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In a science-based futuristic world, individuals (soldiers) are able to become invisible to the human eye while keeping a decent amount of agility. What could be the different possible technologies behind such ability and what would be their downfalls?

For example, I'm thinking solutions could be:

  • they are wearing a suit that displays in the front what it films in the back. An issue with this would be a possible lag in the display.
  • they are wearing a suit made of material that distorts the light so that it goes around them. A downfall of this method would probably be that you see a distortion in the shape of a person, like looking at someone's eyes through correction glasses.

No solution is too crazy as long as it is scientifically sound.

Again, I'm trying to fool humans only, not cameras, dogs, sensors and whatnot.

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  • $\begingroup$ Does the SEP field count? $\endgroup$ – Philipp Nov 2 '14 at 12:21
  • $\begingroup$ absolutely ;) I was hoping someone would mention it! $\endgroup$ – Sheraff Nov 2 '14 at 12:27
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Metamaterial Cloaking

Light (all electromagnetic radiation) is based on the electromagnetic force [2]. This means it has an electric and a magnetic component. Normal matter only interacts with the electrical component. Yet optical metamaterials [3] interact with the magnetic component to prevent material from interacting with the electrical component.

This can be used to design cloaking materials [4]. The picture below shows a normal object on the Left. It changes the pattern representing the electrical component of the light. Thus it is visible. On the Right, we see an object using metamaterial cloaking. The magnetic component of light is used to prevent interaction with the electrical one. The object does not disturb the electrical pattern. Thus it is invisible.

enter image description here

Picture and description from Wikipedia: "Left: The cross-section of a PEC cylinder subject to a plane wave (only the electric field component of the wave is shown). The field is scattered. Right: a circular cloak, designed using transformation optics methods, is used to cloak the cylinder. In this case, the field remains unchanged outside the cloak and the cylinder is invisible electromagnetically. Note that the special distortion pattern of the field inside the cloak."

enter image description here

This graphic from this recent article [6] illustrates the phenomenon somewhat more graphically.

Now it is easier to manipulate lower wavelengths than higher ones. Radio, Microwave, and Infrared can be manipulated this way. Scientists are currently working successfully on Visible Spectrum cloaking and some very small object have already been cloaked this way. Ultraviolet seems plausible, but X-Ray and Gamma seem improbable, as the manipulation of their wavelengths would require molecular and atom scale structures respectively. Although those would make one hell of a nice radiation shield...

So how do we build you an invisibility cloak? That's easy enough. We put you under a flexible version of the Visible light metamaterial and for good measure, we equip the inside of your suit with a Thermal Infrared invisible metamaterial. Now you are invisible in the Visual and Thermal Infrared. Anyone using a Near Infrared or UV camera, seismic detection, echolocation, electric detection a Thermal Infrared laser and a billion other members of the cast of Utility Belt: The Movie could still detect you. But that's beyond the scope of your question, thus not my problem.

Be aware that this technology brings more than Harry Potters cloak. This video [7] is a good summary of its implications.

[2] https://en.wikipedia.org/wiki/Electromagnetism

[3] https://en.wikipedia.org/wiki/Metamaterial#Electromagnetic_metamaterials

[4] https://en.wikipedia.org/wiki/Metamaterial_cloaking

[6] https://www.ft.com/content/c6864c76-de7d-11e7-a0d4-0944c5f49e46

[7] https://www.youtube.com/watch?v=s0UZ6-oeiIE

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    $\begingroup$ A flexible metamaterial would need to change its internal index of refraction at every point in real time to adapt to how the wearer changes shape. This seems very difficult to do (though maybe possible with significant advances in the future). But I wanted to draw attention to the amusing possibility that, rather than an invisibility cloak, people would need to surround themselves with what could be described as a large, rigid, invisible trash can, and sort of shuffle past their enemies. $\endgroup$ – Gilad M Apr 14 at 9:32
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    $\begingroup$ Great point, I wasn't aware that flexibility would be a problem. The trashcan approach just makes it way cooler story wise, as drawback are often more interesting than advantages. This is a real world case of Brandon Sandersons Laws of Magic applyed to technology. $\endgroup$ – TheDyingOfLight Apr 14 at 9:44
  • $\begingroup$ e there also needs to be a gap or hole in it otherwise you can't see out. and there still will be a distortion if the light source is at an angle to the horizontal, caused by a cylinder. you need something more like a ball than trashcan. $\endgroup$ – John Nov 8 at 14:43
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Douglas Adams describes an unique cloaking technology known as the SEP field in his novel Life, the Universe and Everything. This technology persuades the observer that the object under the field is Somebody Else's Problem and doesn't concern them, so they just ignore it.

The Hitchhiker series is of course not hard Science Fiction but rather a parody thereof, but this particular concept isn't actually that strange. When we extrapolate hard sciences like Physics or Biology, then why not also extrapolate soft sciences like Psychology?

A technology which manipulates people psychologically to ignore and forget the subject might not actually be that implausible. It could employ some kind of hypnosis or other neuropsychological effect.

Possible drawbacks could be:

  1. Its effectiveness might depend on the emotional state of the observer. It could be more or less reliable depending on the observers current level of certain neurotransmitters like adrenaline (stress) or endorphine (happiness). Psychoactive drugs which alter the emotional state of the observer might counter the effect reliably. The observer actively looking for the subject and expecting it might also affect the reliably (in any direction).

  2. It might not work on everyone. People with certain mental conditions might be immune to the effect.

  3. It might not work under any condition. When it works visually, a completely unobstructed view and certain light conditions might be required for it to work. When it works through sound, it might not work well when there are other noises around.

  4. It is unlikely to work on animals, except when designed for that specific species.

  5. It will not work on any automatic detection systems which recognize threats automatically without human input (although there might be a chance that advanced neural networks could be vulnerable).

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  • $\begingroup$ This is a good answer but very restricted to one type of invisibility. I wish a few other people had replied :) $\endgroup$ – Sheraff Nov 23 '14 at 17:32
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I cannot comment on the possible scientific approaches, but their downfalls would be all the same: individual invisibility is great for spying, criminal activity and maybe small special operations teams.

Warfare

Any scale of warfare needs a very detailed picture of where the friendlies are. You need to coordinate the troops. You need to evacuate the wounded. You need to make sure there is no friendly in the explosion radius or line of fire of your weapon. You do not want to miss on the morale boost that the feeling of "the man next to me" gives to the troops. In short, what you really don't want is an individual soldier, seemingly all alone, in danger of getting hit by the enemy as well as getting hit by any weapon his buddies may fire, fully aware that he cannot be evacuated and any wound will be his death. So what you need is a way to identify those invisible soldiers. And if that technology exist, it's only a matter of who wins the encryption race. Will the enemy be able to decrypt (or maybe just trace the signal to a specific point in 3D to fire on, that would be enough) the signal in real time and kill the invaders or not. Kind of the Enigma in WWII.

Technology

Generally speaking, technology can come in two variants:

  • Fooling the individual viewer
  • Actually sending a different image

Fooling the viewer by for example somehow manipulating his brain can easily cicurmvented by technology available today. Cameras. If you cannot fool an optical recording device, things like night vision or modern day tank optics will already invalidate the whole concept, because it's short time recording and replay.

So what you need is a device that actually changes the rays of light and transmit a different image to any image processing device, technical or biological. The obvious countermeasure is to emply a different sense for detection. Guard dogs are not exactly the pinnacle of technology, but they work. Ok, lets assume with the invisibility kit came a "smell transmutor". We could still detect moving bodies by air pressure. And radar. And maybe Infrared or Ultraviolet. Or maybe ultrasonic sound. So this invisibility kit needs to be a full blown scientific wonder, or the technology to detect invisible people will be cheaper than the invisibility itself.

Assuming the invisibility is perfect in any way, it would still be vulnerable to things that normally obscure visibility or hinder our senses. Invisible people moving through smoke or rain will be visible by their surroundings. The rain not hitting spots, the smoke moving around columns of "nothingness". Any kind of muddy ground can make technology worth a few millions totally meaningless. Comic-style even in the case of muddy footprints. People will need to be very skilled and specially trained even if they have perfect technology.

Society

And still, if the guy employing such a kit is absolutely not detectable, the world is in danger of failing. How do you prevent crimes with such technology available. For a believable world, you will need a way to control such technology. If this technology is widely available in wartime, what would the civilization look like in peacetime? With all the guys that know that nothing can stop them? Maybe the real drawback of the technology will be, that after war all the veterans able to use that technology will need to vanish without trace because society would fail if they were actually coming back alive.

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  • $\begingroup$ My question is about fooling "the human eye" (or brain, it's part of it anyway), not sensors. $\endgroup$ – Sheraff Nov 2 '14 at 13:36
  • $\begingroup$ Well, the downfall of it would be the fact that even the military today (look at Iraq or Afghanistan) has sensors available that are man-portable, easily usable and cheap. You can even buy them on Amazon . And I guess they would be half-size, half-cost if they did not include actual night-vision. $\endgroup$ – nvoigt Nov 2 '14 at 13:43
  • $\begingroup$ Again. Not sensors. Human eye only. $\endgroup$ – Sheraff Nov 2 '14 at 13:47
  • $\begingroup$ @nvoigt You assume that the observers actually expect the enemy to employ cloaked troops. When cloaked troops are rare (or the common soldier doesn't even know the technology exists), not everyone will be equipped with the detection goggles and even those soldiers who are won't look through them all the time. $\endgroup$ – Philipp Nov 2 '14 at 13:51
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    $\begingroup$ @Philipp Those "detectors" are employed already in todays warfare. Soldiers wear them to see in the darkness, to see in sand storms or to see when they need protection from direct fire (tank or aircraft crews for example have no direct vision anymore, not even using mirrors, they have sensors all the time even today). To assume that in a scientific future people will use less technology than available from online retailers today is not SciFi, that's fantasy (and I'm fine with fantasy, if it's declared that way). $\endgroup$ – nvoigt Nov 2 '14 at 13:56

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