explain xkcd - User contributions [en]

2971: Celestial Event

2024-08-14T14:48:58Z

Murkledroid: Changed link to "nifty tool from NASA" to the page offering the tool for people globally, rather than the page just for North America, Central America and the Caribbean

{{comic
| number = 2971
| date = August 12, 2024
| title = Celestial Event
| image = celestial_event_2x.png
| imagesize = 471x300px
| noexpand = true
| titletext = If we can get a brood of 13-year cicadas going, we might have a chance at making this happen before the oceans evaporate under the expanding sun.
}}

==Explanation==
{{incomplete|Created by a CURSED SHOP THAT APPEARS EVERY FOUR POINT THREE BILLION YEARS - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

This comic was posted shortly after some people reported seeing the aurora in conjunction with the perseids meteor shower.<ref>https://www.msn.com/en-us/weather/topstories/the-2024-perseid-meteor-shower-and-northern-lights-overlapped-in-a-rare-cosmic-display-see-photos-of-the-dazzling-event/ar-AA1oJKKC</ref> It lists several events that are considered special to be witnessed due to their rarity. Total solar eclipses, auroras and comet sightings are all rare events. This year, all three of these events happened in parts of Massachusetts, United States, where the author lives.

Although it might be, conceivably, possible to witness all three at once in a given location (in this case, the author's neighborhood), the odds stack up to make that occurrence extremely improbable. To boot, those events can only be observed with a clear sky (a 50:50 chance), so that too has to be accounted for in the calculation.

"Total eclipse" presumably refers to total solar eclipses, as total lunar eclipses are visible from any side of the earth that the moon is visible. Using [https://eclipse.gsfc.nasa.gov/JSEX/JSEX-index.html this nifty tool from NASA], we can estimate the frequency of solar eclipses for a given location. For example, for the city of Boston, MA, there are 14 total solar eclipses in a 4500 year interval, an average of once every 320 years, which is close to the author's estimate. It must be noted that the author did not include annular eclipses, possibly because it would be harder to witness the aurora and the comet sighting during those. Also, unlike the other factors, the 1/350 implies that a total eclipse can be seen for the whole year, when in fact that variable would be (when generous) a mere hour every 350 years.

17-year {{w|Periodical cicadas|cicadas}} are also special in the sense that a brood will only emerge from the ground once every 17 years. Periodical cicadas recently became the object of media furor in 2024 as a 13-year brood and a 17-year brood happened to emerge together in parts of the US, an event that for the same two broods only happens once every 13x17 = 221 years. This caused a lot of noise and double the amount of dead cicadas after they had mated. Needless to say, having a cicada emergence co-occur with all those previously mentioned events would be extra rare, and thus extra special.

How rare, exactly, is the point of this calculation. The resulting product is the expected frequency that all of them would occur at the same time at that location. The value he calculates is once every 4.3 billion years. This is in the same ballpark as the current age of the Earth, about 4.5 billion years.

There are multiple inaccuracies in this type of calculation (though, given the extravagant nature of the proposed event and the unfeasible time scale, perhaps that hardly matters). Multiplying probabilities only works for random variables that are entirely independent. If nothing else, orbits are (luckily[maybe]
) not random.{{cn}} It also requires that all of the probabilities remain constant over time. In reality, cicadas will not exist for very long compared to the time scale, since Earth will become uninhabitable to complex life within a billion years' time and all life will be extinct within {{w|Future of Earth|4 billion years}}. Also, the moon is moving away from Earth, and total solar eclipses will cease to occur in about [https://www.space.com/37627-total-solar-eclipse-earth-moon-alignment-future.html 600 million years]. Luckily, this is not the time that you are ''always'' going to wait, merely the (usual) period between one occurance and the next. A person starting to wait at a random point in the cycle, and not knowing anything else, would ''on average'' only have to wait ''half'' the time. (If very lucky, it could happen tomorrow, as it hypothetically might have done a bit over four billion years ago; if unlucky, it would indeed be slightly more than four billion years, having most recently happened yesterday; if ''very'' unlucky, the frequencies are slightly less defined, do not actually align as expected for the next conclusion of the cycle and additional billions of years need to be waited until the next example when it 'might' indeed occur as anticipated. Finally, if '''extremely''' unlucky, you will never get a clear sky. Ever.

In order for the math to work out, some unit conversion is necessary. To solve "20 days / 11 years", one can consider 1 year = 365.24 days (a reasonable approximation that accounts for most leap years). For "2 months / 50 years" and "2 months / 17 years", the simplest way is to convert 1 year into 12 months. Unit cancelation works out, and you end up with a number in years that corresponds to the average amount of time between events when all those different things are happening at once. (the implied unit for eclipses is events per year, and that's the unit you get as a result, so the actual length of the eclipse doesn't influence the result much).

In the title text, Randall mentioned swapping 17-year cicada broods for 13-year ones, to have some chance at witnessing the proposed super-event before life on Earth becomes impossible. Massachusetts is near the northern limit of {{w|Periodical_cicadas|Magicicada}} distribution, and only one 17-year brood is established there (and not in Cambridge, MA). However, the introduction of a different brood could, with some help from global warming, be feasible. There are other variables that are possibly available for manipulation: clear skies can be arranged with various methods of {{w|Weather modification|weather control}}; special cicada broods may be bred or bioengineered specifically to coincide with the other factors of the celestial event; and while the aurora cannot currently be manipulated by humans, there may be artificial methods to make it possible.

Earth's oceans may evaporate in about one billion years<ref>https://www.sciencedaily.com/releases/2013/12/131216142310.htm</ref>. In order to beat that, we need to better our odds. Using 13-year cicadas in our calculations reduces the average interval between events to 3.29 billion years. We can lower that further by hoping that we'll have clear skies by then (who knows, we might get good enough at manipulating weather that we can *make* it happen). That gives us an average interval between events of about 1.6 billion years. Which means a larger than 50% chance that we'll get our special super-event to happen within a billion years, therefore beating ocean evaporation. Of course, cicadas may not last that long.[baseless conjecture] [trust the cicadas]

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:Approximate frequency in my area

:Active northern lights: 20 days per solar cycle

:A naked-eye "Great Comet": 2 months every 50 years

:Total eclipse: once every 350 years

:Clear skies: 50% of the time

:17-year cicada emergence: 2 months every 17 years

opening bracket

20 days over 11 years multiplied by

2 months over 50 years multiplied by

1 over 350 years multiplied by

one half multiplied by

2 months over 17 years

closing bracket to the power of -1

equals 4.3 billion years

:[Caption below the panel:]
:Every 4 billion years or so, my neighborhood gets to see a ''really'' spectacular show.

{{comic discussion}}

[[Category:Astronomy]]
[[Category:Solar eclipses]]
[[Category:Time]]
[[Category:Animals]]

2943: Unsolved Chemistry Problems

2024-06-10T08:56:33Z

Murkledroid: Clarity: Changed "being able to design an arbitrary enzyme ... means .. any reaction can be ..." to " [...] would mean ... any reaction could be ...

{{comic
| number = 2943
| date = June 7, 2024
| title = Unsolved Chemistry Problems
| image = unsolved_chemistry_problems_2x.png
| imagesize = 361x386px
| noexpand = true
| titletext = I'm an H⁺ denier, in that I refuse to consider loose protons to be real hydrogen, so I personally believe it stands for 'pretend'.
}}

==Explanation==
{{incomplete|Created by a caffeinated biochemist - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

Every field of research has unsolved problems considered "important" or "significant" that motivate continued research. The scientists at what is apparently the "grand opening" of their new chemistry lab list several real chemistry problems, followed by one also-unsolved-but-less-scientific problem (the p in pH)

'''Arbitrary Enzyme Design:'''

{{w|Enzymes}} are catalytic proteins. Enzyme catalysis is often unique in comparison with other catalysis methods as it is highly specific, or tailored to a specific reaction. As such, enzyme catalysis, besides being the basis of all biochemical processes, is becoming increasing relevant to industrial synthesis processes. As enzymes can be easily synthetically produced through recombinant gene technology, being able to design an arbitrary enzyme for any reaction would mean that effectively any reaction could be relatively easily catalyzed, revolutionizing the chemical synthesis industry.

'''Protein Folding:'''

{{w|Protein|Proteins}} are large molecules that consist of chains of amino acids. These amino acids chains become {{w|Protein Folding|folded}} in extremely complex ways into intricate 3D structures, and the way a protein is folded is of critical importance to its function. Because of the huge importance of proteins to biological life, biologists have devoted significant attention over many decades to the problem of {{w|Protein structure prediction|protein structure prediction}}. This refers to the ability to predict the 3D structure of a protein based on the amino acid sequence, and remains one of the most important problems in computational biology. The ability to predict protein structure purely from amino acid sequence, the so-called "de novo" prediction, is known in computational biology as an unusually difficult problem due to the complexity of amino acid chains. Known as "Levinthal's paradox," the number of possible conformations from the backbone conformations alone is estimated to have in the ballpark of 10^300 different conformations. Despite this, protein folding occurs extremely quickly in reality. Because of this difficulty in sampling conformations, even with optimization, such as secondary structure prediction and Monte Carlo simulation, a "true" accurate simulation is extremely computationally expensive. Because of this, the most accurate solutions, such as AlphaFold, utilize a combination of homology modeling - sampling experimentally determined proteins with similar sequences to infer structural motifs and similarities - and deep learning to accurately guess protein structure.

'''Depolymerization:'''

Polymers are very large molecules formed out of repeating subunits called monomers. Monomers are molecules, typically organic in nature, that can bond with at least 1 other molecule, with chains of 2 or more making long chains or networks called polymers. That process is known as polymerization. Depolymerization is breaking down polymers into the small molecules they were originally made from. This is done through a variety of processes such as using radiation, electrolysis, adding chemicals, and other means. Plastics are the best-known polymers, but cellulose, proteins, and DNA are also technically polymers. The huge number of varieties and mixtures in plastics makes recycling them a huge challenge, and there is increasing concern about plastic waste damaging the environment.

Polymerization is usually exothermic, releasing energy as heat. To reverse this would require adding energy, in a targeted way. Simply ''destroying'' a polymer, by means of highly-reactive chemicals, heat, or radiation, doesn't generally release the monomer molecules to a significant degree; most of the reaction products are highly degraded. Most polymers are made by a process of catalysis, with the small monomer molecules interacting via a catalyst structure, often in liquid form, and the eventual product is usually solid. To reverse this would require getting the catalyst to interact in a very precise way with the solid polymer, and it's relatively difficult for the catalyst structure to get into the proper configuration with the solid tangled polymer molecules.

Another highly-desired depolymerization process would be to convert cellulose into its component glucose molecules. That glucose could then be used for a variety of different purposes, including fermentation to alcohol to use as a fuel. Currently, when plants are grown, much of the solar energy and carbon dioxide they absorb ends up in the form of cellulose rather than as starch, sugar, protein, or other substances that we find useful. Our being able to make use of the cellulose would make farming much more energy-efficient. Some organisms are able to depolymerize cellulose by means of enzymes, but our ability to use similar processes on an industrial scale is still limited. (Those organisms use a complex multi-step biochemical process which essentially "invests" energy into splitting off a glucose molecule, then recoups the investment by metabolizing the glucose.) It's also possible to depolymerize cellulose at high temperature and pressure using nothing more than water and acid, but that process is energy-intensive. It ''might'' be possible to do it with a solar-heated reactor.

'''What the “p” in pH stands for:'''

“p” shows up in pH, pKa, pKb, and other things related to the concentration of H+ ions and OH- ions. The meaning of the "p" in "pH" has been the subject of much dispute. It is sometimes referred to as "power of Hydrogen", but it's not at all clear in English what this means - are we talking about hydrogen fuel cells, or someone's superpower? The connection - which most teachers (original research) do not make - is that this "power" should be understood in the sense of "x to the third power." pH is a logarithmic scale, and the logarithm is the inverse of the exponent, and, in all three languages that pH was first published in, the word for "potency" is used for exponents. The term pH was introduced by {{w|Søren Peter Lauritz Sørensen|Søren Peter Lauritz Sørensen}}, who did not publish his results in English, and more accurately translates as "hydric exponent". The letter p could stand for,in the languages in which Sørensen published: the French 'puissance' , German Potenz, or Danish potens, all referring to the concept of the "exponent" in exponential functions.

'''ρ operator'''

2000 years ago, Romans knew nothing of zero, place-value-notation, nor real numbers. Roman scribes knew integers, integer ratios, and standard oft-used ratios. In modern times, we buy and sell by the pair (pr), dozen (dz), and score (sc). Roman shipping used ρ and ρρ to represent 4/5 and 5/8 sizes. As [https://en.wikipedia.org/wiki/Greek_numerals Ionian numerals] gave way to decimal point use, ρ use became archaic, remaining only in a few fields like music, art, and chemistry.

Over centuries, ρ gradually evolved into a real number operator as log base cube root of 1/2 (musical major third), square root of golden ratio (artistic fractal shape), tenth root of 1/10 (statistical power of effect), or 1/10 (chemical strength). Lacking otherwise infrequently-used greek letter ρ printers began using latin p instead.

'''Title Text: Hydrogen Denier'''

In the title text, Randall claims to be an H+ denier by refusing to consider loose protons to be hydrogen atoms, and as such, the “p” stands for pretend. Part of the joke is Randall's implication that this is a well-known conspiracy theory that he personally buys into (it isn't). The word "denier" is often used as shorthand for other conspiracy theories, such as a "climate change denier" or a "moon landing denier."

Here's a breakdown of this joke:

* H+ is the chemical symbol for a positively-charged atom of hydrogen, the smallest atom on the Periodic Table. Since hydrogen is normally just one proton and one electron, when you take the electron away, you make it positively charged (the + sign in the superscript) and you effectively end up with just a single loose proton. So the shorthand for "loose proton" is to refer to it as an H+ ion.

* pH is taught in high school science class to essentially measure the concentration of extra loose protons in, say, an aquarium. (Different fish prefer slightly different pH levels/alkalinity.) As mentioned earlier, you can interpret the term "pH" to be referring to the "p" of "H" -- the power/potency of H+ ions.

(Note that in reality, lone H+ ions do not exist in water, and instead they glom onto H2O molecules to form H3O+ and H5O2+/(H2O--H--OH2)+. If you don't know what these chemical symbols mean, don't worry about it.)

But as an H+ denier, Randall doesn't consider loose protons to be hydrogen atoms. He has a purist's view of hydrogen, that it is just "pretending" to be hydrogen as soon as it loses an electron. As a denier, he interprets the term "pH" as referring to the concentration of "pretend Hydrogen".

==Transcript==
:[Hairbun stands behind a lectern on a podium speaking into a microphone on the lectern. A Cueball like guy stands to the left and another Cueball like guy and Megan stand to the right. There is a large sign hanging in the background along with some ornaments.]
:Sign: Grand Opening
:Hairbun: Our lab will be working on chemistry's top unsolved problems: arbitrary enzyme design, protein folding, depolymerization, and, of course, the biggest one of all:
:Hairbun: ''Figuring out what the "p" in "pH" stands for.''

{{comic discussion}}

[[Category:Comics featuring Hairbun]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Multiple Cueballs]]
[[Category:Chemistry]]

2846: Daylight Saving Choice

2023-10-27T08:50:17Z

Murkledroid: Some additions and amendments to reflect more accurately differences between DST practices in different countries, including the upcoming difference in dates between the end of DST in Europe and North America

{{comic
| number = 2846
| date = October 25, 2023
| title = Daylight Saving Choice
| image = daylight_saving_choice_2x.png
| imagesize = 231x386px
| noexpand = true
| titletext = I average out the spring and fall changes and just set my clocks 39 minutes ahead year-round.
}}

==Explanation==
{{incomplete|Created by TWO CLOCKS SET ONE HOUR APART - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
{{w|Daylight saving time}} (DST) is a practice usually involving changing the clock one hour ahead for approximately half the year, typically from spring to autumn. Countries nearer the equator do not see significant changes in daylength between winter and summer and so have rarely had a reason to follow this practice. Many countries which used to follow this practice no longer do, and a few now follow year-round DST - however summer-only DST is still used in North America, Europe, and parts of South America and Oceania.

Within countries that still follow this practice, there are frequent arguments (mostly during the 2-3 days surrounding the clock change) over the pros and cons of it. [[Black Hat]] is suggesting that veryone should observe or ignore daylight saving time based on their personal opinion. While it might put an end to the arguments (although this itself is debatable) it would clearly cause disharmonious time. For example, during daylight saving time, an observer would claim it's 9:00 when a non-observer would claim it's 8:00. This would lead to many scheduling errors, delays, and other mistakes, resulting in widespread inconvenience and harm.

The joke here is that, while most options in life can be left to individual choice, clock time is only fully useful if everyone involved agrees on what it means. There may also be a humourous reference to the confusion already often caused around this time when countries do not all begin or end DST on the same date, for example in scheduling calls or online meetings between Europe and North America in the week after publication of this comic.

There are known incidents in which an actual application of Black Hat's proposal
[https://www.sandiegouniontribune.com/opinion/the-conversation/sdut-daylight-saving-time-sunday-2015mar07-htmlstory.html rendered a terrorist plot void]. One of them is a [https://darwinawards.com/darwin/darwin1999-38.html 1999 Darwin Award Winner]

This comic was posted 4 days before the end of 2023’s daylight saving time in most European countries, and 11 days before the end of 2023's daylight saving time in most of North America. If the proposal is actually instituted at this time, those in the Northern Hemisphere who do not like the fuss of changing their clocks would ''remain'' on DST (as {{w|Sunshine Protection Act|has been actually proposed}}), yet those who are happy with it will fall back to non-DST over the winter months. Presumably, unless anyone changes their minds over the 'winter' period, everyone would actually be back in synch for future 'summer's.

The rule, as spoken, does not restrict people to merely choosing whether the daylight offset is personally used during DST periods. It instead seems to ''impel'' them to undertake (or not) the statutary changes according to personal convictions, perhaps contrary to what their convictions actually desire. It is left open-ended ("From now on...") if people from ''both'' mindsets can arbitrarily change their minds in the future. If they can, and act accordingly, this time next year there could be people on three different 'summertime' offsets: zero (change now, but not change later), +1 (steadfast change/no change) and +2 (don't change now, but shift forward in spring). Beyond next year's "fall back" date, there could be people on -1 (fall back, don't spring on, fall back ''further'') and each full year beyond may add additionally positive/negative extremes of offset by those who periodically change their inclinations to only obey ''one'' of the relative imperatives, and a potential {{w|Galton board|standard distribution}} of everyone else between. All this could just be a badly worded explanation of the policy, or even in the wording of the legislation behind it, but the presence of Black Hat at the lectern probably indicates that he fully expects and ''intends'' such a boding and expanding chaos.

The title text suggests splitting the difference by using a constant offset which is the average of the daylight saving offset across days of the year. We do not know if in this system Randall would change his clock for leap year.

==Transcript==
:[Black Hat is speaking at a lectern, flanked by Ponytail and Hairy.]
:Black Hat: From now on, everyone who likes daylight saving time should change their clocks, and everyone who doesn't, shouldn't.

:[Caption below the panel:]
:The government finally decides to put an end to all the arguments.

{{comic discussion}}
[[Category:Daylight saving time]]
[[Category:Comics featuring Black Hat]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Hairy]]