Researchers from Slovakia and Spain have created an "invisibility" — actually a small cylinder — that makes itself and the object inside it undetectable to machines such as MRI scanners and airport security sensors. The research team reported its finding March 22 in the journal Science.
The cylinder, which works by attracting and repelling magnetic field lines at the same time, would not be hard to manufacture, physicist Alvar Sanchez, one of the researchers, said.
Two magnets near each other will have either an attractive or a repulsive force between them, something many people figure out as children by playing with refrigerator magnets. "The thing is, can we surround one of these magnets with something so that this interaction is lost?" said Sanchez, a physicist at the Autonomous University of Barcelona. "The answer is yes, if you use our cloaking device." The new device obliterates that natural force between magnets, rendering a magnetic object inside it invisible to a magnetic detector.
"This is a seminal experimental paper in the evolution of cloaking technology," John Pendry, a theoretical physicist at Imperial College London who came up with the first theories for invisibility cloaks, wrote in an email to InnovationNewsDaily.
In the past, several research groups have created cloaks that hide objects from light waves and microwaves. This is the first time anyone has made a cloak for magnetic fields, though physicists have known theoretically how they should work, Pendry wrote.
Making things invisible to magnetic detectors is generally easier than making things invisible to the human eye. Visible light is made of moving waves that have both magnetic and electric properties. Magnetic fields are a kind of special case of electromagnetic waves, with wavelengths at the longest end of the spectrum. At those lengths, magnetic field lines are motionless and they don't have electrical effects. That means they're simpler to work with than visible light or other electromagnetic waves such as microwaves.
"It's a very special condition that makes the job easier," said Andrea Alù, an engineer at the University of Texas at Austin who created an invisibility tube for microwaves in January.
Sanchez first published his idea in September 2011 for rendering things magnetically invisible. At the time, his plan was a cylinder made of several layers, which would be difficult to create. But he eventually hit on the idea of using just two layers. In his March 22 paper, he used physics equations to show the two-layered device would work theoretically, then made and tested a sample device.
The cylinder is made of different materials on the inside and outside, like a pirouette cookie with a layer of frosting inside. In this case, the inner layer is a superconductor, kept cold with liquid nitrogen. The superconductor repels magnetic fields, blocking an outside sensor from penetrating the layer. The outer layer is made of an iron alloy that attracts magnetic fields.
Sanchez and his colleagues carefully chose the superconductor and the iron alloy to have the right balance of repulsion and attraction so that the device guides magnetic field lines around it in a way that makes the lines look as if they are flowing unperturbed through the device and the object.
Pendry said, "If an object does not disturb the magnetic field lines, then it is invisible, magnetically speaking."
The materials Sanchez and his colleagues used are readily available, so their device can be commercialized. "You don't need to buy very expensive and complicated things," Sanchez said.
The cylinder is a few centimeters long, but theoretically, people could make it any size. Cloaks against electromagnetic waves can hide only small objects, usually not much larger than the lengths of the waves they're hiding from. Because magnetic fields have infinitely long wavelengths, however, a magnetic cloak should be able to hide objects of any size, from microscopic things to cars and trucks, Sanchez and Alù told InnovationNewsDaily.
A cloak like this could be used in MRI scanners, Sanchez and Alù said. It could help protect people's pacemakers from the magnetic fields created inside MRI machines, so they could wear their pacemakers while they're getting scanned.
This also could be a new technique for hiding naval ships from magnetic detection, Sanchez said.
It might find a home in one of the many technologies that use magnetic fields today, such as power plants and computer memory. However, Sanchez was reluctant to give a time frame for how long it might take industries to develop his magnetic cloak for practical use. Perhaps 10 years or less, he said. "This is there for people to develop."
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