Published online 12 April 2006 | Nature | doi:10.1038/news060410-6

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Jumpy eggs caught on camera

Give it a good spin and a hard-boiled egg will start leaping.

In a spin: watch it closely and you might see it jump.In a spin: watch it closely and you might see it jump.© Getty

After two years of work, with a purpose-built steel machine wired up to high-speed cameras, microphones and electronic sensors, a team of Japanese researchers has finally proved that a hard-boiled egg can jump1. All it takes, according to Yutaka Shimomura and colleagues of Keio University, is a good spin.

A spinning egg will spontaneously rise up from lying on its side to standing on its end. Shimomura, along with physicists at the University of Cambridge, had previously worked out why this is so, and predicted that the forces involved could also make an egg leap a tiny bit into the air2.

You wouldn't be able to see it leaping in your kitchen, though; the jumps are expected to be less than a tenth of a millimetre high and last for only a few thousandths of a second.

But does it really happen? To check, Shimomura's team had to make a device capable of spinning an egg perfectly, so they could be sure that the effect wasn't due to an upwards motion introduced by a spin done by hand. They also had to hit a spin rate of 1,800 revolutions per minute. So as not to be too messy and to ensure easy measurements, they used an ovoid-shaped metal egg in the experiments.

Set on spin

The team placed their egg horizontally on a slab of polished copper, started it spinning, and filmed it. Three different methods of detection all showed that the egg made tiny jumps before rising to a fully upright position.

A little bit of light was spotted shining through between the egg and the plate. More persuasively, microphones picked up the quiet cracking sounds of the metal egg hitting the plate in a series of impacts less than a second after the spin's initiation. And a break in physical contact between the two bits of metal caused a change in the electrical capacitance of the copper plate.

Having shown that the metal egg does indeed jump, the researchers moved on to a real hard-boiled egg. Only visual detection by the high-speed camera worked in this case. But since the set-up was the same as for the metal egg, the researchers could be confident that they were observing the same process.

Jump to it

Shimomura and Cambridge physicist Keith Moffatt first explained why a spinning egg stands up four years ago. Friction grips the egg at the pivot point and turns some of the spin into uplift (see '"Maths cracks egg flip": http://www.nature.com/news/2002/020325/full/020325-6.html'). "The egg sacrifices spin energy to achieve its rise," says Moffatt.

Shortly after their prediction that the same forces should also make an egg jump, researchers at Kobe University in Japan produced photographs that seemed to show a spinning egg doing just this3. But it has taken the experiments of Shimomura's team to prove that these jumps are real.

The jumps rely on oscillations of the spinning egg, which in turn are caused by random, small fluctuations in the spinning conditions, such as very slight asymmetry in the egg or roughness of the surface. This makes the egg rise in a slightly jerky fashion, and these jerks can be amplified to produce jumps.

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Shimomura says that the amplification of tiny, random fluctuations is common in other engineering situations, such as turbulent flows of air or water around large objects. So he thinks that the jumping egg is a nice 'toy' example of an important and widespread process.

Now the researchers hope to investigate whether jumping also happens for raw eggs, where the motion is complicated by the sloshing about of the liquid yolk.

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