# ICCS: Time-Dependent Networks

Yesterday, the International Conference on Complex Systems wrapped up with five talks on networks. For me, the most interesting was that by Dan Braha, who spoke about what happens when you analyze a system as a network which changes over time, instead of the aggregate network formed by lumping all the timesteps together. Imagine a system made out of a whole pile of parts. At time $t$, part number $i$ might or might not be interacting with part number $j$, which we could represent as a time-varying matrix $C_{ij}(t)$. Many studies of network-related phenomena obscure the time-dependence part. For example, in a living cell, genes are switching on and off, concentrations of enzymes are going up and down, and all sorts of stuff is changing over time. You can mix proteins A, B and C in a test tube; perhaps A bonds both to B and to C. You’d then draw an interaction network with links connecting A to B and to C — but what if B and C are never present in the cell at the same time?

Braha and company looked at a collection of e-mails sent over 113 days, exchanged among 57,138 users. (The data comes from arXiv:cond-mat/0201476v2, published five years ago in Phys. Rev. E, and were gathered at Kiel University.) A node is an individual e-mail address, and a link is established when a message is sent from one address to another. They found, among other things, that whether or not a particular node is a “hub” changes over time: popular today, an outcast tomorrow. Moreover, a node which is in the top 1000 most connected on one day may or may not be in the top 1000 for the aggregate network. Furthermoreover, when the window of aggregation is gradually increased — from one day to two days, to a week, up to the entire time period — the similarity to the total aggregate network increases, as you’d expect, but without any threshold.

In the last few minutes of his talk, Braha did a brief overview of a related investigation, in which they studied a “social network” derived from Bluetooth devices. If my Bluetooth gizmo is within two meters of yours, we’ll call that a link. The network of Bluetooth devices will naturally change over time, so we can do the same comparison between the graphs observed at short timesteps to the graph formed by aggregating all connections. During the Q&A session afterwards — before I had to, ironically enough, run off to find my cell phone — I pointed out something which it appears Braha hadn’t fully grasped.

# ICCS: Pictures

I seem to have become the official Conference Paparazzus. Some of my photos are standard conference fare. For example, here’s Gregory Chaitin having a chat with his session chair, Bob Savit:

However, every picture I have of Philip Zimbardo looks at least a little. . . evil.

This is, in fact, how many of the people in the audience will remember Dr. Zimbardo:

# Update on the D-Word

Dwight Read is another academic who uses the word Darwinism to refer to evolution by natural selection. During his plenary talk this morning, Read spoke of “Universal Darwinism,” Dawkins’ term for the idea that natural selection is not substrate-specific and can in principle be applied to non-biological things, like cultural memes.

As I mentioned earlier, it’s folklore among the science-blogging community that British academics are more likely to use “Darwinism” in this sense than Americans are. (Over here, hearing the D-word is a pretty sure sign you’re dealing with a creationist, or at least somebody whose knowledge derives too much from creationist sources. I wonder if there’s also a bit of national pride at work.) Read is currently at UCLA, but in 1999 was a visiting professor at the University of Kent, Canterbury.

# ICCS: Emergence in Particle Systems 1

I typed the following notes during Hiroki Sayama‘s presentation on “Phase separation and dynamic pattern formation in heterogeneous self-propelled particle systems.” Unfortunately, I couldn’t get a WiFi signal in the room where Sayama gave his talk, so I’m falling short of the gonzo science ideal, posting about the talk after it was given instead of as it occurs.

Sayama is speaking about particle swarm systems, and the phase-separation and dynamic pattern formation behaviors they exhibit. He adds the novel feature of heterogeneity to the particle system. Research on self-propelled particles goes back to Reynolds (1987), Vicsek et al. (1995), Aldana et al. (2003), Chuang et al. (2006), etc. Reynolds was a computer scientist who created a method for simulating bird flocking, which developed into the simulation which created the bats in the otherwise unremarkable Batman Begins. Vicsek and Aldana were physicists.

These systems show collective behaviors such as random clustering, coherent motions and milling. The same system can exhibit all of these behaviors, depending upon the input parameters. Cranking up the noise can induce phase transitions. Almost all of this work focused on homogeneous particle systems, in which all particles share the same kinetic particles. What, then, would happen if two or more types of self-propelled particles were mixed together?

Sayama works in a framework he calls Swarm Chemistry, which is implemented as a Java applet that can be run online.
Continue reading ICCS: Emergence in Particle Systems 1

# ICCS: Monday Evening

The parts between talks are the best parts of conferences. Sure, it’s great to hear Greg Chaitin deliver his sermon about the ideal realm of pure mathematics being an infinite ocean of complexity, out of which we can only seize finite buckets — but Chaitin writes about that kind of thing, and you can read it for free online. It’s an altogether different experience to discuss during the coffee break Mike Stay and Cristian Calude’s paper, “From Heisenberg to GÃ¶del via Chaitin,” with one of the three men in the title.

Question-and-answer sessions after the presentations can also be quite good. Last night, for example, Barbara Jasny of Science Magazine explained how that publication is adapting to the whizbang modern world. It’s reassuring to hear that at least one person in the publishing community has a common-sense understanding of what cheap, open digital access means: journals can only justify charging prices if those prices reflect the actual value which those journals add. More interesting than that, however, was Jasny’s reaction to the question from Frannie Leautier, former Vice-President of the World Bank and currently head of the World Bank Institute. Leautier asked if Science would publish articles which used cartoons as illustrations (instantly endearing herself to all the Larry Gonick and Sid Harris fans in the audience).

# ICCS: Sunday Morning

The following is my first attempt to liveblog ICCS 2007. I arrived at the Quincy Marriott shortly before 8:30 this morning, having driven south on I-93 from Boston. Unlike the first time I drove out here, I didn’t get lost in Braintree, since I took the left fork at the “Braintree split,” where I-93 undergoes mitosis. These things are important to know.

The morning’s plenary talks began with Diana Dabby (Franklin W. Olin College of Engineering), who spoke about chaotic transformations one can apply to music in order to generate musical variations, as in “Variations on a Theme of Beethoven.” Her scheme begins by breaking the musical performance into a sequence of pitches, denoted $p_i$, and then mapping each $p_i$ to a section of a dynamical trajectory on a chaotic attractor like the Lorentz owl/butterfly mask.