What do Casper the Friendly Ghost, Harry Potter and H.G Wells' Griffin all have in common? The answer, of course, is “the ability to become invisible”.
And these three characters weren’t the first to have such an ability – they were predated by several millennia by a range of mythological characters from the ancient world, such as Brownies and Mazikeen.
But while we’re all familiar with invisibility as it appears in fiction, is such technology possible in the real world?
As it turns out: yes – and the research is rather more advanced than you might think.
Hiding in plain sight
The fictional characters mentioned were all “invisible” when viewed in the part of the electromagnetic spectrum the human eye is sensitive to. Somehow the rays of light emanating from the scenery behind these individuals passed through them unaltered, making them disappear from sight.
Of course, there is another kind of invisibility. Imagine painting every part of yourself black and then hiding in a coal mine. You will be hard to find even if your adversary has a powerful flashlight. The invention of these “stealth coatings” brought invisibility of this kind to the aerospace industry in the 1980s. The result has been the manufacture and use in combat of a series of aircraft designed to absorb or deflect the microwaves emitted by other people’s radar.
This is perhaps not as elegant as Harry Potter’s invisibility cloak (see below) but it’s useful nevertheless.
The imagination of many physicists was galvanised by the claim of the English theoretical physicist John Pendry in 2006 that true invisibility (of the Harry Potter kind) could, in principle, be achieved in the visible part of the spectrum.
How? By bending light around a specially constructed object, and then reflowing the wavefront back together on the observer’s side of the object in order to send it on its way, as if it had never been interrupted.
Actually, Pendry had in mind rather small (nanoscale) objects and the proposal was probably intended more as an illustration of the power of his new “transformation optics” than as a practical suggestion.
Nevertheless, several teams of researchers have considered his proposal and verified that it is feasible, at least for specially designed structures.
And the principles extend beyond visible light. A recent paper by Nathan Landy and David R Smith is the latest of several describing the application to microwaves and there have also been demonstrations of such invisibility with sound waves.
Cloaked in secrecy
So, can cloaking in the visible region of the electromagnetic spectrum be extended to macroscale objects? Will you or I ever be able to don a cloak and disappear from view entirely?
Regrettably, the answer seems to be “no”.
Bending of visible light around an object in the manner required will only work for exceedingly small metallic particles. Why? Because the technique exploits plasmonic phenomena that play out over a few tens of nanometres.
(A nanometre is one billionth of a metre – rather smaller than the height and width of a human body).
However, the concept of “invisibility” in the more general sense still has plenty of space in which to run.
Since “invisibility” can literally depend on your point of view, there has been interest in projecting a deceiving image by other means.
There have been demonstrations in which the surface of an object, a T-shirt or car is coated with a flat or flexible display panel (see video below). The display surface is then fed an image of whatever lies behind the object, which it in turn projects onwards to the viewer.
Naturally this trick only works from certain viewing angles but is nevertheless quite striking when done well.
It would be remiss not to recognise that some creatures in the natural world have evolved a similar ability to appear invisible all on their own.
And then there are small silver fish such as sardines and herrings that can use their highly reflective scales of their scales to appear invisible to predators.
So, where might this all go? While not exactly passé yet, stealth aircraft are already relatively mature, so it is elsewhere we should look for the next big applications of these ideas.
On occasion, while drinking coffee in our Physics tearoom (yes, there are still actually such places!) my colleagues and I have ruminated on these topics.
One thought is to look at ways to mask the blackbody infrared radiation given out by people and vehicles. This is the thermal radiation that objects at some finite temperature above absolute zero must inevitably emit in order to attain thermal equilibrium with their surroundings. This would have undoubted military applications because infrared imaging is used to identify targets.
(Yes, I know it has already cropped up in fiction: in the 1987 movie Predator the hero coats himself with mud to fool the infrared receptors of his alien adversary.
Naturally the thermodynamics of radiant heat transfer cannot be defeated entirely, but perhaps all that is required is to reduce emissivity in that part of the spectrum where the enemy’s detectors operate.
That will work … at least until the enemy figures it out.
Perhaps Harry Potter’s invisibility cloak will remain out of reach for people-sized objects, but there are plenty of other exciting ways in which the physical laws might still be exploited to trick cameras, detectors and eyes.