# What does a person in a light-bending invisibility cloak look like from behind?

So, yesterday I found an invisibility cloak in my attic. It's got an instruction manual with it, and there it was mentioned that it is truly an invisibility cloak: it bends the light (actually, all electromagnetic radiation) around the wearer so that absolutely nothing can detect the wearer when looking at them.

Really nothing?

Of course, I tried it on. And from what I could see of myself in the mirror, I was really invisible. It didn't even have the slight lag my previous invisibility suit (a chamaeleon suit) had in imitating the surroundings.

Then I realized: If the suit is bending the light around me -- how come I am not blind when wearing the thing?

My conclusion: It must let some photons through, ideally only those that will hit my retina to produce a picture in my brain. Things look exactly the same, whether I am wearing the cloak or not, so it can't let only a partial amount through.

But then, those photons that hit my retina can't be bent around me anymore to show the original picture. They're gone in a photochemical reaction.

So: what do I look like from behind? Do I show:

• two floating black spots that are the size and orientation of my retinas?
• two floating red spots (same as above) since my retinas have got blood in them and so should look red and not black?
• An area around the height my eyes would be that is slightly darker than normal, but no black spots because of interference and light scattering? (i.e. surrounding photons make up for some of the lack of those my eyes absorb, but of course, cannot compensate for all of it)
• Or is the number of photons my eyes absorb so minimal that nothing can be detected at all?
• Are we sure your suit doesn't duplicate the photons that hit it, let one through to you and bend the other around? – Annonymus Dec 11 '16 at 8:15
• Hmmm, that is a good question. I'm not sure about it. The instruction manual didn't say anything about it, but neither did it say anything about floating black spots. I want to be on the safe side, so please assume the suit isn't capable of duplicating photons, just bending them – subrunner Dec 11 '16 at 8:29
• It should say, "Nothing can detect the wearer by using visual information or electromagnetic detection of any kind." Obviously if there are pressure-sensitive floor plates you're done for. And for that matter, if there is a fixed-position wall behind you, a continuous radar beacon could probably pick you up (if sensitive) because the wall would appear to change position minutely due to the detour of energy going around you and its consequent delay. There are probably many other similar side-channel attacks; your cloak is fallible. – Wildcard Dec 11 '16 at 23:11
• Assuming that only light directly in front of your retina passes through, you would be invisible from. Behind entirely because all the light will reflect forward with a very small margin reflecting to the sides and none reflecting backward. Of course your retina is translucent, not opaque, so some light does pass through, but this will be absorbed by the rest of your invisible head. Only in front of you and to the sides where light can diffuse through your exposed eyeballs will any artifacts be visible. – Vince Scalia Dec 11 '16 at 23:33
• The harder problem is going to be what they can see from the front; the rear can be extrapolated or compensated for. But those eyes... Those eyes, they... they disappeared. – Mazura Dec 12 '16 at 2:57

Human eyes are really, really good at adapting to various light levels. Response is not linear. So first thing first, if your cloak only passes 10% of light to your eyes, in daylight conditions you will still see pretty well. That means you can pass 90% of light and still see.

As seen on above image, some rays from one point might be blocked by your eye, but at the same time, some rays may hit some part of the retina of guy behind you. Thus, the dimming effect would be even lower. Smoothed on the edges. There would be no hard border. If you want to see it in real life, and have a camera with big front lens, cut a paper circle and put on lens. Then look thorough it. If circle is small, you will hardly see anything wrong with the image. Won't post picture, because, well, it would just look normal. Or paint a little dot with whiteboard marker (not permanent one!) on a window, and then look outside. Probably you wouldn't really notice it, unless you actively look for it.

Wit these two effects, I wouldn't bother putting any info in the manual. No point in making user nervous.

• if your cloak only passes 10% of light to your eyes, in daylight conditions you will still see pretty well. and the great thing is, when light conditions get worse, it can start passing more and more light, because nobody is going to notice a slightly dimmer patch in the dark anyway! – Michael Dec 11 '16 at 15:35
• If technology advanced that far as to bend light, I assume there're pretty good computers to analyse the live video and find even the slightest differences in air transparency. The moment you're invisible in the place you shouldn't be, you're shot right into the head. There are countermeasures, for example, invisible drones with eye decoys, but that doesn't allow you to get away unnoticed anymore. The real thing would require using advanced ultra-sensitive retinas to take the amount of light that wouldn't get noticed, with long exposure times. – polkovnikov.ph Dec 11 '16 at 18:38
• So what? There are dozen ways to detect him anyway. Question was about how would he look like. – Mołot Dec 11 '16 at 18:59
• Okay, thanks for the explanation! I didn't really notice a darkening of the light when wearing the invisibility cloak, but even so it sounds like the floaty black spots will be so small and washed out that it doesn't matter. Takes a huge load off my mind! – subrunner Dec 14 '16 at 17:03

It's almost certain the cloak does not cause the other observer to see two floating spots the size and shape of your retinas. Unless the other observer is standing with their nose inches behind your head, most of the light falling on your retinas comes from objects to which your observer would have a direct line of sight even if you were not wearing the cloak. The only photons that the cloak might need to intercept (and not simply transmit around you) would be the ones that would have struck your corneas in such a way as to pass through the opening in the irises of your eyes.

Assuming the cloak passes exactly those photons to your eyes and bends all others around you, the observer would see two perfectly black spots (black being what you see in the absence of any photons) about the size of the openings in your iris (but slightly larger, because the corneas refract light into the openings).

But the cloak might take less than 1% of all the photons that would have struck your face, and redirect them onto your corneas so that they produce a correct image of the objects in front of you. From the point of view of an observer behind you, there would be a region about the size of your face in which the brightness of the scene was between 99% and 100% of what it should be. This would be hard to detect even with specialized equipment unless things were arranged so that the equipment can predict exactly what the brightness of each part of the scene should have been.

• Thanks for giving me more background info on how my cloak might work! I'm really relieved to know it won't be a problem! – subrunner Dec 14 '16 at 17:05

You would still be invisible!

If you have ever heard of that, we got inside eyes a blind point, called Macula of retina. This is a blind point, and we don't notice that in reality we are missing a point in the images we are seing, however there's a really simple test that allows to detect that.

The cloak could work in a similiar manner, light is bent in a way that the image is slightly deformed in order to cover eyes, this will reduce the luminosity of your "invisible shape" by really a tiny amount (what is the surface of eyes compared to total trasversal cloak surface? 0.5%?)

Of course if someone is looking directly at your eyes there is a slight chance he will see a small deformation around the eyes. However you should not really warry, most people is not able to recognize photo manipulations at all ^^.

The cloak would be really dark inside, apart the eye zone, and even the immediate bottom of the cloak would be really dark (otherwise you could be spotted from the above).

Also the cloak has a eye shut detector, when eyes are closed the light will stop passing through the cloak at all: making it really really dark and making you perfectly invisible.

Now you want to ask yourself if you really want all electro-magnetic waves to be blocked by your cloak:

• If you want to block heat dissipation, you would be invisible even to infrared vision, but the cloak will become quickly very hot inside
• If you want to allow heat dissipation, then you will be visibile to infrared scanners
• You could partially block certain waves so you would be partially invisible to infrared vision but still have a very hot wearing
• You could have ventilation that quickly replace air, in that case the noise becomes the biggest factor.
• Thanks for the info! Yeah, I'm not too sure how the thing works with outgoing electromagnetic radiation (sadly I don't have an infrared camera to check for my heat profile) but one thing you guys have already made more than clear is that 'invisibility cloak' doesn't mean 'being undetectable'. (That said -- I think I could make it work as a radiation suit, since it redirects all electromagnetic waves. Gamma and beta are covered that way, and alpha can be stopped by things as thin as paper -- or the invisibility cloak) – subrunner Dec 14 '16 at 17:09

Your manual is lying: It is not possible to build an invisibility cloak which is not detectable and allows the wearer to see. Especially it is not possible/not advisable to bent the whole EM spectrum.

A passive cloak means the wearer is in total darkness because all photons are bent around him. Even worse, the person feels hotter and hotter because thermal radiation cannot escape. If only a part of photons is bent, the cloak is detectable (perhaps not very good by human eyes, but still detectable). It also is detectable by phase changes (You know those transparent wobbling figures in hot air ? It can be made visible against background). If you are satisfied with Molot's answer, this is the way to go.

The only thing working would be an active cloak. This means it has an energy source and consists of an inner cloak which has emitters on the inside near the eyes showing the outside and allows heat transfer to the outer shell. The outer shell has high density transmitters and emitters at the outside which emit light at a point which is the exact computed composition of if the cloak would be perfectly invisible. This also allows to correct phase changes.

Even such a cloak would be visible in the IR range because you cannot hide thermal emissions without killing yourself. It can also be actively detected by firing strong light pulses (like a stroboscope) because no material can hold a linear response for strong inputs (Plain speak: You are overloading the cloak).

You can also combine both models: In "active" mode it works like my model, allowing you normal seeing without detecting your eyes, but it costs energy. In "passive" mode it works like Molot's model, bending light, but you can only see very dimly and it still allows detection for an attentive observer. For IR you must have something to vent off the heat, so if you include a nearly perfect switchable IR suppressor, you will only hold it out for a very short timeframe (sth in the minute range).

• wobbling figures has nothing to do with phase changes, it is refraction. Please explain what you mean about linear responses. – Innovine Dec 12 '16 at 14:54
• @Innovine Air of different temperatures has a different refractive index. The refractive index is the ratio between vacuum c and phase velocity. Phase velocity is the propagation speed of the phase. So if two light beams travel the same distance in two materials of different refractive index they will have a phase difference (except at specific points) and therefore a phase change. Refraction occurs because light chooses the shortest/longest pathway which is the same if you say that it minimizes/maximizes the number of phase changes. – Thorsten S. Dec 12 '16 at 15:53
• @Innovine If you use an amplifier and increase the amplitude, at a given point the music sound disturbed. The reason is that we are out of the linear response and get harmonic distortions which are not neglible anymore. The same goes with light. Whatever this wonder material of the cloak is, it should also consist of electrons. These electrons will react to high amplitudes with a non-linear response creating nth harmonic waves (so you can create green light with IR !). So illuminate the cloak with very short (no damage) high amplitude light and it will be detected ! – Thorsten S. Dec 12 '16 at 16:00
• Uh, your description of 'active light-bending suit' sounds pretty much like my chamaeleon suit already. You know, like, actively showing on the surface what is behind it. Dang it, so the light-bending cloak really is the stupid outdated model with so many issues that a complete redesign is needed? Pouts. I did notice it getting a bit warm on the inside, but I just thought it was because I kept breathing my own air. – subrunner Dec 14 '16 at 17:15
• @subrunner It's the chamaeleon on steroids. Your old suit sounds like the outdated one which only imitates color and brightness, can possibly only emit color in one direction and takes only the back in account. This model minimizes the distortion from all directions and imitates polarization, so it is really invisible and has not the typical Predator distortion. I think you can combine both of them, I will add a description to my answer. – Thorsten S. Dec 15 '16 at 21:53

There can be a fine weave mesh in the invisibility cloak that lets sufficient light through to the inside for its wearer to see what is in the surroundings. It can be that simple.

Unless there's a "magic" mechanism that creates a total immersion virtual reality for the wearer of the invisibility cloak that they can what is around them.

• Uh - wouldn't the fine weave mesh creat something of a washed-out look when viewed from close-up? Kind of like using gauze as a projection screen -- works marvelously at a distance, but loses its appeal up close? I kind of was with my nose to the mirror and didn't realize anything. But, what do I know, the instruction manual did have some really complicated physics notes that I kind of... skipped. You know, the way everyone reads instruction manuals... – subrunner Dec 14 '16 at 17:20
• @subrunner Fine meshes work well. For example, with security doors. You can look out and see clearly without being seen inside. Also, the light bending properties of the fabric could "conceal" the mesh in front of the wearer, while on the outside fabric bends light to "conceal" the tiny holes in the mesh itself. The cloak's light bending can be used too advantage with a see-through fine mesh. Basically a matter of design. The sort of thing that never gets mentioned in instruction manuals. – a4android Dec 15 '16 at 0:47

Dude, you're asking about two dots the size (diameter) of earphone jacks. We don't see everything our eyes register - our brains 'wallpaper' much of our visual field with what we expect to be there. There's no reason why a magical cloak couldn't add back some light for the visible light our eyes stopped. There's no reason why it couldn't behave the same way as earphones with sound cancellation. Sure there'd be a lag, but on the scale of our response time (tenths of seconds or longer) there's plenty of time to recreate the necessary light "on the other side".

• Interesting point! Yeah, I'm sure that dots or not -- human vision will be fooled. I kind of was thinking about using the thing for... let's say, less than legal purposes, but the previous answers pretty much cured me of that. I kind of keep forgetting that nowadays, security cameras always come together with heat vision, and I'm pretty sure the location I'm thinking about also has the phase-shift-detection-thingy others mentioned. I'm just glad I asked my question here -- would have been a bit embarrassing to get caught so easily... – subrunner Dec 14 '16 at 17:28

Probably something like this. If I didn't understand wrong.

• Cough, science-based tag, cough. Might want to elaborate a bit. The provided screenshot is hardly indicative of what an actual device might do, as the effect there is purely for the player's benefit. – Draco18s Dec 11 '16 at 23:21
• Planar mapping of ground texture from above is not at all helping in not being spotted from few meters of distance, however it could really help hiding from helicopters. – GameDeveloper Dec 12 '16 at 11:29
• @Draco18s other users have been explained above clearly, so I wanted to add a possible situation of cloaking. The idea is "bending light", so I think it'd be something like this in reality. – Alper Özdemir Dec 14 '16 at 10:29
• @DarioOO Maybe a few meters - you can be spotted easily, but you can purely invisible for ranged and no-scoped equipments. This is absolutely not a good idea for getting too close to enemies. However, this is usable for spying and disguising. – Alper Özdemir Dec 14 '16 at 10:32
• I can see where the misunderstanding comes from. This pretty much looks like me in my chamaeleon suit, kind of warping things when looking at me from a close distance. The light-bending cloak didn't do anything like this, it was like being totally invisible when I looked at myself in the mirror. Even when I moved. But people here were so nice and inform me that this kind of invisibility comes with some heavy draw-backs (and ways of detection), so I think I will have to stick to my trusty old chamaeleon suit. Thanks though! – subrunner Dec 14 '16 at 17:33