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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Sunday, November 01, 2015

A Magnetic Wormhole

(From left to right) Carles Navau, Alvaro Sanchez, Jordi Prat-Camps

Authors: Jordi Prat-Camps, Carles Navau, Alvaro Sanchez 

Affiliation: Departament de Física, Universitat Autònoma de Barcelona, Spain.

Link to Superconductivity Group UAB >>

Is it possible to build a wormhole in a lab? Taking into account that large amounts of gravitional energy would be required [1], this seems an impossible task. However, redefining a wormhole into a path between two points in space that is completely undetectable, Greenleaf and colleagues [2] suggested in 2007 a (theoretical) way of realizing an electromagnetic wormhole capable of guiding light through an invisible path. They demonstrated that this is topologically equivalent as if the light had been sent through another spatial dimension. However, such a wormhole required metamaterials with extreme properties, which prevented its construction.

In our work, we have constructed an actual 3D wormhole working for magnetostatic fields. It allows the passage of magnetic field between distant regions while the region of propagation remains magnetically invisible. Our wormhole takes advantage of the possibilities that magnetic metamaterials offer for shaping static magnetic fields [3]. These metamaterials can be constructed using existing magnetic materials that can provide extreme magnetic permeability values ranging from zero - superconductors - to effectively infinity - ferromagnets.
Figure 1: (Left) 3D sketch of the magnetic wormhole, showing how the magnetic field lines (in red) of a small magnet at the right are transferred through it. (Right) From a magnetic point of view the wormhole is magnetically undetectable so that the field of the magnet seems to disappear at the right and reappear at the left in the form of a magnetic monopole. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

The magnetic wormhole requires three properties: (i) to magnetically decouple a given volume from the surrounding 3D space, (ii) to have the whole object magnetically undetectable, and (iii) to have magnetic fields propagating through its interior. The first two properties are achieved by constructing a 3D magnetic cloak. Based on previous ideas [4] such a cloak could be made by surrounding a superconducting sphere with a specially created ferromagnetic (meta)surface, such that the magnetic signature of the superconductor was cancelled by the ferromagnet. For the third property, we use magnetic hoses, also made by magnetic metamaterials, as developed in [5].

The parts composing the magnetic wormhole are shown in fig. 2: a central magnetic hose to guide the magnetic field from one end of the hose to the opposite one, and a magnetic cloak composed of a superconducting-ferromagnetic bilayer to make the hose magnetically invisible.
Figure 2: (a) 3D image of the magnetic wormhole, formed by concentric shells: from outside inwards, an external metasurface made of ferromagnetic pieces (b), an internal superconducting shell made of coated conductor pieces (c), and a magnetic hose made of ferromagnetic foil (d). (e) Cross-section view of the wormhole, including the plastic formers (in green and red) used to hold the different parts. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

Experimental results clearly demonstrate [6] the two desired properties for the wormhole: (i) magnetic field from a source at one end of the wormhole appear at the opposite end (actually as a kind of isolated magnetic monopole), (ii) the overall device is magnetically undetectable (it does not noticeably distort an applied magnetic field, even a non-uniform one).

Besides the scientific interest per se in the realization of an object with properties of a wormhole, our device may have applications in practical situations where magnetic fields have to be transferred without distorting a given field distribution, as in magnetic resonance imaging.

Acknowledgements: We thank Spanish project MAT2012-35370 and Catalan 2014-SGR-150 for financial support. A.S. acknowledges a grant from ICREA Academia, funded by the Generalitat de Catalunya. J. P.-C. acknowledges a FPU grant form Spanish Government (AP2010-2556).

[1] Michael S. Morris, Kip S. Thorne, Ulvi Yurtsever, "Wormholes, Time Machines, and the Weak Energy Condition", Physical Review Letters, 61, 1446 (1998). Abstract.
[2] Allan Greenleaf, Yaroslav Kurylev, Matti Lassas, Gunther Uhlmann, "Electromagnetic Wormholes and Virtual Magnetic Monopoles from Metamaterials", Physical Review Letters, 99, 183901 (2007). Abstract.
[3] Steven M. Anlage, "Magnetic Hose Keeps Fields from Spreading", Physics, 7, 67 (2014). Full Article.
[4] Fedor Gömöry, Mykola Solovyov, Ján Šouc, Carles Navau, Jordi Prat-Camps, Alvaro Sanchez, "Experimental realization of a magnetic cloak", Science, 335, 1466 (2012). Abstract.
[5] C. Navau, J. Prat-Camps, O. Romero-Isart, J. I. Cirac, A. Sanchez. "Long-Distance Transfer and Routing of Static Magnetic Fields", Physical Review Letters, 112, 253901 (2014). Abstract.
[6] Jordi Prat-Camps, Carles Navau, Alvaro Sanchez. "A Magnetic Wormhole", Scientific Reports 5, 12488 (2015). Full Article.

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