TWO CENTURIES ago Hans Christian Oersted, a Danish physicist, demonstrated that the motion of an electric charge produces magnetism. This was the first observation of a wide-ranging phenomenon. The charged clouds of particles which float through the cosmos generate vast interstellar magnetic fields as they go. The sloshing of molten metal in Earth’s core produces the planet’s north and south magnetic poles. Even the firing of nerve cells in a human brain creates a minuscule amount of magnetism.
The ubiquity of such electrically generated magnetic fields does, though, bring problems ranging from the pragmatic to the esoteric. Doctors looking at MRI scans, for example, have to compensate for background magnetism. Meanwhile, experimentalists conducting precision tests may have to build complex shields to obscure the magnetic effect of something as simple as an electric wire running through the wall of their laboratory.
It would be useful, then, to be able to control, limit or shape magnetic fields from a distance. Useful, but apparently impossible. For, in 1842, Samuel Earnshaw, a British physicist, demonstrated mathematically that the maximum strength of a magnetic field cannot lie outside its source. Every such field must, in other words, surround and radiate from the object which generates it. And there matters stood until Rosa Mach-Batlle of the Autonomous University of Barcelona spotted a way around Earnshaw’s conclusions. She has not actually proved him wrong. But she has shown that multiple magnetic fields, each obeying Earnshaw’s theorem individually, can collectively appear to bypass it.
As they describe in Physical Review Letters, Dr Mach-Batlle and her colleagues pulled off their trick in a surprisingly simple way, by arranging 20 straight wires next to one another in the form of a cylinder 40cm tall and 8cm in diameter, with a 21st running through the cylinder’s centre. When they passed electric currents through all 21 wires a complex pattern of magnetic field lines blossomed in the surrounding area, forming shapes which varied with the strength and direction of the individual currents.
By choosing the right combination of currents the researchers found they were able to create a field pattern which emanated from a virtual version of the 21st wire that ran not through the middle of the cylinder but, rather, 2cm outside it. In other words, if the apparatus doing the generating were to be shielded from an observer, Wizard of Oz-style, by a curtain, it would look to that observer as if this field was appearing from nowhere.
Going from Dr Mach-Batlle’s demonstration to something which could be used in practice to manipulate distant magnetic fields will be a long journey. But if that journey can be made, potential applications go way beyond cleaning up fuzzy MRI scans. Remotely cast fields of this sort might be used to steer medical nanobots through someone’s bloodstream to deliver drugs to a particular tissue, or else to guide them towards a malignant tumour and remotely raise their temperature once they have arrived, in order to cook it to death. There are also likely to be applications in quantum computing….
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