HOW KEVIN BACON SPARKED A NEW BRANCH OF SCIENCE

The theory that everyone in the world is six friendships away from everyone else is regarded by many as a myth. So what happens when the theory is put to the test?

The thought that all 6.9 billion people on the planet could be closely connected to one another through their network of friends has a long-held fascination.

For decades, scientists have tried to prove that the world is made up of social networks that are ultimately interconnected.

The theory that there were "six degrees of separation" between everyone - with each degree being a person they knew - entered the mainstream when John Guare wrote a play of that name, followed by a 1993 film starring Will Smith.

The following year, a group of US students came up with the party game Six Degrees of Kevin Bacon, apparently after a late-night viewing of back-to-back films starring the actor. They began to pick random Hollywood names and then connect the actors to Bacon via films in which both had appeared.

IT'S A SMALL WORLD In 1967, Stanley Milgram asked 160 people in Nebraska to get a parcel to a named person in Boston He found that some people have three links of separation, some have 100 and some have none In 1973, American sociologist Mark Granovetter discovered society is made up of clusters of people, with strong and weak links British photographer Andy Gotts spent eight years taking pictures of 115 actors linked by friendship

That sparked a website, a board game and a book, and the phenomenon was born, although many dismissed its accuracy. So how true is it?

Recreating a famous experiment done in the 1960s, 40 parcels were given to people randomly picked around the world, as part of a BBC programme testing the "six degrees" theory.

They then had to try and get the parcel to a scientist called Marc Vidal based in Boston, via someone they knew on a first-name basis.

Three of the parcels made it to Mr Vidal, and on average took six steps to get there.

Organisers of the experiment believe the other 37 chains broke because of the apathy of individuals who failed to send the parcel on.

One that made it started off in the remote village of Nyamware in Kenya, where Nyaloka Auma gave it to her aunty in Nairobi. She then sent it to a friend in New York state, who in turn sent it to another friend in Boston. Eventually it arrived in the hands of the right person, through seven people in total.

FIND OUT MORE... Six Degrees of Separation is broadcast on BBC Two on Tuesday 5 May at 1900BST Or catch up using the iPlayer

Its success, and the fact that two other parcels also reached their destination within six links, proves that the theory can work, even if it does not always.

There have been many other attempts to test how "small" the world really is. Last year, Microsoft examined its instant-messenger network of 30 billion electronic conversations between 180 million people.

Its researchers concluded that any two people are on average apart by 6.6 degrees of separation, meaning that they could be linked by a string of seven or fewer acquaintances.

The six degrees theory has had a huge impact on other areas of science.

When mathematician Steven Strogatz and his PhD student Duncan Watts set out to investigate why some crickets chirp in unison the young Australian got inspired by his father's words in a phone call: "Did you know that you are only six handshakes from any person on Earth?" Watts thought this might explain why his crickets are such good synchronizers but he had no idea that this question would lead him to a major discovery in an emerging branch of research - network science.

WHY KEVIN BACON? Name loosely rhymes with 'separation' Students who invented the game watched two of his films that night Bacon said in an interview that he had worked with everyone

If the phenomenon applies to cricket synchronisation, what are the consequences for the way disease spreads throughout a human population, or the dynamics of markets, he asked.

These sorts of questions seem to have something to do with networks. Teaming up with Strogatz, the two scientists used data from the online trivia game Six Degrees of Kevin Bacon to see if there was a mathematical basis for this complex set of relationships.

They discovered a formula for the invisible links that make the big world small, and another scientist Albert-Laszlo Barabasi examined the importance of hubs in how networks evolved.

A new discipline - network science - was established and other scientists gladly applied these universal laws to other kinds of networks, such as the world-wide web, the growth of cities, global travel, sexual relations between people, wealth and property distribution, and protein molecules in cells.

Professor Alex Vespignani, of Indiana University, says network theory is having a huge impact on predicting how swine flu will spread. His work is being used by the European Community and authorities in the US.

How people go from one place to another - what he calls mobility networks - is at the heart of his predictions, and involve long-range journeys (such as by plane), short-range commutes, and how we move around small spaces like the workplace or the home.

"We can identify the pathways along which the disease will spread and therefore the next places to observe the cases. "That gives you time and where you can focus. With limited resources you need to make choices and focus on certain routes. Network theory can tell you and it's not just about looking at the obvious."

Marc Vidal, the "target" of the parcel experiment, is creating the first "roadmap" of the human cell - how proteins interact - in order to find the origin of disease.

"If I start with my favourite protein and I ask what does it interact with, I'm now back to basically a problem of six degrees of separation - who is connected to whom."

Although scientists may disagree on how much emphasis they give network theory, it is now a well established principle.

And it's all down to Kevin Bacon.

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