explain xkcd - User contributions [en]

2520: Symbols

2021-10-20T15:20:57Z

Jgt: Substitute archived link for bloomberg curie radiation cleanup article

{{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. 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 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 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 which 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 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 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 in 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 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—roughly the number of protons plus neutrons in 1 gram of matter. This is an enormous number, exactly 6.022 140 76 × 10²³, or 602 214 076 000 000 000 000 000. 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 powers 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
{{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 near 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}} to soft tissues than optical-wavelength lasers.

*mSv: You are about to get into an Internet argument
The {{w|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|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 for millennia, but proponents of {{w|Turn (angle)|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 debate over Tau vs Pi was solved by Randall in this compromise: [[1292: Pi vs. Tau]].

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 probably dangerous[https://archive.md/v3dME], hence the no trespassing part.

==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 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}}

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

Talk:2420: Appliances

2021-02-04T13:41:28Z

Jgt: comment on potential for derivative content

This seems related to [[1890: What to Bring]]. [[User:Bubblegum|bubblegum]]-[[User_talk:Bubblegum|talk]]|[[Special:Contributions/Bubblegum|contribs]] 02:55, 4 February 2021 (UTC)

I actually disagree with the stove and washing dishes. In a large pot with water and with some stirring one can remove stains from cloths and kill germs. --[[Special:Contributions/141.101.96.28|141.101.96.28]] 07:08, 4 February 2021 (UTC)

It's interesting that all green squares are mirrored along the diagonal, with one exception, but no yellow ones at all. Usually you would assume that somewhat similar things result in a yellow square on both sides. [[User:Fabian42|Fabian42]] ([[User talk:Fabian42|talk]]) 07:35, 4 February 2021 (UTC)

== A washing mashine can (and does) spin-dry clothes ==

A spin-drying program can usually be selected separately without getting the clothes wet. While this is not effective as a dryer, still much better than nothing.
----

I microwave can boil eggs, you just nweed to wrap them in foil and ensure they are covered by the boiling water. The foil acts as a faraday cage and ensures they only cook by conduction from the outside and the water stops arcing.
You cvan also make scrabled eggs and omlettes. [[User:Arachrah|Arachrah]] ([[User talk:Arachrah|talk]]) 09:31, 4 February 2021 (UTC)
:You seriously suggest putting foil into the microwave? I guess you don't mean tinfoil/aluminum? --[[User:Lupo|Lupo]] ([[User talk:Lupo|talk]]) 09:58, 4 February 2021 (UTC)
:: You can put a foil there IF you know what you are doing. Try dissecting a microwave popcorn bag - its bottom has an aluminium foil inside and acts as a pan. I was surprised by inclusion of egg as okay by Randall though - I thought egg yolks were famous for infrequent but nasty [https://abcnews.go.com/Health/dangers-microwaving-eggs/story?id=51630977 superheating surprises] when microwaved. [[Special:Contributions/162.158.165.116|162.158.165.116]] 10:41, 4 February 2021 (UTC)

----
I didn't get this at all until I read the explanation, as I am red-green colorblind and the shades of red and green that Randall chose appear completely identical to me.
[[Special:Contributions/141.101.104.35|141.101.104.35]] 09:47, 4 February 2021 (UTC)

----

What a microwave does to bread is very weird. It seems to make the bread tougher (i.e., harder to bite through; in one case of a microwaved sandwich I was totally unable to bite off pieces), but doesn't make it stiffer (it ends up kind of like rubber). It doesn't wind up unevenly toasted like in the picture, but rather cooked in a completely different way. [[User:Magic9mushroom|Magic9mushroom]] ([[User talk:Magic9mushroom|talk]]) 11:22, 4 February 2021 (UTC)

----
Using a microwave to dry clothes is generally a bad idea, there is quite a high chance of burns. The most household appliance to quickly dry clothes beside a dryer is actually the freezer. [[User:Perigril|Perigril]] ([[User talk:Perigril|talk]]) 13:20, 4 February 2021 (UTC)

----
Updated transcript from [[1890: What to Bring]]. Considering the increased size of this table maybe something else is wiser? Does anyone know why the three first items are separate, or why the washing machine appears only to wet parts of the shirt? Should the cited text from the comic be written in all caps to match the comic? Should the "with a" between the pairs be replaced with something better to indicate that unlike What to Bring, Appliances does not have an equivalent to the "Should you bring _____ to ______"-text? --[[User:IonIceXIII|IonIceXIII]] ([[User talk:IonIceXIII|talk]]) 13:22, 4 February 2021 (UTC)

----
Attempting to do as well as possible in each square could make an interesting short video series. Stove/oven could probably achieve 6 greens, at least for quality of results if not for the time required [[User:Jgt|Jgt]] ([[User talk:Jgt|talk]]) 13:41, 4 February 2021 (UTC)

Talk:1902: State Borders

2017-10-13T19:33:44Z

Jgt: Formatting fix

Let's be honest- it should ''all'' be Canada. [[Special:Contributions/162.158.74.123|162.158.74.123]] 12:24, 13 October 2017 (UTC)

Could Arizona, New Mexico be a reference to Trump? Like, make the border straighter so it's easier to build a wall? [[User:Herobrine|Herobrine]] ([[User talk:Herobrine|talk]]) 12:35, 13 October 2017 (UTC)
:More likely the joke is that conceding territory to Mexico is about the last thing Trump would do [[User:AnotherAnonymous|AnotherAnonymous]] ([[User talk:AnotherAnonymous|talk]]) 13:04, 13 October 2017 (UTC)

My first thought is to wonder if it would be possible to arrange the map such that all internal borders are "straight lines" that span the entire country, to satisfy as many criteria as possible:
* The number of states remains unchanged
** …and they all get to keep their capitals (probably quite difficult)
*** …or (and?) each state manages to keep either its current population, land area, or coastline length
* Or all internal borders are parallels or meridians
* Or all states have the same land area
** …or population; or population density
* Or if you're allowing more (or fewer) states than the present layout, what's the greatest number of states possible such that they all contain at least one complete city?

Which of those criteria would be the most interesting challenge? And which could you construct an algorithm to solve?
I really should refrain from trying to build those algorithms, because I'm supposed to be working --[[User:Angel|Angel]] ([[User talk:Angel|talk]]) 13:28, 13 October 2017 (UTC)

There are some great videos on YouTube about weird State boundaries. There are some REALLY weird oddities out there. Take for instance the "Give to Canada" piece - that's the Northwest Angle in Minnesota. It's really an accident that it ever ended up in the USA at all, and doesn't make any sense! [[User:Martini|Martini]] ([[User talk:Martini|talk]]) 13:40, 13 October 2017 (UTC)Martini
:I wouldn't call the NW Angle an accident as much as a slightly illogical solution in order to maintain the terms of the original border agreement in the face of the Mississippi River's inconveniently located headwaters. My recollection is that it said roughly: the border goes west of <this> point until reaching the Mississippi river [which all parties assumed continued that far north]. [[Special:Contributions/108.162.216.40|108.162.216.40]] 14:13, 13 October 2017 (UTC)

I believe Randall's overall point is that though a large part of the individual United States have straight boundaries, especially in the West, or other features that are aesthetically pleasing, as in the S Carolina/Georgia/Florida coastline, there are a good number of internal inconsistencies. Many of these (most of the untagged "fixes") can be attributed to the concept that "Rivers make good logical boundaries", but even then, if you look closer, there are some really puzzling bits:
* The "Give To Canada" bit of Minnesota is almost all Indian Reservation land, so that kind of makes sense...
* The "Fix this thing" in Missouri is even stranger than it initially looks - while the notch in Arkansas is caused by the Mississippi River, there is a large bight of land in the middle of the Missouri-owned bit that is actually Kentucky (yes, there's an island of Kentucky that is separate from the main Kentucky state and entirely surrounded by Missouri)
* Not edited, but equally odd is the dip Florida cuts into Georgia near the east coast - there's no apparent town or natural features there to cause that irregularity

I don't happen to think the Arizona/New Mexico bits are political commentary, just "the entire rest of the state is a box, make this a straight line, too." cleanup. I mean yes, it would make wall-building easier, theoretically, but the Chinese showed the world centuries ago that straight lines are not needed to build a big fricking wall. [[Special:Contributions/108.162.238.131|108.162.238.131]] 14:23, 13 October 2017 (UTC)

- While I agree it probably isn't conscious political commentary, its interesting that there are not places the border increases; always concessions, never gains. May take into account its easier to give than take territory? --[[User:Jgt|Jgt]] ([[User talk:Jgt|talk]]) 19:32, 13 October 2017 (UTC)--[[User:Jgt|Jgt]] ([[User talk:Jgt|talk]]) 19:33, 13 October 2017 (UTC)

I'm surprised Randall didn't suggest cleaning up Point Roberts as well [https://en.wikipedia.org/wiki/Point_Roberts,_Washington]. [[Special:Contributions/141.101.107.174|141.101.107.174]] 14:33, 13 October 2017 (UTC)

I'm shocked he didn't support fixing the Idaho/Wisconsin/Montana/Oregon border. That top part should be either given to Montana, or split between Washington and Oregon... I wonder if he left out certain things in order to avoid offending certain groups of people. Like suggesting that Rhode Island and Connecticut should probably be one state, or that Vermont and New Hampshire should be as well. [[User:Kashim|Kashim]] ([[User talk:Kashim|talk]]) 17:03, 13 October 2017 (UTC)

Some of the suggestions are ironic, for example Michigan's upper peninsula actually used to be part of the Wisconsin territory, but it was ceded to Michigan in exchange for the port of Toledo being ceded to Ohio. "why does Florida get Alabama's coastline" is actually because Alabama got part of Florida's coastline so it wouldn't be landlocked. The bit of Nevada that he wants to fix it so Nevada has territory along the Colorado River [[Special:Contributions/162.158.75.250|162.158.75.250]] 17:18, 13 October 2017 (UTC)

Talk:1902: State Borders

2017-10-13T19:32:19Z

Jgt: added comment

Let's be honest- it should ''all'' be Canada. [[Special:Contributions/162.158.74.123|162.158.74.123]] 12:24, 13 October 2017 (UTC)

Could Arizona, New Mexico be a reference to Trump? Like, make the border straighter so it's easier to build a wall? [[User:Herobrine|Herobrine]] ([[User talk:Herobrine|talk]]) 12:35, 13 October 2017 (UTC)
:More likely the joke is that conceding territory to Mexico is about the last thing Trump would do [[User:AnotherAnonymous|AnotherAnonymous]] ([[User talk:AnotherAnonymous|talk]]) 13:04, 13 October 2017 (UTC)

My first thought is to wonder if it would be possible to arrange the map such that all internal borders are "straight lines" that span the entire country, to satisfy as many criteria as possible:
* The number of states remains unchanged
** …and they all get to keep their capitals (probably quite difficult)
*** …or (and?) each state manages to keep either its current population, land area, or coastline length
* Or all internal borders are parallels or meridians
* Or all states have the same land area
** …or population; or population density
* Or if you're allowing more (or fewer) states than the present layout, what's the greatest number of states possible such that they all contain at least one complete city?

Which of those criteria would be the most interesting challenge? And which could you construct an algorithm to solve?
I really should refrain from trying to build those algorithms, because I'm supposed to be working --[[User:Angel|Angel]] ([[User talk:Angel|talk]]) 13:28, 13 October 2017 (UTC)

There are some great videos on YouTube about weird State boundaries. There are some REALLY weird oddities out there. Take for instance the "Give to Canada" piece - that's the Northwest Angle in Minnesota. It's really an accident that it ever ended up in the USA at all, and doesn't make any sense! [[User:Martini|Martini]] ([[User talk:Martini|talk]]) 13:40, 13 October 2017 (UTC)Martini
:I wouldn't call the NW Angle an accident as much as a slightly illogical solution in order to maintain the terms of the original border agreement in the face of the Mississippi River's inconveniently located headwaters. My recollection is that it said roughly: the border goes west of <this> point until reaching the Mississippi river [which all parties assumed continued that far north]. [[Special:Contributions/108.162.216.40|108.162.216.40]] 14:13, 13 October 2017 (UTC)

I believe Randall's overall point is that though a large part of the individual United States have straight boundaries, especially in the West, or other features that are aesthetically pleasing, as in the S Carolina/Georgia/Florida coastline, there are a good number of internal inconsistencies. Many of these (most of the untagged "fixes") can be attributed to the concept that "Rivers make good logical boundaries", but even then, if you look closer, there are some really puzzling bits:
* The "Give To Canada" bit of Minnesota is almost all Indian Reservation land, so that kind of makes sense...
* The "Fix this thing" in Missouri is even stranger than it initially looks - while the notch in Arkansas is caused by the Mississippi River, there is a large bight of land in the middle of the Missouri-owned bit that is actually Kentucky (yes, there's an island of Kentucky that is separate from the main Kentucky state and entirely surrounded by Missouri)
* Not edited, but equally odd is the dip Florida cuts into Georgia near the east coast - there's no apparent town or natural features there to cause that irregularity

I don't happen to think the Arizona/New Mexico bits are political commentary, just "the entire rest of the state is a box, make this a straight line, too." cleanup. I mean yes, it would make wall-building easier, theoretically, but the Chinese showed the world centuries ago that straight lines are not needed to build a big fricking wall. [[Special:Contributions/108.162.238.131|108.162.238.131]] 14:23, 13 October 2017 (UTC)
- While I agree it probably isn't conscious political commentary, its interesting that there are not places the border increases; always concessions, never gains. May take into account its easier to give than take territory? --[[User:Jgt|Jgt]] ([[User talk:Jgt|talk]]) 19:32, 13 October 2017 (UTC)

I'm surprised Randall didn't suggest cleaning up Point Roberts as well [https://en.wikipedia.org/wiki/Point_Roberts,_Washington]. [[Special:Contributions/141.101.107.174|141.101.107.174]] 14:33, 13 October 2017 (UTC)

I'm shocked he didn't support fixing the Idaho/Wisconsin/Montana/Oregon border. That top part should be either given to Montana, or split between Washington and Oregon... I wonder if he left out certain things in order to avoid offending certain groups of people. Like suggesting that Rhode Island and Connecticut should probably be one state, or that Vermont and New Hampshire should be as well. [[User:Kashim|Kashim]] ([[User talk:Kashim|talk]]) 17:03, 13 October 2017 (UTC)

Some of the suggestions are ironic, for example Michigan's upper peninsula actually used to be part of the Wisconsin territory, but it was ceded to Michigan in exchange for the port of Toledo being ceded to Ohio. "why does Florida get Alabama's coastline" is actually because Alabama got part of Florida's coastline so it wouldn't be landlocked. The bit of Nevada that he wants to fix it so Nevada has territory along the Colorado River [[Special:Contributions/162.158.75.250|162.158.75.250]] 17:18, 13 October 2017 (UTC)

Talk:1891: Obsolete Technology

2017-09-18T14:51:43Z

Jgt:

Wasn't DOS still running behind Win95, and integrated into the OS similarly to the Linux shell? [[Special:Contributions/162.158.59.154|162.158.59.154]] 14:48, 18 September 2017 (UTC)

This reminds me of this Raganwald article on Blub: [http://weblog.raganwald.com/2006/10/are-we-blub-programmers.html Are we blub programmers?] Adequate doesn't mean best for the job; this comic presents the other side of the coin, don't upgrade just for upgrade's sake. --[[User:Jgt|Jgt]] ([[User talk:Jgt|talk]]) 14:51, 18 September 2017 (UTC)