explain xkcd - User contributions [en]

2525: Air Travel Packing List

2021-10-08T11:00:53Z

141.101.99.150: /* Table of items */ Still not quite right

{{comic
| number = 2525
| date = October 6, 2021
| title = Air Travel Packing List
| image = air_travel_packing_list.png
| titletext = I know the etiquette is controversial, but I think it's rude when the person in front of me reclines their seat into the bell of my trumpet.
}}

==Explanation==
{{incomplete|Created by an TRUMPETBORNE PARACHUTER - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
This comic is another in a [[:Category:COVID-19|series of comics]] related to the {{w|2019–20 coronavirus outbreak|2020 pandemic}} of the {{w|coronavirus}} {{w|SARS-CoV-2}}, which causes {{w|COVID-19}}.

The comic is about a proposed air-travel packing list, and the humor stems from the fact that many people have not been flying during the pandemic, and thus they might have forgotten what to pack. So [[Randall]] is so kind as to provide a packing list with 20 items. However, apart from the first item, the rest is not something you should or would normally bring on a plane.

Many of the items are already found on the plane, some would seem like they could be useful on a plane, while other could actually be useful in case of a plane crash (but only if you survive). Below in [[#Table of items|the table]] is a quick summary of each item.

The title text reference the idea that there is a trumpet for each passenger provided by the airline; item number 16 on the list. This items also states that you, because of the covid-19 pandemic, should remember to bring your own mouthpiece for the trumpet for safety measure.

The trumpet idea is then combined with the common debate regarding reclining your seat in airplanes. About half of the people think that reclining is rude as it takes up the space of the person behind you. The other half think that seats recline for a reason and the person in a seat has the rights to the space behind them. See for instance this [https://www.youtube.com/watch?v=08A30v8isRs video] about such a debate. Reclining a seat has resulted in actual [https://www.independent.co.uk/news/world/americas/crime/fight-airplane-man-punch-video-b1895402.html physical fights] on board planes.

Here it seems that Randall sides with the anti-recliners, although maybe only in the context of the comic, because he states that the recline would prevent him from playing his trumpet, as the seat hits the bell of the trumpet. The person in front could certainly argue that playing the trumpet behind them would be very annoying, to which Randall could reply that because the trumpet is provided by the airline, he has the right to play it. This would add a new layer to the debate. This could also be Randall's way of arguing against the right to recline a seat, just because it is possible.

===Table of items===
{| class="wikitable"
! Item
! Explanation
|-
|Seat cushion
|This can be used as a flotation device in a crash and is provided by the airline. Some people may also bring their own cushions for comfort.
|-
|{{w|Parachute}}
|Parachutes are normally used to slow down your falling out of the sky to a relatively safe speed in case of a severe problem with your aircraft, and are routinely used as a safety device by (para)glider pilots, test pilots, military aircraft crew and in similar situations when being unable to land safely is a significant concern. A parachute won't be very useful in a typical passenger airplane (even a small one) as there is no way to safely exit such a plane in-flight. Even the airplanes used for {{w|skydiving}} need to be specifically designed or modified for that purpose, such as having wide sliding doors that are unaffected by airflow. However, there were single cases of people being ejected or sucked out of a passenger airplane; in such case a parachute could by arguably useful. Famously, [[:Category:Comics featuring D. B. Cooper|D.B. Cooper]] jumped from an airplane in flight, with a parachute - never to be knowingly seen again.
|-
|Wing glue
|Probably to repair wings in the event of damage, potentially in a crash. Would be tricky (but not necessarily impossible) to apply mid-flight. This the first of several items that are potentially useful to the flight crew or maintenance teams, but would not be useful or appropriate for passengers to bring aboard.
|-
|{{w|Air horn}}
|An air horn uses compressed air to make a very loud noise very easily. This may be important for drawing attention to yourself in the event of a crash. Typically, emergency lifejackets on a plane are provided with a light and whistle for this purpose. The noise of an air horn might prove more effective for this purpose than a whistle, but it would become useless as soon as the compressed air ran out. Its inclusion is probably meant to suggest that the word 'air' in its name indicates that it's designed for use in an aircraft. Using one in a non-emergency situation would infuriate everyone else on the plane.
|-
|{{w|Sextant}}
|In combination with star charts, a sextant can be used to determine your position based on the location of stars in the night sky. Alternately, in combination with an accurate clock, a sextant can be used to determine the position of the sun relative to the aircraft to determine the vehicle's position. In a crash, you could use this to find your way to a safe place, but sextants are rarely used, and most people not trained on how to operate one. GPS will also allow you to find your position, is built into many phones, and is faster and easier to use than a sextant. If you've got a homing beacon, it probably makes more sense to just activate that and wait for help to arrive.

Until the early 1980s, long-range airplanes had a {{w|Air_navigation#Flight_navigator|flight navigator}} who used sextants and {{w|celestial navigation}} to determine the position of the airplane. Interestingly, it was much more accurate than early {{w|inertial navigation systems}}, and the accuracy of celestial navigation is still useful today. What made the sextant redundant was the INS' lower workload - the error accumulated by the INS during a long oceanic flight could always and easily be mitigated by other means, for example with {{w|VHF omnidirectional range|VOR}}/{{w|Non-directional beacon|NDB}} radio beacons.
|-
|Nose plugs and goggles for pressure
|Nose plugs and goggles are commonly used in swimming but would be useless for dealing with cabin pressurization or depressurization. Since your mouth and nose are interconnected, nose plugs would be useless on their own. Trying to hold your breath in a sudden depressurization event will cause lung damage, so nose plugs wouldn't be a good thing, even if you could also seal off your mouth. Goggles would also not be useful. During depressurization, the air would just seep out. During pressurization, they would just become uncomfortable and difficult to remove.
|-
|Airplane shoes
|Airlines typically don't require the use of special footwear for passengers, nor do they provide special shoes. Before emergency egress, certain shoes (like high heels) must be discarded, though. Aircrew are also prohibited from wearing such shoes.
|-
|Navigation crystal
|Mystical form of navigation, presumably either for navigation during flight or to help you get home after a crash.
Crystals that polarize light can be used as a compass [http://www.polarization.com/viking/viking.html].
|-
|Spare batteries in case the plane runs out
|Airplanes will generally use more power than any battery small enough to be easily packed in a bag can provide. They will generally use either 115V AC at 400Hz or 28V DC, both of which are very uncommon outside of aviation. The plane will almost never use its own batteries in-flight anyway, getting its electric power from the main engines, the APU, or, in emergencies, the ram air turbine or similar generating device. The batteries are generally only used on the ground when the engines are not running.
|-
|{{w|Birdseed}}
|So one can attract birds. In practice, this wouldn't work for multiple reasons (high speed, altitude, and sealed windows being some of most obvious ones) and would pose a significant hazard of birds getting stuck in an engine if it did. On the other hand, spreading birdseed before boarding would be seen as misconduct by airport authorities, as it may pose a danger to aircraft by attracting birds. Alternatively, birdseed can be used to attract birds after surviving a crash, e.g. to catch them for food.
|-
|{{w|Homing beacon}}
|Once activated, a homing beacon will send out a continuous radio signal so that rescuers can find your location. These can be very useful in a plane crash, but airplanes already carry them ({{w|Emergency position-indicating radiobeacon}}s), so you don't need to pack one yourself. Incidentally, the {{w|COSPAS-SARSAT}} system for locating distressed airplanes and ships was a cooperation started by the United States and the Soviet Union, and it was an elegant and simple solution that uses the {{w|Doppler effect}} of radio signals for accurate location - long before the {{w|Global Positioning System}}.
|-
|Meteorite antidote
|Meteorites are pieces of space rocks that make it all the way to the ground. They can cause injury but they aren't poisonous{{Citation needed}}, so an antidote would not help.
|-
|USB wing connector
|This is a just a wire connector, but because it has wing in the name is on the list. Alternatively, the plane wings connect by USB, and this can be used to reattach wings. Airplanes usually use the {{W|ARINC 429}} protocol (or, increasingly, TCP/IP, RS427, RS232, or even CANBUS) instead of USB protocols to facilitate electronic communication between flight computers and the engines, for example.
|-
|Emergency siren
|USE IN CASE OF EMERGENCY
|-
|Spare flaps
|Flaps can be moved to adjust the lift/drag ratio of a wing, generally during takeoff and landing. Flaps are very large and mounted on the wing, outside the passenger compartment, so bringing spares would be very difficult and completely useless. Flaps failing to come down can also usually be remedied by just landing at a longer runway.
|-
|{{w|Mouthpiece (brass)|Mouthpiece}} (pandemic restriction; airlines still provide the trumpet)
|A part of a brass instrument like a trumpet. Randall jokes that trumpets are provided on airplanes (which would be very obnoxious to other passengers), but due to the pandemic you cannot use a shared mouthpiece. (You shouldn't share mouthpieces for anything anyway, in general.)
|-
|Luggage ballast
|Likely to make plane more balanced. While balancing weight in a plane is indeed a real problem, it's solved by rearranging luggage and adjusting engine power slightly. Introducing ballast would mean additional weight for no real reason.
Alternatively, while a common passenger issue is to have hand- and/or hold-luggage that exceeds the airline's personal allowance, this person has ''under''weight baggage and does not wish to 'waste' the difference, so bulks it up. (Noting that someone already with the rest of the items on this list is unlikely to suffer this 'problem'.)
|-
|Flag (international flights)
|To identify your country of origin. Other flags are also used to communicate between boats without electricity, in the event the boats are in distress, so they could be used in the event of a crash.
|-
|Decoy tickets
|Maybe these would used as a distraction so you can sneak onto the plane without paying?
But also a typical trope for fictional (and real life?) attempts to evade being tracked or followed. Buy tickets for one destination, that one assumes the opposition will be fully aware of, but also arrange for another set (probably with a 'clean' identity) for your intended destination and switch to using those once in the chaos of the departure-lounge.
|-
|Keys to the plane
|Though some pushed for it after a plane was stolen in the {{w|2018 Horizon Air Q400 incident}}, most commercial planes do not require keys to start the engine(s) like a car does. Likewise, plane doors are not locked with a key. Instead, they are secured with a tamper seal. If a seal is found broken, the plane is thoroughly checked for any wrongdoing.
|}

==Transcript==
:[A lists of 20 items is given in two columns with 10 items in each. Each item is preceded by a checkbox. Most items only take up one line, but in the left column two items take up two and in the right one item take up three, so they take up the same space. Above is a large heading, with an explanation beneath it.]
:<big><big>Air Travel Packing List</big></big>
:<big>If you haven't flown in a while, you might not remember what you need to bring. Use this handy checklist to pack!</big>

:[Left column:]
:☐ Seat cushion
:☐ Parachute
:☐ Wing glue
:☐ Air horn
:☐ Sextant
:☐ Nose plugs and goggles for pressure
:☐ Airplane shoes
:☐ Navigation crystal
:☐ Spare batteries in case the plane runs out
:☐ Birdseed

:[Right column:]
:☐ Homing beacon
:☐ Meteorite antidote
:☐ USB wing connector
:☐ Emergency siren
:☐ Spare flaps
:☐ Mouthpiece (Pandemic restriction; airlines still provide the trumpet)
:☐ Luggage ballast
:☐ Flag (International flights)
:☐ Decoy tickets
:☐ Keys to the plane

{{comic discussion}}

[[Category:COVID-19]]
[[Category:Animals]]

2520: Symbols

2021-09-24T20:50:04Z

141.101.99.150: /* Explanation */ There's a lot of editing needed, but let's start here and wait a while for everyone else to pile in on the rest.

{{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 notation commonly used in various fields of math and science.

d/dx and ∂/∂x are both used to represent derivatives in calculus and related fields. d/dx is most commonly used in introductory and basic calculus, while ∂/∂x is the symbol for a partial derivative, indicating a problem involving multivariable calculus, which is a level of difficulty above single-variable calculus.

ħ (pronounced 'h-bar') is a symbol used for Planck's constant, a universal constant in quantum physics equal to the energy of a photon divided by its frequency.

NA is Avogadro's number, which is used in chemistry for calculating the number of molecules in a mass of substance. Its use implies a chemistry problem where relative concentrations and orders of magnitude are important; if a mistake is made the concentration of a potentially dangerous chemical could be far too strong or too weak.

μm and mK (micrometers and millikelvin) are very small units of length and temperature respectively. Any equipment that is operating in these units will be incredibly finely calibrated and thus very expensive. Millikelvins would measure temperatures barely above absolute zero, suggesting sensitive experiments probing quantum mechanical behavior that would likely only exist in an advanced lab.

nm and eV (nanometers and electronvolts) are also small units of length and energy. Nanometres in particular are commonly used to refer to wavelengths of light, and therefore might be seen when working with lasers, which you should definitely not shine in your eye. Electron volts are a measure of energy in particle physics; particle accelerators produce intense radiation which should definitely not be directed toward your eye.

mSv (millisieverts) are a unit of radiation exposure. Randall's comment may be referring to [https://xkcd.com/radiation/ this chart].

mg/kg and μg/kg (milligrams per kilogram and micrograms per kilogram) are units used to count the proportion of a substance that contains a certain chemical; they are equivalent to 'parts per million' and 'parts per billion' respectively. These might be found when working with particularly dangerous chemicals, where even slight exposure could lead to severe health risks.

Finally, the comic references pi and tau. Pi is defined as the ratio of a circle's circumference to its diameter, while tau is defined as pi times two. Pi is more commonly used as a circle constant and is helpful when working with areas and volumes, but proponents of tau argue that it is more useful in a pure mathematics context as it makes working with radians easier. The joke here is that whichever constant you attempt to use, it will probably be the wrong one for what you are trying to achieve.

This comic discusses different symbols found in equations, and humorously comments on their implications.
*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 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.

*d/dx: An undergrad is working very hard
d/dx is the symbol for a single-variable {{w|Derivative|derivative}} taken with respect to x. This is a mathematical operation that, while difficult, is 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
: Partial derivative. Partial differential equations are typically encountered in higher-level coursework, hence why a grad student would be working hard vs. an undergrad.
;Re
:Reynolds number, used in modelling the flow of fluids. Fluid flow cannot usually be modelled analytically and so a numerical model is necessary.
;(Ta4 - Tb4)
:Temperature component of net radiation loss. See Stefan-Boltzmann's law. This is a joke about how the Sun is a very powerful blackbody radiator.
;NA
:Avagadro's constant. If you are using this and not working in Moles there is a good chance that you will make a mistake of a factor of a power of ten. THis could create dangerous amounts of chemicals
;µm
:Micrometers are small. Any equipment that is accurate to that degree is likely to be expensive
;mK
:millikelvin are small changes in temperature, or represent very low temperatures. Equipment that uses mK is very expensive
;nm
:Describes the wavelength of laser light
;eV
:describes the energy of a particle beam
;mSv
:The amount of radiation absorbed. The pun most likely refers to internet trolls debating the effects of radiation like 5G networks, or fans of the [[2163|Chenobyl series]].
;mg/kg
:The toxicity of a chemical, per kg of body mass. If it is measured in mg/kg it is quite toxic
;µg/kg
:If the chemical is measured in µg/kg it is extremely toxic
;π or τ
:τ = 2π, so it would be easy to make a mistake and use π when τ is correct. Or vice-versa!

The title text refers to two non-SI units of radiation measurement. An area with significant amounts of radiation is probably dangerous, 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}}

2519: Sloped Border

2021-09-24T11:31:28Z

141.101.99.150: /* Explanation */ To be more correct...

{{comic
| number = 2519
| date = September 22, 2021
| title = Sloped Border
| image = sloped_border.png
| titletext = "The slope will be 74° at ground level." "Okay, I think we can hack together a ... wait, why did they specify ground level? It's 74° everywhere, right? ... Oh no, there's a whole section in the treaty labeled 'curvature.'"
}}

==Explanation==
{{incomplete|Created by a SUBSIDING GERRYMANDER - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
Nearly every single country (except some islands and Australia) demarcates an {{w|international border}} with other countries. The borders are established through law, treaty or consensus. Establishing an international border is assumed by present-day customs, immigration and security checks. Some countries (like {{w|Cyprus}}) have established a {{w|buffer zone}} in place of an international border to gain additional protection during a conflict.

In this comic, Cueball and Blondie established a "sloped" international border through a treaty. Usually borders are perpendicular to the ground{{citation needed}} so that all the air(space) above the ground belongs to the same country. This is called {{w|Air sovereignty}}. Thus it suffices to define the border on the earth surface, as 1D lines across the curved 2D surface. The precise definition is that a line from the center of the Earth through the point of the border is drawn. Sloped terrain is immaterial to the border of the air sovereignty which is still vertical, even if not perpendicular to the terrain.

If the borders were sloped (with respect to the horizontal ground level) an airplane would need to know its precise height to decide which country's jurisdiction currently applies. With the help of the {{w|Global Positioning System}} this would be in principle possible, although the height information of GPS is less reliable. (It might be possible to program a computer to use altitude data from the airplane's altimeter along with latitude and longitude data from the GPS and a relevent ground relief database to make an accurate determination.)

The mathematical computation for an angled air sovereignty seems relatively straight-forward at low level and could be expressed with a single line of code or a single equation, although the people acting on the information are likely unfamiliar with code and equations and likely use tools with completely no support for sloped borders. The mention of curvatures in the title text may reveal some emergent problems that need accounting for.

A totally straight line drawn far enough upwards at an angle will find the surface of the Earth curving away beneath it (not even considering terrain undulations) and the angle to the local vertical will reduce as it continues, tending towards vertical after far enough.

Alternately (although it seems this is not the case) the profile of the sloped border may be assumed to remain at a constant angle to the shifting vertical, in which case it describes a certain {{w|Logarithmic spiral|form of spiral}} (if shallow enough it could loop round the Earth).

A third option is that it gains altitude at a constant rate, with respect to the lassage of land below its track, to form a {{w|Archimedean spiral|different spiral}}, in which case it will still loop around the Earth but at an angle that increasingly tends towards horizontal.

While the comic doesn't mention this, such a boundary could be applied subterraneously, i.e. underground. (Although the 'straight' model, if implemented, will only reach the depth at which it is tangential to the radius and then re-emerge through the surface some way distant around the planet.) This would then impact, at practical depths for such things, planning rights for property foundations and, at deeper levels, mining rights for minerals.

Practically an upper-limit to a nation's claim (somewhat below satellites, e.g. the Karman Line) and a lower limit (well before reaching the Earth's mantle) will prevent many of these complications, together with intersections with other (probably vertical) 'territorial volume' borders that will supercede in any compound claims to ownership. - However, it is still ''very important'' to specify exactly which curve (i.e. with respect to what) the boundary is designed to respecting.

"GIS" refers to {{w|geographic information system}}, a tool for capturing and analyzing spatial and geographic data. People developing these systems would be inconvenienced by the border described in the comic.

It is possible this comic is inspired by such boundary disputes as the {{w|Territorial_claims_in_the_Arctic#Beaufort_Sea|Beaufort Sea 'wedge'}} which, while in this case perpendicular to the surface, suffers from alternative interpretations of how to extend it from the shoreline out towards international waters.

==Transcript==
:[Cueball and Blondie are standing on a podium either for TV, or for the photo op when signing a treaty. They are holding a document together between them. In the background are two informational graphics. One intended to be cross-sectional showing a non-vertical dotted line between representative Cueball and Blondie figures, the other a skewed perspective of a similar setup but with two further figures and all but one these now positioned to be intersected by a 'shaded plane' effect so that some or most of their bodies are beyond the sloped boundary, as greetings of some kind are conducted through it.]
:Cueball: With this treaty, we are proud to announce the creation of the world's first '''''sloped''''' international border!

:[Caption below frame:]
:If I'm ever put in charge of a country, I'm going to spend all my time trying to think of new ways to make life a nightmare for GIS people.
{{incomplete transcript|Do NOT delete this tag too soon.}}
{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Blondie]]
[[Category:Multiple Cueballs]]
[[Category: Geography]]

Talk:938: T-Cells

2021-09-23T06:49:50Z

141.101.99.150:

Does anyone have a link to the actual article? Or possibly a proper citation? {{unsigned ip|192.17.144.82}}
:I have added such a link in the explanation. Unfortunately, you have to subscribe to the magazine asterisked in the comic, so the link goes to another one. It also helps to Google "nejm aug 10 2011". Anonymous 04:51, 13 December 2013 (UTC)

Trial appears to have been a success, although the patient now has no B-cells and thus a compromised immune system (will need regular gamma globulin transfusions and the like). [[Special:Contributions/75.103.23.206|75.103.23.206]] 16:54, 13 December 2012 (UTC)

Looks to be this article here [http://www.nejm.org/doi/full/10.1056/NEJMoa1103849] and [http://kiefercon.tumblr.com/]. I'll stick with chemo, thanks. [[Special:Contributions/173.245.54.87|173.245.54.87]] 16:36, 24 November 2013 (UTC)

I know it's a joke, but just in case people are taking this seriously, this is worth a read. [http://scienceblog.cancerresearchuk.org/2013/06/25/no-doctors-did-not-inject-hiv-into-a-dying-girl-to-treat-her-cancer/] The key word should have been "lentivirus", not "HIV". The T cells were modified using a heavily altered lentivirus derived from HIV. The virus shouldn't be referred to as HIV, though it makes for some great headlines. [[Special:Contributions/199.27.128.167|199.27.128.167]] 20:40, 2 December 2013 (UTC)

Before WWII there was an succesful method of curing syphilis with malaria (malariotherapy). Maybe a reference[[Special:Contributions/141.101.96.217|141.101.96.217]] 11:32, 27 March 2014 (UTC)

Interesting. I had heard a "story" some time ago that disease brought to higher latitudes from newly discovered tropical countries laid waste to myriads due to the fact that the climate was cooler. Maybe it was related to the lack of suitable pathogens in the communities.

[[User:Weatherlawyer| I used Google News BEFORE it was clickbait]] ([[User talk:Weatherlawyer|talk]]) 13:09, 23 January 2015 (UTC)

Isn't "HIV virus" redundant? [[Special:Contributions/108.162.245.42|108.162.245.42]] 02:24, 28 September 2014 (UTC)
:Yes, yes it is. Anonymous 20:25, 9 December 2014 (UTC)

Removed the incomplete thingy because I fixed the explanation. Put it back if there are more questions[[User:XFez|XFez]] ([[User talk:XFez|talk]]) 13:32, 21 February 2017 (UTC)

I don't think it's a reference to the little old lady who swallowed a fly - I think it's a reference to the use of introducing invasive species as pest control (like cane toads) and how you might try to control that invasive species by introducing their natural predator [[Special:Contributions/141.101.99.150|141.101.99.150]] 06:49, 23 September 2021 (UTC)