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How somebody could become perfectly invisible by the use of technology without being blind because of the deviation of photons. These past years, a lot of meta-materials or other technologies have shown to be able to bend light and being imperfectly invisbles, for example :

In 2019, a paper-thin material that bend utraviolet and infrared light to make only the background of a man visible and not himself was made. It is achieved by the use of lenticular lenses in the material. It is alas rigid. Source : https://globalnews.ca/news/6110652/bc-company-cloak-of-invisibility/

In 2011, at the Technical University of Denmark a carpet cloak made of dielectric materials was developed. It consist of a « semiconductor manufacturing techniques that involve patterning the top silicon layer of a silicon on insulator (SOI) wafer with nanogratings of an appropriate size and structure ». The design can be tweaked by changing the orientation of the layers, so the wavelength of light affected. According to the scientists, it is a method easier to do than other and can produce a « carpet » large enough to cover entire vehicles. The more layers is added to the metamaterial, the more efficient it is, for me this is a form of drawback in term of weight.
Source : https://newatlas.com/invisibility-cloak-metamaterials-size/18454/

In 2016, scientists at the Iowa State University unveiled their meta-skin cloaking project. For now it only make you invisible against radar detection but they are confident about making it bend ultraviolet and infrared light. Source : https://newatlas.com/meta-skin-invisibility-cloak/42205/

As you see these technologies shows a lot of promises, starting just by the fact that they do not require energy to function but are still in development stage and with specific draw backs. Despite this, by take one of those examples a base, how true invisibility without becoming blind by the deviation of photons could be achieved ?

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    $\begingroup$ In Quake, invisibility didn't affect the eyes. The short, cop-out answer is that you can see just fine with a small spot of non-invisibility to let light reach your eyes. This doesn't meet your "perfectly" criteria, but it's much harder to notice one or two tiny sensor patches compared to an entire person. $\endgroup$
    – Matthew
    Mar 9 at 18:48
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    $\begingroup$ How should the invisibility react to more mechanical interaction, especially sound? $\endgroup$
    – hyde
    Mar 10 at 8:46
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    $\begingroup$ Also interesting to note that H. G. Wells very briefly addressed this issue in The Invisible Man (1897): "I went and stared at nothing in my shaving-glass, at nothing save where an attenuated pigment still remained behind the retina of my eyes, fainter than mist. I had to hang on to the table and press my forehead against the glass." It's never mentioned again but it seems like he put it in there to explain why the invisible man can still see. $\endgroup$
    – Nathaniel
    Mar 10 at 14:56
  • $\begingroup$ The same concern was addressed from a slightly different point of view (oops, pun) in What does a person in a light-bending invisibility cloak look like from behind? $\endgroup$
    – David K
    Mar 11 at 12:55

12 Answers 12

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Transmissions:

A few possibilities:

  • Your person isn't using direct light, but is instead receiving feeds from drones following them around. The transmissions are outside the (very broad) invisibility range, so they get through. Then a computer converts them into visual signals. This means the person can get transmissions in ranges unlikely to be monitored by others (maintaining superior invisibility) but does mean that the drones themselves are detectible unless masked by some lesser degree of invisibility.
  • Your person is using sound, ultrasound or direct non-visible light to "see." The data from this info is translated by computers or special goggles into a visual interpretation of what the surroundings should look like. So sound (sonar) is translated into a visual equivalent by smart software. Non-visible light is shifted into the visual range and displays as visible to the person's eyes. This means the person is independent of outside signals, but the scope of the invisibility has to NOT include whatever the user is seeing (so if infrared, then an infrared scanner can spot them).
  • If NO feedback can get through the barrier, our invisible person CAN'T get this data from outside, and instead functions in a virtual simulation of what their surroundings SHOULD look like. If no feedback is possible, then a visual overlay of what reality SHOULD be displays for the person. This is problematic in a dynamic environment with shifting objects and moving people, but fine for, say, a bank at night where the path was well understood in advance.
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  • $\begingroup$ I love the ultrasound solution. Guess radio-goggles could do the job as well? $\endgroup$ Mar 10 at 9:06
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    $\begingroup$ The nice part with "non-visible light" is that we already have IR goggles. No new technology required. $\endgroup$
    – Graham
    Mar 10 at 12:52
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    $\begingroup$ Btw, blind people can learn to use echolocation to navigate their surroundings without needing any computers: en.wikipedia.org/wiki/Human_echolocation $\endgroup$
    – alexgbelov
    Mar 10 at 16:51
  • $\begingroup$ I was going to suggest non-visible radiation as a means to "see" as well. I also like the idea of drones and VR/AR goggles to get a view of what is around the person inside the invisibility suit. Seeing what we can do with perimeter cameras on cars today there's feasibility of a person in this invisible suit to use sonar and/or IR in the suit to navigate with sensors on a drone and some processing to fill in some color, shapes, and other details. $\endgroup$
    – MacGuffin
    Mar 12 at 17:31
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This is a non-issue: Visual sensors can be so small and absorb such a small portion of light passing through, that if the observer is able to see the seeing pinholes, they will also see the dust in the air and see that there is a "hole" in the dust field.

So, just have pinhole cameras or just passive optics, which are used to see the outside. A fine mesh of holes with random camo-pattern for eyes to see out directly might be slightly more observable from outside, but also much simpler. They could still be in the realm of "if you see these, you see the dust particles bumping into the invisible thing too".

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    $\begingroup$ As a further enhancement, ensure that any such optics are not retroreflective. This is especially important if using the invisibility in a lighted, night-time environment. $\endgroup$
    – Brian
    Mar 10 at 15:51
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    $\begingroup$ This is by far the simplest answer. You don't need some cyberpunk-style wraparound googles to see, you just need two small holes for stereo sight. In most combat situations, two pupil sized holes hovering 5' above a busy background would be practically invisible. $\endgroup$
    – Dan W
    Mar 10 at 17:07
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All the metamaterial based methods are wavelength dependant. Make the shield deflect visible light, be transparent to some wavelengths beyond visible spectrum (e.g. far infrared) and make your vision sensitive to that light (e.g. by wearing night-vision-like goggles, with an illuminaiton source if necessary, or even a radar).

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  • $\begingroup$ Mine's just a bit different/more comprehensive, but the flavor is the same. +1 $\endgroup$
    – DWKraus
    Mar 9 at 19:14
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Have your invisible viewer wear a pair of immersive vr glasses which are entirely inside of the deflection material, then wirelessly send them visual information from cameras located entirely outside of the deflection material.

If the cameras were mounted on drones, they could travel with the viewer as they move around, broadcasting their gps location along with their video feed using an omni-directional radio signal. The viewer could then use their own gps location information to triangulate on each of the cameras and formulate a 3d rendering of their undistorted perspective of the world outside of the deflection.

Taking this a step further, a massive swarm of drones could serve multiple viewers simultaneously so that the presence of drones in any specific location doesn't provide clues as to the presence of viewers.

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  • $\begingroup$ Cool suggestion! The first thing that occurred to me was that "wirelessly" sending the visual info would probably be difficult...wouldn't your wireless signal just get deflected with all the other electromagnetic signals? But, on reflection (ha!), I think one would probably expect any such deflection material to only work over an interval of the electromagnetic spectrum. (In fact, the OP even specifies that) $\endgroup$
    – Qami
    Mar 9 at 19:04
  • $\begingroup$ I am reminded of the wide receiver who was running to catch a football in the air, but instead of looking at the ball he was watching both himself and the ball on the giant TV in front of him. Third person sports in real life! $\endgroup$
    – Willk
    Mar 9 at 19:15
  • $\begingroup$ Henry Taylor, if could do two green check, you would have been the second one. $\endgroup$ Mar 9 at 21:30
  • $\begingroup$ @TimotheLepine, Much appreciated! I happen to agree with your choice. DWKraus's covers more solutions and solves the inherent vulnerability of my solution. If all of my completely visible camera drones get shot down, the invisible viewers would be blind. Combining DWKraus's three methods would let such droneless viewers switch to sonar and/or pre-recorded overlays. The sonar might compromise their invisibility but at least the viewers could escape along pre-recorded paths. $\endgroup$ Mar 9 at 21:37
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The laws of thermodynamics say you can't sense light without absorbing it. So absorb it… then re-emit it. This takes energy, but given that there's a person to be invisible, I assume a tiny light source isn't too much of an energy cost.

In fact, you need to vent energy somehow; humans produce a lot of heat, and I assume you don't want your magical invisibility powers to also be self-immolation powers. This tiny light source doesn't count (in fact, it generates additional heat). Unless you can teleport the heat away, your invisibility is not going to be perfect regardless.

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    $\begingroup$ An example implementation could use light-field detectors and emitters to generate the cloaking field. Light-field detectors already exist, but not in the shape and scale required for a cloaking field. $\endgroup$
    – Fax
    Mar 11 at 13:25
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Linear optics makes this premise impossible. It exhibits time reversal so light going one way equals light going the other way. However, once you step out of linear stuff and break time symmetry, you can have pretty fancy stuff and nearly everything is possible, a lot even with passive components. The tiny little problem is that reaching just the right nonlinear properties is a bit of an issue to put it mildly.

Suppose that almost all light goes around in various linear metamaterials, while there is a little "forward" hole that does not operate the same way and lets light through. Now, in that tiny little hole, light enters fiber and goes around you, getting out on the back side of your device. Fiber leaks a little bit of light towards you, but is amplified, so the end amount of light reaching back side is the same. This amplification is obviously active, but if you are willing to ditch a bit of light, you don't need it, just make fiber dimmed enough.

You can see clearly with that light (or amplify as needed). Now, just make light going only in one direction so your light does not leak out - optical isolators are abundant, typically using Faraday rotator.

So, great ultrasuper cloaking that works at arbitrary wavelengths for arbitrary amount of time, yet lets you see outside... but requires a bit of power to amplify number of those photons to account for those seen by your eyes (or ditches that little bit of light, making you possible to be detected)

Now for issues: 0. Thermodynamics. You are emitting energy (even without cloaking). This requires some heatsinks to store it for a while, or you will be simply leaking light out at other wavelengths. So, cloak and store energy, uncloak and recharge heatsinks, cloak again etc.

  1. Metamaterials for radio are simple enough, but extremely hard for visible or near UV. Impossible for xray - it cannot be based on electrons at all, some sort of neutron-only material would be required to avoid electrons being kicked out of orbitals (material should preferably not be as dense as a neutron star).
  2. These metamaterials do NOT maintain correct phase of the light! They cannot. In theory they can be precisely constructed to get exactly right 2piN phase delay for some wavelengths, so a periodic plane wavefront will look undisturbed after passing through. But assume a sufficiently short light pulse at some wavelength, and the pulse will be lagging a little bit in the center. Fortunately for you, this delay can be small enough to cloak a human-sized object even against high-speed cameras.
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The meta material allows some light to pass inside the invisible shell but does not allow light to be leaked in the other direction (heading outside the shell). Think of a very advanced one way mirror (made out of meta materials).

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Invisibility cloaks use some variety of beam-splitter or "virtual particle" technology to allow some photons to pass into the head area of the wearer so that the world can still be seen/navigated.

Cheap invisibility cloaks (e.g. the ones you get at Walmart) just pass some percentage of the photons which would normally strike the head region of the wearer through, while wrapping the remaining photons around the wearer. This results in a noticeable "head shadow" corresponding to the cranial region of the wearer, which a careful observer can track as it moves.

Better invisibility cloaks use PVP (Paired Virtual Photon) technology, where for each photon which would normally strike the cloak a virtual photon is created on the interior of the cloak which then proceeds onwards to strike the eye, skin, etc of the wearer. In civilian use this technology is only used in the head region of the cloak, as that's where eyes will normally be located, but it has the disadvantage that if the cloak is dislodged or improperly secured it's possible that the "head region" of the cloak may slip to another region of the body which leaves the wearer blind. It also means that if the user looks at him/herself inside the cloak their body is not illuminated, which can make it difficult to find small objects such as knives, spiders, scorpions, etc which may be located on the wearer's body.

The highest-quality military invisibility cloaks implement PVP technology on the entire interior surface of the cloak, which can be turned on or off as needed (except in the head area).

Recent invisibility research is focused on new technological approaches. One promising one, known as Ego Negation, is not really "invisibility" in the traditional sense. Instead, it makes the person using the device seem so dull and boring that no one pays any attention to him/her. Further developments in this area have been paused, however, due to lack of interest.

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He/she could see in infrared or ultraviolet, which would make the eyes invisible while still allowing your invisible person to see. Your person could also become invisible via active camouflage, which wouldn't affect their ability to see.

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It's often assumed that an invisibility shield must be passive, in the sense that if you shined a certain amount of light at it, then that light must be redirected to the other side, so that all of it goes out the other side, creating the illusion of invisibility.

But what if your invisibility technology takes an energy source? (After all, a lot of technology uses energy to function.) Then you can have your sensors detect all the light that hits them, but then it's another piece of technology that calculates what light needs to emanate from which emitters, and another piece of technology still that can emit the light from the proper emitters.

The amount of energy involved in detecting and emitting the light does not have to be equal to the amount of energy reflected/emitted, thanks to your power source.

Once the power source gets used up, though, then you risk discovery.


So when you ask:

How could somebody become perfectly invisible by the use of technology without being blind (because of the deviation of photons)?

don't think along the lines of:

Light is diverted, and what's left goes to the sensors (which is nothing, leaving the user effectively blind).

but rather:

All (or much of) the light goes to the sensors, and then a machine (with its own power source) is able to emit light in the proper way to give the technology-user the illusion of invisibility.

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One of my protagonists is able to become invisible by secreting a substance from her body that refracts light so that it'll go around her. As her eyes are coated by said substance, she cannot see using visible light, but the invisibility solution doesn't refract infrared light, which means that she can see with infrared light. The solution is, make the cloaking device/mechanism not affect all wavelengths of electromagnetic radiation, then give the user the biology/technology needed to see using the wavelengths that aren't affected by the invisibility tech.

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Synthetic Aperture Camera

If you own a modern smartphone with multiple cameras on its back, this is exactly the same technique as that. Multiple cameras combining their images to produce a better result. It's also used by space-telescopes and military-grade radar in a technique called a "Synthetic Aperture".

Your cloak works by bending light around the person across its entire surface.
So your cloak's entire external surface can be treated therefore as a billion extremely low-resolution cameras by siphoning a very small amount of their redirected light, and their results poured into a digital composite image which the user sees.
This would technically make the cloak marginally less effective on its reverse side, but a fractional reduction in the transmitted light would probably not mean much. You may be able to inject light into the system to compensate for the losses in the camera system.

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