In 2015 we published an article entitled “A magnetic wormhole” which aroused a big interest among the general audience. This is not surprising, taking into account that the word “wormhole” is an attractive concept by itself. Although we tried to be very precise in our claims and goals in the research article we wrote, some information that appeared in the media was not that rigorous and some people was a bit puzzled by the results. For this reason, I think there’s no better article to start “translating” than this one.
But let’s start from the very beginning. The first relevant question here is what is a wormhole? And, being even more precise, what is a gravitational wormhole? If we look at Wikipedia we find that a wormhole is “an hypothetical topological feature that would fundamentally be a shortcut connecting two separate points in spacetime”. Notice the word “hypothetical”, because nobody has been able to prove their existence so far, all we have are theoretical predictions. And what does it mean it would be a “shortcut” between separate points in space time? Roughly speaking, this means the separate points would be connected through a path or tunnel that lies out of the regular space in an extra dimension.
I have to admit that I don’t know much about gravitational wormholes but, as far as I know, they would distort space-time in such a way that matter and light could be easily transferred from one point to the other. Definitely, they would be something amazing!
All this is very interesting but, as you may imagine, the idea of creating or building artificially something like a gravitational wormhole is a matter of sci-fi. Nevertheless, in 2007, a group of mathematicians published and interesting theoretical research paper presenting the new concept of electromagnetic wormholes (we’ll name them EMWH). Authors claimed that making use of sophisticated electromagnetic metamaterials (materials with exotic electromagnetic properties created artificially), one could build a device that acted as an invisible tunnel for electromagnetic waves (for light). This invisible tunnel would transfer light between distant regions while, at the same time, most of the tunnel would remain invisible to external observations. Therefore, they demonstrated that such devices would behave as wormholes (only) for light.
From a “visible” point of view, electromagnetic wormholes would produce the following effect. Light would enter through one end and would go out through another distant end, and no one could see the path that light followed between the two. This is sketched in Fig. 2; light coming from a flower goes into one end of the EMWH and reappears at the other end. Looking at the end of the device, the observer will see the light coming from the flower. However, there is no trace of the light that has been transfer between the two ends of the EMWH.
Since we are dealing with an EMWH, this astonishing effect of teleportation only works from the point of view of light. Thus, if one would approach to the device and would start inspecting with his hands the space between the two ends, he would soon discover that there is something visually hidden there. Indeed, one would discover the invisible tunnel that transfers the light from one end to the other (made explicit in Fig. 3).
At this point one could argue that this is just a visual trick and should not be named a “wormhole”. Although this is a fair criticism, authors of the mathematical paper demonstrated that such devices would effectively change the topology of the space with respect to the Maxwell equations (those describing the propagation of light) just in the same fashion a full gravitational wormhole would do with space-time. In any case, the proposal of Greenleaf and colleagues arose a big interest among researchers working in the control of light. Unfortunately, the proposal turned out to be very challenging to realize in practise. The materials that would be needed to shape the light in the required way would be extremely cumbersome and nobody was able to experimentally build one of these electromagnetic wormholes.
In this scenario is where we asked ourselves if we could make, not a fully electromagnetic wormhole for light, but one that behaved as a such only for magnetic fields; a magnetic wormhole. Controlling and guiding magnetic fields can be much simpler than controlling light, in particular, thanks to the existence of many magnetic materials that can be combined to obtain very different and surprising effects. A magnetic wormhole would be analogous to an electromagnetic wormhole but only for magnetic fields; the device should be able to transfer magnetic fields from one point to another distant one, and the path of propagation should be completely magnetically undetectable. In this context, “magnetically undetectable” means that using a sensor that detects magnetic fields (for example a Hall probe), we shouldn’t be able to detect any kind of magnetic field or magnetic material between the two ends of the wormhole.
The second requirement is that this tunnel that transfers the field (and the field itself) has to be undetectable when a magnetic sensor is approached. This represents a big challenge, because magnetic materials are typically very clearly detectable due to their own magnetic nature. The superconductors and ferromagnets forming the hose are not an exception and the hose alone would not fulfil this undetectability condition at all! (see Fig. 4, the Hall probe in blue gives a strong signal when is close to the hose). Therefore, one needs a kind of magnetic cloak that makes the hose magnetically undetectable. Our previous research on magnetic cloaks was very useful to solve this defiant question. We designed a spherical magnetic cloak to magnetically “hide” most of the hose. Ironically, the cloak was also made of superconducting and ferromagnetic materials. In this case, though, the arrangement of the two materials was carefully calculated (we made many numerical simulations) such that the signals originated by the two different materials cancel each other and the whole device (and its content) was undetectable.
And this is how we got the final design. In Fig. 5 I show you an actual 3D render of the different parts of the device; the inner hose is shown in blue and the two parts forming the spherical magnetic cloak are in yellow (the internal superconducting part) and gray (the external ferromagnetic one). Once assembled, the device looked like in Fig. 6 left. Of course, the device was perfectly visible to our eye. However, from a magnetic point of view, the wormhole would behave as shown in Fig. 6 right, transferring magnetic fields from one point to another without leaving any kind of signature in the intermediate space.
Figure 6: On the left we show the appearance of the magnetic wormhole to our eye, transferring the field of a small magnet. On the right we show its behaviour from the magnetic point of view: the volume of the wormhole is undetectable so that magnetic field disappears at one point and reappears at another distant point.
To demonstrate that we made different experiments. Take into account that superconductors have to be cooled down very much in order to get their superconducting properties. That’s why we had to submerge all the device and the measuring sensors in liquid nitrogen (at -196°C!!!). After a series of different calibrations and measurements we could demonstrate the two effects that we were pursuing: the wormhole was transferring magnetic field between its ends and very little signals were measured between. In Fig. 7 I reproduce the relevant results of the article. On one hand the field transmission was very clear (left) and at the same time, the distortion when all the parts where properly assembled was very very small (right, blue surface). So, a magnetic wormhole was created!
Figure 7: On the left we show the magnetic field transferred through the wormhole. On the right you can see the measurements of detectability of the device. Red and green surfaces are measurements when the device was not totally working. When all the parts were properly activated we obtained the blue surface, showing almost no distortion.
At this point you could ask what is the use of a wormhole that only works for magnetic fields. To answer this, first you should thing about the use of magnetic fields in general. And might be surprising to know that magnetic fields are at the basis of many everyday applications and technologies (apart from keeping your magnets stacked to the fridge, of course!). Most of the electric energy that is consumed everyday is produced in generators whose working principle relies on strong magnetic fields. All electric engines (also those in novel electric cars) move thanks to the magnetic fields that are produced in their interior. Hard disks, transformers, magnetic resonance imaging, wireless energy transfer, plasma confinement strategies… all of them are examples of magnetism-based applications. In view of that, the ability to transfer magnetic field to a given point in space without bothering and distorting the rest of magnetic field that are already there seems, at least, an interesting thing. :-)