Lambda \, \dots \begin{cases} \dots > 0 & \text{(Hubble model)} \\ \dots = 0 & \text{(Flat sphere model)} \\ \dots < 0 & \text{(Bright dark matter model)} \end{cases}<br />
</math><br />
This is a parody of equations defining the {{w|Hubble's_law#Derivation_of_the_Hubble_parameter|Hubble Parameter}} <math>H(t)</math> although it looks like Randall has become bored and not bothered to finish his equation. Such equations usually have several <math>\Omega</math> terms representing the contributions of different substances to the energy-density of the Universe (matter, radiation, dark energy etc.). In this context <math>G</math> could be Newton's constant and <math>\Lambda</math> is the cosmological constant (energy density of empty space) although seeing them appear multiplied and on the same footing as <math>H</math> is unusual (the dot is entirely unnecessary). Choosing to make <math>H</math> a function of time <math>t</math> and not of redshift <math>z</math> is also unusual.<br />
<br />
The second section looks like the inequalities used to show how what shape the Universe, based on the value of the curvature parameter <math>\Omega_k</math>. A value of 0 indicates a flat Universe (this more or less what we observe) whilst a positive /negative value indicates an open /closed curved Universe. Randall's choice of labels further makes fun of the field as both a flat sphere and bright dark matter are oxymoronic terms which would involve some rather strange model universes.<br />
<br />
;All truly deep physics equations<br />
:[[File:All truly deep physics equations.png]]<br />
<math>\hat H</math> is the Hamiltonian operator, which when applied to a system returns the total energy. In this context, U would usually be the potential energy. However, there is also a subscript 0 and a diacritic marking indicating some other variable. Much of physics is based on Lagrangian and Hamiltonian mechanics. The Lagrangian is defined as <math>\hat L = \hat K - \hat U </math> with K being the kinetic energy and U the potential. Hamiltonian mechanics uses the equation <math>\hat H = \hat K + \hat U </math>. The Hamiltonian must be conserved so taking the time derivative and setting it equal to zero is a powerful tool. The "principle of least action" allows most modern physics to be derived by setting the time derivative of the Lagrangian to zero.<br />
<br />
==Transcript==<br />
:[Nine equations are listed, three in the top row and two in each of the next three rows. Below each equation there are labels:]<br />
<br />
:E = K<sub>0</sub>t + 1/2 &rho;vt<sup>2</sup><br />
:All kinematics equations<br />
<br />
:K<sub>n</sub> = &sum;<sub>i=0</sub><sup>&infin;</sup>&sum;<sub>&pi;=0</sub><sup>&infin;</sup>(n-&pi;)(i-e<sup>&pi;-&infin;</sup>)<br />
:All number theory equations<br />
<br />
:&#x2202;/&#x2202;t &nabla; &sdot; &rho; = 8/23 (&#x222F; &rho; ds dt &sdot; &rho; &#x2202;/&#x2202;&nabla;)<br />
:All fluid dynamics equations<br />
<br />
:|&psi;<sub>x,y</sub>&#x232a; = A(&psi;) A(|x&#x232a;&#x2297; |y&#x232a;)<br />
:All quantum mechanics equations<br />
<br />
:CH<sub>4</sub> + OH + HEAT &rarr; H<sub>2</sub>O + CH<sub>2</sub> + H<sub>2</sub>EAT<br />
:All chemistry equations<br />
<br />
:SU(2)U(1) &times; SU(U(2))<br />
:All quantum gravity equations<br />
<br />
:S<sub>g</sub> = (-1)/(2&epsilon;&#x0304;) i &eth; (&#x302; &xi;<sub>0</sub> +&#x030a; p<sub>&epsilon;</sub> &rho;<sub>v</sub><sup>abc</sup> &eta;<sub>0</sub> )&#x302; f&#x0335;<sub>a</sub><sup>0</sup> &lambda;(&#x0292;&#x0306;) &psi;(0<sub>a</sub>)<br />
:All gauge theory equations<br />
<br />
:[There is a brace linking the three cases together.]<br />
:H(t) + &Omega; + G&sdot;&Lambda; ... <br />
:... > 0 (Hubble model)<br />
:... = 0 (Flat sphere model)<br />
:... < 0 (Bright dark matter model)<br />
:All cosmology equations<br />
<br />
:&#x0124; - u&#x0327;<sub>0</sub> = 0<br />
:All truly deep physics equations<br />
<br />
{{comic discussion}}<br />
<br />
[[Category:Science]]<br />
[[Category:Physics]]<br />
[[Category:Math]]<br />
[[Category:Chemistry]]<br />
[[Category:Astronomy]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=Talk:2034:_Equations&diff=161412Talk:2034: Equations2018-08-17T18:57:04Z<p>162.158.106.216: </p>
<hr />
<div><!--Please sign your posts with ~~~~ and don't delete this text. New comments should be added at the bottom.--><br />
Is the joke that all of the equations are actually wrong/malformed/meaningless but they sort of look like typical equations for that field? {{unsigned ip|172.68.133.66}}<br />
:Sort of. A bit of dimensional analysis would have helped. ;-) --[[Special:Contributions/162.158.91.221|162.158.91.221]] 07:28, 17 August 2018 (UTC)<br />
<br />
Should we add a column with examples of similar correct equations from the respective fields? Sebastian --[[Special:Contributions/172.68.110.4|172.68.110.4]] 09:33, 17 August 2018 (UTC)<br />
:That would definitely tidy up my attempts to provide context for Randall's versions. The challenge then is working in explanations for the correct equations as well as arguing over which examples should be used. [[User:Exxi|Exxi]] ([[User talk:Exxi|talk]]) 09:45, 17 August 2018 (UTC)<br />
<br />
I don't think the part in parentheses about OH in the Chemistry equation explanation is correct. OH- would mean that it's negatively charged and has nothing to do with unpaired electrons of Oxygen. It would add another horror to the equation, though, as it wouldn't be charge preserving anymore. [[Special:Contributions/162.158.88.230|162.158.88.230]] 09:58, 17 August 2018 (UTC)<br />
<br />
"Redshit". Best typo ever. Please keep it. [[Special:Contributions/172.69.54.177|172.69.54.177]] 10:13, 17 August 2018 (UTC)<br />
<br />
;Deep physics equations<br />
The transcript is wrong here, the last letter is not a <math>\mu</math>, but a "u" with a cedilla: u̧. The math parser refuses to render it, though. [[Special:Contributions/162.158.88.230|162.158.88.230]] 05:54, 17 August 2018 (UTC)<br />
:Looks like it. But I don't think that letter exists even. --[[Special:Contributions/162.158.91.221|162.158.91.221]] 07:28, 17 August 2018 (UTC)<br />
:Is this equation a sort of nod to a Theory Of Everything which unifies quantum mechanics and gravity... H-hat (a Hamiltonian, which in quantum mechanics describes the total energy of a system, and usually runs in to problems describing large systems - such as the entire universe - where gravity or spacetime curvature effects matter) *minus* u0 (the relativistic mass of the whole system at time zero ie. the big bang) gives 0 (no energy everywhere always). Since mass is energy (e=mc^2) and mass is also the sole cause of gravity the two theories cleanly collapse together when mass is zero, and figuring out how to extend the theory to other less clean points on the mass axis is obviously a job for less profound physics? I've no ideas to explain the cedilla. [[Special:Contributions/141.101.98.28|141.101.98.28]] 08:49, 17 August 2018 (UTC)<br />
:It looks to me a little like a parody of the {{w|Wheeler-DeWitt_equation#Hamiltonian_constraint|Wheeler-DeWitt equation}} which (in theory) describes a wavefunction for the entire Universe. [[User:Exxi|Exxi]] ([[User talk:Exxi|talk]]) 09:06, 17 August 2018 (UTC)一<br />
: I'm just thrilled someone found the right character for it. I spent 20 minutes looking for the right u symbol without any luck at all. {{unsigned ip|172.68.143.132}}<br />
Is this poking fun at equation-filled blackboards in movies and cartoons? {{unsigned ip|172.68.254.42}}<br />
:Doesn't seem like it. These equations actually do look like the kinds of equations you would see in these fields. On blackboards in movies you tend to get equations that are pure nonsense. {{unsigned ip|172.68.143.132}}<br />
I think this may also be a reference to Feynman's unworldliness equation, http://www.feynmanlectures.caltech.edu/II_25.html#Ch25-S6 . [[Special:Contributions/108.162.219.220|108.162.219.220]] 17:02, 17 August 2018 (UTC)<br />
<br />
;Table layout at the explanation<br />
That oversized table is really bad layout. We've had this discussion many times before - tables should only be used for small contents. Right now I would run into too many edit conflicts but I'll change it to a proper floating text with small headers for each section. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 11:51, 17 August 2018 (UTC)<br />
:Done, looks much more like a real paper... --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 12:58, 17 August 2018 (UTC)<br />
<br />
;All number theory equation<br />
The explanation for math doesn't seem entirely correct. You can in fact extend the ring of integers (as well as rational and real numbers) with positive and negative infinity, but it won't be a ring anymore. Specifically, the infinities don't have an additive or multiplicative inverse (but 1/infinity = 0); and addition of positive and negative infinity, as well as the product of 0 and either infinity is undefined. However, these properties are not used in the above equation. What we ''can'' use is that <math>\forall n < \infty: n - \infty= -\infty </math>. We would thus have <math>K_n = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-e^{\pi-\infty}) = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-0) = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)i= \sum_{i=0}^{\infty}i\sum_{\pi=n}^{\infty}-\pi= \sum_{i=0}^{\infty}i\cdot(-\infty)=-\infty</math>. Also, how often does one use e and pi in number theory? --[[User:Ycthiognass|Ycthiognass]] ([[User talk:Ycthiognass|talk]]) 12:11, 17 August 2018 (UTC)<br />
:Pi (or any other number) minus infinite is just absurd. You can use the infinite symbol only as a limit but NOT as number in calculations. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 12:33, 17 August 2018 (UTC)<br />
::It is not absurd. Adding the rules <math>n+\infty=\infty\text{ for }n>-\infty,n-\infty=-\infty\text{ for }n<\infty, \pm n\cdot\infty = \pm\infty\text{ for }n>0, \pm n\cdot(-\infty) = \mp\infty\text{ for }n>0,\frac1{\pm\infty}=0</math> gives you a consistent theory that is especially useful when talking about infinite sums and integrals. Would you say the term <math>n-\sum_{i=1}^\infty i</math> is absurd? --[[User:Ycthiognass|Ycthiognass]] ([[User talk:Ycthiognass|talk]]) 14:35, 17 August 2018 (UTC)<br />
:::Of course it's absurd. It is <math>\infty-\infty \neq 0</math> because it could be everything between <math>\infty</math> and <math>-\infty</math>. {{w|Infinity}} is a concept describing something without any bound... And, as you can't divide by zero you can't do the same for infinity. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 17:24, 17 August 2018 (UTC)<br />
:::One more: It is <br />
::::<math>\sum_{i=1}^\infty a_i = \lim_{n\to\infty} \sum_{i=1}^n a_i.</math><br />
:::When this limit exists, one says that the series is ''convergent'' or ''summable''. Otherwise it's called ''divergent'' and has no solution like this one:<br />
::::<math>\sum_{i=1}^\infty i</math><br />
:::Infinite is NO number! --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 17:33, 17 August 2018 (UTC)<br />
<br />
Completely separate from the above, it's probably worth noting that i is also a constant, and as such has the same misconception as <math>\pi</math>. Computer scientists are happy using i for loops/summations, but mathematicians prefer using n. Based off that, it's probably another misconception/joke that n is treated as a constant, while known-constants are used as variables. [[Special:Contributions/108.162.246.149|108.162.246.149]] 17:28, 17 August 2018 (UTC)<br />
<br />
;Chemistry equation<br />
OH should have a charge symbol: OH<sup>-</sup>. The actual reaction would be:<br />
<br />
CH<sub>4</sub> + OH<sup>-</sup> + heat -> CH<sub>3</sub><sup>-</sup> + H<sub>2</sub>O<br />
<br />
The methyl group can dissolve in water, and this is presumably happening in water, so this equation can work, just not the one provided by Randell. Reacting longer alkanes with bases is a way to make soaps, but the methyl group would be too reactive to be used this way. [[User:Nutster|Nutster]] ([[User talk:Nutster|talk]]) 13:13, 17 August 2018 (UTC)<br />
<br />
;Fluid Dynamics equation<br />
I believe the fraction 8/23 in the Fluid Dynamics equation is a Randallesque reference to the fractional approximation of pi = 22/7. It's probably not a coincidence that you get 8/23 from 22/7 if you invert it and add 1 to both the numerator and denominator. [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 15:19, 17 August 2018 (UTC)<br />
<br />
;Gauge theory equation<br />
<s>I think the transcript is missing a left superscript 0 before the turned xi. [[Special:Contributions/172.68.226.16|172.68.226.16]] 16:50, 17 August 2018 (UTC)</s> Ah no, sorry. False alarm. It's just that Randall writes the xi with a funny tail. The same tail is on the non-turned xi earlier. [[Special:Contributions/172.68.226.10|172.68.226.10]] 16:52, 17 August 2018 (UTC)<br />
<br />
<br />
If explainxkcd.com is to make XKCD comics more understandable then this explanation is failing that. I assumed from the beginning that the joke was about the equations being wrong, but the description of the joke is making my head hurt.</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=2019:_An_Apple_for_a_Dollar&diff=1609962019: An Apple for a Dollar2018-08-09T05:23:34Z<p>162.158.106.216: /* Explanation */ punctuation, minor</p>
<hr />
<div>{{comic<br />
| number = 2019<br />
| date = July 13, 2018<br />
| title = An Apple for a Dollar<br />
| image = an_apple_for_a_dollar.png<br />
| titletext = I'd like 0.4608 apples, please.<br />
}}<br />
<br />
==Explanation==<br />
[[Megan]] is about to buy an apple at a grocery store when she is surprised that the price is exactly one dollar. A common practice in pricing items is to deliberately make them slightly less than a round number, such as $1.99 or $1.95 instead of $2, as a psychological trick to make the item seem significantly cheaper than it really is, as "less than two dollars" sounds much less than "two dollars" even though the difference of 0.01 is minimal. Additionally, in most cases in the US, {{w|Sales taxes in the United States|sales tax}} must be taken into account, as it is generally not included in the list price (although, [https://taxfoundation.org/which-states-tax-groceries/ most states] do exempt food sold in grocery stores from sales taxes), so a price rarely comes out to a round value. That it came out to an exact dollar is so strange for Megan that it throws her for a loop. Buying one apple for one dollar feels to her more like a simplified, imaginary ''Idea'' of a transaction (a "{{w|Platonic Ideal}}") than like something that could actually happen in real life.<br />
<br />
Megan likely shares Randall's background in engineering and math. When learning science, engineering, and math in the education system, one studies examples where every number is some round value, and all situations are simplified to the barest essentials so as to demonstrate the ideas being taught. Then, when doing real problems in the real world, one spends the rest of one's life almost never being able to use the simplified tricks demonstrated as examples in school, because when math is used to describe the natural world, nothing is ever a round number unless by design.<br />
<br />
Megan references {{w|Platonic Idealism}}, which is the theory attributed to Plato that abstract or non-physical Ideas represent the purest, most accurate version of reality, but we can only perceive of more flawed versions of Ideas because of our limited viewpoint (as explained in his Allegory of the Cave). Thus we can understand the concept of a perfect circle or a perfect line, even though we have never seen one, and cannot create one. Megan believes she has glimpsed a Platonic Ideal because the absolute concept of currency is it is the exact worth of something in trade. Megan is awed because, if this is true, then she is witnessing the next layer of reality, which Plato often compared to heaven.<br />
<br />
The harsh difference between being able to buy an apple for a dollar at this quaint store, and having to deal with arbitrary decimals and numbers in the rest of life could be touching on Megan's life experience of the world not being what she was prepared for, resulting in her intense response. Regardless if that is true or not, it seems the cashier is unable to figure out how to handle it (or does not want to), and raises the price to an arbitrary non-rounded value, which has the intended effect of halting Megan's outburst. The unexpected resolution of the rising tension is a source of humor in this strip.<br />
<br />
Megan's references refer to common parameters used in solving science or math questions. A {{w|Frictionless plane}} is a scenario from the writings of Galileo to calculate the movement of an object down an {{w|inclined plane}}, since his equations did not account for {{w|friction}}.<br />
<br />
"A train leaving Chicago at 40 mph" refers to common math questions, involving trains and solving for the distance required to overtake said train, although this problem involves the rather unrealistic assumption that the train's velocity keeps constant and doesn't need to accelerate in order to reach its speed. Like the frictionless plane, this is a common simplification that allows the problem to be solved with quite simple techniques, just like having round quantities (e.g. 1 dollar/apple) eases arithmetic problems. See also [[669: Experiment]]. Apples themselves are commonly used as units for math problems, including problems as simple as basic arithmetic.<br />
<br />
The comic repeats a common theme in the strip of engineers and computer scientists trying to apply their technical experience to social situations. In this case, the conversation partner is "normal", and does not respond supportively, which is a common situation in the real world and a possible point of empathy with readers. -- An alternate viable reading is that the conversation partner responds extremely supportively (by cleverly removing the source of Megan's distress, rather than by questioning the validity of Megan's response). This is a possible point of wish-fulfillment for readers.<br />
<br />
It seems that according to the title text, Megan only has (or only wants to spend) one dollar, so she would not be able to buy a whole apple at the new price (0.4608 × $2.17 ≈ $1). Stores usually sell whole apples, so asking for a fraction of one is not likely to work out.{{Citation needed}}<br />
<br />
There are some stores, such as {{w|Dollar Tree}}, that specialize in selling everything in the store for one dollar per item, which would seem to be operating at that ideal... except they usually do charge sales tax on taxable items leading many sales to not be an even multiple of a dollar. If a store were to charge one dollar per item without charging sales tax, etc. separately (i.e. building the sales tax into the price of each dollar item), they might be able to simplify some operations, such as not dealing with coin change as much (though they would still need to accept coins), cashiers would be able to calculate the total in their heads, etc. Customers other than Megan would probably be happier.<br />
<br />
==Transcript==<br />
:[Megan is at the store counter, behind which Ponytail (the cashier) is waiting.]<br />
:Megan: Just this apple, thanks.<br />
:Ponytail: That will be one dollar.<br />
:Megan: Exactly? No tax or anything?<br />
:Ponytail: That's right.<br />
<br />
:[Megan stares at the apple in a frameless panel.]<br />
<br />
:[Scene zooms in on Megan.]<br />
:Ponytail: ...Is that a problem?<br />
:Megan: It's just weird to realize that every other transaction in my life will be more complicated than this.<br />
<br />
:[Scene changes focus to Ponytail behind the counter.]<br />
:Megan: This is like a platonic ideal exchange. An apple for a dollar.<br />
:Ponytail: I see.<br />
<br />
:[Scene changes back to Megan, once again lost in profound contemplation of the apple.]<br />
:Megan: Are we on a frictionless plane? Is a train leaving Chicago at 40 mph? ''Should I solve for something??''<br />
:Ponytail: Okay, apples are $2.17 now.<br />
:Megan: That's... probably better for us both.<br />
<br />
{{comic discussion}}<br />
<br />
[[Category:Comics featuring Megan]]<br />
[[Category:Comics featuring Ponytail]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=2027:_Lightning_Distance&diff=1607222027: Lightning Distance2018-08-01T20:51:44Z<p>162.158.106.216: /* Explanation */</p>
<hr />
<div>{{comic<br />
| number = 2027<br />
| date = August 1, 2018<br />
| title = Lightning Distance<br />
| image = lightning_distance.png<br />
| titletext = The index of radio refraction does have a lot of variation, which might throw off your calculations, so you can also look at the difference in brightness between the visible flash and more-attenuated UV and x-rays.<br />
}}<br />
<br />
=Explanation=<br />
{{incomplete|Created by a RADIO BURST - Update calculations and added values in km and miles. Do NOT delete this tag too soon.}}<br />
The usual trick for determining the distance to a {{w|lightning}} flash is to count the seconds from when you see the flash until when you hear {{w|thunder}}, and divide by five to get miles (or three to get kilometers). This works because the {{w|speed of light}} is essentially instantaneous over the relevant distances, while the {{w|speed of sound}} is 331.2 m/s (1,087 ft/s, 1,192 km/h, or 741 mph, varying a bit based on temperature), or about 1/5 mile per second.<br />
<br />
This comic subverts the usual trick by having Megan describe a highly impractical alternative method. Megan's method is based on the fact that the speed of electromagnetic radiation, which includes light and radio waves, is not truly infinite. The radiation produced by lightning on Earth also has to travel through air, which changes its speed in a fashion which depends on its frequency.<br />
<br />
Wikipedia {{w|List_of_refractive_indices|lists}} refractive index of air at standard temperature and pressure as 1.000277, which equates to a speed of light around 299709.4 km/s (186230.8 miles/s). According to [https://www.fig.net/resources/proceedings/fig_proceedings/fig_2002/Js28/JS28_rueger.pdf this paper], refractive index for radio waves in similar conditions is 1.000428, which equates to a speed around 299664.2 km/s (186202.7 miles/s). This means that to get the distance, the time difference in seconds between visible flash and radio burst should be multiplied by a number around 2.0 billion for kilometers (or around 1.2 billion for miles). Depending on actual conditions (temperature, humidity, etc.), this number can change dramatically. For example, the same calculation at 0&deg;C would yield a multiplier of 13.6 billion for kilometers (or 8.5 billion for miles). More details for the calculations are in the comments below.<br />
<br />
With sufficiently precise instruments, it would theoretically be possible to use this effect to determine the distance to a lightning flash, as proposed by Randall. The joke is that it is impractical for humans, both because we can't measure such small time intervals and because we can't detect radiation outside the visible spectrum without very specialized instruments. For the purpose of the joke, the "5 billion" value used in the comic is a fair rule of thumb which also references the original rule of 5 seconds per mile nicely, though the result will have a huge margin of error.<br />
<br />
The title text suggests another method of calculating distance to lightning. Since the absorption of light is also different in different wavelengths, it would be possible to calculate the difference by comparing the brightness instead of delays. This would, however, require the knowledge about prior relative brightness of lightning, i.e. the spectrum, in the compared wavelengths.<br />
<br />
==Transcript==<br />
{{incomplete transcript|Do NOT delete this tag too soon.}}<br />
:[Cueball and Megan stand on either side of a window, observing a bolt of lightning in a dark sky.]<br />
:Cueball: What's that trick for telling how many miles away lightning is?<br />
:Megan: Just count the seconds between the visible flash and the radio wave burst, then multiply by 5 billion.<br />
{{comic discussion}}<br />
<br />
[[Category:Comics featuring Cueball]]<br />
[[Category:Comics featuring Megan]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=68:_Five_Thirty&diff=16044368: Five Thirty2018-07-25T03:49:03Z<p>162.158.106.216: /* Explanation */ I would like to point out that being just woken up and being late to fall asleep are oft similar.</p>
<hr />
<div>{{comic<br />
| number = 68<br />
| date = February 24, 2006<br />
| title = Five Thirty<br />
| image = five_thirty.jpg<br />
| titletext = The 8th panel is my favorite<br />
}}<br />
<br />
==Explanation==<br />
At 5:30 AM, one's sleep-deprived or prematurely-roused mind sometimes comes up with things that seem like nonsense later.<br />
<br />
None of the twelve panels in this comic seem to have any correlation with one another, each one being its own "story," and none of them really make any sense. It is unknown whether Randall really wrote this comic while awake at 5:30 in the morning, or if he wrote it while completely alert and is trying to pass off his rejected ideas by saying what one's mind may experience when trying to process information at an hour when the person is not used to being awake.<br />
<br />
===Explanations of the individual panels (numbered left to right, top to bottom)===<br />
# "'80s night" is a special theme many nightclubs hold, inviting their guests to wear fashions that were popular in the 1980s while playing dance music from the same period.<br />
# {{w|Jack the Ripper}} was an infamous serial killer in Victorian {{w|England}}. {{w|Jack Black}} is a rock star and actor. "There is no Tuesday" is likely a reference to the line "There is no spoon" in ''{{w|The Matrix}}''.<br />
# There doesn't seem to be too much to this panel that isn't self-explanatory, but it's possible that there's a reference to the ''{{w|Civilization (video game)|Civilization}}'' series of video games, in which it's possible (albeit unlikely) for medieval soldiers to attack and destroy 20th-century military helicopters.<br />
# Stick figures standing upright are indeed drawn without any thought as to which section of their legs are the shins.<br />
# Two Cueballs seem very upset as indicated by them shouting ''shit'' ten times without break, and as indicated at the end, they will continue to do so, as only "sh" of the 11th shit is written. What they should have been alarmed about is the '''third arm''' that seems to have '''grown out''' of the left Cueball's torso. But as it turns out, they are worried about daylight savings ''[sic]'', which comes out in the second line of their shouting. {{w|Daylight saving time}} (DST) was {{w|History_of_time_in_the_United_States#Start_and_end_dates_of_United_States_Daylight_Time|not due}} until April 2nd in the USA in 2006, more than a month after the release of this comic. This seems to be the first to use DST as part of a joke, but it is far from the last time that Randall has made it clear that he is [[:Category:Daylight saving time|not a fan of DST]], which he sometimes directly mocks. It's hard to tell what the two stick figures are actually doing and what the black blobs at the ends of their arms should represent, given that Randall normally doesn't draw hands on his stick figures. One guess is that they're both wearing watches, one on each of their combined five arms. In that case, they are upset because they forgot to adjust all their clocks for DST. Alternatively, the two Cueballs represent two clocks, the right one with only hour-hand and minute-hand, but the left one also with a seconds-hand, and it is these hands that are drawn. In that case, the left Cueball is one hour ahead of the right Cueball, as his shortest arm points down left at around 8 o'clock (with the seconds-hand above, and the minutes-hand at 20 minutes past), and the right Cueball has his hour hand at 7 o'clock and the minute hand also at 20 past. This would explain both the three arms, the "hands" at the end of the arms, and the different length of especially the left Cueball's arm, and finally why they are so upset about one of them forgetting DST.<br />
# "Ointment" may be a reference to the infamous lotion scene in ''{{w|Silence of the Lambs}}'', as the panel appears to be invoking horror movie visuals.<br />
# The farthest left angle is labeled theta. The joke is that finding the cosine, the length of the adjacent leg divided by the length of the hypotenuse, would be difficult as the adjacent leg is poorly drawn and does not resemble a straight line to be measured.<br />
# Lots of jokes have been made out of the template "does liking X make you gay?", where the speaker is afraid that he may be a homosexual. Here, the speaker has apparently transformed into a {{w|mermaid}} at some point. His friend seems to be eager to both turn into a mermaid himself and confirm himself for a homosexual. <p>Another explanation may be that the friend thinks that a man who was a mermaid for five minutes should be homosexual afterwards, because he simply can't imagine something else about it. In this explanation, the friend has no interest in others being gay or not; he just thinks that this may be a realistic progress.</p><br />
# Waving a gun around and declaring that things you hate are "for pussies" is stereotypical "{{w|macho}}" behavior. Possibly, the man with the gun is going to cut the other man's hair with bullets because it's more "macho" than going to the barber.<br />
# This doesn't seem to mean anything whatsoever. However, both of the characters say something irrational: "My hair is bleeding" is irrational because strands of hair can't bleed, and "√3" is an {{w|irrational number}}.<br />
# A bachelor party is a traditionally raucous party that is thrown for a groom on the night before his wedding. Because these parties can be wild (involving drinking and such), this may explain why the figure is upside down.<br />
# Likely a reference to the "{{w|ant on a rubber rope}}" thought experiment. Apparently in Randall's mind, the experiment does not end well for the ant.<br />
<br />
---- <br />
<br />
The title text could actually refer to two different panels. If a person chooses to read the comic left-to-right, top-to-bottom (which is more likely given that this is the order in the official transcript), the eighth panel could be the one with where Cueball asks "Does being a mermaid for five minutes make you gay?" However, if a person chooses to read the comic top-to-bottom, left-to-right, the eighth panel will instead be the one with Cueball hanging upside down shouting "Bachelor party!"<br />
<br />
==Transcript==<br />
:Comics from 5:30 AM<br />
:[A succession of unrelated and completely random panels.]<br />
<br />
:Cueball: It's 80's night at the club. Wanna go?<br />
<br />
:Friend: There is no Tuesday.<br />
:Cueball: Jack the Ripper or Jack Black?<br />
<br />
:[Cueball in this panel is holding a glinting sword.]<br />
:Friend: You crashed my helicopter!<br />
:Cueball: Verily!<br />
<br />
:[A small figure is talking with a larger figure.]<br />
:Figure 1: Basically, neither of us have shins.<br />
:Figure 2: Over and out.<br />
<br />
:[Two men are shown: one with three arms, and another with just two. All arms have round appendages at their ends.]<br />
:Men: shitshitshitshitshitshitdaylightsavingsshitshitshitshitsh<br />
<br />
:[Two figures with pumpkins (carved with faces) for heads.]<br />
:Figure 1: You're out of ointment and out of time!<br />
<br />
:[A diagram of a right-angled triangle, with a theta at the smallest angle.]<br />
:FUCK THE COSINE<br />
<br />
:Friend: Does being a mermaid for five minutes make you gay?<br />
:Cueball: I hope so!<br />
<br />
:[The friend is holding a gun to Cueball's head.]<br />
:Friend: Barbershops are for pussies.<br />
<br />
:Friend: My hair is bleeding.<br />
:Cueball: √3<br />
<br />
:[Cueball seems to be walking on the ceiling.]<br />
:Cueball: Bachelor party!<br />
<br />
:[Warning sign with picture of an ant.]<br />
:WARNING: STRETCHY DEATH<br />
<br />
{{comic discussion}} <br />
<br />
[[Category:Comics featuring Cueball]]<br />
[[Category:Multiple Cueballs]]<br />
[[Category:Math]]<br />
[[Category:Homosexuality]]<br />
[[Category:Animals]]<br />
[[Category:Daylight saving time]]</div>162.158.106.216https://www.explainxkcd.com/wiki/index.php?title=2002:_LeBron_James_and_Stephen_Curry&diff=1583452002: LeBron James and Stephen Curry2018-06-05T19:55:02Z<p>162.158.106.216: Sandwich metric is ambiguous</p>
<hr />
<div>{{comic<br />
| number = 2002<br />
| date = June 4, 2018<br />
| title = LeBron James and Stephen Curry<br />
| image = lebron_james_and_stephen_curry.png<br />
| titletext = The 538TR attempts to capture a player's combined skill at basketball (either real-life or NBA 2K18) and election forecasting.<br />
}}<br />
<br />
==Explanation==<br />
{{incomplete|Created by a Basketball - Please change this comment when editing this page. Do NOT delete this tag too soon.}}<br />
At the time of this comic, the {{w|2018 NBA Finals}} were going on, between the {{w|Cleveland Cavaliers}} and the {{w|Golden State Warriors}} with the Warriors leading 2 games to 0 in a best of seven series. At first glance, the comic looks like an in-depth analysis of two of the star players on those teams, {{w|LeBron James}} and {{w|Stephen Curry}}. The joke is that while comprehensive, all the statistics are completely meaningless - many don't show any correlation, and if there is one, it's extremely unlikely there is any causal link in there. <br />
<br />
The first graph includes a nine-digit {{w|Social Security number}} issued for US citizens which is typically not considered a metric related to athletic ability. As Social Security numbers are essentially random numbers ([https://www.ssa.gov/employer/randomization.html until 2011], there was a geographic correspondence for the first three digits), the graph shows only the {{w|free throw}} percentage of a large number of players, artificially spread vertically. Also note that Social Security numbers are not usually made public, barring security leaks.<br />
<br />
The second graph is a graph of 2018 points per game vs teammate's APGAR score. {{w|APGAR score}} is used to quickly summarize the health of newborn children, with scores of 7 and above indicating an infant has generally normal health; its use to rank adult NBA players is odd, if not improper. This graph indicates LeBron's teammates have an average APGAR score of approximately 2.1. Scores of 3 and below are generally regarded as critically low and possibly requiring medical attention. Low APGAR scores can also be associated with increased risk of neurological disorders such as cerebral palsy. The joke appears to be in giving LeBron's less-than-impressive teammates a low APGAR score.<br />
<br />
The shot map shows from what position Curry's shots were scored compared to other NBA players. It shows that he scored several times from outside the playing field, including twice from the {{w|bleacher}}s (which isn't a legal play), and once from the {{w|locker room}} (which is physically impossible due to multiple walls in between). This references Stephen Curry's propensity to take (and make) shots from well beyond the normal distance required for 3 points.[https://thebiglead.com/2016/02/26/stephen-currys-shooting-percentage-from-28-feet-out-is-mind-boggling/]<br />
<br />
Next is a graph of (team) win percentage vs sandwiches eaten during play. This graph shows no correlation between these 2 metrics. It does indicate that the Golden State Warriors have eaten either 4 sandwiches per game, or 4 sandwiches total during games, over the course of the 2018 season (the graph is unclear on this point).<br />
<br />
In the "2018 total points" table, the highlighted {{w|Golden State Warriors}} and {{w|Cleveland Cavaliers}} represent the teams of Stephen Curry and LeBron James respectively. {{w|Magnetic north}} is the south pole of the {{w|Earth's magnetic field}}. Certain animals use the magnetic field to navigate and align themselves (including migratory birds, bees, and foxes), but there is no evidence that humans are affected by the earth's magnetic field. This means that there is very likely no correlation between orientation of a basketball court and points scored.<br />
<br />
The table at the bottom includes more unrelated comparisons:<br />
<br />
;Have You Heard of Him<br />
: Although both players are well known in their native United States, elsewhere basketball is considered a minority sport. Of the 7 billion people in the world it is likely that less than 2% of the total population will have heard of either player.{{Citation needed}} According to Randall, LeBron James is a more well known player than Stephen Curry. <br />
;President During Most Recent Game 7 Loss<br />
: In the NBA, the top 16 teams qualify for a single elimination play-off to determine the season champion, with each series played as a {{w|Playoff_format#Best-of-seven_playoff|best-of-seven}} series (first to win 4 games). After the fourth game, fixtures are only played as required. Most fixtures are therefore resolved before the last game. Lebron James has participated in seven playoff game 7s in his career (winning 5 of 7), and the last time his team lost a game seven was on May 18, 2008 ({{w|George W. Bush}} was still President). This also highlights that James is an older athlete, yet has been fairly dominant through his career. Stephen Curry's last game 7 loss came at the hands of Lebron James in the {{w|2016 NBA Finals}} ({{w|Barack Obama}} was President). It is notable that both the Golden State Warriors and Cleveland Cavaliers won their respective games 7 in their Conference Finals to make it to this year's NBA Finals.<br />
;Pog Collection<br />
: {{w|Milk caps (game)|Pogs}} were a {{w|fad}} in the 1990s. It is unclear why James would have a "staggeringly large" collection of pogs, besides being 4 years older than Curry.<br />
This might also refer to Player of the Game awards. Lebron James would certainly have a staggeringly large amount of them, while Curry has less, having to share player of the game accolades with his other All-Star teammates.<br />
;Career Average Fed Interest Rate<br />
:The Federal Interest Rate, or {{w|federal funds rate}}, is an interest rate set by the {{w|United States Federal Reserve}}. This rate is increased or decreased periodically based on the health of the U.S. economy. As of the time of publishing, the federal interest rate was targeted at 1.75%. The rate has fluctuated from a high of around 5% to a low of near 0% (during the time of the {{w|Great Recession in the United States|2008 recession}}). James' career average federal interest rate is higher than Curry's, because James began his career before Curry, when interest rates were higher.<br />
;Name Scrabble Score<br />
:Both "lebronjames" and "stephencurry" are worth 22 points in {{w|Scrabble}}. <br />
;Best Sport<br />
: It is claimed that their best sport is basketball. However, although they have chosen basketball as a career, this does not mean they were not better at a sport that does not offer a professional career.<br />
;Height<br />
: Both are listed as over 6 feet tall, which is not at all unusual for professional basketball players. Stephen Curry is listed as 6'3" and LeBron James as 6'8".<br />
;Retirement Year<br />
: In 2027, Stephen Curry will be 39 years old, which is a typical retirement age for NBA players. LeBron James's retirement age is listed as ''Unknown''. This may refer to James's high level of play through his mid-30s, when typical players have a decline in their performance.{{Citation needed}}<br />
;FiveThirtyEight Total Rating<br />
: {{w|Nate Silver}} is a political commentator and founder of the website {{w|FiveThirtyEight}}, which uses and promotes statistical approaches in explaining the world. The site's two major areas of focus are in politics (especially on elections - it became famous for correctly predicting for whom 49 of 50 of the 2008 and every US state would vote for in the 2012 US presidential elections, and though it wasn't as accurate in 2016 it had given Donald Trump a larger chance of Electoral College victory than other mainstream media sources) and sports (Silver first got into statistical analysis via baseball). The presence of both sports-related and politics-related topics in the comic, however related they are (or not) with each other, seems to be a nod towards FiveThirtyEight's content.<br />
: Nate Silver has a much higher 538TR than either Curry or James. As explained in the title text, the 538TR combines basketball skill (either real-life or video game basketball) with election forecasting. This would seem to imply that Silver is proficient at basketball, either the real-life kind, or the video game kind, and that James and Curry are not proficient in election forecasting.<br />
<br />
==Transcript==<br />
{{incomplete transcript|Do NOT delete this tag too soon.}}<br />
:'''NBA Playoffs ''DataDive'' '''<br />
:'''LeBron James and Stephen Curry'''<br />
:'''What makes these superstars so extraordinary?'''<br />
<br />
:[The comic consists of several plots and tables, listed here from top to bottom, left to right.]<br />
<br />
:'''Scatter plot of Social Security number vs Free throw percentage'''<br />
:The Social Security numbers range from 000-00-0000 to 999-99-9999. No pattern discernable, aside from points being a bit denser in the middle of the plot. Steven Curry is marked as a point on the right edge of the plot with a high free throw percentage.<br />
<br />
:'''Scatter plot of 2018 points per game vs Average teammate APGAR score'''<br />
:The APGAR scores range from 0 to 10. Pattern suggests a somewhat positive link between the two factors. LeBron James is marked as having a lot of points, but a low teammate APGAR score of approximately 2.1.<br />
<br />
:'''Shot map'''<br />
:Legend: grey dot for all players, black dot for Stephen Curry<br />
:A diagram of a basketball court is shown with dots placed where players have taken shots at the goal. For the all players category the dots generally cluster next to the goal basket and in front of the three point line. Steven has 3 dots next to the basket (one is behind it), but does cluster next to the three point line. He also has several dots on the other side of the playing field, and outside it, including three in the bleachers and one in the locker room.<br />
<br />
:'''Sandwiches eaten during play vs Win %'''<br />
:A plot that suggests no relation between the factors because practically all dots are in the zero sandwiches column. 2018 Warriors have one dot at around 60 win percentage and 4 sandwiches.<br />
<br />
:'''2018 total points'''<br />
:A table listing teams and their points overall and "When net is within 15° of magnetic north". The rows for the Golden State Warriors and the Cleveland Cavaliers are highlighted, the latter shows an abnormally high score in the magnetic north column.<br />
<br />
:{| class = "wikitable"<br />
! 2018 total points<br />
! Overall <br />
! When net is within 15° of magnetic north<br />
|-<br />
|'''Golden State Warriors'''<br />
|'''9304'''<br />
|'''330'''<br />
|-<br />
|Houston Rockets<br />
|9213<br />
|268<br />
|-<br />
|New Orleans Pelicans<br />
|9161<br />
|219<br />
|-<br />
|Toronto Raptors<br />
|9156<br />
|341<br />
|-<br />
|'''Cleveland Cavaliers'''<br />
|'''9091'''<br />
|'''1644'''<br />
|-<br />
|Denver Nuggets<br />
|9020<br />
|280<br />
|}<br />
<br />
:[A table at the bottom:]<br />
<br />
:{| class = "wikitable"<br />
!<br />
! Stephen Curry<br />
! LeBron James<br />
|-<br />
|Have you heard of him<br />
|Probably<br />
|Yes<br />
|-<br />
|President during most recent game 7 loss<br />
|Obama<br />
|Bush<br />
|-<br />
|Pog collection<br />
|Large<br />
|Staggeringly large<br />
|-<br />
|Career average Fed interest rate<br />
|3.42%<br />
|4.41%<br />
|-<br />
|Name Scrabble score<br />
|22<br />
|22<br />
|-<br />
|Best sport<br />
|Basketball<br />
|Basketball<br />
|-<br />
|Height<br />
|Over 6'<br />
|Over 6'<br />
|-<br />
|Retirement year<br />
|2027<br />
|Unknown<br />
!Nate Silver<br />
|-<br />
|FiveThirtyEight total rating<br>(devised by Nate Silver to combine all metrics into a single stat)<br />
|'''37.4'''<br />
|'''31.8'''<br />
|'''86.6'''<br />
|}<br />
<br />
==Trivia==<br />
This comic was posted the day after the second game in the 2018 NBA Finals between the Golden State Warriors (Stephen Curry's team) and the Cleveland Cavaliers (LeBron James' team). It is the fourth consecutive time the two teams faced each other at the finals, which is unprecedented in major sports leagues in North America. The Warriors won in 2015 and 2017, the Cavaliers won in 2016, and the Warriors are leading the current series 2-0.<br />
<br />
{{comic discussion}}<br />
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[[Category:Sport]]<br />
[[Category:Basketball]]<br />
[[Category:Comics featuring Nate Silver]]<br />
[[Category:Politics]]</div>162.158.106.216