In 1994, a group of students at Albright College suddenly realized that Kevin Bacon made a lot of movies, with a lot of different co-actors. They could connect any actor to Kevin Bacon through common movies. This turned into the Kevin Bacon game, which quickly gained popularity after its debut on the Jon Stewart show. The game can also be played online at the Oracle of Bacon at Virginia, built by Brett Tjaden and Glenn Wasson. Technically, it’s a “cheat machine” for the game, as it gives you the answer right away. The website uses information from the internet movie database to immediately link any actor to Kevin Bacon.
The game is also known as “Six Degrees of Kevin Bacon”. This name comes from the similarity of the game to the concept that any two people are connected by a string of 6 people. In other words “six degrees of separation”. This term was fresh in everyone’s head in the early nineties, as John Guare had written a play called “Six degrees of Separation”, based on the real life story of David Hampton, who had conned multiple people in the eighties by pretending to be the son of actor/director Sidney Poitier. The play was turned into a movie in 1993.
But what does all this have to do with science?
The phrase “six degrees of separation” comes from work done by Stanley Milgram, a social psychologist. In 1967 he coined the term “small world phenomenon” – the hypothesis that anyone can be reached by a relatively short chain of acquaintances. Milgram tested his theory by having random people in Omaha try to reach a target in Massachussets. Participants knew the name, occupation, and general location of the target, and were asked to pass on a letter to one of their acquaintainces who they expected were closer to the target, aiming to eventually reach the final target. The experiment showed that the target could be reached in about six steps. Milgram’s original paper was controversial, however, because he only published the succesful chains, and neglected to report on the letters that never reached their target. These experiments have later been repeated with e-mails, and still show a very short amount of steps between original sender and recipient.
“Small world phenomena” are not just limited to social interactions. Duncan Watts and Steven Strogatz have shown that the phenomenon is a general property of several types of networks. Their version of “small world networks” no longer applies to just people (the network of actors, for example), but also electricity grids, road maps, epidemics, protein interactions, neural networks, and peer-to-peer filesharing networks. Watts and Strogatz developed a model to test whether a network is a small world network or not. If it is a small network, then all nodes can be connected in a small number of steps. In the case of the network of actors, the average number of steps is in fact a lot smaller than six – actually closer to three.
The work of these researchers formed the basis for a new kind of science: network science, a branch of computer science and mathematics. It explains why the Kevin Bacon game works, but also has more useful purposes, such as predicting the spread of epidemics or the effect of a failing power grid. You can’t get funding on explaining the Kevin Bacon game!
Nevertheless, scientists love games, and mathematicians have been calculating their Erdos number for years. Paul Erdos was the Kevin Bacon of mathematicians, having published more than 1400 papers with numerous collaborators. Anyone who was a co-author with Erdos has an Erdos number of one. Their collaborators, in turn, have an Erdos number of two, and so on. The network of mathematicians is a small-world network. It works because papers have multiple authors. Other fields work the same way, and a 2001 paper by Mark Newman shows that the collaboration networks of biomedical scientists, physicists, and computer scientists are also small world networks. The average distance between two biomedical researchers, according to his paper, is 4.6 – larger than that for actors, but still smaller than the mythical 6.
- Six Degrees – Book by Duncan Watts. I got almost all the info from there. It’s a great book: it’s actually a good read, not just informative.
- Small World Networks on Wikipedia – this week’s “collaboration of the week” at Wikipedia.
- The Oracle of Bacon at Virginia – How many degrees is your favourite actor removed from Kevin Bacon?
- Internet Movie Database – source of data for the Oracle
- Six Degrees of Separation – the movie, at the IMDB
- E-mail experiment – New Scientist article about the e-mail experiment used to test “six degrees”.
- Erdos Number Project