# Paper: The Chaos within Sudoku

The Chaos within Sudoku” is a paper about solving Sudoku puzzles with physics. They simulate an imaginary physical system, let it run, and when it stops the puzzle is solved. See the video below:

The thing about Sudoku is that Sudoku is hard. More specifically, when Sudoku is generalized to grids of arbitrary size, it’s NP-complete. What happens when you translate an NP-complete problem to a physical simulation? The authors find chaotic dynamics.  And in the process, they identify the hardest Sudoku puzzle…

# Paper: First observation of gravitational waves

This is a repost from February, when LIGO reported its first observation of gravitational waves.  This is relevant because last month LIGO reported its second observation, also resulting from inspiraling black holes.

Today, the Laser Interferometry Gravitational-Wave Observatory (LIGO) reported the first observation of gravitational waves. You can read about it in The New York Times (warning: autoplay), on Sean Carroll’s blog, or in comic form. I went straight to Physical Review Letters.

As an undergrad, I did some work on LIGO. Specifically, I was a data analyst looking for exactly the kinds of gravitational waves here observed. Anyway, I’m happy to play the role of your local expert, providing some context and answering any questions.

# Paper: Cutting ties leads to cooperation

One field of study that greatly interests me is evolutionary game theory. The central question of the field is: how does altruism evolve? In a naive analysis, it would seem that uncooperative individuals can get ahead of the rest of their species, and uncooperative offspring will come to dominate the population. Nonetheless, in the real world we observe mixtures of cooperative and non-cooperative behavior.

In evolutionary game theory, “cooperation” is understood as a strategy in a two-player game. Most commonly, we consider the prisoner’s dilemma game, where two players each have the choice to cooperate or defect. “Defecting” is a strategy that benefits yourself, but hurts your opponent even more. And so if both players defect, then they’re both worse off than if they had cooperated.

The key to the evolution of cooperation is the ability to react to defectors. In particular, one needs to punish defectors, such that defection is no longer beneficial.

Here I’ll talk about one model that allows for such punishment, based on a paper titled, “Cooperation prevails when individuals adjust their social ties“. As suggested by the title, the mechanism for punishment is to cut off ties with defectors.

Figure 1 from the paper illustrates a network of cooperators and defectors

# Paper: Do memes really go viral?

When a meme becomes popular, we often say that it has gone “viral”. This word suggests that memes become popular by being particularly infectious. In analogy to epidemics, a particularly viral meme hits a critical threshold where it just won’t die, because each new infection spreads it to even more people.  But is this epidemic model actually true?

A paper titled “The Structural Virality of Online Diffusion” puts into question the very idea that popular memes are viral. They point out another mechanism by which memes can become popular: the broadcast. Rather than infecting multiple generations of followers, a meme may become popular simply by infecting one person with a lot of followers.

Figure 1 from the paper. The tree on the left illustrates a broadcast, while the one on the right illustrates viral spreading.

The paper considers a massive amount of Twitter data to determine whether the most popular links are broadcast-like or virus-like. On average, they more resemble broadcasts, but there is a huge amount of variation.

# Paper: Sexual racism among gay men

“Sexual racism” is racial discrimination against potential sexual or romantic partners. Sexual racism is very common among gay & bi men. The hookup app Grindr is notorious for the many profiles that say upfront, “no femmes, no blacks, no asians.” I’ve also heard several terms for gay & bi men who prefer partners of a particular race–“rice queen” being particularly frequent in my area.

Normally, when I talk about gay & bi men, I avoid contrasting straight people, because sometimes straight people complain they have it just as bad, and what do I know? But when it comes to sexual racism, research backs me up in saying it really is worse for gay & bi men.

Many defend sexual racism by saying it is a personal preference. Some say that it is a preference determined by biology, similar to gender preferences. Others say that even if racial preferences are culturally determined, they aren’t amenable to moral condemnation because preferences too difficult to change.

I’m skeptical of these defenses, but I’ve never personally used any dating or hookup services. So today, instead of relying on my personal experiences, I’ll rely on social science. Today I’ll briefly discuss the paper, “Is Sexual Racism Really Racism? Distinguishing Attitudes Toward Sexual Racism and Generic Racism Among Gay and Bisexual Men“.