Invisibility cloaks: Will we ever really have them?

Chinese artist Liu Bolin A number of ways exist to mimic cloaking - like artist Liu Bolin's - but more flexible solutions await

You've seen the films, you've read the popular science stories. It's time to ask: when will you have your invisibility cloak?

Hold your horses. It's looking like a great many things inspired by cloaking science will come to pass before then.

Behind most cloaking ideas is the use of metamaterials - materials whose properties are purposefully designed, defined not by chemistry but by the materials' size, shape and structure.

They are already starting to crop up in consumer technology, and could find widespread use in applications ranging from computers to concert halls.

It is a simple idea whose scientific foundations are dizzyingly complex.

At its simplest, explains John Pendry of Imperial College London, "you just want to grab hold of the light coming from an object, guide light around the hidden thing, and then return it to the path it was going on originally - and that's easier said than done."

Another alternative is to find out what a given object will do to incident light beams and create a cloak that does the exact opposite - that's the aim of what are known as plasmonic metamaterials.

Luckily, there are many ways to pull off the invisibility trick. The metamaterials approach seems to be the most promising, but a number of what might be called optical tricks have also cropped up.

Prof Chris Phillips outlines three of the ways to make things invisible

What started it all was a paper in Science in 2006 by Prof Pendry and colleagues David Smith and David Schurig of Duke University, laying out the theory for the idea.

David Smith says that after the first microwave-region demonstration, also in Science, media interest was immediate and voracious.

"I thought, 'Let's demonstrate where this technology is actually at'. I thought people would look at it and say 'not so interesting, we'll check back in a couple of decades'," he told BBC News.

"We rushed out an experiment to throw cold water on it and it's taken years for the cold water to stick."

Other ways to disappear

optical camouflage, keio university
  • Optical camouflage technology: A modified background image is projected onto a cloak of retro-reflective material (the kind used to make projector screens); the wearer becomes invisible to anyone standing at the projection source
  • The "mirage effect": Electric current is passed through submerged carbon nanotubes to create very high local temperatures, this causes light to bounce off them, hiding objects behind
  • Adaptive heat cloaking: A camera records background temperatures, these are displayed by sheets of hexagonal pixels which change temperature very quickly, camouflaging even moving vehicles from heat-sensitive cameras
  • Calcite crystal prism: Calcite crystals send the two polarisations of light in different directions. By gluing prism-shaped crystals together in a specific geometry, polarised light can be directed around small objects, effectively cloaking them

Invisibility of any sort was just too enticing. The BBC and others noted, for example, when a flat region was hidden under a "carpet", when the carpet hid a 3D object, and when the carpet was dispensed with and a 3D object was hidden in "free space".

Here's the problem: these light-guiding structures need to be not much bigger than the wavelength of the light they're hiding from. For visible light, that's just a few hundred billionths of a metre.

So all these efforts either hide things in wavelengths longer than we can see - radio, microwaves and a bit of infrared - or hide objects so small we can't see them. And they're all stiff as a board.

The eventual outcome, Prof Pendry says, "is going to be very far from what most people would think of as a cloak: something thin and flexible, which you can wrap around yourself and change its shape".

"This sounds like being a wet blanket - or a wet cloak if you prefer - I don't see that ever being a reality. And to be honest, that's not the target of scientists."

But stay with me - the targets of scientists are still pretty interesting.

"There's a whole host of other stuff that's spun off because people have realised that light waves are not the only sort of waves you might want to hide from," Prof Pendry says.

You see, much of the mathematics behind cloaking (called "transformation optics") applies just the same - and is likely to find quicker application - in other contexts.

We've already seen bids to cloak magnetic fields, which could find use in security applications, and even cloaks for heat that might keep future computers' processors cool.

Still on the drawing board, in the form of an unpublished paper, is cloaking buildings and dams from seismic waves.

Colour diagram showing thermally cloaked region (Optics Express) Many of the same principles behind optical cloaking underlie the thermal and magnetic versions

And like many areas of science where mathematics makes a first ruling on what's possible before the engineers arrive, it gets weirder: how about a "spacetime cloak" outlined in the Journal of Optics that can hide events in time?

Some ideas Prof Pendry calls "wacky suggestions and not serious possibilities". Others, such as acoustic cloaks that hide from sound, may actually be easier to pull off.

"An acoustic cloak will work even better than an electromagnetic one," says Andrea di Falco of the University of St Andrews. "The analogy is powerful, and there are some fields where it is possible to do better - you don't have the limitations you have with light."

It's not too hard to imagine, say, metamaterials-enhanced noise-cancelling headphones, or concert halls with flawless acoustics.

But what about microwaves, where the whole effort began - written off "because they're invisible anyway"?

"They're highly visible to cell phones, PDAs, to radar," says Prof Smith. "You wouldn't want to fly in a plane that didn't have good radar that sees in the microwave."

He has hinted here at what will quietly (and presumably invisibly) occupy a lot of metamaterial thoughts in the defence industry: how to make planes and boats and tanks invisible to various wavelengths of light and sound.

Left behind

Back to civilian territory, though: commercially available mobile phones already incorporate simple metamaterial antennas, and metamaterial lenses could hugely boost imaging and microscopes.

And though it seems pedestrian compared to invisible wizards, metamaterials might find their biggest strengths in electronics.

First off, metamaterials' ability to manipulate light's speed could lead to a faster internet.

Acoustic cloaking device Acoustic cloaking could make for better sound in a wide range of places - or no sound at all

Applying the same ideas to electrons and "matter waves", Andrea Alu at the University of Texas at Austin told BBC News, "may lead to a complete transformation of the current field of electronics".

But Prof Pendry says the metamaterial "killer app" is a mix of the two.

"We want to guide light with the same precision as electrons, and make light 'talk' to light in the same way that electrons can - the way that one current can turn another on and off," he says - making for faster, smaller "all-optical" computers.

For none of these researchers, though, is the classic "cloak" the ultimate goal.

As Prof di Falco says: "Of course the cloaking is what catches the imagination of the public but... it's more of a legacy that metamaterials are leaving behind."


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  • rate this

    Comment number 211.

    The Shroud of invisibility cloak....the tomb was not empty. The image remaining on the shroud.....

  • rate this

    Comment number 210.


    How about more advanced solar panels? Also see this quote:-
    'cloaking small objects may be exciting for a variety of applications. For instance, we are currently investigating the application of these concepts to cloak a microscope tip at optical frequencies. This may greatly benefit biomedical and optical near-field measurements,"

    You need to think outside the box.

  • rate this

    Comment number 209.

    This technology is pretty exciting. Whilst the basic stuff that merely captures images and projects them is fairly self-explanatory and will doubtlessly be of use to the military, intelligence services and criminals everywhere the stuff that really excites me are the materials used to bend around an object.

    The range of possible uses sounds great; a range from solar panels to radioactive defense.

  • rate this

    Comment number 208.

    23 Minutes ago

    "I'm no Luddite but seriously 198 Mike and 199 doug - what good is this particular research going to achieve for humanity?
    They never said there goal were achieving more for humanity - does it have to? But as said in the report, many things are developed (such as a faster internet) from all types of research. Does your J.O.B. achieve much for humanity?

  • rate this

    Comment number 207.

    I'm no Luddite but seriously 198 Mike and 199 doug - what good is this particular research going to achieve for humanity? Whether in the name of man's greater understanding of nature or not.


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