Archive for July, 2010

The Zombie Herd

Posted in Research on July 24th, 2010 by David – View Comments

The latest episode of Eureka (great show, by the way) has the employees of Global Dynamics basically transforming into a zombie horde. Well, they’re not really zombies; they’re just affected by a crowd-control pulse gun that backfires and produces the wrong neurotransmitter by accident. In other words, they’re angry.

But an angry mob is not so different from a zombie horde; they both tend to exhibit herd behavior. If you look at pretty much any depiction of zombies, they’re nearly always all ambling (or running, depending on what kind of zombies they are) in the same direction, and not always just because they’re chasing the same person. This is something my basic model doesn’t take into account, but it’d make an interesting addition, and I wouldn’t be surprised if it qualitatively alters the behavior of a zombie model. Compared to pure randomness, herd behavior tends to amplify the actions of certain individuals at the expense of everyone else’s, so the mob (or horde) is able to reach a consensus on some action much more quickly. You can see examples of this effect all over the place, from stock market fluctuations to the structure of the entire universe. (Like they say, life imitates undeath) This is good, because it means herd behavior has been fairly well studied, so there should be plenty of existing research out there to take and apply to zombies.

What is a zombie?

Posted in Research on July 13th, 2010 by David – View Comments

If I’m going to analyze zombie behavior, the first step is to lay down some ground rules. Or axioms, whatever. So the first question is, what is a zombie?

Well, there are a lot of conflicting definitions for that. The original zombie modeling paper actually has a decent overview of how the definition of a zombie has varied through the ages. According to that, the modern "standard" comes from Night of the Living Dead, which portrays zombies as mindless, soulless creatures who move slowly and have a singular fixation on consuming human flesh. For analytical work, this definition of a zombie is kind of nice because it’s easy to model. You can easily program it into a computer; the logic to identify the closest human flesh and walk toward it isn’t very complicated.

On the other hand, there are plenty of counterexamples where the zombies are quick and even intelligent, like Zombieland (which I am a big fan of, by the way). Unsurprisingly, there are models for this sort of thing too. Environmental biologists use them to study predator-prey relations, and fictional mathematicians use them to catch escaping criminals ;-) (Actually, to be fair, so do real mathematicians sometimes — a lot of those plot lines are based on true stories) These sorts of models tend to be more complicated, as you’d expect.

Anyway, for now I think it makes sense to start simple. With that in mind, this will be the First Operational Definition of a zombie (capitalization is totally tongue-in-cheek, but at some point I might expand this into a real zombie classification system). It’s basically a list of characteristics of the simplest possible model that corresponds to something people would think of as a "zombie."

  • Zombies are slow. Specifically, a zombie’s top speed is slower than a person’s top speed. Zombies always move at their top speed, when they are moving at all.
  • Zombies are dumb. A zombie moves in a random path, until it gets within some distance of human flesh, and then it makes a beeline for the flesh.
  • Zombies are formed from (relatively) intact human corpses. This means that any process which produces a zombie must begin with the loss of a living person. The requirement of an intact human corpse implies that it is possible to kill people in such a way that they cannot be zombified.
  • Zombification agent is transmitted by human-zombie contact. I’m going to assume, as is typical in the movies, that there is some infectious agent (like a virus) that causes a human to become a zombie after his/her death. This infectious agent can only be transmitted by some particular kind of contact between a living human and a zombie (e.g. a bite). Of course there has to be an exception, some initial "seed" process that produces at least the first zombie, but I’ll assume that that is relatively localized and short-lived.
  • Zombies can be killed. This one seems to be rather contentious, at least in certain circles, but I think most of the primary sources (movies and books) are in agreement: there is some way to kill a zombie such that it cannot possibly be reanimated. Sort of ties into the intactness requirement from the corpse point.

There’s a problem with this list, though: if zombies are slow and dumb, wouldn’t everyone see them coming and run away? How would you ever get the direct physical contact needed to transmit a zombie infection? Well, you probably wouldn’t, in reality. But since this is a super-simple zombie model, I’m going to make an equally simplifying assumption about people: we’re also dumb. Not that people move toward the closest living flesh, but we do kind of move in random paths. I suspect that if you compared a model of a bunch of people moving randomly with actual data on the movements of people in a large city, you’d find that on a large enough scale, they’re fairly similar, at least for the purposes of modeling infectious disease transmission. And I also suspect that some people in a real major city could be kind of oblivious to a zombie roaming the streets, until it was too late for them. (Not to mention, you know there’d be zombie biologists who would insist on studying live samples)

But anyway, it’s just a model, and just a starting point. What I hope to do in the future is refine the assumptions of this model to make more realistic ones, and thereby get better and better approximations to a real zombie apocalypse.

See, I am not the only one who thinks about this stuff

Posted in Reviews on July 2nd, 2010 by David – View Comments

I was quite pleasantly surprised to see a real physics blog taking on the thorny problem of zombie avoidance. Sure, this is not exactly large-scale dynamics, but you know that keeping humanity alive throughout the apocalypse will, at some point, come down to these close encounters. So if you ever find yourself going one-on-one with a zombie, what should you do?

Well, it’s generally assumed that zombies are slower than people. Otherwise we’d be screwed. Anyway, given that assumption, you can run in a straight line in any direction and eventually outpace the zombie. But if you’re confined by walls, this is a problem. The gist of the Dot Physics post is that running around in a circle is a decent strategy because the zombie keeps chasing toward you (zombies are dumb), but since you’re constantly moving in an orthogonal direction, it never catches you.

Lest you think this is purely theoretical, check out Zombie Assault 2 on AddictingGames. The game is simple: you’re stuck in a building with rooms, and there are a lot of zombies. Kill them before they kill you. I’ve known from experience that one of the better strategies you can take when you’re overwhelmed by the horde is — somewhat counterintuitively — to hole yourself up in a room and just keep moving in one direction around the edges. What happens is exactly what the Dot Physics post shows: the zombies follow you around the room, never catching up to you, and as a bonus, they all get packed into a nice line so they’re easy to pick off.

Seriously, go try it.

Anyway, like I said, I’m really happy to see zombie science making a splash in the wider physics community. I have got to get cracking on this blog.