Mark Buchanan Ubiquity The Science of History Why the World is Simpler than we Think Weidenfels&Nicolson 2000

pg 85 SELF-ORGANISED CRITICALITY Keywords: Enrico Fermi: the first nuclear reactor 1942 - … nothing will reach the critical point all by itself. Tuning is crucial. – magnets 770degrees - sand‑pile game seemed to develop into the critical condition quite naturally - 'self‑organised criticality' - The pile grew, became steeper, and then the avalanches began. At first, they only involved a few grains. As the pile grew, so did the typical size of the avalanches. Ultimately, the pile entered the critical state and was susceptible to avalanches of all sizes, as had Fermi's pile when he tuned it properly. But Bak, Tang and Weisenfeld had not tweaked any knobs to bring it there. The critical organisation welled up on its own. Recognising a miracle when they saw one, they enshrined it with the name 'self‑organised criticality'. For the first time in history, physicists had an example of something in which the spectacular organisation of the critical state seemed to arise completely for free, with no tuning needed. ...In the early afternoon of 2 December 1942, a team of physicists filed down the stairs leading to the squash courts underneath the football stadium of the University of Chicago. It was the occasion of an historic experimental test. In a makeshift laboratory set up in one of the courts, the team had built what was meant to be the world's first nuclear reactor. They had drilled holes into an enormous block of graphite, and inserted long rods of enriched uranium. Enrico Fermi, the project's mastermind, had discovered six years earlier that uranium nuclei, when hit by a neutron, could be made to split apart and release further neutrons. These, slamming into other uranium nuclei, could in principle trigger still further splittings and an avalanche of further neutrons ‑ a self­sustaining nuclear reaction. That was the theory, at least. The theory also held that unless one was careful, the reactor would kick into gear all by itself. After all, even without prodding, a uranium nucleus disintegrates every so often, and emits a neutron. Under the right conditions, even a single such neutron could trigger a runaway chain reaction. To keep his pile from going off before he was ready, Fermi had inserted 'control rods' made of cadmium in among the uranium fuel rods. These kept the brakes on the pile by gathering in neutrons and ensuring that the avalanche triggered by a single neutron would soon die out. But on this day, Fermi was ready to take the brakes off, and see what would happen. A little after 3 p.m., everyone took a deep breath as Fermi grabbed hold of a rope and began sliding the cadmium rods ever so slowly out of the block. The physicist Eugene Wigner stood nearby with a celebratory bottle of Chianti, nervous but hopeful. As the rods inched out, a Geiger counter began registering the occasional click; a bit further, and it began rattling like a machine gun. Fermi had calculated with his slide rule how far he could go before the pile would run away into a catastrophic chain reaction, and at 3.36 p.m., as he approached that point, the Geiger counter went crazy. Fermi stopped pulling the rods out. He had tuned the pile to within a whisper of the critical point, where a single neutron could trigger an avalanche of any size. The message in this story is that nothing will reach the critical point all by itself. Tuning is crucial. Whether in a nuclear reactor or a magnet, it takes effort to create the critical state in which any tiny event can trigger a huge and lasting upheaval. Toss any old piece of iron into a furnace, and it will pass through the critical state as it heats up. But to keep it there within a sliver of 770 degrees Celsius takes tuning: miss by a degree or two, and you won't see the rise of factions. This is why, in 1987, Bak, Tang and Weisenfeld were puzzled, mystified and astounded that their simple sand‑pile game seemed to develop into the critical condition quite naturally. Starting with a flat surface, their computer dropped grains slowly and at random. The pile grew, became steeper, and then the avalanches began. At first, they only involved a few grains. As the pile grew, so did the typical size of the avalanches. Ultimately, the pile entered the critical state and was susceptible to avalanches of all sizes, as had Fermi's pile when he tuned it properly. But Bak, Tang and Weisenfeld had not tweaked any knobs to bring it there. The critical organisation welled up on its own. Recognising a miracle when they saw one, they enshrined it with the name 'self‑organised criticality'. For the first time in history, physicists had an example of something in which the spectacular organisation of the critical state seemed to arise completely for free, with no tuning needed. Moreover, the organisation was resilient. Reach in with your hand and wipe away half the pile. No matter. As grains continue to fall, it will again organise itself into the critical state. No one would expect to wander out into the wilderness and find a magnet in a critical state. For a magnet, that state is extremely special. But if a critical state can arise naturally, it is only a short step to suppose that the sand pile might not be the only thing in nature with this remarkable property. Isaac Newton discovered his laws for the motions of planets, and they turned out to apply to comets, raindrops, falling apples, satellites, and, eventually, to fluids and aeroplanes and almost everything on earth …and beyond earth. Max Planck discovered the roots of quantum theory in trying to explain the colour of the light emitted by hot objects, and his discovery soon spilled out into every other corner of physics. After a great discovery, scientists suddenly see everywhere that which they had never seen before. We already know that self‑organised criticality appears to account for the unruly and unpredictable workings of the earth's crust, which seems to live in a critical state. The slow, inexorable drifting of continental plates is something like the dropping of grains, and has brought the crust into a state in which 'avalanches'‑ in this case involving the rocks slipping past one another along fault lines ‑ come in all sizes. Are there other things in the world, things that seem just as complicated, but which also share the same essential logic of the sand‑pile game? For more than a decade this question has been surrounded by a 'hectic air of controversy'. Physicists still haven't nailed down all the answers, but what they have found is as subtle as it is fascinating.

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