explain xkcd - User contributions [en]

1297: Oort Cloud

2022-05-04T01:59:50Z

Kuilin: Undo revision 233797 by X. K. C. D. (talk)

{{comic
| number = 1297
| date = November 29, 2013
| title = Oort Cloud
| image = oort_cloud.png
| titletext = ...I wanna try. Hang on, be right back.
}}

==Explanation==
The {{w|Oort cloud}} is a hypothesized sphere containing many {{w|small Solar System bodies}}, reaching out to roughly 50,000 {{w|Astronomical unit|AU (astronomical units)}} or nearly one {{w|light-year}} from the sun. Gravitational forces from passing stars or collisions with other objects sometimes perturb one of these bodies enough to let it fall into the inner solar system. When it gets closer to the Sun, which is just a bright dot at that far distance, it warms up and some of its mass is lost as gas and dust, making it more visible as an object commonly referred to as a comet. This is what has happened to a comet called {{w|C/2013 UQ4}}, AKA Comet Catalina. And although this is not what will happen to Catalina, comets that get close enough to the sun may break up entirely.

There seems to be no definitive astronomical definition of the word "comet", and definitions can be challenging and problematic [http://suitti.livejournal.com/56460.html?nojs=1], but in general terms a comet is a celestial object consisting of a nucleus containing a huge amount of ices and dust which, when near the sun, has an atmosphere (called {{w|Comet#Coma and tail|coma}}) and perhaps a 'tail' of ionized gas and dust particles pointing away from the sun.

The comet pictured here upon its return strangely resembles the unusual asteroid {{w|P/2013 P5}}. That object sported six comet-like tails, but it's not a comet. Rather, the six comet-like tails were suspected to be caused by rapid spinning of that object.

Randal has drawn the hapless Oort Cloud object with its tails generally left of frame, i.e. away from the sun. Comet tails point away from the sun regardless of their direction of movement, as they are blown out by the solar wind which moves much faster than the comet. As neither of the other two objects have tails, this lends the picture a comical cartoon-like quality, as when Yosemite Sam is blasted by his own gun and it leaves his moustache tails statically pointing away from the direction of the blast.

{{w|C/2012 S1|Comet ISON}} presumably came from the Oort cloud and reached its closest approach to the sun ({{w|Apsis|perihelion}}) on the day before this comic was published. The comet passed very close to the sun, at a distance of 1,860,000 kilometers or 1,150,000 miles from the centre of the sun. It was thus within one sun-diameter of the surface of the sun itself (diameter of sun = 1,391,000 km). At that distance the temperature, at approx. 2,700 degrees Celsius, vaporizes rock as well as ice and can break the comet apart entirely.

The broken-up object here is presumed to be ISON, and is labeled as such in the transcript, even though Randall hasn't unambiguously identified it. Note that it's not realistic that ISON still would have a tail so far away from the sun.

On December 2, 2013 NASA released a statement that ISON did not survive its close perihelion with the sun. The Comet ISON Observing Campaign posted a delightful biographical sketch (In Memoriam Comet C/2012 S1 (ISON) Born 4.5 Billion BC, Fragmented Nov 28, 2013, age 4.5-billion yrs old) which touches on its early years, retreat to the Oort Cloud, career as a Sungrazer, "dynamic and unpredictable life, alternating between periods of quiet reflection and violent outburst", delicate inner working, and its tragic demise.

The closest approach of ISON to the earth was predicted for December 27, 2013 at a distance at approx. 60 million kilometers or 37 million miles, 170 hundred times more than the moon. The {{w| Hubble Space Telescope}} [http://www.universetoday.com/107407/hubble-looks-but-finds-no-trace-of-comet-ison/ looked for it on December 18] but saw nothing.

This video shows an animation of the encounter at the sun: [http://stereo-ssc.nascom.nasa.gov/browse/2013/11/28/ahead_20131128_cor2_rdiff_512.mpg ISON 28.11.2013].

===Perspective===
Some more information about comets will help put the comic in perspective:
*The surface of cometary nuclei reflects less sunlight than asphalt. Telescopes can't identify or find them until they have a coma.
*An object at a distance of one light-year would only have an orbital speed about 100 meters per second; the speed of the Earth is about 30 kilometers per second.
*One revolution at that distance would last approximately 20 million years.

==Transcript==
:[Three asteroids float in space.]
:ISON: Have you noticed that bright dot in the distance?
:Asteroid: Yeah. What's the deal with it?

:ISON: Dunno. I'm gonna go check it out.

:[Pause while ISON checks it out off screen.]

:ISON (broken up, with multiple tails): Wow. Do NOT go over there.

{{comic discussion}}
[[Category:Astronomy]]

718: The Flake Equation

2022-05-04T01:59:32Z

Kuilin: Undo revision 233791 by X. K. C. D. (talk)

{{comic
| number = 718
| date = March 24, 2010
| title = The Flake Equation
| image = the flake equation.png
| titletext = Statistics suggest that there should be tons of alien encounter stories, and in practice there are tons of alien encounter stories. This is known as Fermi's Lack-of-a-Paradox.
}}

==Explanation==
This strip parodies the {{w|Drake equation}}, which is an method for estimating of the number of detectable extraterrestrial civilizations in our galaxy. The Drake equation starts with the best estimate for the number of stars in our galaxy, then multiplies it by successive probabilities (such as the number of stars with planets, the number of planets which can support life, etc), to ultimately calculate how many civilizations exist. While such a calculation necessarily uses speculative numbers, it gives a good sense of how many civilizations could potentially exist.

The Flake equation presented in this strip provides an estimate about how many false or fake stories ''about'' aliens are likely to exist. It does so in similar manner as the Drake equation, by starting with the entire population, estimating how many people are likely to believe that they've had an alien encounter, and then calculating how likely those stories are to become public. Just like in the Drake equation, exact numbers are unknown, but can be estimated, and the equation in the comic shows [[Randall|Randall's]] guesses about these values. See an [[#Explanations of values|explanations of values]] below.

"Flake" is American slang for a person who is casually dishonest or unreliable, implying that such a person would be likely to imagine an alien encounter. Note that, while the Flake equation includes people who imagine encounters "because they're crazy or want to feel special", it doesn't attempt to include outright lies or deliberate hoaxes.

The final results tells us that there should be about 100,000 stories about aliens that have reliable explanations. (The numbers given in the equation gives 126,000 stories). The data is obviously highly speculative, and as with the Drake Equation, you can plug in your own numbers, but if you keep your guesses realistic, you will most likely get a very large number. This convinces the reader that the fact that there are many stories about aliens does not necessarily mean that many people actually met aliens.

The title text refers to Fermi's Lack-of-a-Paradox. The {{w|Fermi paradox}} refers to the contradiction between high numbers of calculated civilizations and the total lack of verified alien contact with earth. This is related to the Drake Equation, many estimates calculate that there should be large numbers of civilization in the galaxy, and they should have existed for long periods of time, suggesting that humanity should have been contacted by them, or at least seen some clear evidence of their existence. There are multiple explanations for this paradox, but it remains a question of scientific debate. The Lack-of-a-Paradox in this strip, however, is that the math suggests that there should be huge numbers of claimed alien sightings, and that's exactly what we observe.

Another comic parodying this equation is [[384: The Drake Equation]]. The credibility of paranormal reports in general is revisited in [[1235: Settled]], which posits that if such phenomena were real they should have been unambiguously captured on camera by now.

===Explanations of values===
{| class="wikitable"
!Symbol
!Assumed value
!Explanation
|-

|WP
|7,000,000,000
|World population at the time of the creation of the comic, taken as a starting value.
|-
|(CR + Mi)
|1/10,000 + 1/10,000
|Fraction of people who would falsely believe they had been visited by aliens. This is attributed to either a person imagining an encounter, and believing that it was real (due to mental illness or a desire to feel special), or to people misinterpreting something as an alien encounter (this can include possibilities as broad as unusual lights in the sky to actual hallucinations). It is estimated that one person in ten thousand falls into each of these categories, suggesting that one person in five thousand either has or will, at some point, believe they've encountered aliens.
|-
|TK
|1/10
|The fraction of people who believe they have experienced an alien sighting that tell others about their experience. [[Randall]] estimates (rather conservatively) that 90% of people who believe they've encountered aliens will keep quiet about it (likely out of fear of not being believed), and only one in ten will talk about their 'experience'. Multiplying with the previous values we get the of first-hand accounts of alien encounters.
|-
|F0
|10
|Average number of people they tell about their "sightings". Multiplying with the previous values we get the number of people who hear about an alien sighting from the "primary source".
|-
|F1
|10
|Average number of people that they decide to tell about the "firsthand" account. Multiplying with the previous values we get the amount of people who hear a second-hand account of a false story.
|-
|DT
|9/10
|The probability that the details will be slightly adjusted during the retelling process, making the account believable. [[Randall]] estimates that 90% of accounts that are actually shared have detailed changed when they're retold. This is exceptionally common when stories are passed from person to person, it's rare for all the details to survive unchanged. In this sort of case, "not fitting the narrative" implies that some details will be unbelievable, or falsifiable, or will be insufficiently dramatic, and those tend to morph over time (often innocently, as people don't remember the original version perfectly). Multiplying this probability by the previous numbers gives number of believable-yet-false alien sighting stories in circulation.
|-
|AU
|1/100
|The proportion of people who have the willingness and ability to share this story with a broad audience. [[Randall]] assumes that the overwhelming majority of people who hear such stories either have no platform to share stories to more than a handful of people at a time, or aren't willing to share these stories. But enough people in modern times have broad audiences (this number includes people with internet audiences), that it's estimated that 1% of the population both can and wants to share second-hand accounts of alien encounters. The total is now the amount of believable-yet-false alien sightings that are published to a wider audience.
|}

==Transcript==
:{{incomplete transcript}}
:The Flake Equation:
:P = WP × (CR + MI) × TK × F0 × F1 × DT × AU ≈ 100,000
:Where:
::WP = World Population (7,000,000,000)
::CR = Fraction of people who imagine an alien encounter because they're crazy or want to feel special (1/10,000)
::MI = Fraction of people who misinterpret a physical or physiological experience as an alien sighting (1/10,000)
::TK = Probability that they'll tell someone (1/10)
::F0 = Average number of people they tell (10)
::F1 = Average number of people each friend tells this "firsthand" account (10)
::DT = Probability that any details not fitting the narrative will be revised or forgotten in retelling (9/10)
::AU = Fraction of people with the means and motivation to share the story with a wider audience (blogs, forums, reporters) (1/100)
:Even with conservative guesses for the values of the variables, this suggests there must be a ''huge'' number of credible-sounding alien sightings out there, available to anyone who wants to believe!

{{comic discussion}}
[[Category:Math]]
[[Category:Astronomy]]
[[Category:Science]]
[[Category:SETI]]
[[Category:Aliens]]
[[Category:Statistics]]
[[Category:Paranormal]]

970: The Important Field

2022-05-04T01:58:23Z

Kuilin: Undo revision 235605 by X. K. C. D. (talk)

{{comic
| number = 970
| date = October 28, 2011
| title = The Important Field
| image = the important field.png
| titletext = I hear in some places, you need to fill one form of ID to buy a gun, but two to pay for it by check. It's interesting to see who has what incentives to care about what mistakes.
}}

==Explanation==
In this comic, a soldier with a green hat with a black emblem on the front, is using his computer to access an online web interface to launch a missile at a target. The joke is that even though the interface only asks him to enter the target coordinates once, it asks for his email address twice, even though the coordinates are by far the more important detail to get right (launching the missile at the wrong target could result in a disastrous loss of life or property damage).

It is common for online interfaces to force users to type certain details twice, as a form of redundancy checking to ensure that the user really has entered the correct details and hasn't made an error. Some forms even go the extra step of preventing the user from copy-pasting into the second field, which would render it useless as a redundancy check.

This is usually done for email addresses and when creating new passwords, which are used to identify and authenticate users, and are therefore important to get right.

In the title text, [[Randall]] suggests that the presence of redundancy checks can give you an interesting insight into what things people deem to be important. He gives a (supposed) real-life example of a merchant that requires only one form of ID in order to buy a gun, but two forms if you want to pay for it by check - suggesting that the seller is more worried about the safety of their money than the potential danger of giving a lethal weapon to someone untrustworthy.

==Transcript==
:[A soldier with a green military hat, with an black emblem on the front, is sitting in an office chair typing at his computer. Sounds are shown when he types, and the message he reads on the screen is shown above with a zigzag line coming from the computer screen. In the first panel he only uses one hand on the keyboard.]
:Computer: Welcome to the missile launch web interface!
:''Click''

:[Same setting but the soldier types with both hands.]
:Computer: Enter the target's coordinates.
:''Type Type''

:[Same setting as previous panel.]
:Computer: Enter your email address for our records.
:''Type Type''

:[Same setting except the soldier has stopped typing.]
:Computer: Enter your email again, to ensure you typed it correctly.

==Trivia==
*This is the only comic so far to feature a soldier using such a green Hat.
**The green hat with the black emblem implies that he is part of the military. The color code of his hat is #123E0E.
**Until realizing that the hat is green not black it could look like [[Black Hat]].
***Although the hat seems in the first panel to only have a lie going towards the front of the cap, this changes in the later images, making it here look more like a Black Hat type of hat.

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Characters with hats]]
[[Category:Sarcasm]]
[[Category:Email]]
[[Category:Computer security]]