Leading article: From a butterfly's wing to Burger King: it really is chaos out there

There were no butterflies in Burger King in Heathrow's Terminal One in the early hours of yesterday morning, as far as we know. The famous illustration of chaos theory imagines how the fluttering of a butterfly's wings could cause a hurricane a thousand miles away. It is a mathematical speculation intended to help understand the unpredictability of natural systems - how a tiny perturbation could make a difference between one climate pattern or another.

Now we have to invent a new chaos theory to help understand the unpredictability of human systems. Yesterday, a small fire in a burger bar stranded people in Aberdeen, grounded planes in South Africa and snarled up the motorways around west London. Anyone with the time and inclination to listen to or watch global, 24-hour news services could follow the ripples from a relatively trivial event as they spread out in distance and scale. There is a childish pleasure in watching a chain reaction develop in a series of events, some predictable, some unexpected. Of course, the disruption was serious, and inconvenient, and will cost businesses and individuals millions of pounds. Shares in BAA fell 13p before bouncing back as investors realised the losses were covered by insurance. But it was fun to watch the ripples. It was fun in the way that Heath Robinson's unlikely contraptions are fun, or the children's game in which a marble rolls down a slide, tips a see-saw which swings a boot which kicks a cage which wobbles down a pole to catch a mouse.

Because Heathrow is the busiest hub of world air travel, and even though Terminal One mostly handles domestic and European flights, a fire in a burger bar air duct caused global gridlock. Well, if not gridlock, then a premonition of what might happen when the world becomes even more interdependent, more criss-crossed by webs of communication and transport.

With 180,000 people flying in and out of Heathrow every day, the greasy glitch touched all sorts of lives at random. John and Gillian Kernon did not manage to get from Somerset to Nuremberg for the Christmas fair. Our own political editor found himself in the House of Commons when he should have been in Luxembourg annoying the Prime Minister.

And, because there is only one road in and out of Heathrow through the tunnel under the runways, traffic tailed back all round the M25 and along the M4. As a result, there was an eerie calm in the streets of central London, because morning commuter traffic could not get through. Who knows how many people were late for work, how many delivery deadlines were missed, how many millions were lost?

The growing complexity of human systems is understood by terrorists, which was why the IRA, in the days when it knocked on the door of Downing Street with mortar bombs, also launched a mortar attack on Heathrow airport. Between the time when it was fighting a war against the "occupying" British army, and the time it demanded photocopying facilities in the House of Commons, the IRA went through a phase of targeting the nodal points of British social and economic organisation in order to cause maximum disruption. The attempt to blow up six electricity substations around London would have been a bit more than the flap of a butterfly's wing, but it was intended to cause dislocation out of all proportion to the trigger event.

It is salutary to be reminded of the frailty of complex systems. It was a tiny flicker of static electricity which destroyed the TWA jumbo jet over Long Island, according to the latest theory. A newspaper like this one relies on hundreds of computers linked by wires and switches. If one "computer server" crashes, the whole system stops. So the people who know what a server is have devised back-up systems and contingency plans, and another layer of unpredictable complexity is added. There is nothing new in this: in the old days of hot metal and typewriters the print unions understood too well how vulnerable the whole process was at certain links in the Heath Robinsonian chain.

And the butterfly's wing principle of momentous consequences flowing from trivial events has always given Fate the appearance of inevitable and logical unfolding. A few years further back in history, the Romans understood how the genes which shaped Cleopatra's nose started a war and destroyed an empire. Even the First World War seemed something of an overreaction, to say the least, to the anarchist's assassination of Archduke Ferdinand.

What has changed, though, is the speed with which a whole city, or world- wide Internet communications, can be brought to a halt by an escalating series of coincidences. Humanity is engaged in a constant battle to stop the complex systems it creates from seizing up. Traffic lights are computerised and linked to road sensors to keep the jams crawling. New bits are being plugged into the Internet to keep it growing faster than junk e-mail slows it down. New computer programs are being written to override the mistakes made by the old ones.

So the next time someone complains about the traffic and says, "It's chaos out there," point out how the non-linear dynamics of road transport are helping us understand the fuzzy logic of human systems. They will thank you for it.