Now the invisible man can take the heat
The science of invisibility has now thrown its cloak over heat - hiding it in a way that objects close to scorching temperatures remain cool to the touch.
It's another step towards making things invisible in reality - with one Hong Kong physicist leading the quest to, magician-like, make whole objects disappear.
The latest breakthrough came from a French team. So far, tests have cloaked areas of just 300 micrometres, about a third of a millimetre, bending heat around the 'invisible' spot.
Most previous work on invisibility has revolved around manipulating trajectories of waves - light, sound and the waves that travel through the earth and oceans. With heat, no waves are involved.
The French scientists used a 'thermal' cloak to split space into a visible and a dark domain, with the object hidden in the dark domain shielded from heat.
The new approach was released in the journal Optics Express last week.
Sabestien Guenneau, who works with both the University of Aix-Marseille and France's National Centre for Scientific Research, did the study to see if they could control the way heat diffused in a manner similar to those achieved for waves.
'Heat isn't a wave - it simply diffuses from hot to cold regions,' he said. 'The mathematics and physics at play are much different. For instance, a wave can travel long distances with little attenuation, whereas temperature usually diffuses over smaller distances.'
The trick, he said, was to apply the mathematics of transformation optics to the equations describing diffusion. The result, Dr Guenneau and his colleagues found, was a means to shuttle heat around at will.
In the study, the researchers propose a cloak made of 20 rings of material, each with its own 'diffusivity' - the degree to which it can transmit and dissipate heat.
'We can design a cloak so that heat diffuses around an invisibility region, which is then protected from heat,' he explained. 'Or we can force heat to concentrate in a small volume, which will then heat up very rapidly.'
This approach still employed the same fundamental theory, called transformation optics, that was first introduced for light in 2006, when physicists and mathematicians for the first time succeeded in designing invisibility cloaks.
The invisibility cloaks introduced in 2006 work for a set of wave equations - Maxwell equations - that govern the propagation of light and electromagnetic waves, for instance microwaves.
The French team extended the concept for their diffusion equation, which governs the flow of heat from one place to another.
It is very different from the method used by a team of physicists at the Hong Kong University of Science and Technology.
Chan Che-ting, chair professor of physics at the university, said their approach, called scattering compensation, uses artificial materials that are able to scatter light in an opposite fashion to how light is scattered by the original object.
Every object scatters light shone on it to form a pattern. Our eyes detect the pattern and see the object.
Chan uses metamaterials - man-made substances that possess properties not found in nature - that are able to scatter light in this opposite way. Both scattering effects simply cancel out each other. Thus, no light can now be detected and the original object is apparently invisible.
He gave an example: X + (-X) = 0. 'The X is scattering due to the object we want to hide. Our cloak is the (-X) that undoes the scattering due to that object. The hidden object cannot be seen by the outside world, but it can see the outside world,' he said. 'And so it has an advantage.'
Along with Lai Yun, Chen Huanyang and Zhang Zhaoqing, he published the idea in Physical Review Letters in 2009. His team has even gone a step further to create other forms of illusions, for instance, 'design cloaks that make an object look like another object'.
'We are just at the beginning. We now know how to make things invisible in principle, but in practice, there are still many challenges to overcome,' said Chan, also a director of the university's William Mong Institute of Nano Science and Technology.
For instance, 'the operational bandwidth is very narrow - you can make an object invisible at one frequency but it remains visible at other frequencies'. Expanding the operational bandwidth is very difficult for light, and the artificial materials that can do invisibility cloaking are very difficult to make.
The interest in accomplishing the 'mission impossible' is shared by many scientists in different fields. 'Physicists have come up with innovative ideas. Mathematicians have developed the mathematical tools needed to design these novel materials. Scientists in nanoscience and nanotechnologies have invented the fabrication techniques needed to make these artificial materials for cloaking. Engineers are trying to make real samples and worry about applications,' he said.
'Solving these problems is part of my mandate as a professor to generate new knowledge. These research activities are very exciting as they satisfy my quest for curiosity. This new knowledge can be the foundation of future technologies,' he said.
It is this curiosity which has, for several years, prompted Chan and his team to ask: 'Can we make an object invisible? Can we create an optical illusion so that an apple would look like a banana? Can we create 'optical tracker beams' as those depicted in the movie Star Trek?'
For those who, for instance, want a bite of that apple, the idea may seem dreadful. But Chan said: 'It is not scary if we look at the bright side of the possibilities.
'When we are making something invisible or making an apple look like a banana, what we are doing is changing the way light is scattered or absorbed by an object,' he said.
'The same technology can protect us from harmful or unwanted radiation. If a coating can enhance the absorption for light, it will facilitate light harvesting. If a coating can enhance the absorption for sound, it will be a potent sound absorber. If a material can focus waves better, it can help us see better. There are a lot of potential applications that are good for mankind.'