Cloaking device

This Cloaking Device Could Hide Us From Alien Life

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IN BRIEF

Emitting a continuous 30 MW laser for about 10 hours, once a year, would be enough to hide us from aliens, at least in visible light.

OFFERING A CHOICE

Stephen Hawking has often cautioned humanity against broadcasting our presence to alien life. He noted that any civilization with which humanity could communicate is likely to be much older and much more technologically advanced than ours.

In short, they could easily kill us and strip-mine our planet for parts, if they chose to do so.

Photo Credit: ESO / G. Hüdepohl

Hawking isn’t the only scientist to share this concern. However, now, astronomers at Columbia University in New York could have the answer to staying hidden from potential other-worldly threats. Professor David Kipping and graduate student Alex Teachey suggest humanity could use lasers to conceal the Earth from the searches of advanced extraterrestrial civilizations.

To help clarify, astronomers try to find other Earth-like planets by looking for the dip in light when a planet moves directly in front of the star it orbits. If a far-off extraterrestrial is using the same method, our visibility could be masked by controlled laser emission, with the beam directed at the star where the aliens might live. When the Earth moves in front of the Sun, the laser would be switched on to compensate for the dip in light.

“There is an ongoing debate as to whether we should advertise ourselves or hide from advanced civilizations. Our work offers humanity a choice,” says Kipping.

KEEPING UP APPEARANCES

According to the authors, in order to mask our presence, emitting a continuous 30 MW laser for about 10 hours, once a year, would be enough to eliminate the dip, at least in visible light. A chromatic cloak, effective at all wavelengths, is more challenging.

“Alternatively, we could cloak only the atmospheric signatures associated with biological activity. This should make the Earth appear as if life never took hold on our world,” said Teachey.

But what if aliens already know about laser cloaking and are doing it themselves? That might sound a little bit conspiracy-theory heavy, but the scientists have considered this possibility. They propose that the Search for Extraterrestrial Intelligence could be broadened to search for artificial transits in order to help us find alien life.

Which leaves us asking: Would we really want to seek out a civilization that doesn’t want visitors?

Researchers design first battery-powered invisibility cloak.

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Researchers at The University of Texas at Austin have proposed the first design of a cloaking device that uses an external source of energy to significantly broaden its bandwidth of operation.

Andrea Alù, associate professor at the Cockrell School of Engineering, and his team have proposed a design for an active cloak that draws energy from a battery, allowing objects to become undetectable to radio sensors over a greater range of frequencies.

The team’s paper, “Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces,” was published Dec. 3 in Physical Review Letters. Alù, researcher Pai-Yen Chen and postdoctoral research fellow Christos Argyropoulos co-authored the paper. Both Chen and Argyropoulos were at UT Austin at the time this research was conducted. The proposed active cloak will have a number of applications beyond camouflaging, such as improving cellular and radio communications, and biomedical sensing.

Cloaks have so far been realized with so-called passive technology, which means that they are not designed to draw energy from an external source. They are typically based on metamaterials (advanced artificial materials) or metasurfaces (a flexible, ultrathin metamaterial) that can suppress the scattering of light that bounces off an object, making an object less visible. When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency to radio-wave detectors. They can suppress 100 times or more the detectability at specific design frequencies. Although the proposed design works for radio waves, active cloaks could one day be designed to make detection by the human eye more difficult.

“Many cloaking designs are good at suppressing the visibility under certain conditions, but they are inherently limited to work for specific colors of light or specific frequencies of operation,” said Alù, David & Doris Lybarger Endowed Faculty Fellow in the Department of Electrical and Computer Engineering. In this paper, on the contrary, “we prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks. I believe that our design helps us understand the fundamental challenges of suppressing the scattering of various objects at multiple wavelengths and shows a realistic path to overcome them.”

The proposed active cloak uses a battery, circuits and amplifiers to boost signals, which makes possible the reduction of scattering over a greater range of frequencies. This design, which covers a very broad frequency range, will provide the most broadband and robust performance of a cloak to date. Additionally, the proposed active technology can be thinner and less conspicuous than conventional cloaks.

In a related paper, published in Physical Review X in October, Alù and his graduate student Francesco Monticone proved that existing passive cloaking solutions are fundamentally limited in the bandwidth of operation and cannot provide broadband cloaking. When viewed at certain frequencies, passively cloaked objects may indeed become transparent, but if illuminated with white light, which is composed of many colors, they are bound to become more visible with the cloak than without. The October paper proves that all available cloaking techniques based on passive cloaks are constrained by Foster’s theorem, which limits their overall ability to cancel the scattering across a broad frequency spectrum.

In contrast, an active cloak based on active metasurfaces, such as the one designed by Alù’s team, can break Foster’s theorem limitations. The team started with a passive metasurface made from an array of metal square patches and loaded it with properly positioned operational amplifiers that use the energy drawn from a battery to broaden the bandwidth.

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“In our case, by introducing these suitable amplifiers along the cloaking surface, we can break the fundamental limits of passive cloaks and realize a ‘non-Foster’ surface reactance that decreases, rather than increases, with frequency, significantly broadening the of operation,” Alù said.

The researchers are continuing to work both on the theory and design behind their non-Foster active cloak, and they plan to build a prototype.

Alù and his team are working to use active cloaks to improve wireless communications by suppressing the disturbance that neighboring antennas produce on transmitting and receiving antennas. They have also proposed to use these cloaks to improve biomedical sensing, near-field imaging and energy harvesting devices.

New invisibility cloak type designed

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A new “broadband” invisibility cloak which hides objects over a wide range of frequencies has been devised.

Despite the hype about Harry Potter-style cloaks, our best current designs can only conceal objects at specific wavelengths of light or microwaves.

At other frequencies, invisibility cloaks actually make things more visible, not less, US physicists found.

Their solution is a new ultrathin, electronic system, which they describe in Physical Review Letters.

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If you want to make an object transparent at all angles and over broad bandwidths, this is a good solution”

Andrea Alu University of Texas

“Our active cloak is a completely new concept and design, aimed at beating the limits of [current cloaks] and we show that it indeed does,” said Prof Andrea Alu, from the University of Texas at Austin.

“If you want to make an object transparent at all angles and over broad bandwidths, this is a good solution.

“We are looking into realising this technology at the moment, but we are still at the early stages.”

Passive vs Active

While the popular image of an invisibility cloak is the magical robe worn by Harry Potter, there is another kind which is not so far-fetched.

The first working model – which concealed a small copper cylinder by bending microwaves around it – was first demonstrated in 2006.

Left: uncloaked sphere. Right: Same sphere covered with a plasmonic cloak
The sphere on the right is “cloaked” but actually scatters more radiation than when bare (left)

It was built with a thin shell of metamaterials – artificial composites whose structures allow properties which do not exist in nature.

Cloaking materials could have applications in the military, microscopy, biomedical sensing, and energy harvesting devices.

The trouble with current designs is they only work at limited bandwidths. Even this “perfect” 3D cloak demonstrated last year could only hide objects from microwaves.

At other frequencies the cloak acts as a beacon – making the hidden object more obvious – as Prof Alu and his team have now demonstrated in a new study in Physical Review X.

They looked at three popular types of “passive” cloaks – which do not require electricity – a plasmonic cloak, a mantle cloak, and a transformation-optics cloak.

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

All three types scattered more waves than the bare object they were trying to hide – when tested over the whole range of the electromagnetic spectrum.

“If you suppress scattering in one range, you need to pay the price, with interest, in some other range,” Prof Alu told BBC News.

“For example, you might make a cloak that makes an object invisible to red light. But if you were illuminated by white light (containing all colours) you would actually look bright blue, and therefore stand out more.”

A cloak that allows complete invisibility is “impossible” with current passive designs, the study concluded.

“When you add material around an object to cloak it, you can’t avoid the fact that you are adding matter, and that this matter still responds to electromagnetic waves,” Prof Alu explained.

Instead, he said, a much more promising avenue is “active” cloaking technology – designs which rely on electrical power to make objects “vanish”.

Active cloaks can be thinner and less conspicuous than passive cloaks.

Alu’s team have proposed a new design which uses amplifiers to coat the surface of the object in an electric current.

This ultrathin cloak would hide an object from detection at a frequency range “orders of magnitude broader” than any available passive cloaking technology, they wrote.

Nothing’s perfect

Prof David Smith of Duke University, one of the team who created the first cloak in 2006, said the new design was one of the most detailed he had yet seen.

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This does not necessarily preclude the Harry Potter cloak”

Professor David Smith Duke University

“It’s an interesting implementation but as presented is probably a bit limited to certain types of objects,” he told BBC News.

“There are limitations even on active materials. It will be interesting to see if it can be experimentally realised.”

Prof Smith points out that even an “imperfect” invisibility cloak might be perfectly sufficient to build useful devices with real-world applications.

For example, a radio-frequency cloak could improve wireless communications – by helping them bypass obstacles and reducing interference from neighbouring antennas.

“To most people, making an object ‘invisible’ means making it transparent to visible wavelengths. And the visible spectrum is a tiny, tiny sliver of the overall electromagnetic spectrum,” he told BBC News.

“So, this finding does not necessarily preclude the Harry Potter cloak, nor does it preclude any other narrow bandwidth application of cloaking.”