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Can a see-through sight be built that is small enough to be mounted on something the size of a sniper rifle?

It does not need to be based on X-rays; ultrasound, mesons, neutrons... everything that exists is fair game.

The sight must be capable to see through a normal brick wall and function at a distance of about a 100 meters.

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    $\begingroup$ 100m for a sniper rifle? what's the point of that..? $\endgroup$ – dot_Sp0T Mar 26 '18 at 8:04
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    $\begingroup$ @dot_Sp0T Sniping is not about shooting at extreme distance (that is the job of the marksman). Sniping is about avoiding detection before (and preferably after) taking the shot. Now granted detection is made harder for the opposition if the shooter is at extreme distance, but it is not a requirement. $\endgroup$ – MichaelK Mar 26 '18 at 8:10
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    $\begingroup$ @MichaelK I think "marksman" is in this context usually used in the military meaning of the word, which usually means shorter ranger than with snipers. A "sniper rifle" would indeed generally be intended for ranges far in excess of 100m. Even with a "designated marksman rifle" effective ranges in excess of 150 metres would be expected I think. I am thinking the question is about a specialized short range weapon for shooting concealed targets. It would be issued to one designated soldier with some training per unit like DMRs, so he just thought it should be of similar size to be useful? $\endgroup$ – Ville Niemi Mar 26 '18 at 9:11
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    $\begingroup$ I'm not completely clear on what is the point of a rifle scope that lets me see through walls I can't shoot through anyway... $\endgroup$ – David Richerby Mar 26 '18 at 19:48
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    $\begingroup$ @rmunn The question says "brick wall" not drywall. $\endgroup$ – David Richerby Mar 27 '18 at 7:13
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Not believable at all

Basic lesson on how vision works:

  1. Some source of emission (a light-source) produces particles, normally these are photons. These particles radiate in some or all directions.

  2. These particles either pass through, or reflect off of an object. When that happens some pass are absorbed, while others reflect/pass through with ease. In very simplified terms we say that "the light takes on colour of the object in question".

  3. Next the particles are received by a detector (eyes, a camera, a CCD plate, etc). This detector captures the light and produces an image that can be perceived by humans.

The order of things is important here: From light-source, onto target, then into detector.

Now here comes the problems for your idea:

You have the special light-source and the detector in the same location.

This means that the only way this special light can go from the light-source and then into your sight, is through reflection. But you said you do not want reflection, because what you see — if the light reflects — is the brick wall, not the target. You want the special light to pass through the wall, and hit the target, and then make it to your sight.

The only way you can do this is if you have some special light-source behind the target, that shines this special light through the target and the wall, so that you can see it in your sight.

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    $\begingroup$ yup that's exactly the problem, there are solution though, some waves will only partially reflect and part of it will make it through. Ultrasounds and radar can be used like that, but I'm not sure to what extent $\endgroup$ – SilverCookies Mar 26 '18 at 8:32
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    $\begingroup$ @SilverCookies Radar has the problem that is is RAdio Detection And Ranging. The longer wavelength means that radio waves can more easily penetrate into visually opaque materials. However that also means a complete loss of resolution. Radar can only see big things. Ultrasound has the problem of extreme attenuation, at best is it 5 dB per meter. Which means that at 100 meters you would be transmitting ultrasound at a bone-pulverising 500 dB x 2 to get anything back to the receiver. (The loudest sound ever recorded was the Saturn V moon rockets... at 204 dB). $\endgroup$ – MichaelK Mar 26 '18 at 8:39
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    $\begingroup$ If you are going to shoot dragons of the "Look at the size of that thing!" category, from 100 meters away, then it could be done. But you will need two carts with you: one to haul the radar (because such radars are bulky), and one to haul your steel balls. But you asked for a reality check so here goes: no-one would ever develop radar sights for a rifle to look through walls because you can only see materials — huge materials — with that. For that kind of target, you do not use firearms, you use recoil-less rifles. $\endgroup$ – MichaelK Mar 26 '18 at 8:55
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    $\begingroup$ @Raditz_35 It is getting extremely situational, and extremely static. No-one would ever make such an apparatus for a sniper. This is where the reality-check must say "No.... not feasible", because now we are into territory akin to when Mythbusters are "Replicating the results by any means necessary". Yeah sure, we may be able to get something out of this, but the effort is so astoundingly large that it is entirely impractical. $\endgroup$ – MichaelK Mar 26 '18 at 13:28
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    $\begingroup$ According to your logic, the back scatter X-Ray machines that airport screeners use to see weapons concealed under passenger's clothing should not exist. I'm not suggesting that one of those machines would be of any use if you attached it to a rifle, but they do just what you said could not be done: They emit radiation, and some of that radiation penetrates clothing, and some of the radiation that penetrates the clothing is reflected back from skin and/or from concealed weapons, and some of the reflected radiation again penetrates the clothing, and finally is "seen" by the detector. $\endgroup$ – Solomon Slow Mar 26 '18 at 16:23
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As long as you are flexible with how you define "sight"

As others have pointed out, the a gun sight or sniper scope requires photons from the target to reach the user's eye - structures such as a brick wall block photons, therefore an "x-ray sight" is not possible.

An actual x-ray machine uses an active source of high energy photons - known as x-rays - on the far side of an object to provide enough photons for imaging.

However:

If you expand your definition of scope or sight from aiding the eye to replacing it, then you could be in business.

A sniper scope could actually be a display screen, radio receiver, and precision GPS device mounted on the top of a rifle. Provided you have a very precise location for your target - say through drones or other soldiers providing a live, GPS enabled feed - the device could create a synthetic view of what is on the other side of the wall.

This would enable a user with a high powered rifle to shoot through walls, giving an x-ray vision sort of feel.

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    $\begingroup$ +1: The usefulness of Augmented Reality for infantry combat is often underappreciated in SciFi. $\endgroup$ – Philipp Mar 26 '18 at 14:42
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    $\begingroup$ @Philipp It is used in plenty of sci-fi video games, for Identify Friend Foe systems, wall hack mechanics, objective markers and a number of other things the player's character can see. Not just talking about the game HUD elements the player can see. The above 3 are all explicitly referred to by non-playable characters in Call of Duty: Advanced Warfare as well as other games. $\endgroup$ – Nzall Mar 26 '18 at 15:32
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    $\begingroup$ @Nzall The usefulness of HUD elements even in those games where you have no in-universe justification for them is yet another reason why I find it strange that it is so under-utilized in other media. $\endgroup$ – Philipp Mar 26 '18 at 15:59
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    $\begingroup$ Fire a camera with your camera-drone gun, then aim your sniper rifle at things near there. $\endgroup$ – Peter Cordes Mar 27 '18 at 5:55
  • $\begingroup$ Or have close-air support drones cropdust the area with microcams $\endgroup$ – Tacroy Mar 27 '18 at 18:50
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You actually have a couple options:

Microwaves can help you see through concrete

The ability to see through walls is no longer the stuff of science fiction, thanks to new radar technology developed at MIT’s Lincoln Laboratory.

You may be thinking that this should be impossible. That's because:

At first, their radar functions as any other: Transmitters emit waves of a certain frequency in the direction of the target. But in this case, each time the waves hit the wall, the concrete blocks more than 99 percent of them from passing through. And that’s only half the battle: Once the waves bounce off any targets, they must pass back through the wall to reach the radar’s receivers — and again, 99 percent don’t make it. By the time it hits the receivers, the signal is reduced to about 0.0025 percent of its original strength.

But they managed it:

The Lincoln Lab team’s system may be used at a range of up to 60 feet away from the wall. (Demos were done at 20 feet, which Charvat says is realistic for an urban combat situation.) And, it gives a real-time picture of movement behind the wall in the form of a video at the rate of 10.8 frames per second.

Muon can help you see through mountains

Muon are produced in star cores and can pass through planets. The Earth itself in its whole is practically transparent to them. Well, most of them pass through, anyway. A very small fraction of them do collide with matter, and can thus be detected with special machinery. Some scientists in Japan used this to build a muon detector, and when pointed at a volcano, they could see where it had different densities. Kinda like an X-ray:

Images of Mount Asama published in 2007 astonished the world’s scientists. They revealed for the first time, the interior of an active volcano, impossible to see with the naked eye (Figure 1). These images, published by Professor Hiroyuki Tanaka of the Center for High Energy Geophysics Research at the Earthquake Research Institute, the University of Tokyo, were made possible by volcano muography, a technique devised at the University of Tokyo in the 1990s.

This is passive detection. However, since muons also come from outside the solar system, it just might be possible that you don't need to bring your own source even if the sun is behind you.


Right now, both technologies require big machines (there is a picture of the one used by MIT in the first link), and the resolution and rate of capture/frames are... well... for the purpose you want them. But technology usually evolves towards being faster, more accurate, and miniaturized. In real life we have to give it some more time, but in your stories people could already have both technologies made portable enough for a rifle scope.

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    $\begingroup$ You are conflating muons with neutrinos: it would be very hard to see anything as insignificant as a mountain with neutrinos. $\endgroup$ – richardb Mar 26 '18 at 17:26
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    $\begingroup$ Muon neutrinos are not muons in the same way that tiger sharks are not tigers. $\endgroup$ – richardb Mar 26 '18 at 19:09
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    $\begingroup$ Muons are different from muon neutrinos. "Volcano muography" used muons (somehow), not muon neutrinos. Yet more proof that naming things is hard and rarely done well. Like tiger sharks. If that name led you to believe that it's somehow possible to mate tigers and sharks, you'd be mistaken. It's really just an expensive way to drown some tigers, and lose a limb or two. Regarding the OP's question, muons probably won't help you see a person through a wall, unless the person is radioactive. $\endgroup$ – HopelessN00b Mar 26 '18 at 20:11
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    $\begingroup$ @Renan it's not corrected. Muons aren't "almost massless particles" that pass through a planet's worth of matter without interaction; that's true of neutrinos. Muons are, in fact, more akin to extra-massive electrons. $\endgroup$ – hobbs Mar 26 '18 at 23:13
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    $\begingroup$ @HopelessN00b Hey! Muon neutrinos are named very aptly. They are the neutrinos from the same lepton generation as muons. Actually, neutrinos would also help you see through things.... but then your detector is going to be a little large. $\endgroup$ – Clumsy cat Mar 27 '18 at 6:24
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Plausible

Based on the products already available it doesn't seem out of the world to create something like this a bit more powerful to get a range of 100 m out of it.

It basically works with ultra wide band radar pulses, which penetrate normal walls (like Wifi) but gets reflected by other stuff like water and metal.

There is also this military research report for through the wall surveillance available, which is now close to 20 years old. Especially in RADAR technology there were some impressive advancements in the last years, so for me this sounds plausible.

You will not get a crisp full HD image of your target, but with enough training you could be able to tell what is what.

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  • $\begingroup$ The problem with this is that it is active, at the same time your sniping. Then you are actively broadcasting your location, so a antisniper unit can now see you and your advantage is dead. Offcourse if the enemy has no such tech its fine, this is how the near infrared scopes of WW2 worked and nobody but germans could see it since they didnt have the tech. But this can only work for a very short while. But who is to say some world couldnt have a second active source of radiation that you could use passively. $\endgroup$ – joojaa Mar 27 '18 at 14:21
  • $\begingroup$ @joojaa have you read the paper? The pulses are formed in a way that it looks like random noise, so it is difficult to detect the presence as it is just an elevated background noise (on average). So the detector hardware needs to be equally advanced to distinguish the signal. Yes it is active and yes it has the same drawbacks as any active system - I'd imagine that a very high up the ladder person would have defensive systems on par, but maybe some other targets don't. And there is always that quirky guy who doesn't want all that security stuff around. $\endgroup$ – Arsenal Mar 27 '18 at 14:52
  • $\begingroup$ @Arsenal how much for one of those "Ranging and Localization Development Kits"? I don't want to waste their time with a quote (and get lots of calls). I assume I can't pick one up from Grainger on the cheap. $\endgroup$ – kevingreen Mar 27 '18 at 19:14
  • $\begingroup$ @kevingreen I have no idea on the pricing, sorry - I neither work for them or with their products. $\endgroup$ – Arsenal Mar 27 '18 at 20:48
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Completely realistic - assuming there is a signal source inside the building.

Already today, police can detect the location of individuals inside of a building using nothing more than the wi-fi access point signals their internet connection is generating inside the home.

It isn't a stretch to imagine some kind of wall punching weapon coming into play that renders those individuals inside the construct of whatever building they are in by overlaying blueprints with live sensor data.

I would use a gun that can sense the wi-fi signals in order to tag individuals, then fires a bullet which expands into a micro-flight assassin bug to make the kills. Maybe it burrows into the targets head via his ears and explodes as a small shaped charge.

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  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Sec SE - clear Monica's name Mar 26 '18 at 20:09
  • $\begingroup$ You don't even need a "micro-flight assassin bug", just a bullet of caliber and energy big enough to go through a wall. $\endgroup$ – user31389 Mar 27 '18 at 13:44
  • $\begingroup$ Yeah - but it is much cooler to think of an assassin bug making its way through ac vents into the targets safe room, dropping onto his head, entering his ear canal, then with a small puff - the guy keels over dead with smoke wafting out of his ear! $\endgroup$ – Rodney P. Barbati Apr 16 '18 at 22:49
  • $\begingroup$ Actually - watch the Black Mirror on NetFlix for an episode featuring exactly this kind of tech! $\endgroup$ – Rodney P. Barbati Apr 16 '18 at 22:52
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tl;rd

A single scope that can see through walls? Unlikely (without some serious future-tech hand-waving). But an emitter/receiver system placed on either side of the target might be feasible.

How/why

Some people have touched on this topic but no one has really delved into it. In order to "see" anything using just a scope, some form of EM radiation has to strike a material and be reflected back to the observer to be detected. The reason our eyes evolved to see the very narrow "visible light" part of the spectrum, is because it's the most reliable form of radiation that can be reflected and absorbed by a range of materials that are common in our environment. Shorter wavelengths tend to get absorbed more, and longer wavelengths tend to pass right through common substances (notably, people). So what does this imply?

  1. A passive scope that merely collects EM radiation from the environment isn't going to detect much beyond what we already can do, with any level of precision. So this is unlikely to do plausibly.
  2. A transceiver that would emit radiation and detect the reflections would rely on a form of radiation that is reflected by some part of the human body, but not absorbed (much) by any other materials in the way. It would act like a flashlight. While this has the advantage of being able to target the part of the spectrum you want, it's going to be highly unlikely to penetrate concrete, bounce off a person, then penetrate that concrete again and arrive at your scope intact enough to give an image. (One of the cited examples, WiFi, would have to be blasted to get an appreciable response, and that would wreak havoc with the target.)

But what about placing the transmitter on one side of the target, and the receiver on the other? Then you take the reliance on reflections out of the picture. You just emit something that will pass through most of the materials in the way, and measure on the other side what bits of it got absorbed. That massively simplifies the problem (though it would still require some advanced tech). This is basically how imaging systems like X-rays work.

So would X-rays work in this case? Probably not, because concrete would absorb them pretty handily, and they would require a lot of energy to travel 200m anyway. Your best bet is going to be something that's heavily absorbed by water, because that's the material that is very prominent in humans but not the objects we interact with all the time. But you've got to be careful because most of the radiation that is absorbed by water and not concrete is high-energy, so you're risking the well-being of anyone else in the way. Alternatively you could revisit the WiFi end of the spectrum (closer to radio waves) because now you're just looking for absorption, not reflection, so you could use a lower-powered signal. It would still have to be pretty strong to get real-time readings however, so it could cause some disruption in the target area. You'd either need to risk that and very quickly pick off your target, or image them over a longer period of time and base your shot on their routine instead of a real-time view. Either could make for interesting plot points. Note that metal objects and thicker walls would give you less visibility or more noise, so again it would come down to research, planning, and patience, but that's something snipers are famous for anyways.

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Your question sounds like the gunsight in the movie Eraser.

The plot revolved around an attempt to sell a number of these guns to a foreign power for a rather low price. The sight alone would have hospitals salivating to get ahold of one of them at ten times the price asked per gun. :)

They were utterly unbelievable at the time, and even today they would require large sensors to do a fraction of what would be needed.

The problem is that currently, nothing works that way at a distance from a small sensor (yet). It is conceivable that in the future a sight that sees through walls could be built around radar. It might be that it would require multiple devices spread apart to get a good signal resolution. (Neat sci-fi movie scene: sniper sets up and activates his system, and a dozen small anti-gravity drones fly off to either side to give him the resolution. We see his screen start out as just static, and getting more clear as the drones get into place.)

Of course, one of the main problems of any active gunsight is that they could be detected once they are common enough to be used. For example, with radar or x-rays, a simple machine the size of a cellphone should be able to detect that such sights are being used against them.

And then, of course, even if you can see clearly through a brick wall, you still have to have a bullet powerful enough to go through that wall without being deflected so that you hit your target, but I suppose that is another question.

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  • $\begingroup$ you solve that last problem with your conveniently co-invented handheld railgun, of course. $\endgroup$ – ths Mar 27 '18 at 9:15
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Maybe.

Passive Radar

The concept would be similar to passive radar. Passive radar uses natural and artificial radio signals to spot aircraft. Radio waves such as the ones for cell phones, AM/FM radio etc. They bounce off the aircraft and the angle of the bounce can be detected by an array of dishes. Unfortunately waves in that spectrum only travel through air and not solid objects.

Cosmic Rays

You'd need something that passes through solid objects. Muons have been used to see inside stone structures.

Nagoya University used cosmic rays to see inside a pyramid. They wanted to check for empty space but they needed to do it in a way that would not compromise the structure.

The sensors they had were passive and slowly collected hits and formed a picture. Unfortunately it took months for the experiment to yield. That does't make it very useful for an immediate combat use. The resolution wasn't very good either, seeing small objects is out of the question.

Conceptually there's a possibility there. Depending on how much you would want to bend physics rules, or assume extremely advanced technology.

Hodoscope

Maybe extremely sensitive and compact Hodoscopes have been invented. The name already works too.

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  • $\begingroup$ A hodoscope has limited ability to tell you anything about position beyond did or did not hit the detector (you can generally get a very rough and noisy idea of how far along the paddle the hit was, but the noise is such that this is only useful in a statistical sense) and essentially no angular sensitivity at all. They are parts of a segmented detector: you use them in arrays to build up a position and direction sensitive detector. $\endgroup$ – dmckee Mar 27 '18 at 19:01
  • $\begingroup$ @dmckee Like a wire chamber with multiple plates tied to a computer and LCD? $\endgroup$ – kevingreen Mar 27 '18 at 19:09
  • $\begingroup$ Each paddle in a hodoscope array is comparable to a wire in a wirechamber. Note that I say "paddle" because traditional these things were several inches (or even a foot) across and feet long, but with the development of SiPMTs it has become practical to use thick fibers (a couple of mm diameter) as the scintilating component of a hodoscope, so the arrays can be much finer grained. However, the more elements in your array the more computational demanding the data processing. $\endgroup$ – dmckee Mar 27 '18 at 19:18
  • $\begingroup$ I suggested a hodoscope because it may be easier to visualize and describe. A fictional mini super computer creating a 3d render out of data from a detector array sounds like a lot to describe. It's sci-fi technology regardless, I think it's up to the writer to decide how 'hard' the sci-fi should be. $\endgroup$ – kevingreen Mar 27 '18 at 19:35
  • $\begingroup$ The thing is that the word "hodoscope" means a piece of scintillator wrapped in opaque materials with a light detector on one or both ends (and optional light guides). It doesn't mean something that gives you a picture, which seems to be how you are trying to use it here. $\endgroup$ – dmckee Mar 27 '18 at 19:52
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Very very unrealistic. X-Rays are used because they have two properties:

  • they are good at passing through human flesh undisturbed. (The scope would not be able to see most of the human target).
  • they are attenuated by bone and hard objects. (The scope would not be able to see through walls).
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    $\begingroup$ Key word here being through. If the scope produces x-rays... those x-rays will just pass through thing and either become absorbed or just continue into the wide blue yonder.... they will not be reflected back to the scope. $\endgroup$ – MichaelK Mar 26 '18 at 12:32

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