The urge to become invisible goes back a long way. Hunters and soldiers have been finding ways to camouflage themselves for centuries, but scientists are edging closer to truly making things invisible.
Modern stealth tech can not only shield planes from radar, but can also conceal high heat signatures from infrared cameras and stop sound waves from being heard. So how close are we to developing invisibility technology?
Hiding in plain sight
We see objects because when light interacts with them the light is absorbed and reflected. Transparent objects, such as windows, allow light to pass through them almost undisturbed. For an invisibility cloak to hide an opaque object, it would need to redirect the light around it instead.
One of the earliest proof-of-concept cloaking devices was developed in 2006 by engineers at Duke University. This experimental device was made up of a copper cylinder that was ‘hidden’ by deflecting microwaves around it, making it appear almost as if it wasn’t there to a microwave detector.
It was made from a metamaterial – a structure made up of a periodic array of features (an arrangement that repeats at regular intervals in three dimensions – think of a lattice) that give it unusual properties.
This early cloaking device only worked for microwaves – electromagnetic radiation with relatively long wavelengths. Making an object invisible to visible light, which has a much shorter wavelength, is a tougher challenge.
That’s because at the nanoscale, quantum effects start to take over, for one thing, but also because the cloak only worked for a specific wavelength of microwaves. For a cloak to be truly invisible to all light, it would need to conceal itself from all the colours of the rainbow.
Plus, there’s the fact that it only worked for a small object; not something the size of a human.
A later breakthrough came in 2018, when researchers at Harvard and the University of Waterloo, Canada, demonstrated a device made from an array of metalenses (flat surfaces that use nanostructures – in this case, titanium-based ‘nanofins’ – to focus light) that was able to bend a broad range of visible light wavelengths around it.
A step closer, but a true wearable invisibility cloak still remains out of reach.
“The thing that everybody wants, an invisibility cloak that you could wear… they’re stuck on being able to make materials anything like that flexible,” says Simon Horsley, an associate professor of theoretical physics at the University of Exeter.
“Today’s materials would be like a cylinder you’d put around yourself. But if you want something you could move around in, that’s a whole other design problem.”
Stealth in the sky
Although we may still be some years off developing a true invisibility cloak, some things can already make objects effectively invisible to other wavelengths: the design principles and materials that hide military jets (such as the stealth fighter, below) from radar, for example.
Radar works by sending out a burst of radio waves and listening for any waves reflected back by objects it encounters. In this way, the radar receiver can calculate how far away an object is.
Being made of metal makes aircraft very good reflectors of radar signals and therefore very easy to detect. But two things can help an aircraft to become invisible.
The first thing is its shape. The rounded shapes found on passenger aircraft are excellent at reflecting radar, because no matter what angle the transmitted signal hits the aircraft at, some of it will always bounce back to the receiver.
This is why stealth aircraft are almost entirely constructed using flat surfaces and sharp edges – they’ll still deflect radar signals, but not straight back towards the receiver.
The second thing is to avoid making the aircraft out of electrically conductive materials, such as metals and carbon fibres, to improve its radar transparency.
If that’s not possible, the aircraft can be coated in a special radar-absorbent material. These paints absorb the radio waves and convert their energy into heat, instead of reflecting them back.
One example of such a coating is ‘iron ball paint’, which contains microscopic iron spheres whose resonant frequency matches those of typical radar. When a radar signal hits the aircraft, the balls resonate and convert its energy to heat, which dissipates into the environment.
In this way, a stealth aircraft can effectively be disguised as a small bird. Such materials are only effective within certain wavelengths, however, and the increasing computing power of radar detection systems means it’s increasingly difficult for aircraft to hide from them.
Of course, it’s not just radio frequencies that stealth aircraft must be designed to evade. They’re often painted matte black and fly at night, and the pilots are guided to altitudes where contrails are less likely to be formed, making them more difficult to spot in the skies.
The intense heat produced by their engines is also an issue.
This can be partially mitigated by injecting cool ambient air into the hot exhaust and using a slit-shaped tailpipe to maximise the mixing of hot exhaust with cooler ambient air. Some designs even position the exhaust above the wing to hide it from observers below.
Next-generation electromagnetic metasurfaces also promise an even more effective method of redirecting incoming electromagnetic waves, though the challenge of avoiding detection by a broad range of wavelengths remains.
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It's getting hot in here
For soldiers, staying camouflaged to infrared tech is a problem. The human body naturally gives off around 200W of infrared radiation (IR), or heat – that’s about the same power as three household lightbulbs – and is easy to detect with the right equipment.
A simple and cheap IR invisibility cloak is an aluminium foil blanket. Shiny metals like this have almost zero emissivity – the measure of how well the material gives out thermal radiation.
These work surprisingly well for a short period of time, but soon body heat accumulates inside the blanket and is easily detected. Also, depending on the emissivity of the environment where you’re hiding, the blanket may show up as a cold spot to the camera.
A more effective camouflage that Prof Coskun Kocabas and a team from the University of Manchester are working on actively adapts to the environmental background, like a chameleon.
"Our initial motivation was: can you make smart surfaces that can mimic these animals?" Kocabas says.
The answer is yes, with graphene-based devices. “By changing the optical properties of graphene, you can make adaptive surfaces that you can use to camouflage from visible, infrared and even microwave radiation,” he says.
The material harnesses the optical properties of incoming light interacting with the electrons on its surface. As a 2D material, graphene is unique in that it has a lot of mobile electrons on its surface. This is what makes it so incredibly electrically conductive.
“Fundamentally, you don’t need the material, you need electrons. If you can control the electrons on the graphene’s surface, you can change reflectivity, absorption and thermal radiation. Graphene is the platform giving you these tunable optical properties,” says Kocabas.
This is done by squeezing ions in between graphene layers – a process called ‘intercalation’.
By doing this, Kocabas’ team can alter the mobility of electrons on the graphene surface, allowing them to control the optics of the material – including its emissivity.
In 2022, the team made a wearable jacket containing 42 graphene patches that acted like pixels in a display. On the battlefield, this could be used to match the background emissivity, rendering the wearer invisible to an IR camera.
But graphene was only isolated 20 years ago and there are still challenges around integrating 2D materials with bulk 3D materials. But, once solved, such wearable devices that can cloak both infrared and visible spectrum radiation could take us one step further to real-life invisibility cloaks.
Be seen, but not heard
When it comes to being silent, nature has us humans beat. Take the African cabbage tree emperor moth, for example. As it’s nocturnal, it’s not bothered about being seen by its predators in infrared or visible light. Instead, its main problem is being detected by bats using echolocation.
So this tiny insect has evolved the perfect acoustic invisibility cloak in its wing scales and fur that effectively absorbs a bat’s ultrasound calls, preventing reflection and, therefore, detection.
This ingenious mechanism was discovered by Prof Marc Holderied at the University of Bristol and is the first known naturally occurring acoustic metamaterial.
Acoustic metamaterials are structures that can control how sound waves move through their structure. But, instead of it being the electrons around regularly spaced carbon atoms that interact with the light waves, in acoustic metamaterials, the periodic array of structures interact with sound waves.
“This allows us to play around with the structure, geometry and materials,” says metamaterials researcher Dr Felix Langfeldt, an associate professor at the University of Southampton. “We can put them all together in a periodic structure and it can reflect, refract or absorb certain frequencies of sound very strongly.”
The genius of these structures is that they can absorb sound waves in much lower frequencies than conventional materials, such as foams, and can also be made much thinner.
"Imagine a paper-thick sheet that blocks sound the same as a concrete wall," says Langfeldt. Such structures could be used to silence irritating noises, such as ventilation systems or, as Legfeldt is looking into, on aircraft.
But acoustic metamaterials don’t just dampen noise or vibration. They can also be used to redirect vibration (if you wanted to redirect an earthquake around a building’s foundations, for example) and to harness the energy of vibration.
Dr Gregory Chaplain, a senior lecturer at the University of Exeter, is studying this advanced technology.
“There’s a lot of wasted energy from vibrations in a car – the sound, for example, can be really annoying,” he says. “If you can localise where that energy goes by directing it using metamaterials, and put something there that can siphon off that energy, then you can scavenge it.”
Such systems could be used to harvest ambient vibrations to power small devices such as sensors, particularly in hard-to-reach places like bridges, nuclear reactors or on aircraft.
In theory, then, the metamaterial cities of the future could be quiet, energy- harvesting, safe from seismic activity and, maybe one day even invisible.
About our experts
Prof Simon Horsley is an associate professor of theoretical physics at the University of Exeter. His work has been published in a number of journals, including articles in Scientific Reports, Physical Review Letters and Physical Review Research.
Prof Coskun Kocabas is a professor of 2D device materials and engineering at The University of Manchester. He has been published in the likes of Composites Science and Technology, Science and ACS Applied Nano Materials.
Dr Felix Langfeldt is an associate professor in the Faculty of Engineering and Physical Sciences at the University of Southampton. His work has been published in Journals such as Acoustics Today, The Journal of The Acoustical Society of America and Journal of Sound and Vibration.
Dr Gregory Chaplain is a senior lecturer at the University of Exeter who specialises in wave propagation using structured materials and metamaterials. He is published in journals including New Journal of Physics, Communications Physics and Physical Review Materials.
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