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The year is 2020, 98% of the world population are normal people with normal skills, two percent (2%) are people who have the ability to become invisible at will.

The majority of the invisible people live in peace with us, following the law. But part of this 2% are criminals, and we must find a way to prevent them committing crimes.

  • Robbery
  • Murder
  • Etc...

One more ability the invisible people have is that, everything that their skins are touching also become invisible (That includes clothes, other people, objects). Floor and Walls are not affected.

Body heat and so on occurs the same way as normal people body.

How to prevent invisible people from committing crimes?

Our technology is the same as we have in today's world.

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    $\begingroup$ Do laser beams / infrared beams (like sensor-based doors and home alarm systems) "see" them? What about heat sensors? $\endgroup$ – CaM Aug 15 '17 at 15:25
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    $\begingroup$ Dogs can smell the invisible scum and I'm pretty sure they can receive special trainning to bite invisible things $\endgroup$ – jean Aug 15 '17 at 16:50
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    $\begingroup$ A witness would have to pick up one criminal from a line up of 5 invisible people... $\endgroup$ – Alexander Aug 15 '17 at 16:55
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    $\begingroup$ How do these invisible people see, given that light passes right through their eyeballs? Is this invisibility magical in nature (e.g, allows their retinas to see without capturing photons)? Can they see their own bodies, and anything else they're touching? Can they see other invisible people? How large an object can they cloak simply by touching it? (e.g. could they cloak a bag by touching it? A car? Train?) What about objects comprised of many components? Is it just the component they touch that becomes invisible, or does the entire (conceptual) object? $\endgroup$ – Pak Aug 15 '17 at 20:49
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    $\begingroup$ We are unable to prevent visible people from committing crimes, much less invisible people. $\endgroup$ – Eric Aug 16 '17 at 15:50

21 Answers 21

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Spencer Muise already mentioned that they could be invisible to our visible spectrum, but could be still visible as a source of heat or other kind of radiation (maybe the process of becoming invisible itself leaves some kind of electromagnetic signature).

Another thing that would work are, quite simply, pressure plates. Invisibility can't hide the fact that people have a weight, so, covering the floor of some sensible building (e.g. a bank, or a jewelry) with pressure plates, or other weight-detecting technolgy, would serve as alarm against robbery. This could be quite expensive and require some major restructure on some buildings, but it would probably be a good option.

If you think about it, a lot of conventional measures of security would still work. If you have a bank with a safe that opens only at certain hours (to allow employees to make deposits) you could try to enforce some kind of policy to prevent invisible people getting in and out. After all, locked doors still work well against invisible robbers. Each sensible building could have "private quarters" where only allowed people, based on facial recognition, can enter, along with a mantrap ( Mantrap ): a door that doesn't let more than one person in at a time. An invisible robber could still try to sneak behind a regular employee, but would risk the door closing on himself (pain and detection involved).

Also, I suppose invisible people still emit and produce sounds, so that's another range of sensor that could be used at their disadvantage. Moreover, even in the case that their body heat cannot be detected, they still need to breathe. Air in a empty room is usually colder than air with a person breathing it - in certain occasions and/or weather conditions, this could serve as an alarm.

You noticed I've only talked about robbery thus far, because robbery - at least if we are talking about "big, stylish robberies" are actually easier to avoid than petty crimes or murder. As Ash pointed out, you can't "prevent" crime, you can either dissuade it and prosecute it. An invisible assailant could rob whatever he pleases in an open street, taking it by force by the hand of unsuspecting strangers. Still, in a world where invisible criminals are a thing, the other 97% of the people would probably like to have stronger security measures - or maybe they would stop carrying wallets, at least.

I'd have other ideas, but I'll leave you those two cents.

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    $\begingroup$ @ShawnV.Wilson It varies, but in many cases yes. Automatic doors will often use a pressure plate, an interrupted beam ("electric eye"), or microwave/ultrasonic/passive IR detector (more common these days - it's the thing mounted above the door). And, if the invisible person is only invisible in the visible spectrum, they'll foil absolutely none of those. $\endgroup$ – Sebastian Lenartowicz Aug 15 '17 at 18:20
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    $\begingroup$ simple bead curtains can make it easier to see people entering restricted areas. $\endgroup$ – John Aug 15 '17 at 18:39
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    $\begingroup$ "only allowed people, based on facial recognition" -- you're going to have the Invisibility Rights Campaign furious about that. You're discriminating against invisible people just because some of them are criminals. $\endgroup$ – Jules Aug 16 '17 at 7:48
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    $\begingroup$ @Jules They are not invisible all the time, they can become invisible at will. So just ask them not to do that when facing the camera. $\endgroup$ – Reinstate Monica Aug 16 '17 at 8:52
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    $\begingroup$ @Jules it's the same as today. At least in my country, we are not allowed to wear masks, helmets, scarfs or other things that completely cover the head in public buildings and banks for security reasons. $\endgroup$ – Reinstate Monica. Aug 16 '17 at 10:44
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You don't, you can't, same as you can't prevent normal people from committing crime, if you're asking how do we detect and prosecute crimes committed by invisible people that's a whole different thing but the answer is the same, the same way you do with normal people; just with a couple less tools. Invisibility precludes primary witnesses both human and machine but in no way removes DNA, trace, footprints etc... it also doesn't stop people noticing things are missing or people are dead.

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    $\begingroup$ I was going to say the same thing. Most people could, in theory, commit crimes and get away with them. Most people don't. Except speeding. Man, I would love to touch my car and make it invisible as I drove through those speed cameras! :-D $\endgroup$ – dmm Aug 15 '17 at 21:04
  • $\begingroup$ Of course you can prevent normal people from committing crime... just not all the time. This question is asking about ways to prevent invisible people from committing crimes in some new ways, which are impossible/unpractical for non-invisible people. $\endgroup$ – hyde Aug 17 '17 at 11:42
  • $\begingroup$ @dmm I'm pretty sure the last thing you want are invisible speedsters near speed-traps. Not only do they feel free to speed, no one can see the 1t+ projectile approaching at 200 km/h where 100 km/h is allowed. At OP: Do dead invisibles remain invisible, and do they keep whatever they're touching invisible? That's one heck of a cleanup. $\endgroup$ – J_F_B_M Aug 17 '17 at 16:03
  • $\begingroup$ @J_F_B_M I'm reminded of the episode of X-Files where they find the Jinn. $\endgroup$ – Ash Jul 10 at 11:36
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Luckily for us, we have created machines that exploit all of the non-visible aspects of objects. There are several interesting ways to deter invisible people from committing crimes.

Infrared

Infrared cameras allow us normal humans to see the effects of hot objects in any circumstance. Your invisibles may give off heat, thus this is an effective method. Using advanced algorithms in target finding systems, it would even be possible to have a machine or weapon with two cameras, one visible light and one infrared, that compares the two images. If an object is present in just the visible light camera view but no in the infrared, then the machine/weapon may engage with that object to whatever effect you desire.

Make sure the police are equipped with infrared goggles and the like. Even normal people will likely start wearing infrared countermeasures simply to make sure they're aware of what's around them.

Seismic

Humans may no be really heavy, but they still have mass. Each time a human, visible or not, takes a step, he will impart force to the ground. Using that, we can take advantage of seismic sensors in areas that shouldn't have foot traffic. If those sensors detect motion, they set off an alarm and alert other people an invisible person is there.

Physical deterrents

Even an invisible human can't jump really far or fly. If you put something high enough or far enough away that a normal human can't reach it, you've succeeded in making sure invisibles can't get to it either.

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As Always, nothing is safe from a skilled thief who really really wants it. Not in the real world, not in this fantasy world. The best we can hope for is to make it really hard to steal (which means someone has to be really motivated) and recovery after it's been stolen. I'm also assuming heist movie level theft here because that usually makes for more interesting reading :)

OK, now that we have that little caveat out of the way, there are tons of ways to rig security around things that do not rely on sight. Humans have a bias toward that sense because that is how we evolved, but we have other senses.

Smell: Most animals, as far as I know, produce pheromones. These can be detected. For a high tech alarm system use a clean room type system to make sure the chamber for whatever valuable Dingus you happen to be trying to protect is free of any lingering pheromones. Once closed, the chamber is sealed off and sensors enabled to detect anything scent related, triggering an alarm. Bonus points if you can use the same system to evacuate the chamber with the would-be thief inside, suffocating them.

Hearing: I'm putting all sorts of vibration testing in this category along with various uses of ultrasensitive microphones. Here is a more specific idea. Echolocation type pings akin to sonar. Anything changes in the soundscape of a room, like maybe a person moving about, triggers an alarm. Your evil mastermind bonus to this one is again, the room seals, and our would-be thief is bombarded with super high decibel noise that would deafen them, or just the "Brown Note" (yes I know the brown note was busted on mythbusters, but I'm having fun here so hush)

Touch: this is where the whole range of pressure sensors go. Everything from the Indiana Jones Idol pedestal to The floor that Tom Cruise sweat on in Mission Impossible. Pressure sensors are pretty common, so this should be easy to do in a ton of creative ways.

Taste: This one kind of goes with Smell, since it is all about chemical detection. I really don't know what do do with this, unless you make the person trying to access your treasure chamber, I dunno, eat scotch bonnet peppers and then the High tech tongue analyzes the sweat as an identity key. Pretty farfetched though.

The long and the short is that there are a lot of modern-day tech things we can do that do not rely on Visible light to aid in our security efforts. In fact, we could even use the idea of checking in with our other senses to set the story and just about any tech level or time frame, from the bronze age to modern day.

Smell: Dogs. They bark when something doesn't smell right.

Hearing: Dogs again. Their sense of hearing is a lot sharper than ours.

Taste: Dogs yet again, Hungry, angry dogs will eat intruders.

Touch: No dogs here, but things like bead curtains in doorways make it hard to pass through silently. Put spiky bits along the tops of high walls, generally, make it hurt to get to whatever you are protecting.

Finally, since I just watched Ghost in the Shell, How about shallow water? Even someone who is invisible will make ripples in the water as they move or even just stand there. Those ripples will be visible. Disturbed water will make noise.

So your invisible people have just a small edge over normal thieves. Testimony based on what someone saw will become circumstantial, so securing convictions will require an advance on other forensic tools. They will be harder to trace in the immediate time after a crime unless you have a dog who can give chase based on scent. So yes, advantage, but not as much as you think

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    $\begingroup$ +1 for water. Didn't thought of that, but I like it. Just a small waterfilm in a corridor, a guard... No way to get through (unless you're a movie heister of course). $\endgroup$ – J_F_B_M Aug 17 '17 at 15:59
  • $\begingroup$ @J_F_B_M At first I thought of water in an artfully styled garden, then I realized that your typical mythical ninja would just jump from ornamental feature to to ornamental feature. then I realized that a big, open plane of shallow water gives ninjas nothing to hide on. $\endgroup$ – Paul TIKI Aug 17 '17 at 16:23
  • $\begingroup$ @PaulTIKI: I think there is precedent to ninja-fu on a large body of water. I need to rewatch Hero to be sure. (Plus it's a beautiful movie.) $\endgroup$ – Codes with Hammer Jul 31 at 13:06
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If we are thinking at OUR world, i.e. not an idealized world where everybody is smart and scientifically minded, I would bet that the first type of response would be simply some sort of law forcing the Invisibles to make their presence known to everybody all the time when in the public space.

Imagine the following scenario:

  • average Joes already generally uncomfortable with the idea of invisible people around ("how do we know what they are doing?")
  • first random act of crime from one of the Invisibles: huge media campaign, public outrage ("See, I told you they are all thieves/rapers/etc?")

  • public hysteria follows. Politicians pressed for quick answers pass laws to force the Invisibles to make their presence known. From now on the simple act of being somewhere and invisible without declaring it can be prosecuted. Some populist-type politicians propose that you have to wear a tracker all the time which beeps automatically to make people aware of your presence. Like people with leprosy in the ancient times who had to ring a bell when passing on the street

  • most of honest Invisibles outraged with support of NGOs but nobody cares as they are only 2%

until...

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    $\begingroup$ Also, on a separate note, it might be worth considering that even if the politicians mentioned create a law that those people must wear a tracking device/beeper, people will break the law. Especially the criminals. $\endgroup$ – Cullub Aug 15 '17 at 20:44
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    $\begingroup$ @Cullub, I am European. But let's not make this into a discussion on politics, shall we? Following your comment I have replaced "right wing" with "populist-type" (which can be both right or left). Coming to your second point, I agree that people will still break the law, but as Ash pointed out before me, this happens even with normal people. If we have not figured out yet how to completely prevent crime from normal people, how can we expect (in less than 3 years, remember. The story is set in 2020) to prevent crime from Invisible people ? $\endgroup$ – AleAve81 Aug 16 '17 at 5:21
  • $\begingroup$ If these people can become invisible at will, wouldn't the law just be "Don't become invisible in public?" $\endgroup$ – Brian McCutchon Aug 17 '17 at 5:29
  • $\begingroup$ from the OP it sounds like they are permanently invisible ? $\endgroup$ – AleAve81 Aug 17 '17 at 7:43
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To propose some different security mechanisms:

If doors are affected you can easily detect an invisible person indirectly by detecting if a door (or any other suitable object) becomes invisible in all relevant spectra. Point a laser at the door and ring an alarm if it hits the receiver on the other side of the door (basically the opposite of a standard photoelectric sensor). Bonus if you place the emitter on the inner side of the door so the laser-beam cannot be seen from the outside.

Another possible method is ultrasound. As sound waves are a physical phenomenon related to air and not an optical method you can use it to see even persons who are transparent to any electromagnetic wave. The range is a bit limited, but to cover a whole room should be easy. They can't look through anything though. Even a very thin glass-pane is opaque to ultrasound.
Positive is that we already mass-produce ultrasound-detectors, e.g. for use by roboters.

And, as already mentioned, mechanical sensors like tripwires, pressure plates and so on.


This is of course only useful to protect properties. Nobody can prevent an invisible person to simply snatch a bag that is standing around. But then again you can't stop a normal person as well. Good thieves know when no one is paying attention and will not be seen (the bad ones are seen once and then land in jail).

To protect small things I can imagine that trackers will become very popular.

You also have to imagine how difficult it is for an invisible chased person to escape. Nobody will evade them, and followers can easily track the trail of vanishing and reappearing persons as the invisible person accidentally touches persons and objects.

Invisible Assassins though... They can get much closer, can carry equipment in supervised areas... Luckily we don't have that, right? Right?


Police will probably tend to use nets and other area-based attacks to engage invisible offenders. I imagine tactical equipment like shields to be equipped with an "ultrasound-camera" to get an idea of the position, combined with pepper spray or other sprays. Laws will probably be adjusted to make it illegal to become invisible in public space or at least in the presence of police officers. Resisting this will allow excessive use of material not usually deployed against civilians.


TL;DR: Invisible persons probably have an edge when committing crimes, but in a lot of situations this can be offset by technology. There are areas where invisible criminals definitely have an unfair advantage, but their visible counterparts are apparently as efficient with a bit more preparation. You can't stop them, but you can adapt.

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I can imagine a political/social upheaval that would require everyone to have an RFID chip implanted at birth (i.e. before you learn to turn invisible). Readers at all or important portals would identify who has just entered. If there is a discrepancy (RFID reports someone, but cameras or other detectors do not) it would set off an alarm.

If you have the chip removed or disabled, you would be subject to prosecution, unless you can prove it happened by accident or defect.

Your requirement that floors and walls aren't affected would need a reasonable explanation. What is the difference between a wall and another person? Density? total mass? What about the other person's clothes, i.e how far does the touching effect spread? If the invisible touches a person who touches another etc, at what point does the effect fail?

On the other hand, if walls would be affected, imagine the possibilities for peeping!

EDIT: I meant to include this link: http://www.dailymail.co.uk/sciencetech/article-4203148/Company-offers-RFID-microchip-implants-replace-ID-cards.html

There are companies now using RFID to replace other authentication methods (passwords, swipe cards, etc.)

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  • $\begingroup$ I like this method, as it avoids the racism inherent in a lot of other answers, and isn't a giant anklet you have to wear for life. You'd still have issues with criminals (even visible people) removing them while committing crimes, then re-installing them afterward so they can buy a burger to celebrate, so there'd need to be tamper-resistant mechanisms built in. Also, it would help track visible criminals as well as invisible ones (well, the diamonds disappeared while these 72 people were in the building, there's your suspect list). $\endgroup$ – MichaelS Aug 17 '17 at 4:58
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When you have mutants committing crime, then you may also have other mutants trying to prevent those crimes.

Give your Invisibles the ability to sense other Invisibles better than normal people. Most Invisibles will not be criminals, and there will also be those Invisibles who are ready to take a stand against the evil and corrupted of their own clan.

The sensory ability need not to be perfect vision. Here are some ideas:

  1. Invisibles can always tell when another Invisible is in the vicinity.
  2. Or, Invisibles can see other Invisibles as mists/point-light-sources/ghostly figures.
  3. Or, upon becoming invisible, their vision alters to infrared and microwave bands, making them able to see the body heat of Invisibles. This will make it disadvantageous for someone to become invisible, because the world would have lot less color and resolution in those high-wavelength bands.
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This is kind of a different angle but what if the powers were endowed by some sort of deity with the caveat that if the power is ever used for ill then the one that committed the crime loses their power. This could be communicated with some sort of mark that would be obviously visible to everyone thus would be viewed as social outcasts or at least a lower level of society.

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    $\begingroup$ Normally, I would claim an answer like this doesn't really fit with the spirit of the question, but after reviewing the tags and the phrasing, I can't really claim OP was looking for a scientific or practical solution, as opposed to a spiritual/magical one. I might recommend beefing your answer up a bit just for the sake of being thorough, but in my personal opinion it conveys all the necessary information as-is, so it's up to you. Oh, and welcome to Worldbuilding! :) $\endgroup$ – MozerShmozer Aug 15 '17 at 21:58
  • $\begingroup$ Thanks @MozerShmozer I am still new here but I like to try to think outside the box a little bit and push the limits! :) $\endgroup$ – Justin Morrison Aug 16 '17 at 16:04
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You can use paintings, to make them visible, or water spraying

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  • $\begingroup$ That wouldn't work. "Everything that their skins are touching also become invisible", so if you spill a bucket of paint on them, the paint also turns invisible. $\endgroup$ – Philipp Aug 15 '17 at 18:48
  • $\begingroup$ I think this answer is somewhat compelling. If the rules were permitted to bend enough to allow a technology to be researched that stopped the "invisible on contact" situation such as a special paint, you could have lots and lots of those little retail-clothing tags that eject paint when tampered with instead of removed through the appropriate tool. $\endgroup$ – BlackVegetable Aug 15 '17 at 19:06
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    $\begingroup$ This is completely strange,if they touch a building,it becomes invisible? $\endgroup$ – Phaidonas Gialis Aug 15 '17 at 19:24
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    $\begingroup$ I think the water spraying would still work for entry/exit points. Although I'd think something less... wet... would be in order. Either we see the water/fog/etc. wrap around the invisible person, or it suddenly becomes invisible. Both cases give us confirmation an invisible person is entering or leaving the area. I don't think this is a full answer, but I think it's on the right track. $\endgroup$ – MichaelS Aug 17 '17 at 4:43
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What about the reverse? If ordinary sensors can see only visible people, put them on your automatic doors, so that they'll only open if they SEE visible people.

A fingerprint reader wouldn't need a database of fingerprints. If it can see your finger, you're in.

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  • $\begingroup$ How would you stop the invisible people from tailgating behind visible people entering a building legitimately? $\endgroup$ – Philipp Aug 15 '17 at 18:51
  • $\begingroup$ Drat! You guessed my plan! <*Vanishes*> $\endgroup$ – Shawn V. Wilson Aug 15 '17 at 18:54
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    $\begingroup$ @Philipp Turnstiles. $\endgroup$ – ths Aug 16 '17 at 12:27
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One thing that I don't see mentionded but feel compelled: Humans are actually quite capable of echolocation, and even though the most proficient are blind, it is quite capable of being learned by sighted individuals too. In fact, studies have shown that there is no difference in recognizing distance or general shape of an item in three dimensional space and at least one individual was proficient enough by childhood (and self taught methods) that he was able to operate a bicycle and and convince his parents he still had his sight.

It might become a form of self defense. It would allow individuals so trained to identify and attack an invisible agent with something of an equal playing field. Again, the only reason most humans do not use it is because we rely on sight. And all that stuff, yeah, that's gonna help... but when the chips are down and someone who you can't see wants you dead, it's a nice fall back technique.

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Subcutan electromagnetic receiver/sender, probably combined with remote controlled taser

Because the invisibility gives perpetrators too much of an advantage, each person able to turn invisible gets a receiver/sender under their skin which gives away their position and tracks their movement. The receiver can use a multitude of frequencies (GPS, Wifi & special radio towers triangulation) and if all fails, use inertial navigation to get a raw approximation where the person was.

The protection is multi-fold. First the tracking allows to retrace the steps which the person has made. If (see later) the tracking fails, the failed tracking is automatically assumed to be a proof because the person is the only one which lacks tracking (so that in the own interest the person handles the tracking device with care). If two or several smartasses lack tracking at the same time, everyone of them is considered guilty.

Second some areas are geo-fenced or, if people are afraid that an invisible enemy entered their area, activate the EM defense. In both cases invisible people will be internally tasered by the chip if they enter the area or are inside the area.

Traps triggered by invisibility.

Use their ability against them. A laser is inside the door/door handle/wand/floor and is reflected by the opaque material. Once a invisible person touches the material and turns the material invisible, the laser is not reflected anymore and can trigger a multitude of nasty traps: Gas grenades, trap doors etc.

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There are devices in today's world that you can attach to your mobile phone to take thermal images. Such things would for sure expose invisible people as you described above, and I'd assume that such technology would sell at a much higher rate in your world. With a given demand people would be willing to write certain apps that are specialized to detect such people, using thermal imaging or other devices.

In addition invisible people still fill the room they are standing in, so they'd be pushing things away and can be touched and felt. This turns even simple house-hold items into detection-devices against invisible burglars. Shop owners would probably add a kind of curtain to their front door tied to a bell to be alerted if someone invisible stalks into the room. Window displays could be covered with transparent hardcover that you'd need to pull up to take the product. Guard dogs would still be able to smell invisible people and could become a more common sight.

Banks or similar institutions with enough money available would have a hallway filled up with solid fog, that would make anyone invisible trying to walk through it immediately visible. More advanced research would try to find materials that are sticky but don't become invisible, and then cover areas with that "anti-invisible paint". Or in reverse someone could invent something like a laser-pointer that emits light, that is refractioned by the invisibility effect, and turn them into a shining disco ball.

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I've read quickly the other posts and I've seen a lot of good ideas, so here's mine.

Depending on the degree of dystopia your world is set, you could have some kind of authority that forces people who can turn invisible to wear an ankle monitor, just like the ones used in the USA. In addition to the GPS tracking, you could imagine that those monitors detect if the wearer turns invisible (laser trigger, if he is visible the laser is stopped by the ankle, if he's invisible the laser goes right through it and triggers the device, even if the device itself is invisible). Once triggered, it could either alert the police or emit a loud beeping sound or whatever.

That device could prevent petty crimes, but also is difficult to manage : how do you know a person has that ability in the first place? What if they are not detected, and hide that power?

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  • $\begingroup$ Welcome to WorldBuilding Ellynas! Excellent point. If you have a moment please take tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Sec SE - clear Monica's name Aug 16 '17 at 12:55
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    $\begingroup$ Good idea. It may not make a difference if they hide their power; if they are caught in a crime they get the anklet. Do we need to prevent honest citizens that can turn invisible from doing so? It is a natural form of protection, after all, say in the event of a bank robbery or escaping a mugging or other assault. Should somebody be denied their natural ability to protect themselves? Should martial artists be prevented from using their skills to protect themselves from crime? IRL a reasonable amount of suspicion can warrant being held without bail before trial. could apply to the anklets, too. $\endgroup$ – Amadeus-Reinstate-Monica Aug 16 '17 at 13:06
  • $\begingroup$ That's a valid point, but that why I used the word dystopia. In my mind, people don't like differences, especially if that difference provides a privilege or advantage. OP said it was year 2020, three years from now. I could easily imagine escalation in violence and crimes from invisibles and politicians trying to turn that into fear to gain power over control. That anklet could be an "early solution" to that problem. But it depends of each person's point of view now. $\endgroup$ – Ellynas Aug 17 '17 at 7:35
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I guess the most important part of this is the mechanism by which they become invisible. Can they be seen in other spectrums? If so, cameras may still be able to pick them up. Or if they still emit heat, then something like FLIR could be used to track them.

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  • $\begingroup$ Maybe track them and see where they are. But how we will be able to identify them? Arrest them after they comitted the crimes? $\endgroup$ – Gustavo Gabriel Aug 15 '17 at 15:32
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    $\begingroup$ Well, if they are in a room where no people should be, you can always lock the room and trap them inside. $\endgroup$ – Reinstate Monica. Aug 15 '17 at 16:27
  • $\begingroup$ I mean yeah, you generally arrest someone in the process of committing a crime or after they've committed it. Preemptively arresting people could cause problems. $\endgroup$ – Spencer Muise Aug 15 '17 at 16:28
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Microphones and signal processing to detect the location of heartbeats. Computer-generates an image of the room with everyone in it that the guards can compare to what their eyes see.

Room contains light fog. Hole in the fog means an invisible person.

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Manufacture and sell 'infrared contact lenses'. This technology is under development is entirely possible. The contact lenses will allow the other 98% of the population to see the invisible people. Source

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  • $\begingroup$ I don't think the technology in your source is anywhere near being able to show infrared imagery; it's just a detector. Also, you'd have the problem that wearing the lens means you're constantly seeing infrared from everything not invisible too. This kind of thing wouldn't likely be used by regular people as it would be too annoying. Still, it could be useful for bouncers and guards in some high-profile areas. $\endgroup$ – MichaelS Aug 17 '17 at 4:54
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Let's say you have a vault with valuables which needs to be protected. Litter the hallway leading up to it with microphones, pressure plates, air current sensors, whatever you can imagine that would detect a human (or anything out of ordinary) without relying on the visible spectrum.

Then, you might not even care if anyone walking the path might get soaked or dusted. Perhaps because he would normally be in a bunny suit - it could be a vault containing hazardous materials. A biolab for example.

So, once "something" is detected but the cameras don't show a picture, flood the hallway with dust or an aerosol. Then you'd surely see the silhouette of the trespasser.

For added measure, make the aerosol flammable. A fuel/air mixture for example. But even simple flour or iron shavings floating through the air coud suffice.

Then, if you really wish to keep said trespasser away from your vault, provide a spark.

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  • $\begingroup$ Welcome to Worldbuilding, Anonymous, a fuel/air explosion might, just might blow the vault open, There is also damage to property. Plus laws against unauthorized explosions in public places or private property. Locking all the doors and windows when an invisible intruder is detected might be more effective and sensible too. $\endgroup$ – a4android Aug 16 '17 at 6:13
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Simple, you have infrared cameras placed in public spaces.

Due to the fact that you mentioned that body heat occurs like it does in normal people the cameras will be able to see them.

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I think the best way is to find the physiology of invisible people and what makes them invisible,taking in mind all the parameters that people have said, I assume for example that there is science in this world.If so, the only that you have to do is have some tracking mechanism. If someone does a crime, there are always somehow hints of the actions.Police when finds a murdered man, at first the murderer is like an invisible man, they don't have apparent hints of who is he.They track down the hints.I think the same could be with the invisible men.I don't think the point is to have physical devices.Like surveillance systems of any kind. If the invisible men just appeared, and there is no history, thus research on them, the questions are more, they just made invisible? They born that way? It is like a mutation?

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protected by L.Dutch - Reinstate Monica Jul 10 at 6:50

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