explain xkcd - User contributions [en]

2520: Symbols

2021-09-25T03:40:02Z

Mathferret: In pi/tau, fixed a typo: changed "constants" to "contexts"

{{comic
| number = 2520
| date = September 24, 2021
| title = Symbols
| image = symbols.png
| titletext = "röntgen" and "rem" are 20th-century physics terms that mean "no trespassing."
}}

==Explanation==
{{incomplete|Created by an INTERNET ARGUMENT - Please change this comment when editing this page. Do NOT delete this tag too soon. Bare-bones explanation is in, but needs much more detail.}}

This comic refers to elements of (mostly mathematical or engineering) notation commonly used in various fields of math and science. Each piece of notation is presented as "symbolizing" not what it specifically means, but a typical ''context'' in which it might be encountered. Many of the individual descriptions sounds look like verbiage that might be found on informational or warnings signs or placards, although typically with a silly edge.

*d/dx: An undergrad is working very hard
d/dx is the symbol for a single-variable {{w|Derivative|derivative}}. This is a mathematical operation that, while difficult, is one of the most basic operations in calculus and consequently well within the reach of an undergraduate student, particularly in science. Thus, an equation with this operation would be one that would cause an undergraduate student to work very hard.

*∂/∂x: A grad student is working very hard
The replacement of the standard "d" letters with the curly letters "∂" denotes the partial derivative, which generalizes the ordinary derivative to multi-variable calculus. Problems with partial derivatives, especially partial differential equations, can be extremely challenging—appropriate for hard graduate-level work.

*ħ: Oh wow, this is apparently a quantum thing
ħ (pronounced "h-bar") is a symbol used for (the reduced) Planck's constant, a universal, fundamental constant in quantum physics. ħ is equal to the energy of a photon divided by its frequency, and angular momentum in quantum mechanical systems is measured in quantized integer or half-integer units of ħ.

*Re: Someone needs to do a lot of tedious numerical work; hopefully it's not you
The Reynolds number (which is actually usually denoted by "Re," not "Re" as it appears in the comic) is the most important dimensionless group in fluid mechanics. Named for Osborne Reynolds, Re characterizes the relative sizes of inertial and viscous effects in a moving fluid. Large values of Re are indicative of turbulent flow, which cannot usually be solved for analytically, and so numerical modelling is necessary. Accurate numerical studies of high-Reynolds-number flows are notoriously difficult to create and program.

*(Ta4 – Tb4): You are at risk of skin burns
The Stefan-Boltzmann law says that a perfectly absorbing ("black body") source emits electromagnetic radiation with a power per unit area of σT4, where σ is a known constant and T is the absolute temperature. The quantity (Ta4 – Tb4) thus appears in any calculation of purely radiative energy transfer between two bodies, one at temperature Ta and the other at Tb. When radiative transfer is large enough to be the most important form of heat interchange, it is normally also large enough to sear the skin with thermal or ultraviolet burns.

*NA: You are probably about to make an incredibly dangerous arithmetic error
NA, or Avogadro's number, is the number of molecules in a mole of a substance—roughly the number of protons and neutrons in 1 gram of matter. This is an enormous number, approximately 6.02 × 1023. Adding up molecular weights and converting between grams and moles of several substances is a lot of arithmetic on a scale where intuition won't help you catch mistakes. Working with NA, it is easy to make errors of one or more factors of ten without noticing. If this kind of error is made in the calculation of the stoichiometrically correct amount of a reagent in a chemical reaction, it is possible to accidentally create dangerous amounts of unwanted chemical products.

*µm: Careful, that equipment is expensive
Micrometers are a very small unit of distance. Any equipment that is operating in these units will be incredibly finely calibrated and thus very expensive.

*mK: Careful, that equipment is very expensive
Millikelvin temperatures are very cold, barely above absolute zero, suggesting sensitive experiments probing quantum mechanical behavior that would likely only exist in an advanced laboratory. Any equipment that works down a mK temperatures is likely to be very expensive.

*nm: Don't shine that in your eye
Nanometers are most frequently seen in the listed wavelengths for lasers. Pointing a visible or infrared laser at someone's eye is notoriously dangerous; the tightly-focused coherent light can cause permanent damage very quickly.

*eV: Definitely don't shine that in your eye
Electron volt energies are typical of moderate-energy particle beams, produced by accelerating electrons (or protons) over macroscopic voltages. These particle beams can be {{w|Anatoli Bugorski|even more damaging}} to soft tissues than optical-wavelength lasers.

*mSv: You are about to get into an Internet argument
The millisievert is a unit of radiation dose absorbed. It is actually a very small dosage, but the joke refers to Internet trolls debating the effects of low-dose radiation sources, such as 5G wireless networks. Randall's comment may also be referring to [https://xkcd.com/radiation/ this chart].

*mg/kg: Go wash your hands
This unit measures the dose of a drug or other chemical in milligrams per kilogram of body mass. If the appropriate dose - or worse, the lethal dose - is measured in mg/kg (parts per million), then the substance may be quite toxic.

*µg/kg: Go get in the chemical shower
A unit 1000 times smaller than mg/kg. If a dosage is measured in micrograms per kilogram (parts per billion), it is probably causing chemical rather than biological reactions.

*π or τ: Whatever answer you get will be wrong by a factor of exactly two
π is defined as the ratio of a circle's circumference to its diameter, while τ is defined as 2π (the ratio of the circumference to the radius). π has been used as the primary constant for describing the circumference and area of circles for millennia, but proponents of τ claim that τ is actually more natural in most contexts, since it makes working in radians more straightforward. The joke here is that whichever constant you use, it will probably be the wrong one (off by a factor of two, one way or the other) for the formula you are trying to use.

The title text refers to two non-SI units of radiation measurement. In the mid-20th century when they were in use, the dangers of radiation weren't as well understood as today, so an area with radiation that was noteworthy back then is probably dangerous,{{citation needed}} hence the no trespassing part.

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

;d/dx
:An undergrad is working very hard
;∂/∂x
:A grad student is working very hard
;ħ
:Oh wow, this is apparently a quantum thing
;Re
:Someone needs to do a lot of tedious numerical work; hopefully it's not you
;(Ta4 - Tb4)
:You are at risk of skin burns
;NA
:You are probably about to make an incredibly dangerous arithmetic error
;µm
:Careful, that equipment is expensive
;mK
:Careful, that equipment is very expensive
;nm
:Don't shine that in your eye
;eV
:Definitely don't shine that in your eye
;mSv
:You are about to get into an internet argument
;mg/kg
:Go wash your hands
;µg/kg
:Go get in the chemical shower
;π or τ
:Whatever answer you get will be wrong by a factor of exactly two

{{comic discussion}}

1755: Old Days

2016-11-04T15:46:59Z

Mathferret: correct grammar: if ... would have to if ... had

{{comic
| number = 1755
| date = November 4, 2016
| title = Old Days
| image = old_days.png
| titletext = Lot of drama in those days, including constant efforts to force the "Reflections on Trusting Trust" guy into retirement so we could stop being so paranoid about compilers.
}}

==Explanation==
{{incomplete|More explanation required on computer programming. Fill out the explanation column in the [[#Table of statements|table]] that lists all the statements.}}
This comic is showing a conversation between (young) [[Cueball]] and (old) [[Hairbun]] about computer programming in the past, specifically the {{w|compilers}}. Cueball, having a faint idea of just how difficult and byzantine programming was "in the old days", asks Hairbun to enlighten him on the specifics. Hairbun promptly seizes the opportunity to screw with his head.

While her initial explanation that code needed to be compiled for multiple architectures is correct, Hairbun's claims rapidly grow ridiculous to the point where the improvement from {{w|C (programming language)|C}} to {{w|C++}} was that C++ finally supported {{w|floppy disks}} but just punched holes in them rather than using {{w|punch cards}} "like C used". (A notch in the side of 5.25" floppy disks indicates when the disk could be written. Though many floppy disks were intended to have only a single side with data, many people used a hole punch to notch the opposite side of the disk, allowing a drive to write data to the other side of the disk in a single sided drive.)

Hairbun tells Cueball a tall tale about how hard it was back in the '''old days''', making it sound like some of the programming languages used today (C, C++) were written on punch cards and that you had to ship your code in the mail to a computer company ({{w|IBM}} in this case) to compile your code, which would take from four to six weeks. If there was a simple error, you would have to ship it again for another compilation.

Nothing of what she tells Cueball makes any sense, but it is clear from Cueball's final ''Wow'' that he falls for it, ready to believe anything the old Hairbun tells him about how horrible it was to program in the olden days. It is true that it was tough and slow to program on punch cards, which were actually used for an extended period of time, but there is nothing in the rest of Hairbun's story that accurate, except that it was a big deal when the floppy disk was invented.

In the title text, Hairbun continues her musings on the old compiler days, stating that there was ''a lot of drama in those days''. Specifically the references ''[http://www.win.tue.nl/~aeb/linux/hh/thompson/trust.html Reflections on Trusting Trust]'' a famous 1984 paper by {{w|UNIX}} co-creator {{w|Ken Thompson}} in which he described a way to hide a virtually undetectable backdoor in the UNIX login code via a second backdoor in the C compiler. Using the technique in his paper, it would be impossible to discover the hacked login by examining the official source code for either the login or the compiler itself. Ken Thompson may have actually included this backdoor in early versions of UNIX, undiscovered. Ken Thompson's paper demonstrated that it was functionally impossible to prove that any piece of software was fully trustworthy.

Hairbun claims that one of the dramas she refers to was that people tried to force Ken Thompson to retire, so everyone could stop being so paranoid about compilers. In reality, any coder who created the first version of a compiler (or a similar critical component) could inject a similar backdoor into software, so it would be false safety. Even if no one else had thought of this, then Thompson's paper was there for any future hacker to see. Though the problem was (claimed to be) solved in {{w|David A. Wheeler}} Ph.D dissertation "[http://www.dwheeler.com/trusting-trust/ Fully Countering Trusting Trust through Diverse Double-Compiling (DDC)]".

==Table of statements==
{|class="wikitable"
!Statements
!Concepts used
!Explanation
|-
|Compile things for different processors
|Compilers convert code from a human-readable programming language into a binary code that can be directly executed by computer processors.
|Many popular modern programming languages are either interpreted - meaning that they run directly from source code - or compile to an intermediate bytecode, like Java or common Python implementations. Programs written in such languages are portable across processor architectures - x86 to ARM, for example. Lower-level languages must take into account the features available on a given processor architecture and operating system. Before that, programs needed to compile directly into the native machine language for each processor they were intended to run on.
Native machine language is dependent on processor architecture. Therefore different processors designed around different architectures will not run the same compiled code (unless the architectures are compatible; AMD64 processors will run i386 code natively, for example.) If the same code needs to be run on multiple architectures, it must be compiled separately for each supported architecture.
|-
|To compile your code, you had to mail it to IBM. It took 4-6 weeks.
|Similar to sending Kodachrome slide film to Kodak to be developed.
|While IBM has released multiple compilers, they sent the compiler to you, you did not send the code to them. There is some kind of truth in the statement, though: When programming on mainframes, programmers submitted their source code in the evening for compilation over night. When there was an error in the code, they did not get a compiled version of it back, and had to resubmit their code. Sometimes there were time slots available for compilation, and in universities, students will have to wait for their next time slot for another try.
|-
|Before garbage collection, data would pile up until the computer got full and you had to throw it away.
|A {{w|Garbage collection (computer science)|garbage collector}} is a piece of the software that cleans the {{w|RAM}} of data that is no longer being used in the execution of a program.
|Garbage collection is a form of memory management that generally destroys objects or frees up memory once a program no longer needs it. In languages without automatic memory management, like C, the program itself must keep track of what memory has been allocated, and free it once it is no longer needed. If the program does not, it can end up trying to use more memory than the computer has, and may crash. This was, however, a temporary condition. In the worst case, a simple reboot will clear the computer's memory.
|-
|Early compilers could handle code fine, but comments had to be written in assembly.
|A {{w|Comment (computer programming)|comment}} in programming is a text written in natural language that is meant to explain some feature of the source code. {{w|Assembly}} is a low-level programming language.
|Comments, in code, are portions of one or more lines that are ignored by the compiler. They are commonly used to explain or comment on the code itself. But sometimes the comments are written in a certain way to compile documentation automatically from it.
|-
|C could only be written on punch cards. You had to pick a compact font, or you'd only fit a few characters per card.
|{{w|C (programming language)|C}} is a programming language. A {{w|punch card}} is a primitive form of storing data; it stored data in {{w|binary language}} with holes in a paper or cardboard card where a hole meant a 1 and the absence of a hole meant a 0.
|While punch cards were used through the late 1970s and early 1980s to enter programs and data in COBOL, FORTRAN and other early languages, the use of punch cards and punch card machines had been replaced by a {{w|text editor}} long before C (or C++) was developed as a language.

Also, as punch cards store characters in binary, there is no font involved and they store up to fixed limit of characters per card (80 characters in the most common format.)
|-
|C++ was big because it supported floppy disks. It still punched holes in them, but it was a start
|{{w|C++ (programming language)|C++}} is a programming language. A {{w|floppy disk}} is a (more advanced than punch cards but still old) form of storing data magnetically.
|Hairbun says that the improvement from C to C++ was the C++ finally supported floppy disks, but then it turns out that in C++ the floppy disks were just used instead of punch cards. So the programing was to make holes in floppy disks rather than punch cards. This would of course not be an improvement as floppy disk are not as easy (actually very difficult) to make holes in, compared to punch card, which are made for this purpose and then the whole concept of using floppy disk to store data magnetically is ignored (you could not re-use the floppy again). In any case, a hole punched in a floppy disk would render it useless.
|}

==Transcript==
:[Cueball and Hairbun are standing together and Cueball is talking to her.]
:Cueball: What were things like in the old days?
:Cueball: I hear that you had to ... compile things for different processors?
:Hairbun: Yeah

:[Same setting in a slimmer panel, now Hairbun is replying.]
:Hairbun: To compile your code, you had to mail it to IBM.
:Hairbun: It took 4-6 weeks.

:[Close-up of Hairbun from the waist up.]
:Hairbun: Before garbage collection, data would pile up until the computer got full and you had to throw it away.

:[Same setting as in the first panel with Hairbun gesturing toward Cueball raising one hand palm up.]
:Hairbun: Early compilers could handle code fine, but comments had to be written in assembly.

:[In a frame-less panel Hairbun is seen from the front, with both arms out to the side with both hands held palm up.]
:Hairbun: '''C''' could only be written on punch cards.You had to pick a compact font, or you'd only fit a few characters per card.

:[Exactly the same setting as the first panel, but with Hairbun doing the talking.]
:Hairbun: '''C++''' was big because it supported floppy disks.
:Hairbun: It still punched holes in them, but it was a start.
:Cueball: Wow.

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Hairbun]]
[[Category:Programming]]

1678: Recent Searches

2016-05-09T15:42:22Z

Mathferret: Capitalized Turing in 2 places

{{comic
| number = 1678
| date = May 9, 2016
| title = Recent Searches
| image = recent_searches.png
| titletext = autoexec code posted by verified twitter users
}}

==Explanation==
{{incomplete|Needs an explanation for the individual queries.}}

The comic refers to the practice of finding answers to computer problems using {{w|Google}}. It shows a list of search queries, each of which suggests the author is perversely misusuing or overextending some computer technology or technologies. The overall impression is of someone technically sophisticated enough to shoot themselves in the foot, and who does not learn any larger lessons despite doing so repeatedly. It is unlikely any of the searches would give useful answers, because no two people would be perverse in these extremely specific ways. The title text is another possible entry in this list.

{| class="wikitable" border="1" cellpadding="4" style="border-collapse: collapse;"
|-
! width=20% | Search
! Explanation
|-
| Google translate syntax highlighting
| {{w|Syntax highlighting}} can be used when editing source code to make the code more readable and easier to understand. {{w|Google Translate}} is used to translate text from one spoken/written language to another. The joke here is that syntax highlighting doesn't make sense in the context of translating spoken/written languages.
|-
| Autodetect mixed bash zsh
|
|-
| CPU temperature sensor limits
|
|-
| GIF to XLS
| .GIF (Graphic Interchange Format) is a file extension used to store images and highly compressed videos. .XLS is thefile extension for Microsoft Excel spreadsheets. The joke is that the complete difference between the two types of file makes any kind of conversion all but impossible, so the search is useless.
|-
| Clock speed jumper sample rate
|
|-
| Clean reinstall keybinding
|
|-
| Cron job to update crontab
| {{w|Cron}} is a utility that allows you to schedule commands or scripts to be run periodically. These scheduled jobs are read from a ''crontab'' file. A job that updates the crontab (therefore creating new jobs, removing old ones or editing existing ones) is highly unusual and unlikely to be what you actually want to do.
|-
| fsck Chrome extension
| fsck is a low level tool that would require a lot of effort to run through a browser with no gain.
|-
| Recursive font
|
|-
| Regex matching valid ebnf
| EBNF refers to {{w|Extended Backus–Naur Form}}
|-
| Hardlinks Turing complete
|
|-
| Opposite of safe mode
|
|-
| Predictive touchpad
|
|-
| Google docs from bootloader
| Google docs relies on programs and libraries much more complex than a bootloader could run.
|-
| Hardware acceleration red channel only
|
|-
| autoexec code posted by verified twitter users
|
|-
|}

==Transcript==
:[Coloured and styled as the logo]
:Google

:[Drop down box, implying recent searches]
:Google translate syntax highlighting
:Autodetect mixed bash zsh
:CPU temperature sensor limits
:GIF to XLS
:Clock speed jumper sample rate
:Clean reinstall keybinding
:Cron job to update crontab
:fsck Chrome extension
:Recursive font
:Regex matching valid ebnf
:Hardlinks Turing complete
:Opposite of safe mode
:Predictive touchpad
:Google docs from bootloader
:Hardware acceleration red channel only

:[Bold, below page outline]
:I have no idea why my computers are always broken

{{comic discussion}}

1546: Tamagotchi Hive

2015-07-04T13:58:25Z

Mathferret: simultaneous should be simultaneously

{{comic
| number = 1546
| date = July 3, 2015
| title = Tamagotchi Hive
| image = tamagotchi_hive.png
| titletext = The Singularity happened, but not to us.
}}

==Explanation==
{{incomplete|Explain the comic and the title text}}
A part of the "[[My Hobby]]" series, this describes a distributed computing network using an automated system to simultaneously run trillions of Tamagotchis. As with most of the "My Hobby" series, the concept would work, and is closely connected to real world activities, but twisted enough to make it inherently absurd.

A {{w|Tamagotchi}} is a keychain-sized virtual pet simulation game from 1996. Ostensibly for children, they had appeal for people of all ages. The characters are colorful and simplistically designed creatures based on animals, objects, or people. Beginning with the 2004 Tamagotchi Plus/Connection, a second wave of Tamagotchi toys emerged, featuring a different graphic design by JINCO and gameplay which elaborated upon the first generations. However, the story behind the games remained the same: Tamagotchis are a small alien species that deposited an egg on Earth to see what life was like, and it is up to the player to raise the egg into an adult creature. The creature goes through several stages of growth, and will develop differently depending on the care the player provides, with better care resulting in an adult creature that is smarter, happier, and requires less attention. Gameplay can vary widely between models, and some models, such as TamagoChu, require little to no care from the player. Tamagotchi has a large fan base.

{{w|Distributed computing}} is a field of computer science that studies distributed systems. A distributed system is a software system in which components located on networked computers communicate and coordinate their actions by passing messages. The components interact with each other in order to achieve a common goal. Examples of distributed systems vary from {{w|Service-oriented architecture|service-oriented architecture}} based systems to {{w|multiplayer online games}} to {{w|peer-to-peer}} applications. Distributed computing is often used for tasks that require resources which would otherwise be impossible or prohibitively expensive to manage with single computers. This may include large {{w|Bitcoin network}} mining operations, the {{w|Worldwide LHC Computing Grid}} or, yes, running trillions of simultaneous Tamagotchis using an AI protocol.

The singularity in the title text refers to {{w|technological singularity}} which would result in an {{w|intelligence explosion}} where artificial intelligence would take over. The image and the title text resemble the scenario in {{w|The Matrix}}, but the implication is that the author takes care of a population of virtual creatures rather than an AI running over the human population.

==Transcript==
:My Hobby
:[A tree graph of Tamagotchis.]
:Running a massive distributed computing project that simulates trillions and trillions of Tamagotchis and keeps them all constantly fed and happy

[[Category:My_Hobby]]

{{comic discussion}}