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.
Basic lesson on how vision works:
Some source of emission (a light-source) produces particles, normally these are photons. These particles radiate in some or all directions.
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".
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.
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.
You actually have a couple options:
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 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.
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.
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.
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.
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?
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.
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.
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.
Very very unrealistic. X-Rays are used because they have two properties:
