Parity Conservation
Parity Conservation
[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.

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 likely 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.
