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Some Levity in Physics

David Schneider

Seeing the April 1997 issue of Physics World, a publication of the Institute of Physics in the U.K., many readers chuckled at what they thought was an April Fool's joke: the report of an antigravity device being used, among other things, to levitate a frog. The appearance of a subsequent notice in a British tabloid, The Sun, probably did little to dispel suspicions of a hoax.

Yet the scientists involved in the work, led by Andrey Geim of the University of Nijmegen in The Netherlands, were being quite serious when they announced that they had floated a live frog using a powerful electromagnet. That a living creature could be lifted in this manner came as a complete surprise even to many knowledgeable physicists. But the flying frog was merely a demonstration of a fundamental property of ordinary materials—and of a technique now being used to perform "low-gravity" biological experiments that might otherwise have demanded a ride on the Space Shuttle.

Most everyday materials—including water and living tissues—are weakly magnetic. They are said to exhibit diamagnetism, a slight tendency to become magnetized in the direction opposite to an applied magnetic field. A diamagnetic object placed in an intense magnetic field that is configured to diminish in strength with height will experience an upward force. So the field of a sufficiently powerful electromagnet can balance the downward tug of gravity—at least over a small volume.

Although materials normally recognized as magnetic (including iron, permanent magnets and so-called paramagnetic substances) can also be lifted with an electromagnet, they cannot hover in midair, unless some feedback mechanism adjusts the surrounding magnetic field constantly to maintain a fixed position. But the levitation of diamagnetic materials is inherently more stable. If the floating diamagnetic object rises slightly, the magnetic force on it diminishes, and it settles back down; if it falls, it automatically gets a boost from the increased magnetic force exerted below its equilibrium position.

James Valles, a physicist at Brown University took advantage of such diamagnetic levitation to perform a biological experiment well before news of the technique had filtered into the mass media. A colleague of his at Brown, Humphrey Maris, had been levitating helium droplets with this technique to address questions of fundamental physics. So Valles decided to try to float the embryos of a frog, which are affected by gravity in easily observable ways. Although his calculations showed that it should be possible to levitate water (or living cells), when he first proposed to do so using a strong electromagnet at the Massachusetts Institute of Technology, his idea was not immediately embraced. Valles recalls that the people he approached at MIT needed first "to check me out to see if I was a quack."

More recently two other physicists, Mark Meisel of the University of Florida in Gainesville and James Brooks of Florida State University in Tallahassee, arranged another biological experiment in simulated low-gravity conditions. Meisel and Brooks, along with several other colleagues, are studying how plants fare without the pull of gravity, an effort they hope will help illuminate why plants tend to grow poorly in space.

Meisel was aware of studies showing that Arabidopsis (a mustard plant, which he calls "the lab mouse for plant growth") becomes stressed by even short periods without gravity: His wife, Anna-Lisa Paul, a horticultural scientist at the University of Florida, had participated in that work, taking plants up with her on NASA's "vomit comet," an aircraft that briefly achieves low-gravity conditions by flying in a parabolic arc. Meanwhile, Brooks had been experimenting with diamagnetic levitation ever since he saw Geim's flying frog on CNN. So together they arranged to float Arabidopsis seedlings using one of the powerful electromagnets at the National High Magnetic Field Laboratory in Tallahassee.

Their preliminary tests showed that the Arabidopsis plants became stressed after just a few hours of simulated zero-g. But control specimens grown at the same time reacted to a strong magnetic field of uniform intensity—that is, one that did not exert a levitating force. So this work highlights a fundamental difficulty: Investigators must be careful to distinguish between the results of counteracting gravity and the effects of intense magnetic fields.

Despite such complications, others are now exploring the use of diamagnetic levitation for their own biological studies. Although Eric Beaugnon (currently at the crystallography lab of the Centre National de la Recherche Scientifique in Grenoble) and a colleague had reported in Nature a full eight years ago that organic materials can be made to float in this way, the press attention focused on Geim's frog can be credited with sparking renewed interest in the technique. "I spent a lot of time, probably a half a year of my life, to popularize this," says Geim. He was immediately rewarded with a flood of correspondence from scientists and engineers inquiring about levitation. Strange as it is to have newspaper reports prove more influential with physicists than a Nature article, Geim understands the virtue of making science appear exciting. "You see a frog that levitates in midair," he notes, "and you remember this for your whole life." –David Schneider

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