Imagine a smartphone that remains "cloaked" until its owner whispers a pass phrase. It's an incredible concept, but could it be that the ultimate smartphone security feature is invisibility?
If you think that sounds crazy, how about rendering an entire car or a space ship invisible? Even crazier, it's rapidly becoming science fact rather than science fiction. And that's because it's not only been shown to be possible within the laws of physics, but it's being worked on by scientists and researchers right now.
It wasn't always the case. Even 10 years ago, cloaking devices were still thoroughly shackled to the realm of science fiction and thought to defy the laws of optics.
But in 2006, scientists from the UK and US created an incredible new material that appeared to do just that.
And this week researchers at Cambridge University have announced the discovery of a new technique that could one day be used to render invisible anything from an iPhone to a space station.
So what techniques could we use to render objects invisible and when can we expect an invisible smartphone?
The most obvious method of making something invisible is to use what's often called optical - or active - camouflage.
By photographing the scenery behind an object and projecting that image on to its front, you'll render it partially invisible.
This was the concept behind James Bond's invisible Aston Martin in Die Another Day. But while this method can actually be quite effective from one angle, there are obvious weaknesses when it comes to viewing the "invisible object" from positions. The technology is still finding some excellent practical uses, though.
For example, you can potentially project the view from underneath a plane onto the cockpit floor so that a pilot can have a much better idea of where the runway is. In the same way, you could project the ground underneath a car onto the hood so that offroad drivers can more effectively navigate tricky terrain. The transparent cockpit idea could also be used to eliminate car blind spots.
It's a realistic and promising method of seeing through objects but almost completely useless for making objects such as an iPhone invisible. For that, you're going to need an understanding of quantum mechanics and the ability to manipulate materials at the smallest of scales...
When you think about it, invisibility isn't actually that strange. Have you ever wondered what air looks like? How about glass or other clear fluids?
Many things are invisible to the human eye, and it's all to do with the way their atoms are arranged. Many gases and liquids are invisible because their atoms are spaced far enough apart for the wavelengths of visible light to travel through without being disturbed.
Water is visible only because of the way it bends and distorts light as it passes through, and it's this bending, or refracting, of light that holds the key to invisibility. Hold that thought.
If atoms hold the key to invisibility, it makes sense that we will need to manipulate them in new and innovative ways. And we first developed the ability to do this back in the early 1980s. Using the 1981 Nobel Prize-winning Scanning Tunneling Microscope, scientists are able to not only take pictures of but also manipulate individual atoms. In 1990, this technology caused waves and hit the international media when it was used to spell out "IBM" using 35 individual xenon atoms - a watershed moment for nanotechnology.
Using this technique, scientists are attempting to to construct materials and even machines using individual atoms as building blocks. And it's research in this field that seems most likely to pay off when it comes to invisibility.
Surely the most promising application of nanotechnology when it comes to invisibility is in the production of what scientists call metamaterials.
Thought to be prohibited by the laws of optics until less than a decade ago, metamaterials have properties that are not found anywhere in the natural world and have the potential to one day render objects completely invisible even to the human eye.
Metamaterials are made by rearranging the building blocks of a material in sophisticated arrays so that its overall index of refraction (the extent to which light is bent as it passes through) is negative rather than positive. By doing this, you can potentially bend light around an object and out the other side - a cornerstone in the quest for an invisibility cloak.
In 2006, scientists from the US and UK created a material from copper and other metals that was able to bend light around a cylinder in such a way as to render it almost completely invisible to microwave radiation. This stunning experiment proved the concept and sparked a new race to build metamaterials that can manipulate different kinds of light.
So the laws of physics don't prohibit light from being bent around an object, but how do we create a material that could render an iPhone invisible?
The problems arise from the fact that the crystals inside the metamaterials must be smaller than the wavelength of the light you're attempting to bend. With a wavelength of around 3cm, creating a material to interact with microwaves it turns out is pretty straight forward. However, to play with visible light in the same way, you're talking many different wavelengths between 380 and 800nm, one nanometre being a billionth of a metre - about the length of five atoms side by side. Quite a challenge.
How to make an iPhone invisible: 1.
Scientists all over the world are now racing to be the first to create a metamaterial that can bend visible light, and many hope that the computing industry is well placed to lend a hand.
Photolithography is a complex technique that silicon chip manufacturers like Intel and AMD already use to fabricate components that contain billions of microscopic transistors. Ultraviolet light is used to etch the components onto silicon wafers.
It is hoped that a new generation of computer chips will use light instead of electricity to process information, making them faster and far more efficient. These new chips will need billions of crystalline transistors each with a slightly different index of refraction. And that means research into invisibility cloaks can piggyback on work in this field.
"Metamaterials may one day lead to the development of a type of flat superlens that operates in the visible spectrum," says Ames Labratory Senior Physicist Costas Soukoulis. "Such a lens would offer superior resolution over conventional technology, capturing details much smaller than one wavelength of light."
While progress in this field is ongoing, there are still huge obstacles to be overcome.
Photolithography has been shown to be capable of stacking photonic crystals that bend light in two dimensions, but the process is far more complicated when it comes to bending in three dimensions as you'd need to cloak a real-world object like an iPhone. The process might not be capable of it.
Plus of course, bending more than one wavelength of light around the same object is another enormous challenge in itself, potentially requiring multiple layers of different types of metamaterials.
The military applications of rendering objects invisible are fairly obvious. And as such, military customers are footing the bill for some of the research. Imagine a scary future where the NSA could mount a camera in a room and then cloak it - they'd be able to spy on people without them knowing.
"Ah ha", you say, "but if you're bending light all the way round an object as to make it invisible, how would any light get into the object so that it can observe its surroundings?" That's a good question, and the answer could be plasmonics.
Plasmonics is another potential candidate for becoming the technology behind tomorrow's super-efficient computer chips, and it's also lending a hand with research into invisibility. Instead of using just light to perform calculations as do photonic crystal-based chips, plasmonics uses a combination of light and electricity - photons and electrons.
It works by using the interaction of photons with the electrons in metal nanostructures to induce oscillating electrical currents on the surface of both the metal and the semi-conducting silicon layer underneath, producing scattered light waves. By tuning the structural geometries of these materials, the light waves from the metal and semiconductors will cancel each other out, rendering any object underneath invisible.
"It seems counter-intuitive, but you can cover a semiconductor with metal - even one as reflective as gold - and still have the light get through to the silicon," says Associate Professor Mark Brongersma at Stanford University. "As we show, the metal not only allows the light to reach the silicon where we can detect the current generated, but it makes the wire invisible, too.".
Part of the brilliance of this metamaterial is that, by effectively making it out of light itself, it's far easier to make in larger quantities than other candidate metamaterials.
"We have controlled the dimensions in a way that hasn't been possible before," said Dr Valev, who worked with researchers from the Department of Chemistry, the Department of Materials Science & Metallurgy, and the Donostia International Physics Centre in Spain on the project. "This level of control opens up a wide range of potential practical applications."
So we've seen that while invisibility is in its infancy, progress is being made. And one day scientists could feasibly create an invisibility cloak. So when will we see an iPhone that can render itself invisible?
Well obviously the answer, unfortunately, is never. By the time this technology reaches maturity, we'll likely be far beyond using smartphones as everyday gadgets. But the latest news from Cambridge University does at least show that regular progress is being made towards what is now considered a realistic goal.
The invisible future, it seems, is just around the corner.