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Duration: 18:24

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References:
https://mynasadata.larc.nasa.gov/sites/default/files/inline-images/Electromagnetic_Spectrum_Diagram%20flipped_FINAL.png
https://en.wikipedia.org/wiki/Fresnel_equations
https://pubs.aip.org/aip/jap/article/129/23/231101/286411/Optical-cloaking-and-invisibility-From-fiction
https://arxiv.org/pdf/1306.0863.pdf
https://www.youtube.com/watch?v=oJb9RnAVDuE


Resources:
https://arxiv.org/
https://ui.adsabs.harvard.edu/

Notes/Transcript:

What is light? How do we think about it and define it in physics?
In the most simple terms: light is an energy that is radiated from a source. You may have heard of wave-particle duality, meaning that you can describe light as a particle source (a photon) or a wave (electromagnetic). For this case, let’s stick to the electromagnetic wave formalism, and ignore the quantized photon particle. So if it’s a wave, what is it made of? The electromagnetic wave is the culmination of an oscillating magnetic field and an oscillating electric field. We can think of this overall EM wave as one regular wave represented by our favorite plot, the sine wave. There are lots of details I can go into about how different light waves look and what that means physically, but the most important detail now is the wavelength (lambda). The wavelength (which is a unit inverse to frequency, for my frequency-domain brethren) of a light wave gives you the information about what part of the EM spectrum that wave is in, with large light waves being in the radio part of the spectrum (light waves with wavelengths on the order of 1000 m ) and large wavelengths being in the gamma ray part of the spectrum (on the order of 10^-12 m). These regimes are all good to know for other reasons (i.e. blocking out radio waves, studying radio astronomy, looking for gamma ray bursts from space), we should just focus on the fact that the optical range, the part we can see with our human eyeballs, is in the nanometer range. So when we talk about light, just think about a silly little sine wave that obeys all the wave mechanics we expect it to (and if you don’t know what that means, it’s ok! I’m handwaving to save some time.)
What is a waveguide?
So now we have a light wave! What can we do with it? One such thing we can devise in a lab is a waveguide. It is exactly what it sounds like: a thing that guides (light) waves. An example of a waveguide that we use on a regular basis could be an optic fiber cable, or a cable that can guide an optic (light) signal carrying data to a receiver that you want it to. A sound based waveguide example is a stethoscope used to listen to a human’s insides. A waveguide works by limiting the transmission of the wave to keep it within the guide. Light naturally emits radially, as in it goes away from its source in a spherical 3d motion. What the guide does is that it blocks its path so that it can’t escape every which way it wants to go, and is essentially only allowed to travel in the path of the guide. Light transmission is an aspect of both a specific light wave and material that can be controlled in laboratory conditions using the Fresnel Equations.

So now with the background laid out, what is cloaking and is it real? If so, how does it work?
To answer the first question: is cloaking real? … Yes, mostly. Cloaking tech has been a pretty hot field of photonics and material science for the past twenty years or so. There’s obviously a lot of reasons why an effective tool to make something like a person or jet invisible would be valuable to many (and let’s be honest, the militaries of the world). And for the most part it is being realized slowly. There have been some amazing demonstrations of effective cloaking technology from the past twenty years and it seems like the field is really getting to the point they want it to (though I have no idea about labor intensity or cost for large-scale cloaking devices which I imagine would be the limiting factor as of today).
The general idea of optical cloaking (again, I’m only talking about cloaking that tricks your eyes, not your sensors) is to bend light around the object you’re trying to cloak. The idea is that we only see things that a light beam has reflected off of and is absorbed into our eyes. So if the object you’re trying to hide never reflects light then we can’t see it. The idea then is to have the light from behind the object able to move in front of it so we see the background instead.
This is possible with current technology, BUT it tends to be dependent on a series of factors including temperature, angle of incidence (it may work when looking at it 90 degrees straight on, but not when you’re looking at it from a 20 degree angle), geometry (flat planes vs spheres, etc.), amongst other things. Though in essence all a cloaking device is a waveguide that can show the image from behind an object to in front of it. How one builds that waveguide has a world of possibilities to explore.