explain xkcd:Sandbox

2023-04-18T11:26:31Z

Beanie: trying to make this more text-efficient

__NOINDEX__
Make changes, try things out, or just have fun with the wiki here! Just leave everything above the line alone, please.
[http://www.explainxkcd.com/wiki/index.php?title=explain_xkcd:Sandbox&action=edit&oldid=91667 clear sandbox]
------

[[Cueball]] [[Knit Cap]]

[[Cueball|Cueball]] [[Knit Cap|Knit Cap]]

explain xkcd:Sandbox

2023-04-18T11:26:11Z

Beanie: trying to make this more text-efficient

__NOINDEX__
Make changes, try things out, or just have fun with the wiki here! Just leave everything above the line alone, please.
[http://www.explainxkcd.com/wiki/index.php?title=explain_xkcd:Sandbox&action=edit&oldid=91667 clear sandbox]
------

[[Cueball]] [[Knit Cap]]

[[Cueball|Cueball]] [[Knit Cap|Knit Cap]]

2022-09-06T13:01:32Z

Beanie:

I was going to edit the above description, but it was taking too much time to edit it into a suitable format, so here's the long version.

In the beginning was UNIX. And it was good. And it was written by some very clever people.

One of the first very useful tools they wrote was '''{{w|ed (text editor)|ed}}''', a "line-editor" (i.e. it works one line at a time). It uses some simple commands, and was created to work on very-old-school teletype machines, where you type a command, and '''ed''' types a response back.

It was a lovely bit of code. Using very little the way of resources, it allowed you to create a text document of any length, including source code in whatever language you wanted to program in.

Eventually, a more sophisticated version called '''{{w|ex (text editor)|ex}}''' (short for EXtended) was written by a clever man named Bill Joy. While it has some great improvements over ed, it was still a line-editor.

The trouble was, using a line-editor like '''ed''' or '''ex''' requires you to have a very good {{w|mental model}} of the document you are creating. Unfortunately, humans aren't very good at this, so they constantly need to refresh their mental model by printing out big chunks of the document (or program) they are working on. This took a LOT of paper using teletypes.

Eventually, teletypes were replaced with {{w|Computer_terminal#Dumb_terminal|terminals}}. This saved a lot of paper. But the people who created the terminals began making them smarter than teletypes, so that magic character sequences could be used to move the cursor around, rather that simply going character-by-character across the line, then scrolling down to the next line, and so on. This opened up a whole new world.

The very clever Bill Joy took advantage of these magic character sequences to create his wonderful "full-screen" text editor '''{{w|vi}}'''. '''vi''' was the "VIsual mode" of '''ex'''. With '''vi''', the user could see a screen-full of text at once. Entire forests were saved.

'''Emacs''' was developed at the same time as '''vi''', using the same magic characters, and was also a full-screen text editor. I've never used it, so I can't speak to its merits, but there are many people who still find it more useful than any GUI they've tried.

On the one hand, '''vi''' and '''emacs''' are more sophisticated tools, and thus take longer to learn to use than '''ed'''. However, once you learn to use them, they make writing code EASIER, and they are therefore considered a less praise-worthy way of writing code by those concerned with defining what a "Real Programmer" is. (In other words, those programmers suffering from {{w|testosterone poisoning}}.)

Using '''cat''' to write a program looks like this: (Note that the $ is the prompt provided by the computer. The rest is typed by the user. And the ^D means the user held down the control key while typing the letter "d".)

<blockquote>
'''$''' cat | cc
</blockquote>
<blockquote>
''The user types C code here, and ends with ^D. Assuming all goes well, the compiler silently finishes after creating the executable program '''a.out''' in the user's current working directory.''
</blockquote>

The reason this is considered a more praise-worthy way of coding is that, in those early days, doing this meant that your code was lost the instant you typed it. If you made a mistake, you would have to type the whole thing again. So doing this for code of any sophistication was considered an act of courage, confidence, and conviction. (I myself did it several times, for the fun of it, when no-one was watching, though never for a program that took more than about 30 lines of code. I was delighted that it worked all 3 times, but since I love to write re-usable code, this wasn't really something I wanted to keep doing.)

NOW PAY ATTENTION. '''VI IS NOT VIM!''' '''{{w|Vim (text editor)|Vim}}''' was written in 1991, long after more sophisticated {{w|Shell (computing)|shells}} were created that made it possible to copy and paste text from one part of the screen to another. This ability greatly reduced the risks of using '''cat''' to pass your source code directly to the compiler, so it was no longer a praise-worthy stunt. Thus the line "Real programmers use vim" was NEVER considered true by any UNIX programmer.

Whether this was a mistake of the author, or the character (possibly Megan?) is unclear. It seems possible that it was a simple typo, but since I've never seen one in the strip before, I'm somewhat skeptical.

--[[User:MisterSpike|MisterSpike]] ([[User talk:MisterSpike|talk]]) 07:12, 17 June 2013 (UTC)

:<code>cat | cc</code> doesn't work on my system. My <code>cc</code> is simply a symlink to <code>gcc</code>; what's yours? --[[User:Lucaswerkmeister|Lucaswerkmeister]] ([[User talk:Lucaswerkmeister|talk]]) 10:16, 7 August 2013 (UTC)

::One can also use
::<blockquote>
::'''$''' cat | gcc -xc -
::</blockquote>
::The program will be an a.out file.{{unsigned ip|200.131.199.28}}

Some comments:

I would never claim to be an emacs expert, but I'm reasonably proficient in it. Command of the form M-x (whatever) are a way of calling commands (or, really, arbitrary functions in the emacs code) by name. SO 'M-x butterfly' means that there is a function named "butterfly" somewhere, but that it has not been assigned a keyboard shortcut (or it has, but you're calling it the long way).

Also, three are still advanced Linux programmers today who swear by vim or emacs being superior to IDEs. There are specific technical reasons for this: emacs is basically an IDE construction kit that's incredibly easy to extend and customize, and is more customizable than pretty much any other program in the history of software with the exception of a Smalltalk installation. And vim has highly evolved commands to give experts a superhuman typing and editing speed when coding.

So when someone claims that "real programmers use vim," they are claiming that RIGHT NOW, vim is the best possible editor for developers of sufficient competence. There's a community of very smart people that basically thinks this.

[[User:Tess|Tess]] ([[User talk:Tess|talk]]) 04:19, 15 December 2013 (UTC)

What is "Meta"? As in "M-butterfly"?[[Special:Contributions/108.162.219.180|108.162.219.180]] 23:20, 27 April 2015 (UTC)
When I read this, I started up emacs and tried this... until I realized that there was no butterfly key... --[[Special:Contributions/108.162.215.58|108.162.215.58]] 00:51, 13 May 2015 (UTC)

"Meta" is binded to "Alt" in modern keyboard. "Meta" is referring to the "Meta" key in early {{w|Space-cadet keyboard|LISP keyboard}}. --[[Special:Contributions/173.245.62.93|173.245.62.93]] 18:58, 19 May 2015 (UTC)

I assume this cartoon was inspired by an earlier [http://ars.userfriendly.org/cartoons/?id=20091201 User Friendly cartoon], in which Miranda ends an editor one upsmanship discussion by saying: "Well, I edited the inodes by hand. with magnets." See also [http://dilbert.com/strip/1992-09-08 this classic Dilbert cartoon]. [[User:Espertus|Espertus]] ([[User talk:Espertus|talk]]) 21:56, 13 August 2015 (UTC)

Real programmers don't use any negative calls to sqrt(), of course. --[[Special:Contributions/199.27.128.114|199.27.128.114]] 22:07, 7 September 2015 (UTC)

August 1984 "Real programmers use cat as their editor." http://web.cecs.pdx.edu/~kirkenda/joy84.html --[[Special:Contributions/141.101.104.180|141.101.104.180]] 00:21, 18 October 2015 (UTC)

Could the title text be a reference to the novel "Heechee Rendezvous" by Frederik Pohl, in which an alien species causes the universe to start to contract, in order to provoke a new Big Bang which would lead to "improved" natural constants? --[[Special:Contributions/162.158.114.220|162.158.114.220]] 09:37, 2 December 2015 (UTC)

The title text is more likely a reference to the end of the book [https://en.wikipedia.org/wiki/In_the_Beginning..._Was_the_Command_Line "In the Beginning Was the Command line"], where programming / simulation develops to creating worlds on a single command line, specifying natural constants (in all their precision). [[User:Hrabbey|Hrabbey]] ([[User talk:Hrabbey|talk]]) 13:23, 30 March 2018 (UTC)

There has been a [[explain_xkcd:Community_portal/Proposals#Merge_Cueball_.26_Rob|community portal discussion]] of what to call Cueball and what to do in case with more than one Cueball. I have added this comic to the Category:Multiple Cueballs. In this comic it cannot be said clearly that any of the four/five (if the guy with the butterfly in hand is not the same as the one speaking off-panel about it) is more correctly called Cueball than any of the others. But typically the one named Cueball is either the protagonist or the one with the interesting parts, or the one with the punch line. In this comic it would be the e-mac Cueball, as he makes the butterfly guy loose out to e-mac. It may thus be OK to list him as Cueball, and the others as something else. So I changed that some time ago in the explanation, and transcript. But then I also made sure that it was clear that the other guys also looks like Cueball. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 21:20, 20 December 2015 (UTC)

'''Real Programmers''' use MS Paint: http://i.imgur.com/QlGpd.gif [[User:Luc|Luc]] ([[User talk:Luc|talk]]) 01:40, 3 July 2016 (UTC)

I don't know which came first but this quote reminded me of this comic http://bash.org/?6130[[Special:Contributions/172.69.34.228|172.69.34.228]] 04:20, 3 May 2021 (UTC)

It occurs to me that, during the Apollo space program, the core-rope memory which contained the program code for the ship's onboard guidance computer was, quite literally, hand-woven. While it was non-magnetic, a needle was used to thread a single strand of wire through a frame containing an array of ferrite cores. Similar to then-conventional core memory, except that wires selectively did or did not pass through any given core to permanently set their state as 0 or 1. - Thraddax, 02 June 2021.

This is extremely late but I'm assuming he meant <code>vi</code> but confused the two. [[User:Beanie|Beanie]] [[User talk:Beanie|talk]] 13:01, 6 September 2022 (UTC)

2600: Rejected Question Categories

2022-03-31T21:01:36Z

Beanie: minor but it is still needed

{{comic
| number = 2600
| date = March 30, 2022
| title = Rejected Question Categories
| image = rejected_question_categories.png
| titletext = You can click to preorder to get a copy of What If? 2 when it comes out 9/13, assuming we all make it past the spider situation(?) on Tuesday(?).
}}

==Explanation==
{{incomplete|Created by a SPIDER-CREATED WEAPON OF MASS DESTRUCTION LAUNCHED AT A VOLCANO - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
[[Randall]] recently announced, in [[2575: What If? 2]], that he is publishing a new ''[[what if?]]'' book based on reader-submitted questions. This comic is another [[:Category:Book promotion|promotion]] of the book, and the entire comic is a link to his [https://xkcd.com/what-if-2/ what if? 2] page on xkcd.

This comic shows the [[#Question categories|categories of questions]] he claims to have received, but rejected to use in his book, giving an example question for each category. In typical xkcd fashion, these begin out by being plausible, although often unlikely to have been submitted as a ''what if?'' question, moving in to more and more absurd types of questions, especially with the last question, that appears to be a combination of all previous categories and is therefore marked "?????"

The title text refers to the launch date of the book September 13, rendered in the American style 9/13. This format can be confusing to non-Americans, although usually not when the date is larger than 12, since it would then appear to reference the 9th day of the 13th month. This "13th month" was, however, referenced in the first comic about the book: [[2575: What If? 2]]. See also Randall's take on the date format, {{w|ISO 8601}}, in [[1179: ISO 8601]].

Randall then continues the title text by referencing the second to last category with vague ominous questions. The example question here asks if there is need to worry about spiders after Tuesday. So Randall notes that the release date, 5.5 months after the release of this comic, is of course assuming we all make it past both spiders and next Tuesday (2022-04-05).

Tuesday has been notably referenced in [[277: Long Light]], [[564: Crossbows]], [[1099: Tuesdays]] and most notably in [[1245: 10-Day Forecast]] where it seems that the last day of forever will by a Tuesday. Normally Tuesday is the second day of the week, however the Tuesday of the week following the publication of this cartoon could be the last day of the last week ever.

==Question categories==
{| class="wikitable"
|'''Category'''
|'''Question'''
|'''Explanation'''
|-
| People cheating on homework
| What if I made a pendulum by hanging a rock on a 2.75 meter string? What would its period be in seconds? (Show your work)
| This appears to be a physics student trying to get out of doing their work by presenting the question for Randall to answer in ''What If'' (which is a common occurrence on question-and-answer sites such as Stack Overflow). Most school homework requires the student to "show their work", i.e. write out their process of solving the question, which will usually go towards marks for the right methodology and basic understanding even if they make a wrong turn along the way and end up with an incorrect answer; the student including this caveat is a giveaway as to their tactic (along with the boring nature of their inquiry).
|-
| Medical advice
| What if you got a scratch and the next day your hand looks like this [''📎 attachment'']? Should you see a doctor or what?
| Randall is not a physician, is not qualified to give medical advice, and will not answer medical questions. This was a problem on early Wikipedia which, for a time, required all articles covering medical topics to point to a {{w|WP:MEDICAL|Medical Disclaimer}}. This was also an issue covered in the 1st ''what if?'' book, under Weird and Worrying Questions from the Inbox, where a reader asks about the effects of a toxin on the excretory system. If your body is not behaving in the way you expect, consulting with a medical professional is, in general, a good idea.
|-
| Personal
| Why don't the squirrels in my yard like me???
| Randall has no way of knowing why the wildlife in a given area dislike a specific person, especially if this hints at a specific animosity/belligerence and not just general avoidance of humans as a perceived threat. These kind of questions are common on sites like reddit, and are usually covered by general explanation of animal behaviors unless the asker supplies more information in the comments. However, the choice of squirrels are likely not random since [[:Category:Squirrels|Squirrels]] are a recurring theme on xkcd and has often been [https://www.explainxkcd.com/wiki/index.php/Category:Squirrels#Uses_in_other_xkcd_media: used in what if?] (sometimes as {{what if|105|placeholder}} for a potentially horrible image).
|-
| Spam
| Do you want to meet lonely singles in your area tonight?
| This is a common online popup advertisement and is generally considered undesirable. An unavoidable side effect of Randall having an email address published where the public can find it is that spammers have just as much access to it. Most people do not respond to spam emails, and Randall is no exception.
|-
| Phishing
| Have you recently been the victim of phishing? To check, log in to your account by clicking here.
| Again, because Randall's What-If email is publicly available, phishers have access to it and can probe the recipient for information. Randall is experienced enough to recognize a phishing attempt and ignore it. This type of phishing scam, in which the scammer requests your personal data for the purported purpose of checking if it has been "stolen" thereby "stealing" it himself, has become more common as people have become aware of phishing, and phishers have thus embraced the possibility of exploiting a new niche in the layers of naivete.
|-
| Requests for help with a crime
| Using modern science, what would be the fastest way to get through this bank vault door? [''📎'' blueprints]
| No, Randall is ''not'' going to help a criminal commit breaking and entering or robbery, thank you very much. A common problem with heist movies is that they show a believable{{Citation needed}} method for breaking in to a vault which people then try to replicate in real life. Such movies often portray an expert in some other field being "smart enough" to craft the plan, sometimes unwittingly.
|-
| Unanswerable
| Why am I me and not someone else
| This question is existentialist. Hundreds of the greatest scientific minds are unable to answer this question, why would someone who draws comics on the internet be able to?
|-
| Vague
| What is going to happen? (Be specific)
| Some humor is derived from the request for specificity in the question, which completely lacks any sort of specificity of its own.
|-
| Vague and ominous
| Will I have to start worrying about spiders after Tuesday?
| Possibly a [[:Category:Red Spiders|red spiders]] reference. Could also be a reference to [[1688: Map Age Guide]] where some dangerous event involving spiders is set to happen in 2022.
|-
| ?????
| Hi, we're lonely singles in your area, and we're wondering what would happen if we shot a nuclear bomb into a volcano! Click here to log in and tell us ...
| If nothing else, this entry is clearly made up by Randall, as it appears to be a combination of as many prior categories as he has room for, specifically Spam, Phishing, and an already-answered question, before getting cut off by the bottom of the panel.

"Would dropping a nuclear bomb in a volcano stop an eruption?" was a question featured in the ''first'' book. Randall answered with an emphatic "No!"
|}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
In ''What If? 2'' (xkcd.com/whatif2), I answer ridiculous questions sent in by readers about everything from volcanoes to spaceships to soup.

Here are a few of the common types of questions that I did ''not'' answer:

[In separate boxes for each category]

[In row 1:]

People cheating on homework: What if I made a pendulum by hanging a rock on a 2.75 meter string? What would its period be in seconds? (Show your work!)

Medical advice: What if you got a scratch and the next day your hand looks like this [''📎 attachment'']? Should you see a doctor or what?

Personal: Why don't the squirrels in my yard like me???

[In row 2:]

Spam: Do you want to meet lonely singles in your area tonight?

Phishing: Have you recently been the victim of phishing? To check, log in to your account by clicking here.

Requests for help with a crime: Using modern science, what would be the fastest way to get through this bank vault door? [''📎'' blueprints]

[In row 3:]

Unanswerable: Why am I me and not someone else

Vague: What is going to happen? (Be specific)

Vague+Ominous: Will I have to start worrying about spiders after Tuesday?

?????: Hi, we're lonely singles in your area, and we're wondering what would happen if we shot a nuclear bomb into a volcano! [partially cut off horizontally:] Click here to log in and tell us

{{comic discussion}}

[[Category:Book promotion]]
[[Category:Physics]]
[[Category:Squirrels]]
[[Category:Spiders]]
[[Category:Nuclear weapons]]

2600: Rejected Question Categories

2022-03-30T20:48:00Z

Beanie: don't have time to write an explanation for each but this is a start :d

2520: Symbols

2022-03-29T21:07:33Z

Beanie: /* Explanation */ oop error

{{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 VERY EXPENSIVE EQUIPMENT - Please change this comment when editing this page. Do NOT delete this tag too soon. A 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, see [[#Symbols|below]].

Many of the individual descriptions look like verbiage that might be found on informational or warnings signs or placards, although typically with a silly edge.

The title text refers to two non-SI units of radiation measurement, {{w|Roentgen (unit)|röntgen}} and {{w|Roentgen equivalent man|rem}}. 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 [https://archive.md/v3dME probably dangerous], hence the no trespassing part.

Later [[Randall]] made a similar comic, [[2586: Greek Letters]], regarding the use of Greek letters in math.

===Symbols===
'''d⁄dx: An undergrad is working very hard''' d/dx is the symbol for a single-variable {{w|Derivative|derivative}}. This is one of the basic operations in {{w|calculus}} and consequently is ubiquitous in the work of undergraduates in the sciences. A hard-working undergraduate in the relevant fields would churn through exercises using this symbol.

'''∂⁄∂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. Although PDEs would typically be first taught at an undergraduate level, difficult partial derivatives would be encountered in graduate-level work.

'''ħ: Oh wow, this is apparently a quantum thing''' ħ (pronounced "h-bar") is a symbol used for (the reduced) {{w|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.
Classical physics appears as a limit of quantum physics if all "actions" (quantities of dimension energy * time, momentum * length, or angular momentum) are much larger than ħ. Conversely, you can also formally set ħ=0 to get classical results from quantum formulae. This means that effects that are proportional to some power of ħ cannot be explained classically, and instead are "a quantum thing".

'''Rₑ: Someone needs to do a lot of tedious numerical work; hopefully it's not you''' The {{w|Reynolds number}} (which is 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 retrieved analytically, and so numerical modeling is necessary. Accurate numerical studies of high-Reynolds-number flows are notoriously difficult to create and program.

Alternatively, Rₑ could stand for electronic {{w|transition dipole moment}} in a molecule. This appears in quantum-mechanical calculations of transition probabilities and also includes a lot of unpleasant numerical work. Rₑ is also a term used for the radius of the Earth at mean sea level, though this is not necessarily a complex term in and of itself.

Another alternative is that Rₑ could refer to Relative Error, a measurement of precision or accuracy. Used often in the analysis of scientific data and numerical analysis.

'''(Ta⁴ - Tb⁴): You are at risk of skin burns''' The {{w|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 the 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 {{w|Avogadro's number}}, is the number of molecules in a mole of a substance, approximately the number of carbon atoms in exactly 12 grams of carbon-12. This is an enormous number, exactly 6.022 140 76 × 10²³, or 602 214 076 000 000 000 000 000. Working with NA, it is easy to accidentally divide by it instead of multiplying or vice versa, leading to erroneous and nonsensical answers such as ~10-23 molecules (even though you can't have less than 1 whole molecule) or ~1046 moles (>1043 (>1043 to to 1045 kilograms, depending on the chemical) of a substance.

'''µm: Careful, that equipment is expensive''' {{w|Micrometre|Micrometer}}s are a very small unit of distance. Micrometers are commonly used to measure wavelengths in the infrared, and infrared detectors are very expensive, compared with visible wavelength counterparts. Of course, micrometers are used as a measurement of distance in other contexts, but any distance-measuring device capable of accurately measuring micrometer distances would also be expensive. Similarly, tools used to create or calibrate items within micrometer tolerances can also be expensive.

'''mK: Careful, that equipment is very expensive''' {{w|Kelvin}} is a temperature scale roughly speaking similar to Celsius, but taking absolute zero as its zero point instead of the freezing point of water (rigorously speaking, its definition is now {{w|2019_redefinition_of_the_SI_base_units#Kelvin|based on the Boltzmann constant}}). {{w|Millikelvin}}s (1/1000 of a Kelvin) are used for high precision temperature work. Frequently this is used in processes of cooling temperatures to nearly absolute zero - such as superconductors or other quantum effects that occur when atoms are almost still. This is suggesting that the symbol appears on a sensitive experimental system probing quantum mechanical behavior that would likely only exist in an advanced laboratory. Any equipment that works down at mK temperatures, or at least to mK precision and accuracy, is likely to be very expensive.

'''nm: Don't shine that in your eye''' {{w|Nanometer}}s are 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''' {{w|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 (and are probably a direct reference to Anatoli Bugorski)}} to soft tissues than optical-wavelength lasers.

'''mSv: You're about to get into an Internet argument''' The {{w|millisievert}} is a unit of radiation dose absorbed. It is 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|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 1/1000 times the size of mg/kg. If a dosage is measured in micrograms per kilogram (parts per billion), any accident probably requires whole-body decontamination procedures.

'''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 the ratio of a circle's circumference to its radius (and is therefore equal to 2π). {{w|pi|π}} has been used as the primary constant for describing the circumference and area of circles millennia ago, but proponents of {{w|Turn (angle)|τ}} claim that τ is 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 debate over Tau vs Pi was solved by Randall in this compromise: [[1292: Pi vs. Tau]].

==Transcript==
:[A list with 14 different scientific constants/symbols are shown. Next to each symbol is a description. Above the list is a heading and beneath that a subheading.]
:::::<big>Symbols</big>
::::And what they mean

:::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're 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}}

[[Category:Math]]
[[Category:Physics]]
[[Category:Chemistry]]
[[Category:Biology]]
[[Category:5G]]

2520: Symbols

2022-03-29T21:02:58Z

Beanie: ...there is a range

{{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 VERY EXPENSIVE EQUIPMENT - Please change this comment when editing this page. Do NOT delete this tag too soon. A 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, see [[#Symbols|below]].

Many of the individual descriptions look like verbiage that might be found on informational or warnings signs or placards, although typically with a silly edge.

The title text refers to two non-SI units of radiation measurement, {{w|Roentgen (unit)|röntgen}} and {{w|Roentgen equivalent man|rem}}. 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 [https://archive.md/v3dME probably dangerous], hence the no trespassing part.

Later [[Randall]] made a similar comic, [[2586: Greek Letters]], regarding the use of Greek letters in math.

===Symbols===
'''d⁄dx: An undergrad is working very hard''' d/dx is the symbol for a single-variable {{w|Derivative|derivative}}. This is one of the basic operations in {{w|calculus}} and consequently is ubiquitous in the work of undergraduates in the sciences. A hard-working undergraduate in the relevant fields would churn through exercises using this symbol.

'''∂⁄∂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. Although PDEs would typically be first taught at an undergraduate level, difficult partial derivatives would be encountered in graduate-level work.

'''ħ: Oh wow, this is apparently a quantum thing''' ħ (pronounced "h-bar") is a symbol used for (the reduced) {{w|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.
Classical physics appears as a limit of quantum physics if all "actions" (quantities of dimension energy * time, momentum * length, or angular momentum) are much larger than ħ. Conversely, you can also formally set ħ=0 to get classical results from quantum formulae. This means that effects that are proportional to some power of ħ cannot be explained classically, and instead are "a quantum thing".

'''Rₑ: Someone needs to do a lot of tedious numerical work; hopefully it's not you''' The {{w|Reynolds number}} (which is 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 retrieved analytically, and so numerical modeling is necessary. Accurate numerical studies of high-Reynolds-number flows are notoriously difficult to create and program.

Alternatively, Rₑ could stand for electronic {{w|transition dipole moment}} in a molecule. This appears in quantum-mechanical calculations of transition probabilities and also includes a lot of unpleasant numerical work. Rₑ is also a term used for the radius of the Earth at mean sea level, though this is not necessarily a complex term in and of itself.

Another alternative is that Rₑ could refer to Relative Error, a measurement of precision or accuracy. Used often in the analysis of scientific data and numerical analysis.

'''(Ta⁴ - Tb⁴): You are at risk of skin burns''' The {{w|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 the 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 {{w|Avogadro's number}}, is the number of molecules in a mole of a substance, approximately the number of carbon atoms in exactly 12 grams of carbon-12. This is an enormous number, exactly 6.022 140 76 × 10²³, or 602 214 076 000 000 000 000 000. Working with NA, it is easy to accidentally divide by it instead of multiplying or vice versa, leading to erroneous and nonsensical answers such as ~10-23 molecules (even though you can't have less than 1 whole molecule) or ~1046 moles (>1043 (>1043 to to 1041 kilograms, depending on the chemical) of a substance.

'''µm: Careful, that equipment is expensive''' {{w|Micrometre|Micrometer}}s are a very small unit of distance. Micrometers are commonly used to measure wavelengths in the infrared, and infrared detectors are very expensive, compared with visible wavelength counterparts. Of course, micrometers are used as a measurement of distance in other contexts, but any distance-measuring device capable of accurately measuring micrometer distances would also be expensive. Similarly, tools used to create or calibrate items within micrometer tolerances can also be expensive.

'''mK: Careful, that equipment is very expensive''' {{w|Kelvin}} is a temperature scale roughly speaking similar to Celsius, but taking absolute zero as its zero point instead of the freezing point of water (rigorously speaking, its definition is now {{w|2019_redefinition_of_the_SI_base_units#Kelvin|based on the Boltzmann constant}}). {{w|Millikelvin}}s (1/1000 of a Kelvin) are used for high precision temperature work. Frequently this is used in processes of cooling temperatures to nearly absolute zero - such as superconductors or other quantum effects that occur when atoms are almost still. This is suggesting that the symbol appears on a sensitive experimental system probing quantum mechanical behavior that would likely only exist in an advanced laboratory. Any equipment that works down at mK temperatures, or at least to mK precision and accuracy, is likely to be very expensive.

'''nm: Don't shine that in your eye''' {{w|Nanometer}}s are 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''' {{w|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 (and are probably a direct reference to Anatoli Bugorski)}} to soft tissues than optical-wavelength lasers.

'''mSv: You're about to get into an Internet argument''' The {{w|millisievert}} is a unit of radiation dose absorbed. It is 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|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 1/1000 times the size of mg/kg. If a dosage is measured in micrograms per kilogram (parts per billion), any accident probably requires whole-body decontamination procedures.

'''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 the ratio of a circle's circumference to its radius (and is therefore equal to 2π). {{w|pi|π}} has been used as the primary constant for describing the circumference and area of circles millennia ago, but proponents of {{w|Turn (angle)|τ}} claim that τ is 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 debate over Tau vs Pi was solved by Randall in this compromise: [[1292: Pi vs. Tau]].

==Transcript==
:[A list with 14 different scientific constants/symbols are shown. Next to each symbol is a description. Above the list is a heading and beneath that a subheading.]
:::::<big>Symbols</big>
::::And what they mean

:::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're 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}}

[[Category:Math]]
[[Category:Physics]]
[[Category:Chemistry]]
[[Category:Biology]]
[[Category:5G]]