Invisibility, the abilities in fictional movies is actually possible. Not the power stuff and all, but the use of invisibility cloaks or wears. True harry porter comes to mind but it is totally different from what we think or watched in the movie. Not magic or spells but making an entity not perceivable by a person
Researchers in the Cockrell School of Engineering at The University of Texas at Austin have been able to quantify fundamental physical limitations on the performance of cloaking devices, a technology that allows objects to become invisible or undetectable to electromagnetic waves including radio waves, microwaves, infrared and visible light.
The researchers' theory confirms that it is possible to use cloaks to perfectly hide an object for a specific wavelength, but hiding an object from an illumination containing different wavelengths becomes more challenging as the size of the object increases.
Andrea Alù, an electrical and
computer engineering professor and a leading researcher in the area of cloaking
technology, along with graduate student Francesco Monticone, created a
quantitative framework that now establishes boundaries on the bandwidth
capabilities of electromagnetic cloaks for objects of different sizes and
composition. As a result, researchers can calculate the expected optimal performance
of invisibility devices before designing and developing a specific cloak for an
object of interest. Alù and Monticone describe their work in the journal Optica.
Cloaks are made from artificial
materials, called metamaterials, that have special properties enabling a better
control of the incoming wave, and can make an object invisible or transparent.
The newly established boundaries apply to cloaks made of passive
metamaterials—those that do not draw energy from an external power source.
Understanding the bandwidth and size
limitations of cloaking is important to assess the potential of cloaking
devices for real-world applications such as communication antennas,
biomedical devices and military radars, Alù said. The researchers' framework
shows that the performance of a passive cloak is largely determined by the size
of the object to be hidden compared with the wavelength of the incoming wave,
and it quantifies how, for shorter wavelengths, cloaking gets drastically more
difficult.
For example, it is possible to cloak
a medium-size antenna from radio waves over relatively broad bandwidths for
clearer communications, but it is essentially impossible to cloak large
objects, such as a human body or a military tank, from visible
light waves, which are much shorter than radio waves.
"We have shown that it will not
be possible to drastically suppress the light scattering of a tank or an
airplane for visible frequencies with currently available techniques based on
passive materials," Monticone said. "But for objects comparable in
size to the wavelength that excites them (a typical radio-wave antenna, for
example, or the tip of some optical microscopy tools), the derived bounds show
that you can do something useful, the restrictions become looser, and we can
quantify them."
In addition to providing a practical
guide for research on cloaking devices, the researchers believe that the
proposed framework can help dispel some of the myths that have been developed
around cloaking and its potential to make large objects invisible.
"The question is, 'Can we make
a passive cloak that makes human-scale objects invisible?' " Alù said.
"It turns out that there are stringent constraints in coating an object
with a passive material and making it look as if the object were not there, for
an arbitrary incoming wave and observation point."
Now that bandwidth limits on
cloaking are available, researchers can focus on developing practical
applications with this technology that get close to these limits.
"If we want to go beyond the
performance of passive cloaks, there are other options," Monticone said.
"Our group and others have been exploring active and nonlinear cloaking
techniques, for which these limits do not apply. Alternatively, we can aim for
looser forms of invisibility, as in cloaking devices that introduce phase
delays as light is transmitted through, camouflaging techniques, or other
optical tricks that give the impression of transparency, without actually
reducing the overall scattering of light."
Alù's lab is working on the design
of active cloaks that use metamaterials plugged to an external energy source to
achieve broader transparency bandwidths.
"Even with active cloaks,
Einstein's theory of relativity fundamentally limits the ultimate performance
for invisibility," Alù said. "Yet, with new concepts and designs,
such as active and nonlinear metamaterials, it is possible to move forward in
the quest for transparency and invisibility."
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