Physics always seems to want to come out and play. Just when this most technical of sciences starts to become impossibly arcane, it goes goofy on you, as it did last year with the announcement that physicists at the University of California, Berkeley, had developed a tiny working model of an invisibility cloak. This week, the physics magic shop announced yet another wonder: levitation. Really.
The ability to levitate objects is not an entirely new thing in physics. Lower the temperature of certain metals and ceramics far enough (–459°F is a good number to shoot for), and they carry electromagnetic charges far more efficiently and for a far longer time than they otherwise would. When the metals are magnetized, they become so powerful that their ability to repel one another can actually allow them to lift heavy objects off the ground. That's the elegant principle behind some kinds of magnetically levitated (maglev) trains. (See the 50 best inventions of 2008.)
But maglev takes a load of hardware and a ton of power and is useless for small, simple kinds of engineering. The new breakthrough — achieved by a joint team of researchers from the National Institutes of Health (NIH) and Harvard University and published in a paper in the journal Nature — provides an alternative.
Everything in the universe — metals, gases, dogs, doughnuts — is made of materials with positive and negative charges. Opposite charges attract each other; identical charges repel each other. What prevents us from sticking to anything with an opposite charge is that all these forces have to be properly aligned before you can see them at work. "The materials are in motion, but sometimes the dance of the charges allows them to fall in step," says NIH physicist Adrian Parsegian, one of the authors of the paper. "When that happens, you get attractive forces."
What makes things trickier still is that not all attraction is equal. Some materials are drawn much more powerfully together than others — particularly on the nano (billionth of a meter) scale. And that difference can be exploited. In the Nature experiment, the research team began by placing a microscopically small sphere of gold on a glass surface. Gold and glass get along well enough and under the right circumstances will attract. But what they both like a whole lot more is a liquid called bromobenzene. When the researchers introduced a little bromobenzene to the other two materials, they both began drawing so much of it that the gold began to rise above the glass. In effect, it levitated on a thin bromobenzene film.
O.K., it's not Houdini. The microscopic pas de deux isn't even visible to the naked eye. Still, the phenomenon is not as uncommon as it might seem. Every time you ice skate, you experience something similar, as the shared properties of skate blade against ice create a thin film of water of a very particular thickness on which you, after a fashion, levitate. What makes the Harvard and NIH work so promising is its nano scale.
Increasingly, nanoengineers are working to develop medical devices, batteries, electrical switches and more made up of microscopic parts that float above one another on thin films of other materials. This increases efficiency, reduces friction and allows the hardware to be built to finer tolerances and tinier sizes. Design them small enough, and you can put them in microscopically tiny places machinery could never go before. "When you understand the forces you're manipulating," says Parsegian, "you can design efficiently at the nanometer scale."
By any measure, that's important stuff, though as with last year's invisibility cloak, it doesn't portend magical applications in the everyday world — and won't for a long, long time. "If you're looking for a free trip for your body on quantum levitation, you're not going to get it with this," says Parsegian. Even at its most fanciful, physics, it seems, can play around for only so long before it gets back to serious work.