Guest post by Jeremy Bailin, Assistant Professor of Physics & Astronomy, University of Alabama!
Let’s talk about matter/anti-matter annihilation in the early Universe.
Okay, so that’s not normally what you’d expect in a GJS blog. 
There’s no relation between feminism or sexuality and particles interacting with anti-particles shortly after the Big Bang to turn into gamma rays, right?
Right. And that’s kind of the point.
Let me back up a minute. I am an astronomy professor in the Department of Physics and Astronomy at the University of Alabama. I most often teach our 150-student intro astronomy class for non-scientists who need to fulfill their natural science breadth requirement. Lots of these students are scared to death of math and of science, and so it is really important to build their intuition for important (and sometimes complicated and counterintuitive) astrophysical concepts using analogies to everyday situations where their intuition can point them in the right direction.
I pride myself in coming up with good analogies. In a good analogy, the correct answer in the analogy should be obvious, and the connection between the analogy and the actual physics should be easy to see. The expansion of the Universe is like the scene from Vertigo where the bottom of the tower stretches away from Jimmy Stewart. The “demotion” of Pluto is like an alien visiting a pet store and mistaking the first cat it sees for a strange dog. And, until this semester, matter/anti-matter annihilation in the early Universe was like boys and girls at a dance.
First, the real physics:
In the early universe, when it was extremely hot, there were almost equal numbers of particles and anti-particles. Particles and anti-particles could come together and annihilate, turning into photons (particles of light, in this case gamma rays), and there was enough energy that photons could turn back into particle/anti-particle pairs. But as the universe cooled down, the photons didn’t have enough energy anymore to turn into particle/anti-particle pairs. Meanwhile, particles and anti-particles kept on annihilating with each other as long as they could. It turns out that there were a few more particles than anti-particles, so the extra particles of matter had nothing to annihilate with and are still around today. Those few extra particles of matter are, in fact, the ones we are made of!
Editor’s note: This is basically the best picture ever of anything in the universe. And that is funny because it IS a picture of the universe. I think. OK I NEVER TOOK PHYSICS. (image from Wired.)
The analogy is that there are a bunch of boys and girls at a dance, who each want to pair up with someone of the opposite sex. While the dance is still going on, boy/girl pairs are forming — just like particle/anti-particle pairs coming together to turn into a photon — and coming apart at the end of each song — just like photons turning back into particle/anti-particle pairs. But once the music stops, all of the boys and girls pair up until they run out of potential partners. It turns out that there are more girls than boys in the population, so after all of the couples have gone, there are a few extra girls left.
By the metrics I listed, this is a great analogy. It is intuitively obvious that there will be a few girls leftover at the end of the dance. And the connection between forming couples and forming particle/anti-particle pairs is easy to see. It’s perfect!
It’s also awfully heteronormative.
When I use this analogy, some fraction of the class will be inwardly sighing as a situation where they do not exist is assumed as the norm [note: Thank you to Andrea Leistra for this turn of phrase!]. And during that moment, when I am trying to excite them with the idea that for every 1,000,000,000 anti-matter particles there were 1,000,000,001 matter particles, and *they themselves* are made up of those extra 1-in-a-billion particles, which I think is one of the coolest things in the entire course… they are not paying attention. Even if I wasn’t motivated to change it because I think that avoiding heternormative analogies is a good goal (which I do), I would be motivated to do it as a teacher. Now, if I concluded that there really isn’t another good analogy, then I would keep it — it’s not offensive. But if I can come up with another good option, then I should!
However, I was stumped. So, of course, I asked Facebook I got a number of good suggestions, but most had one of two problems:
The “leads and follows” problem: In ballroom dancing, people either learn to dance as the “lead”, who leads the dance moves, or the “follow”, who follows. Traditionally, for mixed-sex dance partners, males are the leads and females are the follows, but the reverse is perfectly possible, and for same-sex dance partners there is obviously no particular mapping between the sex and the category. This preserves the connection between the physics and the analogy perfectly — there are still two distinct types of dance partners, but there is no longer any connection to sex. However, it fails the intuitive criterion because it is too esoteric; there are likely a large number of students who don’t have enough experience with ballroom dancing to be familiar with the terminology (editor’s note: and this is the real tragedy of our educational system). If the analogous situation is not intuitively obvious, it’s no good.
The “extra sock” problem: Anyone who has ever done laundry in their life has been confounded by the fact that, when you take your laundry out of the dryer and fold it, and put all of your socks into pairs, there is always one left over. This is definitely intuitive, but fails the other criterion — it is much less analogous to the physics because there is only one type of sock (i.e. left and right socks aren’t different).
In the end, there were two analogies that were left standing. One is a zipper: the Universe began as a very, very long zipper, and as it cooled it got zipped up, with every left zipper tooth joining with a right zipper tooth. However, it turns out that the zipper was longer on one side, so after it has been zipped as far as it could go, there was still a little left on one side. The other good analogy is musical chairs: while the music is playing, people (particles) are in front of first one chair (anti-particle), then another, and so on. But once the music stops, and all of the people sit on any chair they can find that’s free, there are some people left over. I think these are both just as good as the original analogy and are not in the slightest heteronormative!
So, is there a relation between sexuality and matter/anti-matter annihilation in the early Universe? There shouldn’t be. And now, in my class, there isn’t.
[Thanks to everyone who commented on the facebook thread that spawned this post.]
