separation". If you read it you will realize that it could help explain why
financial markets move they way they do.
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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.
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 aunt in Nairobi.
Margaret Owino then sent it to a friend in New York state who used to live
in Nairobi, 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.
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.
Swine flu spread
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.
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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