Out of sight

When the first “invisibility shield” was created in 2006, the joke was that it looked like anything but: a set of nested circular plastic strips imprinted with metal foil, it was conspicuously not what it seemed to claim. The catch was that the device made objects placed in its centre invisible only to microwaves, not visible light. To work at visible wavelengths (which are much shorter), the components of the shield—the little printed foil circuits—needed to be too small to see, and so very tricky to make. Harry Potter’s cloak seemed a long way off.

But this exotic corner of materials science has advanced at dizzying speed. Now we have the real item: a shield that hides large objects from the naked eye. As the picture below shows, it’s still a little rough and ready, but it basically does the job: the roll of pink paper continues under the transparent crystal, but it looks—though this is hard to convey fully in a single image—completely absent.

Invisibility is all about bending light. The light coming from behind the object must be bent around it and then reunited in front, so that it seems to have passed right through. But although bending light is familiar enough from refraction—which kinks a straw in a glass of water—no ordinary material will send it on a route like that. Instead, researchers have figured out how to make light follow new rules using so-called metamaterials, assemblies of little metal wires that manipulate the electromagnetic fields of which light consists.

That was how the first microwave shield was made, and clever microfabrication techniques have been previously used to create tiny cloaks that hide small objects from visible light. The tricks learned from these projects now allow one to dispense with such fiddly business and use simpler materials. The key is a mathematical method called transformation optics: which describes how to warp the space through which light travels (mathematically equivalent to what black holes do by means of extreme gravity) so that the hidden object lies outside of it.

That’s the approach a team from Singapore and MIT has used here to make a shield from a block of the humble mineral calcite, which has the crucial property that light travels through it at different speeds in different directions. Wedges of calcite can be tailored to make light appear to bounce off a surface beneath them, as though any intervening object were simply not there. A group from Britain and Denmark recently did much the same trick, also using calcite.

As well as invisibility shields, these methods of moulding light offer all kinds of other seemingly fantastic possibilities: hidden portals in walls (Harry Potter’s platform 9¾ at King’s Cross station?), magnifying cloaks, and “light wormholes” that open up a view of the outside world within a bit of empty space on your desk. It’s no surprise that much of the funding in this area comes from the military.

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Dark matter: wimps versus machos

This year is officially the Year of Chemistry, but the stuff that could make the biggest splash doesn’t appear in any periodic table. Hopes are up that in 2011 we’ll at last get an inkling of what dark matter is. It seems to outweigh all the conventional matter in the universe by about fivefold, yet no one has seen it or knows what it is made of. We know it must be there because of its gravitational tug, evident in the way galaxies rotate without disintegrating. But the particles of dark matter don’t seem to interact with ordinary matter in any other way.

Or almost not. Schemes for detecting dark matter rely on a belief—not much more than that—that dark-matter particles will very, very rarely bump into ordinary particles and leave a trace of the impact. “Theories” of dark matter amount to little more than mathematised guesswork: you can invent any stuff that fits the bill. Hence weakly interacting massive particles (Wimps), one of the leading candidates and yet merely entities tailored to fit the requirement of being massive and “dark.” Alternatively, there are massive compact halo objects (Machos), which are simply lumps of ordinary matter in the extended “halos” of dim materials reaching beyond the visible edges of galaxies that are too dark to see in telescopes. Perhaps because they are the boring solution, Machos have fallen from favour; Wimps involve as yet unknown physics, so they are far cooler.

Experiments to detect Wimps look for telltale flashes of light from their hypothetical collisions within large volumes of some material buried deep underground, where the detectors are shielded from false alarms caused by cosmic rays. The Cryogenic Dark Matter Search experiment, in a 713-metre mineshaft in Minnesota, was rumoured to have seen a Wimp in late 2009, but the official report dampened excitement by admitting that its two candidate detection events weren’t “statistically significant”—a way of saying that they could be anything.

Several other projects hope to claim the prize. One sits beneath Gran Sasso mountain in Italy, where detectors are looking for collisions of Wimps in a tank of supercold liquid xenon. Another lurks 1,100 metres below the Yorkshire moors in the Boulby potash and salt mine. And IceCube, a project in Antarctica in which detectors are arrayed around a 1-cubic km block of the ice sheet, hopes to spot collisions of Wimps, though its main purpose is to study neutrinos (which are almost as elusive).

But the Large Hadron Collider at Cern in Geneva could conceivably trump the lot by actually making dark matter—according to one hypothesis, these might be new exotic particles predicted to appear in the high-energy collisions.