A team of Spanish researchers has successfully constructed a magnetic "wormhole," according to an open-access paper published this week in Scientific Reports. While it's not about to jump a spaceship across the galaxy, the setup is at least analogous to the wormholes of science-fiction and Einstein's theory of general relativity. If it has a real-world application at all, it's not it cosmic transit, but in MRI machines.
The experiment certainly looks cool. It consists of a small sphere with a magnetic dipole on one end. This is basically a tiny closed loop of wire. Current racing around this loop produces a magnetic field, manifested as field lines. The trick is that the field emerges from the other side of the sphere, seeming to jump from to pole to pole. So, it's as if the sphere has cloaked the the field and allowed it to tunnel to the other side. You see the sphere as a spatial object, but the field is hidden.
"Constructing an artificial gravitational wormhole connecting two distant regions in the universe is an apparently unrealizable challenge," the Spanish group, led by Autonomous University physicist Alvaro Sanchez, write. "Large amounts of negative gravitational energy would be required, which makes impossible its realization with present technology."
In 2007, a group of physicists led by Rochester University's Allan Greenleaf proposed a wormhole experiment that would apply to electromagnetic waves—light itself—rather than just magnetism. Greenleaf's device would enable "a singular transformation of space in which an infinitesimally small hole has been stretched to a ball," he wrote then in paper describing the setup. "The result would be a deformation of the topology of three-dimensional space: a wormhole.
Topologically speaking, it would be as if three-dimensional space had a protruding "handle," like a coffeecup. We see the space, but not the handle. Only the EM waves being cloaked by the device would be allowed to experience this bonus dimension. This seems a bit of a headfuck, but it's helpful to understand that it's not the space itself being rearranged, but a specific field within the space, which is electromagnetism.
Alas, as Sanchez and co. write, Greenleaf's wormhole would require conditions nigh impossible to create a lab. But the idea here is similar: the seeming breaking and piecing together of a magnetic field in just such a way that, to us, the field is hidden in the coffeecup handle, but it is most assuredly there. It is a model of warped space.
The cloaking involved in such a device is not as simple as it might seem. It's a matter of not just hiding the field, but hiding the device that's doing the cloaking.
"Our device is a tunnel magnetically connecting two distant points in space, with the tunnel magnetically invisible," Sanchez told me. "Connecting two distant points is already difficult since magnetic fields always decay very rapidly with distance."
"This is achieved by a magnetic hose at the core of our device," he explained. "But a magnetic hose can be easily detected by a magnetic field, so we needed to construct a 3D magnetic cloak to surround it and making the whole device magnetically invisible."
That's where it gets tricky. How do you mask a field and mask the mask itself? The answer was a series of thin stacked layers. The sphere actually consists of two spheres, the outer one consisting of a ferromagnetic metasurface, while the inner sphere is a superconducting layer. Rolled up inside of the spheres is spirally wound ferromagnetic sheet.
The researchers were able to verify the cloaking effect by placing the device within a powerful external magnetic field. Magnetic probes were placed at wormhole's exit and alongside it. At the exit, they registered a magnetic monopole, while the second probe as it was moved back and forth along the device found no magnetic "leaks."
As Sanchez tells Physics World, the device is unlikely to lend much insight into the wormholes we're more accustomed to, which involve topological distortions of space itself rather than fields within that space. "Scientists are very good at manipulating magnetic fields," he said, "but we don't have the same mastery over gravitation."
So, perhaps it's more fair to think of this as tunneling rather than wormholing, but that's not quite it either. A spatial tunnel that is, say, traveling through a mountain or under a river doesn't hide the very thing its tunneling through as well. We see the space involved, whereas with the magnetic wormhole, while we may see the space, the cloaking does not occur with respect to 3D space (Euclidean space) but with respect to magnetic fields.
"It is the combination of both properties—transmission and cloaking—what makes our device acting like a spatial wormhole," Sanchez explained, "since the overall effect is having a magnetic field disappearing on one point and appearing in another as it field had traveled through another spatial dimension."
While it won't be warping spacetime itself, the technology may have some use in the real-world where it's desirable to cloak a magnetic field, as the current study explains.
"These ideas may be applied in devices requiring the local application of magnetic fields in a particular magnetic background that should not be distorted," the paper notes. "One particularly relevant application along this line could be in magnetic resonance imaging. Using the ideas in this work, one could foresee ways to apply a magnetic field locally to a patient, without distorting the homogeneous magnetic field in the region. They could be useful, for example, in medical operations using simultaneous MRI imaging."