Published by Australian Academy of Science
A quiet revolution – the science of complex systems Box 3 | It's a small world after all

Many complex systems are scale-free networks in which some nodes have many links, many nodes have just a few links, and the remaining nodes lie somewhere in between. In this network there is no clear average number of links per node. The internet can be represented as a scale-free network and so can the human population.

One of the properties of a scale-free network is that all nodes are only separated by a handful of links – everything is connected. That’s easy to appreciate for computers, but does it apply to humans?

Many researchers believe it does and have postulated that any person on the planet is only separated from any other person by only six degrees of separation. In other words, anyone on earth can be connected to any other person on the planet through a chain of acquaintances that has no more than four intermediaries. This is sometimes referred to as the small-world phenomenon and underscores the connected nature of the human world.

While the six degrees of separation concept has never been proved absolutely, the concept of the small-world phenomenon has played a valuable role in developing reliable communication protocols for the internet and ad-hoc wireless networks.

Related sites

• Six degrees of separation: Urban myth? (Psychology Today, USA)

• Email experiment confirms six degrees of separation (New Scientist, 7 August 2003)

Box 1. Complex systems, resilience and ecological sustainability

Box 2. Building reliable networks