Become a Superhero: Invisibility is now a Reality

 

 

 

As children, we obsess over our favorite superheroes and the superpowers we wish we could have. We dress up as Spiderman for Halloween and pretend we have superhuman reflexes like Wonder Woman. Some powers, like telekinesis, are out of the reach of science. But what if some are not? In the past decade, researchers have begun developing technologies that render objects invisible to the human eye. Maybe in the near future, we’ll be able to make ourselves invisible too!

Right now, I’m already envisioning the day when my Harry Potter costume has a real invisibility cloak, but there are many more applications of invisibility technology that are just as cool but much more practical. Invisibility can be used in stealth technology, which has military applications, like invisible planes and satellites. There are also applications of invisibility in security; invisibility can strengthen the encryption of secure devices by making parts of computers or other electronic hardware invisible.[1]In the medical field, invisibility gloves could let doctors performing surgeries look through their hands and see their work better. In communications, the technology that makes invisibility possible (which I will explain soon!) can reduce the scattering of waves and improve the quality of ground and aerospace transmissions.[2]

So how are we really going to create invisibility? First, we have to understand how we see normal objects. Light allows us to see the world around us, and it is made of electromagnetic waves that have properties like frequency (how tightly packed a wave’s peaks and valleys are) and amplitude (how tall the wave is). Our eyes interpret these properties to create the images we see; for example, we see different frequencies as different colors: red has a low frequency and blue has a high frequency. If you hold a red block in front of you, the reason that you see it as red is because of how the light waves that hit the block interact with its surface. Depending on what the block is made of, the material will absorb some frequencies of light and reflect others, giving rise to a color. For a red block, the surface will reflect only the frequency that corresponds to red, and absorb all others. Our eyes detect that red light waves have been reflected, and so we see a red object.

 

Red light waves have long wavelengths and low frequencies while blue and purple have short wavelengths and high frequencies

 

So in order to make objects seem invisible, scientists are trying to find ways to keep your eyes from detecting that light is interacting with an object, whether that be through directing light waves around an object so that they never interact with it (method 1), changing light waves directly after they bounce off a surface so they seemlike they never hit it (method 2), or changing the frequencies of light that hit a surface so that no waves will be reflected off of it to give away its presence (method 3).

These are the three main paths that scientists are taking to achieve invisibility. The first method uses basic optics to make objects disappear from your line of sight, like an optical illusion. The second method achieves invisibility by creating physical materials, like cloaks, that render objects they’re placed over invisible. The third method creates ‘digital’ invisibility, which makes objects look invisible through a filter.

Redirecting Light

 

The first and simplest method of creating invisibility, developed by researchers at the University of Rochester, harnesses reflection and refraction of light to direct light around 3D objects so they appear invisible. The researchers positioned a series of four lenses in a way that directs light around objects placed between them, so that if you look through the lenses, the object will not appear. In the image above, a person’s hand is placed in between the lenses. However, when you look through the lenses, you do not see the hand, only the background; this happens because the light reflected off the background travels through the lenses and around the hand, bypassing it so it never interacts with the hand.[3]

A more complicated version of this system uses a digital lens and display. First, the researchers capture images of a background scene from many different angles. Then, they put a filtering device (essentially an iPad) in front of the scene that displays what the background should look like from your particular viewing angle – based on calculations done in the filtering device – and omit any objects in the new image that were not part of the original background scene. This essentially makes the new object invisible to the viewer. By carefully placing lenses over the screen of the iPad, researchers can even make this method work for multiple viewing angles at the same time.[4]

Though this method of creating invisibility seems relatively simple and more like an optical illusion than the magic of invisibility that we imagine, it has the benefit of being the only method of invisibility that can currently render multi-colored, macroscopic 3D objects invisible to the human eye. As we will see, the next two technologies are much more high-tech, but they do have some limitations.

Invisibility Cloaks

The second method of creating invisibility relies on the property of light called ‘phase’. Two waves traveling next to each other are “in-phase” if their crests line up (see the image below). When two light waves hit the same object and reflect off of it, they will usually end up out-of-phase with each other. When your eyes detect this change from in-phase to out-of-phase, they conclude that an object is present. So if the waves could be made to go back in-phase, the object would disappear from view.

On the left, the waves are in phase. On the right, they are out of phase.

In the past ten years, many research institutions have developed physical cloaks that make objects invisible. Through a collaboration between the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California Berkeley, scientists built one of these prototypes. They created a microscopic ‘cloak’ that will conform to the shape of the object it is placed on. The cloak works by reconstructing the phase of the waves. When light hits the cloak and the waves go out of phase with each other, the particles of the cloak interact with the light waves and make them go back into phase with each other. Thus, your eyes never process the change in phase, and they do not notice the object there – the object is now invisible to the human eye. (There are some really cool images of invisibility in action in the paper that the Berkeley researchers published. You can view the paper here.) Cloaks like these are being made from materials called ‘meta-materials’ that can reconstruct the phase of waves, making the light essentially ‘pass-over’ the surfaced unchanged. The researchers at Berkeley used gold nanoparticles to make their cloak.[5]

The two major limitations of this method of creating invisibility are that the cloak can only make one specific wavelength of light invisible (so an object will only be fully invisible if it is all the same exact color) and that the cloak is currently microscopic. However, the researchers at Berkeley say that their design for an invisibility cloak has the best chance for being scalable to macroscopic objects compared to other designs for similar physical cloaks.[5]Other institutions that have developed these microscopic cloaks include Queen Mary University in London and Ben-Gurion University in Israel.[6], [7]

Spectral Cloaking

Another way to render objects invisible is digitally, through a lens. The Canadian National Institute of Scientific Research in Montreal has developed a device that uses lenses to filter out specific colors of light. The device has two lenses, and the object you are making invisible is placed between them. It works by changing the frequency of the light that passes through the lenses.

Remember how we see objects because particular frequencies (colors) of light are reflected off of them into our eyes, while other are absorbed? The lenses in the device will ‘bump’ the light that passes through the lenses up or down to another frequency that we know the object will absorb. So when the object is placed in between the lenses, all the light that hits it will be absorbed, leaving no light to be reflected – and the object appears invisible.[8] According to one of the researchers, Jose Anzana, when the object is viewed through the lens, you are able to see behind it as if it were not there.[9]

Similarly to the physical cloak, a limitation of this kind of device is that currently, the device can only filter specific programmed frequencies of light, rather than all visible light. However, the researchers say that theoretically, the device can be developed to filter out whole spectrums of light.[9]

Though the technology for invisibility is still developing, in the past decade there has been immense progress in this field, and researchers are very hopeful. I suppose I may have to wait another decade before my Harry Potter costume is truly complete, but hopefully the day is nigh when we will get to realize one of our childhood dreams.

References:

  1. (2015, September 17). Making 3-D objects disappear: Researchers create ultrathin invisibility cloak – Phys.org. Retrieved October 29, 2018, from https://phys.org/news/2015-09-d-ultrathin-invisibility-cloak.html
  2. Howard, J. (2016, July 20). Scientists get closer to creating real-life invisibility cloak – CNN. Retrieved October 29, 2018, from https://www.cnn.com/2016/07/20/health/invisibility-cloaks-research/index.html
  3. [University of Rochester]. (2014, Sep 25) The Rochester Cloak [Video File] Retrieved from https://www.youtube.com/watch?v=vtKBzwKfP8E
  4. [University of Rochester]. (2016, May 19). The Rochester Digital Cloak: A New Age of Invisibility [Video File] Retrieved from https://www.youtube.com/watch?v=QF8qkoIWH64
  5. Ni, X., Wong, Z.J., Mrejen, M., Wang, Y. & Zhang, X. (2015, September 18). An ultrathin invisibility skin cloak for visible light.Science 349(6254), 1310-1314.
  6. Howard, J. (2016, July 20). Scientists get closer to creating real-life invisibility cloak – CNN. Retrieved October 29, 2018, from https://www.cnn.com/2016/07/20/health/invisibility-cloaks-research/index.html
  7. Pinnington, R. (2017, November 14). Stunning scientific breakthrough: Researchers discover how to make things invisible  – Daily Express. Retrieved October 29, 2018, from https://www.express.co.uk/news/science/879226/invisibility-cloak-science-research-breakthrough-metamaterials
  8. Cortes, L.R., Seghilani, M., Maram, R., & Azana, J. (2018, June 28) Full-field broadband invisibility through reversible wave frequency-spectrum control. Optica 5(7), 779-786.
  9. Elliot, J.K. (2018, June 28). Full ‘invisibility cloak’ is possible in the real world … – Global News. Retrieved October 29, 2018, from https://globalnews.ca/news/4302166/invisibility-cloak-technology/

 

 

 

 

 

 

 

Advertisements