You've probably heard that every human on earth is separated from every other by six people, each of whom is connected in some way to the next one. This factoid became the basis for the title of a hit Broadway play, Six Degrees of Separation, and for a parlor game, Six Degrees of Kevin Bacon. Now, a new study ventures a digital take on the same idea, and concludes that no two Web pages are more than 19 clicks apart.
Of course, any Web page could link to any other page via a direct link, but the study supposes that no specific links are created, and that only existing links are used. The study has been published in the scientific journal Philosophical Transactions of the Royal Society by physicist Albert-Laszlo Barabasi, who is affiliated with both the Department of Physics at Northeastern University and the Harvard Medical School.
Barabasi, a network  expert, estimates that there are more than a trillion documents on the Web, embedded in an estimated 14 billion Web pages. Barabasi counts every image or video as a document, as well as the page itself and other files.
Nodes of the Web
Barabasi built a simulated model of the Internet, which led him to believe that the 19-connections rule will still hold even as the Web continues to grow. He found that most of the pages on the Web are linked to only a few other Web pages, but a relatively few places on the Web -- such as search engines, indexes or large news sites -- are highly connected. These are the nodes of the Web, acting essentially as Kevin Bacon does in the game referencing his name -- a circuit switcher through which connections are made from most pages to other pages.
Barabasi believes that this self-organization of the Web has taken place because, not unlike the six connections theory about human relationships, the Web is a human construct that tends to group itself into related communities. His research indicates that the links between pages, when seen as patterns across the Web, are not random but are the result of such social groupings.
Web Vulnerabilities
Whether looking at a small portion of the Web or across the entire ecosystem, Barabasi found that the same 19-link rule of connectedness often applies. One thing this structure reveals is the potential vulnerability of Web, since taking out key nodes could cripple the entire Net's connectedness.
The six degrees of separation concept proposes that every person in the world is only six or fewer people removed from anyone else. Social psychologist Stanley Milgram originated the six-degrees concept in 1967, based on a study of 296 volunteers.
A 2011 study by Facebook and the University of Milan, when Facebook had a mere 800 million users, found that 99.6 percent were connected through five other users to all other users, while 92 percent were connected by only four others. In any single country, the average was three people-connections.
The Kevin Bacon game is intended to show a connection of six people or less between that actor and virtually every other actor in Hollywood.
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