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T.WO CENTURIES ago Hans Christian Oersted, a Danish physicist, showed that the movement of an electric charge produces magnetism. This was the first observation of a far-reaching phenomenon. Particle-charged clouds that float in the cosmos generate vast interstellar magnetic fields as they move. The lapping of molten metal in the Earth’s core produces the planet’s north and south magnetic poles. Activating nerve cells in a human brain also creates a miniscule amount of magnetism.
The ubiquity of such electrically generated magnetic fields, however, brings problems ranging from the pragmatic to the esoteric. Doctors watching MRI scans, for example, must compensate for background magnetism. Meanwhile, experimenters conducting precision tests may have to build complex shields to obscure the magnetic effect of something as simple as an electrical wire running through their laboratory wall.
It would therefore be useful to be able to control, limit or model magnetic fields from a distance. Useful, but apparently impossible. Because, in 1842, Samuel Earnshaw, a British physicist, proved mathematically that the maximum strength of a magnetic field cannot be found outside its source. Each of these fields must, in other words, surround and radiate from the object that generates it. And things remained until Rosa Mach-Batlle of the Autonomous University of Barcelona found a way around Earnshaw’s conclusions. It didn’t actually prove him wrong. But it proved that multiple magnetic fields, each of which individually obey Earnshaw’s theorem, can collectively seem to circumvent it.
As they describe in Physical Review Letters, Dr. Mach-Batlle and her colleagues came up with their makeup in a surprisingly simple way, by arranging 20 straight strands next to each other in the form of a 40cm high cylinder and 8cm in diameter, with a 21 ° running through the center of the cylinder. As they passed electric currents through all 21 wires, a complex pattern of magnetic field lines blossomed in the surrounding area, forming shapes that varied with the strength and direction of the individual currents.
By choosing the right combination of currents, the researchers found that they were able to create a field pattern emanating from a virtual version of the 21st wire that did not cross the center of the cylinder but, rather, 2 cm outside it. In other words, if the generating apparatus were shielded from an observer, Wizard of Oz style, by a curtain, it would appear to that observer as if this field appeared out of nowhere.
Going from Dr Mach-Batlle’s demonstration to something that could be used in practice to manipulate distant magnetic fields will be a long journey. But if this journey can be made, the potential applications go far beyond blurry cleaning MRI scans. Remote-fired fields of this type could be used to guide medical nanobots through someone’s bloodstream to deliver drugs to a particular tissue, or to guide them to a malignant tumor and remotely raise their temperature once they arrive, in order to cook it to death. There are also likely to be applications in quantum computing. Many quantum computer projects rely on trapping atoms at precise points in space, a difficult feat that this sleight of hand could simplify.
The trick still requires refinement. To achieve such desired applications, the team must be able to sculpt complex magnetic fields in three dimensions. Currently, limited to emulating the field generated by a single electric wire, they cannot. But it is worth remembering that Oersted’s original experiment, from which all electrical engineering ultimately descended, was even simpler. It involved only a battery, a magnetic compass, and a single wire. Large oaks grow from small acorns.■
This article appeared in the Science and Technology section of the print edition under the title “Out of scope on the left”
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