Text: http://www.nature.com/nsu/021209/021209-8.html Wobbling wire defies gravity Engineers could learn from motions that stand curtain cable upright. 11 December 2002 JOHN WHITFIELD Rapid vibrations make the wire wave violently. © Royal Society "It shouldn't work, but it stands upside down," says Tom Mullin. The vibrating piece of curtain wire in his laboratory has baffled physicists for several years. Now Mullin's team has explained the uncanny hardware. Their insights might find a use in bridge building, or laser technology. Held at one end, plastic-coated curtain wire stands upright if it is moved rapidly up and down a little. When vibrating slowly, the wire flops over. But if it is moved faster, it begins to wave violently. The wire stays upright because waving and flopping cancel each other out, says Mullin who works at the University of Manchester, UK. Once waving takes over, the two types of instability amplify one another, often leaving the wire bent out of shape. "What used to be two fairly small effects suddenly become large effects," explains Mullin's colleague Alan Champneys, a mathematician at the University of Bristol, UK. Amplified instabilities might occur elsewhere, he says, for example in the cables of bridges. Champneys and his colleagues' mathematical explanation of the levitating wire1 might give engineers new a way to work out whether their structures would be capable of such violent behaviour. The standing wire is an example of a phenomenon called parametric oscillation, in which one aspect of a physical system - in this case gravity - is changed rapidly. Some crystals can send laser beams into parametric oscillations, splitting them in two and allowing their colour to be varied. The same equations govern the waves on a cup of coffee Tom Mullin University of Manchester "Parametric oscillations stabilize things that normally you'd think of as unstable," says Mullin. In theory, he says, an upside-down cup of coffee could stay full if it was vibrated fast enough. Parametric oscillations can also make pendulums stand vertically, although the maths used to explain this is very different. To get a rope to stand upright, as in the Indian rope trick, would require impossibly fast vibrations.

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