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 apperance in xkcd, the first being 555: Two Mirrors.
The remark on parity conservation and cobalt-60 is almost surely a reference to the Wu experiment. In 1956, physicist Chien-Shiung Wu and her team at the National Bureau of Standards used cobalt-60 to show that the 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 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 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.
- [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.
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Would it be possible to mirror the light particles bouncing off a mirror in an experiment similar to what cueball is trying to do? Donthaveusername (talk)
- Not sure exactly what you are asking, but photons (and gluons, and Z-bosons, and if they exist, presumably gravitons) are their own anti-particle, so photons are the same regardless if the source is matter or antimatter. https://van.physics.illinois.edu/qa/listing.php?id=1153 for more info.18.104.22.168 04:25, 26 September 2020 (UTC)
- No, gluons and anti-gluons are distinct and carry different charges. Only particles with no charges can be their own antiparticle.22.214.171.124 06:52, 5 October 2020 (UTC)
- I'm also not exactly sure what you mean, but if you're asking about using a mirror to conduct an experiment in reality, the answer is no. Particles in our world will either pass through a mirror or reflect off of it. Either way, they're still in our world. Mirrors are of use when we want to see how reflection works (assuming the mirror reflects the particles concerned). The benefit to enlisting Bloody Mary's help here seems to be that she is located in another location inside or connected to the mirror, which is why she has to perform the measurements; the measurements can't be performed outside her secondary universe. The experiment here confirms whether her universe and our universe work in the same way. Nathan (talk) 06:39, 26 September 2020 (UTC)
- I guess what I'm trying to say is like, imagine the image on the mirror, but mirrored onto our side of the mirror, so you get a near 2d hologram. That was a terrible job of explaining, but maybe it offered some insight? Donthaveusername (talk)
- The mirror of an image in a mirror, is precisely the original object being reflected. Is this what you mean? Do you mean kinda see only the parts of a person standing in front of a mirror, that are reflected in the mirror?
What do you imagine this looking like? 126.96.36.199 23:19, 27 September 2020 (UTC)
- I don't understand physics very well, but my simplistic understanding is that the electric and magnetic field components of the photons that are normal to the surface of the mirror are indeed actually mirrored. I don't believe the orientation of the photons, like that filtered by 3d glasses to separate the eyes, is mirrored. I could be wrong. 188.8.131.52 16:30, 26 September 2020 (UTC)
- The "orientation" you speak about is called polarization and I suspect mirror destroys it. -- Hkmaly (talk) 18:53, 26 September 2020 (UTC)
I am wary about "an entire anti-person would annihilate a normal person if they touch"-type stuff. It was a trope used in '60s cartoons that there'd be an anti-matter world and only representative being/item A and representative being/item anti-A touching would create mutual (or not, if even more laughably plot-driven in favour of one of them surviving) vanishing of both... And often with just vanish-in-smoke. Whereas we all know that any matter meeting any anti-matter (notwithstanding that 'all electrons and positrons are the same electron bouncing back and forth in time) will annihilate, and if the energies produced don't yet actively push the non-fingertip (or breath, or just space-suit glove on anti-spaceship airlock handle) counter-matters apart there's going to be more annihilation after the first fizzle. 184.108.40.206 10:59, 26 September 2020 (UTC)
- Not just cartoons; see also the ST:TOS episode 'The Alternative Factor', 1967, which got the whole "antimatter", "individuals", "destroy the universe" stuff laughably, painfully wrong. BunsenH (talk) 15:14, 26 September 2020 (UTC)
- Although it's not technically the same thing, I found myself reminded of the climax of the movie TimeCop while I was adding that to the explanation. I don't think 100% of the matter and antimatter in two opposite-matter people would annihilate when they touch, because the contact surface is indeed quite small, but clarifying that went further into the technical weeds than I wanted to go. When a nuclear weapon goes off, for instance, not all of the fuel is consumed, but that detail is usually overshadowed by the explosion. Captain Video (talk) 16:28, 26 September 2020 (UTC)
- Using "overshadowed" for something accompanied by big amount of gamma radiation is funny. But yes, ANY antimatter annihilates ANY matter and if you touch hand of person from antimatter your hands annihilate but rest of you would likely be thrown in opposite direction and cooked by radiation. Assuming, of course, you are in vacuum, as otherwise either you or him would be annihilated by the (anti)air. -- Hkmaly (talk) 18:53, 26 September 2020 (UTC)
- I've removed the stuff about "Dr. Edward Anti-Teller" and added a paragraph about the bad SF about antimatter people. BunsenH (talk) 20:16, 26 September 2020 (UTC)
- Fair enough, but I'll post it here for posterity. I first encountered this poem ages ago in a book called A Random Walk in Science. It might be too trivial for the comic explanation, but it always makes me smile. Captain Video (talk) 23:19, 26 September 2020 (UTC)
- I remember that book, from my university library. Category: "Science: anecdotes, facetiae, etc.", as I recall, which led me to several other fun books such as Worm Runners Digest. I still have a few photocopies from them, a few decades later; my M.Sc. supervisor didn't understand them. The sequel, More Random Walks In Science, begins with a photo of an "impossible" cube. Nowadays it would be handwaved as a digital fake, but it appeared to be a very-carefully-constructed gimmick physical object. BunsenH (talk) 03:04, 27 September 2020 (UTC)
- I only just saw this, but: I didn't know there was another book! I'll have to see if I can find a copy. Captain Video (talk) 04:05, 30 September 2020 (UTC)
- Timecop wasn't even antimatter, it was (supposedly) the same matter (which it wouldn't be, with lifetime elemental recycling through most bodily tissues, so it needs a form of Ship Of Theseus teleology built into the universe - close to being a 'soul'-based argument). And... It's been a while since I last saw it (not sure I ever saw the sequel that I expect they made), but didn't it not so much explode the person (both of them, in a laser-guided bit of plot-karma) as kind of mush them into a weird biofractal special-effect? More related to the critically damaged T1000 than anything with an E not far off the mc²ness of total matter-energy conversion. 220.127.116.11 22:54, 26 September 2020 (UTC)
- I saw it once, about 12 years ago, on VHS, and all I remember is Jean Clade Van Damme saying "same matter cannot occupy same space" as he pushes two copies of the same guy together and they turn into the stuff inside a lava lamp. I'm not exactly jumping at the chance to rewatch it. Captain Video (talk) 23:19, 26 September 2020 (UTC)
I am skeptical that any joke about "party conversation" is intended. BunsenH (talk) 15:14, 26 September 2020 (UTC)
It looks like Randall has been exposed to a variant of this myth where bloody mary murders the person saying her name. This would explain the preceding comic about bloody Mary too: the ghost could be leaping to murder the person incanting their name. It would be good to add to the article a reference to this interpretation of the myth if anybody is excited about it. 18.104.22.168 16:30, 26 September 2020 (UTC)
Ooohh, a comic about the same topic as my worldbuildingSE question, nice! https://worldbuilding.stackexchange.com/q/135950/32102 Fabian42 (talk) 16:54, 26 September 2020 (UTC)
I'm kind of doubtful of the interpretation that Cueball intends to "create a real Bloody Mary on his side of the mirror," since she'd then be in the same universe as he is and would get the same result if she does the same experiment. At the same time, this is one of the perils of having a wiki that exists purely to explain the jokes in a comic strip: The editors might not even agree what the joke is. I wonder if my childhood self would respect adult me after learning this is what I do with my free time on Saturday nights. Captain Video (talk) 23:38, 26 September 2020 (UTC)
- Saturday nights, Tuesday nights and Thursday nights... ;) 22.214.171.124 23:55, 26 September 2020 (UTC)
- I can't make sense of that interpretation of Cueball's intentions either. As for what my childhood self would think about my doing stuff like this on a Saturday night... he/I would think it was amazingly cool. Writing up interesting explanations of geeky subjects for people who were equally geeky but less well-informed? Science cartoons? With computers? So cool. BunsenH (talk) 03:04, 27 September 2020 (UTC)
Am I the only person who saw a Mary / Marie (Curie) pun? 126.96.36.199 17:48, 27 September 2020 (UTC)
Is there any reason to use cobalt over another radioactive element? I recall in the long ago the cobalt bomb was supposed to be a world wrecker. Was there some meaning similar to this that led to its inclusion here?
[EricB] 19:49, 29 September 2020 (UTC]