Its plain to see that many of us have a fascination with invisibility.
Whether youre one of the millions of sci-fi aficionados or just someone whos curious about concepts that seemingly elude life on Earth, chances are youve wondered whether invisibility is possible. Greg Gbur says it is, and that it can be used in ways to benefit humankind.
An associate professor in the Department of Physics and Optical Science at UNC Charlotte, Gbur gives presentations on the subject. His Ph.D. research, completed in 2001, was on crude forms of theoretical invisibility that people were considering at the time. I was a hipster physicist, doing this stuff before it was cool! he said.
The possibilities of invisibility got dramatic new energy via two theoretical papers that appeared back-to-back in the journal Science in 2006. We asked Gbur about how invisibility works, how recent findings have given the concept momentum, and where it all could be headed.
Q: Can there actually be invisibility, or is it just a relative thing like fooling electronic monitors?
Its interesting that up until 2006, it was generally thought that perfect invisibility was impossible. There were mathematical theorems that suggested you couldnt make something perfectly invisible because something would always end up being visible in the end.
But research published in 2006 (by Ulf Leonhardt of Scotlands University of St. Andrews, English physicist John Pendry, David Schurig of N.C. State University and David Smith of Duke University) suggested that those earlier theories were too simplified in looking at too limited a possible class of, say, materials that would be used to make an invisible object. (Smiths group at Duke demonstrated an invisibility cloak that rendered an object nearly invisible to microwaves.)
Q: When you say materials, what do you mean?
The original cloaks were envisioned as spherical objects made out of a material (where) light that hits the cloak would be guided around the interior cloaked region and come out the other side as if it had just gone straight through. Thats sort of the water going around the boulder in the stream analogy. But the cloak itself is invisible, and the object within the device, regardless of its nature, is completely hidden.
Q: What are the cloaks made of?
For years, such cloaking devices were thought to be impossible as no material in nature exists that would have the necessary properties. However, scientists are now working on constructing so-called metamaterials, which are not found in nature. Ordinary materials get their optical properties from the behavior of atoms; metamaterials get their optical properties from a manipulation of the arrangement or structure of the atoms on a scale comparable to a billionth of a meter.
Q: Can we make these cloaks?
The original 2006 designs are limited in that they can work perfectly only for a limited range of colors; for example, you could design a cloak to hide you from red light, but it would be visible for blue light. Much of the research since then has involved coming up with new designs that sacrifice perfection in order to function for a broader range of colors, hopefully getting to the visible range.
A number of crude experimental demonstrations of the principles of cloaking have been made, though none of the objects is invisible in any sense of the word. My new analogy on the state of cloaking physics: We have a wheelbarrow and are thinking how cool an interstate highway system would be.
Q: Why should we care about cloaking?
One thing I should make clear is that were still at this stage in cloaking physics where things are really mostly theoretical. Theres nobody running around wearing cloaking devices that you cant see. As Ive kind of put it as a joke before, if theres such a thing as a perfect cloaking device, I havent seen it yet.
Obviously, the military is interested in the ability to make planes, tanks, etc., perfectly invisible. Stealth bombers are designed to be hard to detect by radar but are quite visible to the eye.
An even more interesting aspect of cloaking is the ability to protect objects. Just as we can, in principle, cloak objects from light waves, researchers have suggested that we could cloak buildings from damaging earthquake waves by putting structures underground around the buildings. Also, it has been suggested that offshore platforms could be cloaked from damaging water waves by undersea structures. It is also, in principle, possible to shield objects from magnetic fields, which could be used in protecting the delicate electronics used to drive medical MRI machines.
Q: What else can we do?
If we can design invisible objects, it follows theoretically that we can make any object look like any other object creating perfect optical illusions!
It has also been shown that objects can be cloaked/illusioned externally: We can hide someone by putting a cloak next to them, instead of around them.
Combining these tricks, researchers have shown theoretically that it might be possible to make an illusion device that would allow you to see through a wall (making an illusion of a hole in the wall) or, conversely, make an illusion of a wall over an opening in the wall a secret passage!
Q: What are potential drawbacks when altering the forces of nature in this way?
(In the case of protecting a building from an earthquake), it raises interesting questions of, well, if you just guide the earthquake wave around your building and it hits the next building, somebodys going to get really mad.
So it sort of interestingly adds this possible long-term future legal/political implications: What sort of other implications does it have if we can get away with doing this?
Q: What is your future role in this field?
Im interested in illusion optics: If you can make something, in principle, perfectly invisible, that also means you can design objects that in principle look like anything you want. It also turns out that you can design cloaking devices that can be outside the objects you want to hide. So I can have a cloaking device that cloaks me without even being around me. (The hole-in-the-wall example.)
All of these ideas are still mostly theoretical, and there isnt a lot of exploration of the limits of this. Im sure there are going to be a lot more technical and ethical obstacles, even beyond the ones were already aware of. All of these things are probably farther in the future than we need to worry about directly at the moment.