explain xkcd - User contributions [en]

2364: Parity Conservation

2025-11-27T13:28:26Z

2A00:23C6:5D52:7C01:82B7:2D8B:80C6:8ED8:

{{comic
| number = 2364
| date = September 25, 2020
| title = Parity Conservation
| image = parity_conservation.png
| titletext = Bloody Mary is made of antimatter. It explains so much.
}}

==Explanation==

{{w|Bloody Mary (folklore)|Bloody Mary}} is a legend of a ghost, phantom, or spirit conjured to reveal the future. She is said to appear in a mirror when her name is chanted repeatedly. This is why Cueball says he said her name three times. This is her second appearance in xkcd, the first being [[555: Two Mirrors]].

The remark on {{w|Parity (physics)|parity conservation}} and cobalt-60 is likely a reference to the {{w|Wu experiment}}. In 1956, physicist {{w|Chien-Shiung Wu}} and her team at the {{w|National Bureau of Standards}} used {{w|cobalt-60}} to show that the {{w|weak interaction}} breaks parity: beta particles leave the decaying nucleus in the direction opposite to nuclear spin.

One of the results of this is that it becomes possible to differentiate between the concepts of left and right on a purely technical level, even if the person (or distant alien) you're talking to can't see you. When we say that "parity is not conserved," we mean that the concepts of left and right are not purely symmetrical across all areas of physics. As Richard Feynman put it, this means that "nature's laws are different for the right hand and the left hand, that there's a way to define the right hand by physical phenomena."

It seems as if Cueball is trying to "hand" Bloody Mary his experimental apparatus either physically (as he is asking her to take the cobalt-60 "before [she] come[s] out of the mirror"), or perhaps by reflecting it onto her side. Because Bloody Mary exists in mirrors, her world is implicitly a mirror of ours. This would allow her to conduct mirror physics experiments, such as whether or not the beta particles leave the cobalt-60 in the same direction as they do in our universe.

The title text references {{w|antimatter}}. In physics, antimatter is like a mirrored version of matter — mirrored in charge, parity, and time — composed of antiparticles rather than particles. Antimatter and matter spontaneously {{w|Annihilation|annihilate}} each other when they meet, releasing extremely high-energy radiation. Therefore, Bloody Mary being made of antimatter explains why she kills people when she comes out of the mirror. (Bloody Mary would also be annihilated in such an interaction, so the fact that she keeps coming back may be attributable to her being a ghost.)

There have been a lot of science fiction-y stories featuring antimatter people; often, these are duplicates of "regular"-matter people. The stories often show unrealistic ideas of what would happen if matter and antimatter versions of people met. Sometimes, the duplicates simply disappear; sometimes, if the plot requires it only ''one'' may disappear. Or sometimes the entire universe is destroyed. In reality, what would happen is that the matter and antimatter would mutually annihilate, as pairs of subatomic particles, creating enormous radiation and heat. It's likely that only a small fraction of the matter and antimatter would actually come into contact, rather than being propelled apart by the explosion. Indeed, if the duplicates are in their versions of air, the air and anti-air particles would interact first! Even in interstellar space, an antimatter alien would give off significant radiation from collisions with matter particles. In these stories, it's often presumed that the corresponding duplicates of people can annihilate only each other, but can safely touch anything else. In reality, the matching is at the subatomic level: any proton with any antiproton, any electron with any antielectron (or "positron"), etc.

==Transcript==

:[Cueball is standing in front of an oval wall mirror hanging over a small table. He's holding a tube connected to an electronic device. A face is dimly reflected in the mirror.]

:Cueball: Listen, I know I said your name three times.

:Cueball: But before you come out of the mirror and murder me, can you hold this cobalt-60 and take some measurements?

:Cueball: See, I'm researching parity conservation...

:[Caption below the comic]:
:It took some negotiating, but I've finally become the first person to coauthor a paper with Bloody Mary.

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

1303: Profile Info

2025-11-27T11:13:50Z

2A00:23C6:5D52:7C01:82B7:2D8B:80C6:8ED8:

{{comic
| number = 1303
| date = December 13, 2013
| title = Profile Info
| image = profile_info.png
| titletext = It's ok, they'll always let you opt out! Like they did with the YouTube real name profile thing.
}}

==Explanation==
On most websites people are forced to create an account to get proper support, be it technical support or simply ordering products. This usually consists of the user's name, email address, phone number, and also the user's home address if ordering a product that must be shipped by mail. It's not uncommon for the websites/companies to then use that information for presenting new advertisements in the near future, or even sell it to others for ''their'' schemes.

In 2013 (publishing of this comic), a recent trend was to allow harvesting of profile pictures and real names, mostly by automated processes with little to no human interaction. The personal information is collected in context with other information, such as the purchase or product review history, and shown to people (typically friends and contacts) who are viewing similar products.

The title text references the fact that most of these companies have an "opt out" option so that your name won't be used, but then emphasizes that {{w|YouTube}} (a subsidiary of {{w|Google}}) at the time of this comic forced YouTube user accounts to be tied to {{w|Google+}}. Google+ required the use of the 'first name' and 'last name' convention typical of western cultures, where one cannot 'opt out' (though these requirements did allow for the abbreviation of names). However, this has not stopped people from using names that aren't their own, but using names like "Barack Obama" and "Chuck Norris". Some similar websites allowed the use of aliases in their initial terms of use, but then later changed their TOCs to prohibit use of "false" names. YouTube was one such system; after the merge with Google+ for authentication, both sites automatically linked your false-name account with your real name account, in some cases banning and blocking people with suspected false name accounts.

To try to put a stop to his own information being used, [[Cueball]] sets his last name to "If-you-see-this-name-in-an-ad-give-the-product-a-one-star-review-Smith", a name which includes a phrase that would negatively affect any marketer's attempts to advertise an online product.{{Citation needed}} This name would pass though most harvesting software as-is, and may very well end up being used in such ads, unless some very clever software is able to detect sentences as part of names or similar. In fact much spam is stopped by identifying emails through {{w|Honeypot (computing)|Honeypot}} accounts, among other methods.

Hence engineering part of your profile could be a winning strategy to signal to your friends that your information is harvested without your express knowledge.

==Transcript==
:[Cueball is sitting at a desk, using a desktop computer. He is filling in a form on a webpage.]
:First Name: John
:Last Name: If-you-see-this-name-in-an-ad-give-the-product-a-one-star-review-Smith

:[Caption below the panel:]
:How to deal with companies harvesting your profile for marketing

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Sabotage]]

2345: Wish on a Shooting Star

2025-11-27T11:11:25Z

2A00:23C6:5D52:7C01:82B7:2D8B:80C6:8ED8:

{{comic
| number = 2345
| date = August 12, 2020
| title = Wish on a Shooting Star
| image = wish_on_a_shooting_star.png
| titletext = Congratulations to whoever wished for revenge on a forest near the Tunguska River, a 1980 Chevy Malibu in Peekskill NY, Alabama resident Ann Hodges, every building in Chelyabinsk with glass windows, and the non-avian dinosaurs.
}}

==Explanation==
This comic was published at the annual peak of the {{w|Perseids}} {{w|meteor shower}}. It is a common practice to make a wish when one sees a shooting star, in hopes that the wish comes true. This comic consists of a [[:Category:Venn diagrams|Venn diagram]] showing what things are commonly wished for upon seeing a shooting star, and what things the shooting star may cause. Shooting stars, as they are actually meteors, can only cause changes to physical phenomena, such as radio noise or the appearance of the sky as they burn up in the upper atmosphere. The only thing that is shared between the potential wish side of the diagram and the shooting star caused side is revenge. This would occur when a shooting star actually hits the planet, becoming a meteorite. This is frequently highly destructive, given the high speed of falling meteors. As such, it would be possible for the meteorite to hit something that someone for some reason or another wished revenge upon. However, given the massive surface area of the planet, the likelihood that someone's revenge would be "granted" by a meteorite would be very low (although not quite as low as [[2328: Space Basketball|dunking a meteorite through a basket]]). The title text makes fun of this by detailing several incidents where a meteorite landed and caused damage.

List of things that were damaged by meteorites (from title text):
* On June 30, 1908 ({{w|Gregorian Calendar}}, Russia still used the {{w|Julian Calendar}} and was recorded there as June 17), an {{w|airburst}} caused by the {{w|Tunguska event|breakup of a 100-meter falling meteorite or comet}} with the energy of some 30 megatons of {{w|TNT}} flattened some 80 million trees over 830 square miles (2,150 km<sup>2</sup>) of land in central {{w|Siberia}} near the {{w|Podkamennaya Tunguska|Tunguska River}}. Due to the remoteness of the area, no people were confirmed dead in the incident.
* On October 9, 1992, a meteorite {{w|Peekskill meteorite|totaled a 1980 Chevrolet Malibu in Peekskill, New York}}.
* On November 30, 1954, a {{w|Sylacauga (meteorite)|fragment of a meteorite}} passed through the roof of a house and struck a woman named Ann Hodges. She survived. Only [https://www.telegraph.co.uk/news/science/space/5511619/14-year-old-hit-by-30000-mph-space-meteorite.html two other people] are known to have been [https://www.space.com/earliest-evidence-meteorite-killed-person.html hit by meteorites.]
* On February 15, 2013, a meteorite {{w|Chelyabinsk meteor|exploded in an airburst}} over Chelyabinsk, Russia, creating a shockwave that shattered windows in the area.
* The {{w|Cretaceous–Paleogene extinction event|extinction of the dinosaurs}} around 66 million years ago is believed to be caused by an {{w|Alvarez hypothesis|impact of a comet or asteroid}}. Randall mentions "non-avian" dinosaurs, as birds are dinosaurs, though the general public not trained in science often doesn't realize this. No humans were alive to wish for the extinction of the dinosaurs,{{citation needed}} but perhaps the wish was made by some ancestral mouse-like mammal (or else wishes may violate causality).

If there is a "message" to this comic strip, it could be similar to those of [[1024: Error Code]] and [[2247: Weird Hill]]: that we shouldn't bother wishing for things that shooting stars can't give us, but should instead take time away from our temporal concerns and just relax and appreciate their beauty. Or maybe the message is that, if you ''must'' wish on a shooting star, you should wish for revenge, because that's something that might come true. Of course, as the title text makes clear, meteorites don't really land according to our designs and schedules, and if you're close enough to a shooting star to see it, and you wish for it to avenge you, and it ''is'' big enough to hurt someone, you're probably at risk yourself.

Meteorites were most recently mentioned in [[2328: Space Basketball]]. Randall has discussed strange and "impossible" wishes in [[1086: Eyelash Wish Log]].

==Transcript==
:[A Venn diagram is shown:]

:[The left circle is labeled "Things people wish for" and contains:]
:Love
:Money
:Fame
:Health
:Power
:Luck
:Success

:[The right circle is labeled "Things shooting stars can cause" and contains:]
:Radio noise
:Dust and ionized gas in the upper atmosphere
:Infrasound
:Cool lights in the sky

:[The only item in the overlapping section is:]
:Revenge

{{comic discussion}}
[[Category:Comics featuring real people]]
[[Category:Astronomy]]
[[Category:Genie]]
[[Category:Venn diagrams]]