explain xkcd - User contributions [en]

2426: Animal Songs

2021-02-18T13:02:48Z

172.68.132.5: /* Explanation */

{{comic
| number = 2426
| date = February 17, 2021
| title = Animal Songs
| image = animal_songs.png
| titletext = Dr. Fauci is not permitted to have a cat, because as director of the National Institute of Allergy and Infectious Diseases, his petting one would be considered giving aid and comfort to an allergen.
}}

==Explanation==
{{incomplete|Created by DR. FAUCI IN PRIVATE. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}
Another joke about professionals not being so professsional in private; e.g. [[2401]], [[1463]]

{{w|Dr. Anthony Fauci}} is the government medical science guy{{Citation needed}} (Director of the {{w|National Institute of Allergy and Infectious Diseases}}) who was largely responsible for informing the public in the United States on how to avoid spreading {{w|SARS-CoV-2}} in the beginning of the {{w|COVID-19 pandemic}}. The comic shows him singing a made-up song to his pet fish in order to memorise his daily routine, which Randall notes is a bizarre habit for an adult presented to the public as professional and having a prosaic "grown-up" manner.

Singing is reported to be a successful way of enhancing neural connectivity and engaging the mind's attention: there are several cases in medicine of speech-impaired patients being able to communicate by singing because the ability is controlled by a different section of the brain. In other cases, music can reactivate dormant neurons (cf. Ilene Woods, the voice of Cinderella in the 1950 Disney film: as she suffered from dementia near the end of her life, her nurses realised that she would respond when her song "A Dream Is A Wish Your Heart Makes" was played to her).

The title text may reference the definition of treason in American law, which includes "giving aid and comfort" to an enemy; in this case, being the director of the National Institute of Allergy and Infectious Diseases, allergens such as cat fur could be considered "enemies."

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:[Dr. Fauci is putting on a white lab coat and dancing. A fish tank stands on an end table to the right. Inside, there is a fish looking at him as well as a seaweed-like plant and a small castle. Music notes are scattered about Fauci's speech.]
:Dr. Fauci: ♫ ''Putting on my doctor coat'' ♫

:[Dr. Fauci is buttoning the coat. He is now to the right of the fish tank.]
:Dr. Fauci: ♫ ''It's the coat I wear'' ♫

:[Dr. Fauci is back to the left of the fish tank, looking himself in a mirror, and touching his face.]
:Dr. Fauci: ♫ ''so they know how good a doctor I am'' ♫
:Off-panel voice: Dr. Fauci? The press conference is in five.
:Dr. Fauci: Be right there!

:[Caption below the panel:]
:It's nice to think about how serious and important people probably ''also'' absentmindedly sing made-up songs to pets.

{{comic discussion}}

[[Category:COVID-19]]
[[Category:Comics featuring real people]]
[[Category:Animals]]

Talk:593: Voynich Manuscript

2021-02-11T22:11:07Z

172.68.132.5: ghost post of the obvious but overlooked

I feel that the title text is not well enough explained, but I don't know if it's enough to add an incomplete tag. [[Special:Contributions/108.162.254.163|108.162.254.163]] 03:37, 2 January 2014 (UTC)

[http://www.santa-coloma.net/voynich_drebbel/voynich.html It has been proposed] that the VM is a token artifact for Francis Bacon's utopian book ''{{W|New Atlantis|New Atlantis}}''. Under this theory, it would be akin to a prop replica made in relatively recent times. --[[User:Ishldgetoutmore|I Should Get Out More]] ([[User talk:Ishldgetoutmore|talk]]) 14:43, 18 March 2014 (UTC)

I feel like the [http://en.wikipedia.org/wiki/Codex_Seraphinianus Codex Seraphinianus] should be mentioned, but I'm not sure how it would fit in. [[User:Leafy Greens|Leafy Greens]] ([[User talk:Leafy Greens|talk]]) 17:05, 16 November 2014 (UTC)

Alternately, Megan could be attempting to distract Cueball from his line of questioning about where she got the book, by suggesting they play a game (possibly with the book), to steer the conversation away from the difficult question of where she got it. [[Special:Contributions/162.158.75.232|162.158.75.232]] 15:59, 28 June 2017 (UTC)

Why is Harry Potter part of this explanation? The idea of wolfsbane keeping werewolves at bay isn't anything like that new. --[[Special:Contributions/172.69.33.11|172.69.33.11]] 18:42, 27 August 2018 (UTC)

Am I the only one that noticed that Druids and Dicotyledons was likely abbreviated as D&D, much like Dungeons and Dragons is today?

2035: Dark Matter Candidates

2018-08-23T19:37:57Z

172.68.132.5: /* Explanation */ fix template and superscript problems

{{comic
| number = 2035
| date = August 20, 2018
| title = Dark Matter Candidates
| image = dark_matter_candidates.png
| titletext = My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.
}}

==Explanation==
{{incomplete|Every section needs to be filled and explained. Do NOT delete this tag too soon.}}
{{w|Dark matter}} is a hypothetical, invisible form of matter used by the vast majority of astronomers to explain the far too high apparent mass of objects at large scales in our universe. In galaxies, stars are orbiting faster than the gravitational force of the sum of the masses of visible matter in the galaxy could cause, and entire galaxies are observed moving much faster around each other than their visible masses could explain. In galactic collisions, the mass can appear to separate from the visible matter, as if the mass doesn't collide but the visible matter does. A small handful of galaxies have been observed to not have this property, suggesting that it is a *thing* that a galaxy can have more or less of and is separable from. At scales of our solar system, those effects are too small and can't be measured. In cosmology, dark matter is estimated to account for 85% of the total matter in the universe.

This comic gives a set of possibilities for what dark matter could possibly be, charted by mass from smallest (given in {{w|Electronvolt#Mass|electronvolts}}) to largest (given in kilograms). Masses in the range 10-15 kg to 10-3 kg are given in grams together with appropriate prefixes, while the ton takes the place of 103 kg.

Only massive objects ranging from subatomic particles up to super massive ones are covered in this comic. There are also {{w|Dark matter#Alternative hypotheses|alternative hypotheses}} trying to modify general relativity with no need of additional matter. The problem is that these theories can't explain all different observations at once. Nonetheless dark matter is a mystery because no serious candidate has been found yet.

The joke in this comic is that the range of the mass of the possible particles and objects stretch over 81 powers of ten, with explanations suggested by astronomers covering only some portions of that range. [[Randall]] fills the gaps with highly absurd suggestions.

;Axion
An {{w|Axion|axion}} is a hypothetical elementary particle that might be a component of dark matter.

;Sterile neutrino
{{w|Sterile neutrino|Sterile neutrinos}} are hypothetical particles interacting only via gravity. It's an actual candidate for dark matter.

;Electrons painted with space camouflage
{{w|Electron|Electrons}} are fundamental particles which compose the outer layers of atoms. A large number of electrons in the galaxy would be relatively easy to detect, as they not only interact with light (which dark matter does not appear to), but also have a strong electric charge. Presumably, space camouflage is a positively-charged coating which prevents electrons from interacting with light. (Needless to say, this is not an actual candidate for dark matter.) The mass of an electron is about 0.5 MeV which fits well into the graph.

;Neutralino
A {{w|Neutralino|neutralino}} is a hypothetical particle from {{w|Supersymmetry|supersymmetry}}, it is an actual candidate for dark matter.

;Q-ball
In theoretical physics, a {{w|Q-ball}} is a stable group of particles. It's an actual candidate for dark matter.

(In billiards, a cue ball is the white (or yellow) ball hit with the cue in normal play. In addition, [[Cueball]] is the name explainxkcd uses for the most common xkcd character.)

;Pollen
{{w|Pollen}} is a joke candidate, though people with seasonal allergies may suspect that the universe is genuinely made up entirely of pollen in the springtime.

;No-See-Ums
{{w|Ceratopogonidae|No-See-Ums}} are a family (Ceratopogonidae) of small flies, 1–4 mm long, that can pass through most window screens. Another joke candidate.

;Bees
Insects of the clade {{w|Bee|Antophila}} are major pollinators of flowering plants. In recent years {{w|Colony collapse disorder|bees have been disappearing}} at an alarming rate; {{w|The Stolen Earth|Doctor Who explained}} that they are in fact aliens leaving Earth prior to a Dalek invasion.

;8-balls
In pool, the {{w|Pool (cue sports)|8-ball}} is a black ball numbered 8. It's a pun with Q-ball/cue ball. Unless undetected aliens have discovered billiards and become addicted to it, 8-balls are found only on Earth and are, hence, unlikely dark matter candidates. The 8-ball is also a popular unit of sale for black market pharmaceuticals like cocaine, where it stands for 1/8th of an ounce (3.5g). This doesn't make sense as a dark matter candidate either -- unless dark matter is hard to detect because it's illegal & trying to avoid the cops

;Space Cows
Cows are {{w|Bovinae|bovines}} extensively farmed on Earth for milk and meat. Although there is folklore concerning cows {{w|Hey diddle diddle|achieving circum-lunar orbits}}, not to mention their appearance on a {{w|Shindig (Firefly)|beloved space western TV show}}, as Muppet cow [http://muppet.wikia.com/wiki/Natalie Natalie] in the Sesame Street News Flash (and [https://tvtropes.org/pmwiki/pmwiki.php/Main/SpaceWestern others less-remembered]), they have yet to be found elsewhere in the Universe. In the television show "Too Close for Comfort", one of the characters is the cartoonist of a comic strip called "Cosmic Cow". {{w|Spherical cow|Spherical cows}} have also been used (humorously) by physicists needing to simplify some source of mass in a given problem.

;Obelisks, Monoliths, Pyramids
While those human constructions are huge on a human scale, they're negligible at universe-scale. It would take a large number of such constructions, distributed through space, to replicate the effects of dark matter; while a scenario could be envisioned where enough such constructs existed, with properties and distribution allowing them to match observations, this is obviously not a likely explanation.
They often show up in fiction and pseudo-scientific literature as alien artifacts generating immense unknown power out of nowhere, with the most famous and influential example being the three monoliths from {{w|2001: A Space Odyssey (film)|2001: A Space Odyssey}} (with the largest having a mass of about 500,000 tonnes).

;Black Holes ruled out by:
{{w|Black hole|Black holes}} are known to occur in sizes of a few solar masses (about 1030-1031 kg) as remnants of the core of former big stars, as well as in quite large sizes at the centers of galaxies (millions or even billions of solar masses). But recent gravitational wave detections indicate that black holes at 50 or 100 solar masses also exist, though their origin is still not understood. Randall doesn't mention this but some astronomers hope that these could fill at least a part of the gap. While black holes are widely reported to be ruled out as a candidate for dark matter for various reasons Randall has listed, such constraints are based on "monochromatic" mass distributions -- meaning that all such black holes are assumed to have the same mass -- which is considered physically implausible for populations of merging bodies which are known to have vastly different masses.[https://arxiv.org/pdf/1709.07467.pdf][https://arxiv.org/pdf/1705.05567.pdf] (That this is a common practice in cosmology may be part of the reference to "buzzkill" astronomers.) He rules out all black holes in the range of approximately 1010 kg to 1033 kg even when below some gaps at the bars appear.

Except the last item, all range below the mass of the sun (2x1030 kg) while the smallest known black hole is about four solar masses.
* Gamma rays: If dark matter were black holes of this size, the black holes could be evaporating by the predicted {{w|Hawking radiation}}, and we'd see a buzz of gamma rays from every direction if many of those objects would exist. Nonetheless this radiation is still hypothetical and not been observed on any known black holes. Furthermore those objects would very small because the Schwarzschild radius of a 1012 kg black hole is approximately 148 fm (1.48×10−13 m), which is between the size of an atom and an atomic nucleus.
* GRB lensing: {{w|Gamma-ray burst|Gamma-ray bursts}} (GRBs) are the brightest events in the universe and have been observed only in distant galaxies. While gravitational microlensing (see below) is an astronomical phenomenon, it doesn't make much sense here. GRBs are short (milliseconds to several hours) and are often detected only by space-borne sensors for gamma-rays -- rarely at any other wavelengths. Measuring lensing effects would be very difficult. This [https://arxiv.org/abs/1406.3102 paper] discusses the probability of detecting lensing effects caused by {{w|Dark matter halo|galactic halo objects}} among the known GRBs given sufficient objects to represent the missing mass.
* Neutron star data: {{w|Neutron star|Neutron stars}} aren't black holes, but they're also very small highly compact objects at about 1.4-2.16 solar masses. While black holes can't be observed directly, neutron stars are detectable in many wavelengths. The number of them gives a clue about the number of black holes close to the mass of the sun, a number which is far too low to make up dark matter.
* Micro lensing: {{w|Gravitational microlensing}} is a gravitational lens effect, (the path of radiation is changed by passing through space bent by nearby mass). This was predicted by Einstein's {{w|General Relativity|Theory of General Relativity}} and was first confirmed in 1919 during a solar eclipse, when a star which was nearly in line with the sun appeared more distant to the sun than usual. Astronomers have found many so called {{w|Einstein ring|Einstein rings}} or Einstein crosses where a massive object in front of other galaxies bends the light toward us. Those massive objects may be black holes, but the number is far too low to explain dark matter.
* Solar system stability: Our {{w|Solar system|solar system}} is 4.5 billion years old and has been very stable since shortly after its formation. If not, we wouldn't exist. If dark objects at 1024 kg - 1030 kg (mass of Earth up to mass of Sun) accounted for dark matter and were distributed throughout galaxies, there should be many of them in the vicinity of our solar system and the system wouldn't be stable at all.
* Buzzkill Astronomers: Black holes above a certain size are thought by some astronomers to be impossible to miss, due to the effects they have on nearby matter. At the mass of some 1030 kg there must be many supernova remnants we still haven't found. Black holes of about 1035 kg have long been considered dark matter candidates by a minority group of cosmologists,[https://arxiv.org/pdf/1001.2308.pdf] and the Milky Way's first discovered intermediate mass black hole falls in this range.[https://www.nao.ac.jp/en/news/science/2016/20160115-nro.html]

;Maybe those orbit lines on space diagrams are real and very heavy
Diagrams of our solar system (or any planetary system) often show lines representing the elliptical paths the planet takes around its sun. These lines don't show real objects, though. Astronomers just draw them on pictures of the solar system to show where the planets move. If you draw a line on a map to give someone directions, that line isn't an object in real life; it's just on the map. If these lines were real, they would be ''huge'' (Earth's would be 940 million km long (2π AU) and Neptune's would be 28 ''billion'' kilometers long). [https://www.youtube.com/watch?v=0fKBhvDjuy0 Powers of Ten (1977)] gives a good sense of just how large these orbit lines need to be in order to be visible in space diagrams. If these orbit lines were also very dense, they would have a huge mass and could possibly account for the missing 85% of the mass in the universe. But they would also constantly be impaling the planets, including the Earth, which would probably be a problem. Their mass would also affect planetary motions in ways which we would detect. A related worry about space travel was expressed in previous centuries; it was thought that the planets were embedded within {{w|Celestial spheres|crystal shells}} (spheres or Platonic solids), and a rocket into space could smash the shells and send planets plummeting to Earth. Another joke candidate.

;Title text
The title text refers to the fact that space is just vast emptiness where a little bit of dirt could be overlooked. Actually the mean density of detectable matter in the universe, according to NASA, is equivalent to roughly [https://map.gsfc.nasa.gov/universe/uni_matter.html 1 proton per 4 cubic meters]. And because this matter is mostly located in galaxies -- and inside there in stars and clouds -- the space between is even more empty. For comparison, one gram hydrogen consists of {{w|Avogadro constant|6.022 x 1023 atoms}}. Like at home wiping with a cleaning cloth in which we can see the dirt that wasn't clearly visible on the surface we have wiped, Randall believes that some few atoms more per cubic meter could stay undetected in the same way. This isn't true because in the space between galaxies astronomers can detect matter as it spreads over thousands or millions cubic light years. Atoms can't hide; there is always radiation.

==Transcript==
:Dark matter candidates:
:[A line graph is shown and labeled at left quarter in eV and further to the right in g together with some prefixes.]
:[The labels read:]
:µeV, meV, eV, keV, MeV, GeV, TeV, 10-18kg, ng, µg, mg, g, kg, TON, 106kg, 1012kg, 1018kg, 1024kg, 1030kg

:[All items are shown in bars ranging between two approximately values:]
:< 1 µeV - 10 meV: Axion

:1 eV - 10 keV: Sterile neutrino

:0.5 MeV (exactly): Electrons painted with space camouflage

:10 GeV - 10 TeV: Neutralino

:100 TeV - 10-17 kg: Q-ball

:1 ng - 100 ng: Pollen

:0.1 mg - 1 mg: No-See-Ums

:10-1 g (exactly): Bees

:10 g - 100 g: 8-balls

:100 kg - TON: Space cows

:TON - 109 kg: Obelisks, monoliths, pyramids

:109 kg - 1033 kg: Black holes ruled out by:
::109 kg - 1013 kg: Gamma rays
::1013 kg - 1017 kg: GRB lensing
::1015 kg - 1022 kg: Neutron star data
::1021 kg - 1030 kg: Micro lensing
::1024 kg - 1030 kg: Solar system stability
::1030 kg - 1033 kg: Buzzkill astronomers

:1033 kg - >1036 kg: Maybe those orbit lines on space diagrams are real and very heavy

{{comic discussion}}

[[Category:Science]]
[[Category:Physics]]
[[Category:Astronomy]]
[[Category:Line graphs]]

2035: Dark Matter Candidates

2018-08-23T19:36:39Z

172.68.132.5: /* Explanation */ exclusion constraints are based on monochromatic mass distributions

{{comic
| number = 2035
| date = August 20, 2018
| title = Dark Matter Candidates
| image = dark_matter_candidates.png
| titletext = My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.
}}

==Explanation==
{{incomplete|Every section needs to be filled and explained. Do NOT delete this tag too soon.}}
{{w|Dark matter}} is a hypothetical, invisible form of matter used by the vast majority of astronomers to explain the far too high apparent mass of objects at large scales in our universe. In galaxies, stars are orbiting faster than the gravitational force of the sum of the masses of visible matter in the galaxy could cause, and entire galaxies are observed moving much faster around each other than their visible masses could explain. In galactic collisions, the mass can appear to separate from the visible matter, as if the mass doesn't collide but the visible matter does. A small handful of galaxies have been observed to not have this property, suggesting that it is a *thing* that a galaxy can have more or less of and is separable from. At scales of our solar system, those effects are too small and can't be measured. In cosmology, dark matter is estimated to account for 85% of the total matter in the universe.

This comic gives a set of possibilities for what dark matter could possibly be, charted by mass from smallest (given in {{w|Electronvolt#Mass|electronvolts}}) to largest (given in kilograms). Masses in the range 10-15 kg to 10-3 kg are given in grams together with appropriate prefixes, while the ton takes the place of 103 kg.

Only massive objects ranging from subatomic particles up to super massive ones are covered in this comic. There are also {{w|Dark matter#Alternative hypotheses|alternative hypotheses}} trying to modify general relativity with no need of additional matter. The problem is that these theories can't explain all different observations at once. Nonetheless dark matter is a mystery because no serious candidate has been found yet.

The joke in this comic is that the range of the mass of the possible particles and objects stretch over 81 powers of ten, with explanations suggested by astronomers covering only some portions of that range. [[Randall]] fills the gaps with highly absurd suggestions.

;Axion
An {{w|Axion|axion}} is a hypothetical elementary particle that might be a component of dark matter.

;Sterile neutrino
{{w|Sterile neutrino|Sterile neutrinos}} are hypothetical particles interacting only via gravity. It's an actual candidate for dark matter.

;Electrons painted with space camouflage
{{w|Electron|Electrons}} are fundamental particles which compose the outer layers of atoms. A large number of electrons in the galaxy would be relatively easy to detect, as they not only interact with light (which dark matter does not appear to), but also have a strong electric charge. Presumably, space camouflage is a positively-charged coating which prevents electrons from interacting with light. (Needless to say, this is not an actual candidate for dark matter.) The mass of an electron is about 0.5 MeV which fits well into the graph.

;Neutralino
A {{w|Neutralino|neutralino}} is a hypothetical particle from {{w|Supersymmetry|supersymmetry}}, it is an actual candidate for dark matter.

;Q-ball
In theoretical physics, a {{w|Q-ball}} is a stable group of particles. It's an actual candidate for dark matter.

(In billiards, a cue ball is the white (or yellow) ball hit with the cue in normal play. In addition, [[Cueball]] is the name explainxkcd uses for the most common xkcd character.)

;Pollen
{{w|Pollen}} is a joke candidate, though people with seasonal allergies may suspect that the universe is genuinely made up entirely of pollen in the springtime.

;No-See-Ums
{{w|Ceratopogonidae|No-See-Ums}} are a family (Ceratopogonidae) of small flies, 1–4 mm long, that can pass through most window screens. Another joke candidate.

;Bees
Insects of the clade {{w|Bee|Antophila}} are major pollinators of flowering plants. In recent years {{w|Colony collapse disorder|bees have been disappearing}} at an alarming rate; {{w|The Stolen Earth|Doctor Who explained}} that they are in fact aliens leaving Earth prior to a Dalek invasion.

;8-balls
In pool, the {{w|Pool (cue sports)|8-ball}} is a black ball numbered 8. It's a pun with Q-ball/cue ball. Unless undetected aliens have discovered billiards and become addicted to it, 8-balls are found only on Earth and are, hence, unlikely dark matter candidates. The 8-ball is also a popular unit of sale for black market pharmaceuticals like cocaine, where it stands for 1/8th of an ounce (3.5g). This doesn't make sense as a dark matter candidate either -- unless dark matter is hard to detect because it's illegal & trying to avoid the cops

;Space Cows
Cows are {{w|Bovinae|bovines}} extensively farmed on Earth for milk and meat. Although there is folklore concerning cows {{w|Hey diddle diddle|achieving circum-lunar orbits}}, not to mention their appearance on a {{w|Shindig (Firefly)|beloved space western TV show}}, as Muppet cow [http://muppet.wikia.com/wiki/Natalie Natalie] in the Sesame Street News Flash (and [https://tvtropes.org/pmwiki/pmwiki.php/Main/SpaceWestern others less-remembered]), they have yet to be found elsewhere in the Universe. In the television show "Too Close for Comfort", one of the characters is the cartoonist of a comic strip called "Cosmic Cow". {{w|Spherical cow|Spherical cows}} have also been used (humorously) by physicists needing to simplify some source of mass in a given problem.

;Obelisks, Monoliths, Pyramids
While those human constructions are huge on a human scale, they're negligible at universe-scale. It would take a large number of such constructions, distributed through space, to replicate the effects of dark matter; while a scenario could be envisioned where enough such constructs existed, with properties and distribution allowing them to match observations, this is obviously not a likely explanation.
They often show up in fiction and pseudo-scientific literature as alien artifacts generating immense unknown power out of nowhere, with the most famous and influential example being the three monoliths from {{w|2001: A Space Odyssey (film)|2001: A Space Odyssey}} (with the largest having a mass of about 500,000 tonnes).

;Black Holes ruled out by:
{{w|Black hole|Black holes}} are known to occur in sizes of a few solar masses (about 1030-1031 kg) as remnants of the core of former big stars, as well as in quite large sizes at the centers of galaxies (millions or even billions of solar masses). But recent gravitational wave detections indicate that black holes at 50 or 100 solar masses also exist, though their origin is still not understood. Randall doesn't mention this but some astronomers hope that these could fill at least a part of the gap. While black holes are widely reported to be ruled out as a candidate for dark matter for various reasons Randall has listed, such constraints are based on "monochromatic" mass distributions -- meaning that all such black holes are assumed to have the same mass -- which is considered physically implausible for populations of merging bodies which are known to have vastly different masses.[https://arxiv.org/pdf/1709.07467.pdf][https://arxiv.org/pdf/1705.05567.pdf] (That this is a common practice in cosmology may be part of the reference to "buzzkill" astronomers.) He rules out all black holes in the range of approximately 1010 kg to 1033 kg even when below some gaps at the bars appear.

Except the last item, all range below the mass of the sun (2x1030 kg) while the smallest known black hole is about four solar masses.
* Gamma rays: If dark matter were black holes of this size, the black holes could be evaporating by the predicted {{w|Hawking radiation}}, and we'd see a buzz of gamma rays from every direction if many of those objects would exist. Nonetheless this radiation is still hypothetical and not been observed on any known black holes. Furthermore those objects would very small because the Schwarzschild radius of a 1012 kg black hole is approximately 148 fm (1.48×10−13 m), which is between the size of an atom and an atomic nucleus.
* GRB lensing: {{w|Gamma-ray burst|Gamma-ray bursts}} (GRBs) are the brightest events in the universe and have been observed only in distant galaxies. While gravitational microlensing (see below) is an astronomical phenomenon, it doesn't make much sense here. GRBs are short (milliseconds to several hours) and are often detected only by space-borne sensors for gamma-rays -- rarely at any other wavelengths. Measuring lensing effects would be very difficult. This [https://arxiv.org/abs/1406.3102 paper] discusses the probability of detecting lensing effects caused by {{w|Dark matter halo|galactic halo objects}} among the known GRBs given sufficient objects to represent the missing mass.
* Neutron star data: {{w|Neutron star|Neutron stars}} aren't black holes, but they're also very small highly compact objects at about 1.4-2.16 solar masses. While black holes can't be observed directly, neutron stars are detectable in many wavelengths. The number of them gives a clue about the number of black holes close to the mass of the sun, a number which is far too low to make up dark matter.
* Micro lensing: {{w|Gravitational microlensing}} is a gravitational lens effect, (the path of radiation is changed by passing through space bent by nearby mass). This was predicted by Einstein's {{w|General Relativity|Theory of General Relativity}} and was first confirmed in 1919 during a solar eclipse, when a star which was nearly in line with the sun appeared more distant to the sun than usual. Astronomers have found many so called {{w|Einstein ring|Einstein rings}} or Einstein crosses where a massive object in front of other galaxies bends the light toward us. Those massive objects may be black holes, but the number is far too low to explain dark matter.
* Solar system stability: Our {{w|Solar system|solar system}} is 4.5 billion years old and has been very stable since shortly after its formation. If not, we wouldn't exist. If dark objects at 1024 kg - 1030 kg (mass of Earth up to mass of Sun) accounted for dark matter and were distributed throughout galaxies, there should be many of them in the vicinity of our solar system and the system wouldn't be stable at all.
* Buzzkill Astronomers: Black holes above a certain size would be impossible to miss, due to the effects they have on nearby matter.{{cn}} At the mass of some 1030 kg there must be many supernova remnants we still haven't found. Black holes of about 1035 kg have long been considered dark matter candidates by a minority group of cosmologists,[https://arxiv.org/pdf/1001.2308.pdf] and the Milky Way's first discovered intermediate mass black hole falls in this range.[https://www.nao.ac.jp/en/news/science/2016/20160115-nro.html]

;Maybe those orbit lines on space diagrams are real and very heavy
Diagrams of our solar system (or any planetary system) often show lines representing the elliptical paths the planet takes around its sun. These lines don't show real objects, though. Astronomers just draw them on pictures of the solar system to show where the planets move. If you draw a line on a map to give someone directions, that line isn't an object in real life; it's just on the map. If these lines were real, they would be ''huge'' (Earth's would be 940 million km long (2π AU) and Neptune's would be 28 ''billion'' kilometers long). [https://www.youtube.com/watch?v=0fKBhvDjuy0 Powers of Ten (1977)] gives a good sense of just how large these orbit lines need to be in order to be visible in space diagrams. If these orbit lines were also very dense, they would have a huge mass and could possibly account for the missing 85% of the mass in the universe. But they would also constantly be impaling the planets, including the Earth, which would probably be a problem. Their mass would also affect planetary motions in ways which we would detect. A related worry about space travel was expressed in previous centuries; it was thought that the planets were embedded within {{w|Celestial spheres|crystal shells}} (spheres or Platonic solids), and a rocket into space could smash the shells and send planets plummeting to Earth. Another joke candidate.

;Title text
The title text refers to the fact that space is just vast emptiness where a little bit of dirt could be overlooked. Actually the mean density of detectable matter in the universe, according to NASA, is equivalent to roughly [https://map.gsfc.nasa.gov/universe/uni_matter.html 1 proton per 4 cubic meters]. And because this matter is mostly located in galaxies -- and inside there in stars and clouds -- the space between is even more empty. For comparison, one gram hydrogen consists of {{w|Avogadro constant|6.022 x 1023 atoms}}. Like at home wiping with a cleaning cloth in which we can see the dirt that wasn't clearly visible on the surface we have wiped, Randall believes that some few atoms more per cubic meter could stay undetected in the same way. This isn't true because in the space between galaxies astronomers can detect matter as it spreads over thousands or millions cubic light years. Atoms can't hide; there is always radiation.

==Transcript==
:Dark matter candidates:
:[A line graph is shown and labeled at left quarter in eV and further to the right in g together with some prefixes.]
:[The labels read:]
:µeV, meV, eV, keV, MeV, GeV, TeV, 10-18kg, ng, µg, mg, g, kg, TON, 106kg, 1012kg, 1018kg, 1024kg, 1030kg

:[All items are shown in bars ranging between two approximately values:]
:< 1 µeV - 10 meV: Axion

:1 eV - 10 keV: Sterile neutrino

:0.5 MeV (exactly): Electrons painted with space camouflage

:10 GeV - 10 TeV: Neutralino

:100 TeV - 10-17 kg: Q-ball

:1 ng - 100 ng: Pollen

:0.1 mg - 1 mg: No-See-Ums

:10-1 g (exactly): Bees

:10 g - 100 g: 8-balls

:100 kg - TON: Space cows

:TON - 109 kg: Obelisks, monoliths, pyramids

:109 kg - 1033 kg: Black holes ruled out by:
::109 kg - 1013 kg: Gamma rays
::1013 kg - 1017 kg: GRB lensing
::1015 kg - 1022 kg: Neutron star data
::1021 kg - 1030 kg: Micro lensing
::1024 kg - 1030 kg: Solar system stability
::1030 kg - 1033 kg: Buzzkill astronomers

:1033 kg - >1036 kg: Maybe those orbit lines on space diagrams are real and very heavy

{{comic discussion}}

[[Category:Science]]
[[Category:Physics]]
[[Category:Astronomy]]
[[Category:Line graphs]]

Talk:1867: Physics Confession

2017-07-24T12:08:14Z

172.68.132.5:

[[Special:Contributions/172.68.141.190|172.68.141.190]] 07:54, 24 July 2017 (UTC)F1rst P0st http://www.explainxkcd.com/wiki/index.php/269:_TCMP

Fuckin' ice skates, [https://youtu.be/_-agl0pOQfs?t=1m52s how do they work]? [[User:OldCorps|OldCorps]] ([[User talk:OldCorps|talk]]) 11:26, 24 July 2017 (UTC)

Any relation to #1489? They're both about things physicists don't understand. http://www.explainxkcd.com/wiki/index.php/1489:_Fundamental_Forces

Talk:1810: Chat Systems

2017-03-13T18:44:06Z

172.68.132.5:

"Wall (bathroom)" might be a reference to the Spaceballs movie. President Skroob is using the bathroom when he gets a video call from one of his officers. "Ahh! I told you never to call me on this wall! This is an unlisted wall!" [[Special:Contributions/172.68.54.112|172.68.54.112]] 16:31, 13 March 2017 (UTC)

From the explain section (User: Cosmogoblin): See [https://www.dropbox.com/s/8xpd3iggd47x51q/1810.ods this spreadsheet on Dropbox] for a list of each person in the diagram, as a basis for more complex analysis.--[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 16:48, 13 March 2017 (UTC)

Man, IRC is not old. I remember using it at college in 1996... Oh, wait.{{unsigned ip|172.68.26.65}}

I'm pretty sure that says '''Wall (Unix)''', not '''Wall (Linux)'''. --[[Special:Contributions/162.158.79.131|162.158.79.131]] 17:16, 13 March 2017 (UTC)

He left off the chat tab on wikipedia :o){{unsigned ip|172.68.86.100}}

[https://en.wikipedia.org/wiki/Euler_diagram#/media/File:Euler_and_Venn_diagrams.svg Humorous diagram comparing Euler and Venn diagrams]--[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 18:06, 13 March 2017 (UTC)

To whom are the individuals unique to some sets talking to? eg those in Apache Request logs, and wall (unix) and wall (bathroom)? I suppose there is no reason to assume anyone is receiving their messages.......[[Special:Contributions/108.162.241.160|108.162.241.160]] 18:37, 13 March 2017 (UTC)

This seems related to #1254, and maybe #1789 as well. Randall really has a problem with his friends' bizarre methods of communicating.[[Special:Contributions/172.68.132.5|172.68.132.5]] 18:44, 13 March 2017 (UTC)