explain xkcd - User contributions [en]

815: Mu

2020-09-02T23:28:47Z

Jammo2000: Removed unnecessary quote marks

{{comic
| number = 815
| date = November 5, 2010
| title = Mu
| image = mu.png
| titletext = As the CoKF approaches 0, productivity goes negative as you pull OTHER people into chair-spinning contests.
}}

==Explanation==
The title of the comic, "Mu", refers to the symbol μ. This letter of the Greek alphabet is commonly used in mathematics and physics in many cases and here it denotes the {{w|coefficient of friction}} which describes the ratio of the force of friction between two connected bodies.

Desk chairs usually have the ability to turn and some chairs spin more easily than others. A desk chair which spins easily could be described as having a low coefficient of friction. The horizontal axis of the chart ranges from very easy to spin on the left, to very difficult to spin on the right. The comic shows that if the chair is too difficult to turn it is annoying and impacts productivity. However, if it is too low spinning one's chair becomes more fun than working.

The title text notes that if your chair spins too easily, you can actually hurt other people's productivity by spinning competitively.{{Citation needed}}

In {{w|classical mechanics}} the {{w|angular momentum}} can be transferred to other objects when a rotating object does not have any friction and is rotating very fast. For example, when a {{w|reaction wheel}} inside a spacecraft changes its speed, it turns the entire satellite around. None of this is intuitive, as shown in this [http://www.youtube.com/watch?v=NeXIV-wMVUk video].

==Transcript==
:[Cueball spins in circles on a chair next to a desk. A graph of productivity vs Coefficient of friction of desk chair shows a curve that drops off very quickly as the coefficient of friction approaches zero, with the productivity becoming negative at low values. It plateaus in the middle of the graph, and then begins to drop less steeply as coefficient of friction increases above the optimal point.]
:Cueball: Wheeeeeeee

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Line graphs]]
[[Category:Physics]]

2283: Exa-Exabyte

2020-03-25T15:05:25Z

Jammo2000: Added link to comic #2091 for citation that large numbers are hard to imagine

{{comic
| number = 2283
| date = March 20, 2020
| title = Exa-Exabyte
| image = exa_exabyte.png
| titletext = To picture 10^18, just picture 10^13, but then imagine you connect the left side of the 3 to close off the little bays.
}}

==Explanation==
{{incomplete|Created by 10 EXA-EXABYTES OF APPLES. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}
This is Randall's first comic in over a week not overtly part of his [[:Category: COVID-19|COVID-19 series]]. It could still be a deliberate allusion to the biology and complexity behind the Coronavirus outbreak, or, if not a deliberate allusion, its theme of biological complexity could have been inspired thereby.

This is a comic about the difficulty of picturing or understanding large numbers. As mentioned in the comic, an {{w|exabyte}} is 1018 bytes, while an "exa-exabyte"—not a common word, but one that makes sense if you apply the principles of {{w|metric prefix}}es—is 1036 bytes. 1036 is properly given the name undecillion (in short scale, and sextillion in long scale).
According to [https://www.nytimes.com/2015/07/21/science/counting-all-the-dna-on-earth.html a 2015 article] by ''The New York Times'', researchers estimate that there are about 5 * 1037 DNA {{w|base pair}}s on Earth (50 trillion trillion trillion). So [[Miss Lenhart]]'s claim of 10 exa-exabytes—1 * 1037 bytes is a reasonable approximation ({{w|Fermi estimation}}). (The estimate was 5 plus or minus 4 * 1037. There are 4 possible base pairs, or 2 bits per pair, a byte is 8 bits.)

These numbers are larger than most people can imagine. Even much smaller numbers such as a billion (109) or a trillion (1012) are [[2091: Million, Billion, Trillion|hard to imagine.]] For instance:
* 1 billion seconds is equal to 31.7 years; 1 trillion seconds is equal to 31,688.74 years.
* [https://medium.com/@alecmuffett/a-billion-grains-of-rice-91202220e10e 1 billion grains of rice] weigh approximately 34,447 lb (15,625 kg).

Wikipedia has an article on the {{w|exabyte}} and one on large numbers which describes {{w|Orders of magnitude (numbers)#1018|various things close to 1018}}.
* [https://abc7news.com/science/possibly-habitable-planet-found-100-light-years-away/5821548/ TOI 700 d], a potentially habitable Earth-like {{w|exoplanet}} is 100 light years away, which is about 1018 meters.

[[Cueball]] expresses his difficulty in visualizing a number even as large as ''one'' exabyte (1018 bytes).

[[Megan]] trivializes the problem away by describing an exabyte as 10 apples, with "18 smaller apples, floating next to them and a little above", representing the notation 1018 using apples for digits. This is entirely unhelpful, as using apples in a [https://en.m.wikipedia.org/wiki/Unary_numeral_system base-1] enumeration offers no obvious advantages over base-10 in understanding exponents; Megan's bad advice & Cueball's seemingly ready acceptance of it causes Miss Lenhart to yell out "No!" in frustration.

The title text further trivializes the problem of visualizing large numbers by suggesting that you can visualize 1018 as a number by simply visualizing the similar-looking number of 1013 with some extra lines drawn to turn the 3 into an 8. Changes in exponents can cause huge changes in the value shown, and this is no exception: Changing that 3 into an 8 changes the value by a factor of 100,000.

Randall has previously discussed the difficulty of large numbers in [[2091: Million, Billion, Trillion]], [[1894: Real Estate]], and [[558: 1000 Times]].

[[1605: DNA]] also discusses how "hard" biology is.

==Transcript==
:[Miss Lenhart is holding a pointer, and is pointing it towards a blackboard behind her, while she addresses her student Cueball who is sitting on a chair at a desk to the left of her, holding his hands on his knees.]
:Miss Lenhart: Biology is hard because there's so ''much'' of it. Earth hosts about 10 exa-exabytes worth of DNA.

:[In a frame-less panel, the panel has panned to the left and is now showing Miss Lenhart holding the pointer to her side, but without the blackboard. In front of her is now both Cueball and Megan sitting at their desks. Cueball has taken one hand on to the table. Megan has both hands folded on the table in front of her.]
:Cueball: What's an exa-exabyte?
:Miss Lenhart: It's 1036 bytes.
:Cueball: How do I picture '''''that?'''''
:Miss Lenhart: Imagine you had an exabyte of data, but each byte ''contained'' an exabyte of data.

:[Zoom in on Cueball's head. A starburst to the right indicates Miss Lenhart's voice from off-panel.]
:Cueball: I can't even picture what an exabyte is.
:Miss Lenhart (off-panel): It's 1018 bytes.
:Cueball: But how do I picture 1018?

:[Zoomed out to showing Megan, Cueball, and Miss Lenhart along with the blackboard. Megan has raised a hand palm up. Cueball is looking back at her over his shoulders. Miss Lenhart is forming a closed first with her empty hand, the one without the pointer.]
:Megan: Imagine you had 10 apples.
:Megan: Now imagine 18 smaller apples, floating next to them and a little above.
:Cueball: Cool, got it.
:Miss Lenhart: '''''No!'''''

{{comic discussion}}

[[Category: Comics featuring Miss Lenhart]]
[[Category: Comics featuring Cueball]]
[[Category: Comics featuring Megan]]
[[Category: Biology]]

2274: Stargazing 3

2020-03-03T18:45:10Z

Jammo2000: This is the brightest star i'm currently talking about is always true.

{{comic
| number = 2274
| date = February 28, 2020
| title = Stargazing 3
| image = stargazing_3.png
| titletext = If we can destroy enough of the lights in our region, we may see more comets, but that's a risk we'll have to take.
}}

==Explanation==
{{incomplete|Created by a TERRIFIED LAMP. The explanation, for the most part, doesn't exist. Do NOT delete this tag too soon.}}

This is the third in the Stargazing series. The first [[1644: Stargazing]] appeared four years earlier and the second [[2017: Stargazing 2]] one and a half years earlier.

{{w|Vega}} is a star in the constellation of {{w|Lyra}}. It does indeed have {{w|Magnitude (astronomy)|magnitude}} 0.03 and is the brightest star mentioned in this comic. Vega is only the {{w|List of brightest stars|5th brightest star}} (outside of the Sun), as {{w|Sirius}} is the brightest visible star. The phrase <q>It's the brightest star I'm currently talking about</q> is an example of the technically correct but not at all useful information that is typical of the Stargazing series. The phrase is true no matter what, because any star one talks about is the brightest star one is talking about, as any brighter star becomes the one talked about when mentioned.

{{w|Polaris}} is indeed the star over the North Pole, and is commonly called the North Star or the Pole Star. It is the brightest star in the constellation Ursa Minor, but there are about fifty other stars that are as bright as it is (magnitude 2), so it's not really remarkable apart from being the pole star, as the host says. Despite the fact that being the pole star is "all it has going for it," it is nevertheless very important because it is used for navigation, as it appears fixed in the night sky. It hasn't always been and won't always be the pole star, however, as Earth's axis precesses in a 26,000 year cycle.

{{w|Comets}} are comparatively small clumps of rock and ice, seen mostly by the long, lit 'trail' of particles the heat of the sun causes to be ejected, and the solar wind then spreads outward in thin glowing lines that ''can'' be larger and more visible even than the constellations they are seen in front of - at least during the brief phase of their closest approach to the sun. Many comets have usually highly elliptical orbits around the sun and so they are seen "every few decades". Yelling at comets is believed to be an ineffective way to make them go away.{{Citation needed}} The host may dislike comets because of their history in superstition of being seen as a sign of doom. This provides humor because typically this superstitious fear was caused by a lack of understanding, and it would be expected that a stargazing host would be informed on and therefore unafraid of comets. No actual astronomers are bothered by comets{{Citation needed}}, but some are upset about [https://skyandtelescope.org/astronomy-news/starlink-satellites-astronomy/ satellite megaconstellations] such as {{w|SpaceX Starlink|SpaceX's Starlink}}. In that case, astronomers are not yelling at the satellites, but at the companies that launch them.

{{w|Light pollution}} is indeed a problem with stargazing. Light pollution is the presence of artificial light in the night sky, which makes it very difficult to see stars. Stargazing in remote locations is remarkably different than in populated cities. Light pollution was previously discussed in [[2121: Light Pollution]]. Light pollution does not actually make the "sky go away", but it does affect how humans can see stars or other astronomical features in the sky.

The host advocates an active approach to resolving light pollution—rather than lobbying for reductions in artificial lighting, as the {{w|dark-sky movement}} does, she intends to lead her audience in destroying artificial lights. Older lightbulbs are usually glass bulbs filled with inert gas (for incandescent bulbs) or high-pressure gases (for e.g. {{w|sodium-vapor lamp}}s) and so are easy to destroy with any blunt impact, thus accounting for the host's mention of "throwing rocks at them". Modern LED lights, however, are much more robust, which is why she is handing out crossbows to achieve greater projectile energy. An "[http://sonic.net/~rknop/php/astronomy/classes/a103/sum2006/info/angdist.shtml#xbow astronomy crossbow]" is a tool used to measure the angular distance between stars. They cannot shoot real {{w|crossbow bolt}}s, but any type of crossbow or other weapon could be used to destroy lights and "preserve" the sky. (Speaking of astronomy tools that have weapon-related names, there is a type of telescope called a "{{w|Sun Gun Telescope|Sun Gun}}", but it is only meant to be used during the day to enable groups of people to view the Sun safely. It is probably best that this host's show is taking place at night, or else she might cause even more trouble.)

The title text mentions that the destruction of lights, which they advocate for throughout this comic, would allow them to see more comets. While this is true, they seem to perceive comets as a risk for no apparent reason, perhaps related to their habit of yelling at comets to go away.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:[In a dark panel, a TV host stands in front of a group of people: Science Girl, Ponytail, and Cueball; the panel is inverse-colored, i.e. white text and drawings on black]
:Host: Welcome back to Stargazing.
:Host: There are no new stars since last time, but you came back for some reason.

:[Zoomed out on the same scene, the host is now with Science Girl, Ponytail, Cueball, Megan, and White Hat. The host is pointing upwards with her left hand.]
:Host: That star is Vega. At magnitude 0.03, it's the brightest star I'm currently talking about.
:Host: That one is Polaris. It's over the North Pole, which is all it has going for it.

:[A frame-less white panel, zoomed in on the host, who is now pointing upwards with her right hand.]
:Host: That's a comet. Some of them come back every few decades, no matter how much I yell at them.
:Host: But stargazing isn't all fun yelling. We face a problem even worse than comets: light pollution.

:[Back to a dark panel, the host now has a big bag of crossbows. The bag has a logo of a crossbow with stars around it. She has taken out one of them and is holding it in her right hand.]
:Host: The sky is going away because people keep shining lights at it. The new LEDs are even worse - they're too blue, and you can't turn them off by throwing rocks at them like with the old ones.
:Host: Luckily, I brought these astronomy crossbows.
:Host: Take one, then let's fan out and look for lamps.

{{comic discussion}}

[[Category:Comics with inverted brightness]]
[[Category:Astronomy]]
[[Category:Stargazing]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring White Hat]]
[[Category:Comics featuring Megan]]
[[Category:Comics featuring Science Girl]]
[[Category:Crossbows]]

2274: Stargazing 3

2020-02-28T23:35:14Z

Jammo2000: