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In the near future of some plotline, the cops have been issued some sort of ranged weapon to replace thier projectile pistols. I'm specifically looking for some sort of plausible explanation for a ranged weapon of the future with the following requirements:

1) Ability to shoot through wall(s) of physical material without damaging said wall - however, must still be able to hurt biological life forms

1.5) Doesn't "pierce" through life forms

2) Delivers at least enough (energy/force/magnetic-waves/etc) to kill a human (instantly) with one shot to the head or torso area

3) Must not be "shotgun" style - as in, it can be aimed and fired accurately (like a pistol)

3.5) Causing explosions is not an option

4) Must be able to shoot a minimum of 100 feet

5) If I shoot at a person behind 6 walls, the person should die, and all 6 walls remain intact - this should hold true no matter how many walls are between you and the target.

Assume infinite energy is available. Would this be possible at all? Perhaps a radiation gun? The major problem I see with the design of this weapon would be trying to get through the wall without damaging it, but still being able to kill the guy behind the wall.

For clarity's sake:

  • Wall materials consist of the following: Wooden walls, cement walls, drywall, glass walls.

  • In terms of not damaging the wall, strictness would be on the level of 10000 on a scale of 1 to 100. The reason we don't want to damage the walls is to make the weapon as untraceable as possible.

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    $\begingroup$ @DaaaahWhoosh I think most walls in the world have a perpendicular wall on top. You know, a ceiling. $\endgroup$ – Samuel Mar 20 '15 at 19:49
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    $\begingroup$ So...basically Samus's Wave Beam? $\endgroup$ – Zibbobz Mar 20 '15 at 20:15
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    $\begingroup$ I can do that with my finger. It's a temporal weapon. I just point it at someone, think "die!" and sometime in the next ninety years they die horribly, all wrinkled and shrivelled. I know you said "quickly". On a terrestrial scale ninety years is barely a flicker. On a galactic scale it's infinitesimal. $\endgroup$ – Peter Wone Mar 21 '15 at 2:19
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    $\begingroup$ What happens if someone points their gun at the ground and pulls the trigger? How do they train, if a backstop won't work? $\endgroup$ – cpast Mar 21 '15 at 3:47
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    $\begingroup$ Actually there already is a device like that used for medicinal purposes Extracorporeal shock wave lithotripsy. It focuses an acoustic pulse to kidney stones. The pulse is weak enough not to damage surrounding tissues, but at the focal point it's strong enough to break kidney stones, create potentially very dangerous cavitations etc. You could use the same principle to create a weapon. (I don't have enough reputation for a full answer.) $\endgroup$ – Petr Pudlák Mar 21 '15 at 21:16

18 Answers 18

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Essentially, you want something that has only a small effect on what it's passing through for almost everything it passes through - and then suddenly has a major effect at and only at the target point. Unfortunately, there aren't really many ways to do that effectively within the scope of 'realistic' physics.

A carefully tuned frequency (using either sonic or electromagnetic waves) is one option, but it's going to be rather hit-and-miss in terms of not affecting the environment. It would probably be possible to find an EM wavelength that interacts strongly with water but only weakly with stone/concrete/glass - microwaves more-or-less do this. That's great... until you shoot through a wall made of damp wood, and the flash-heated steam blows the timber apart as well as your target. Or until there's another bag of water person standing behind your target, and your gun happily kills them as well.

Sound waves are going to have similar problems. A gun which emits sound waves that resonate strongly with the target's skull will neatly blow his head apart - and won't hurt anyone else unless they have a skull of exactly the same size and shape as the target. Unfortunately, because the target frequencies do vary based on size and shape, you have to know exactly how said target's skull is shaped, and the angle he's holding his head at, and what kind of damping effects his hat/hair will have, etc. etc. And even then, you're still facing occasional collateral damage when the frequencies in question also set up a freakish resonance with the left rear wheel of a passing truck, or the hard drive in the central computer of the bank across the road, or...

There aren't really any other convenient methods of making something react disproportionately with one target and ignore anything else, but there's still the next best thing: take something that reacts with everything equally, and focus it at the target location. The most practical open-air method of doing that is multiple beams that converge at the target point.

That's going to be difficult from a man-portable weapon, especially since the kind of discrimination you're looking for will require a large number of beams, coming from very different angles. If your police force can put up some supporting infrastructure, however, it would be plausible to make a man-portable target designator, which then sends a signal triggering a number of radiation beams to all target the same point. Placing the beam emitters on towers around a city would be relatively cheap and easy; placing them on orbiting satellites gives you nation-wide and/or global coverage (and is easier to do secretly), but massively increases the engineering difficulties of building a sufficiently precise emitter. Get a decent computer coordinating the beams and you can play tricks with the exact frequencies to set up interference patterns that tighten the killzone even further.

An alternative that could plausibly be man-portable - at the expense of being only mostly based in 'realistic' physics - is two converging beams, one of neutrinos and one of anti-neutrinos. They'll pass through pretty much anything without causing any damage, but mutual annihilation at the point where the beams cross will release a significant amount of radiation. It's still going to be a messier weapon than an ordinary pistol, since there's nothing containing the radiation once it is released, but it should be possible to tune it so that 'collateral damage' means 'sick' instead of 'dead'.

Another option is firing through a wormhole, higher dimension, hyperspace, or anything else along those lines. There's not much more I can say about this - since they aren't really based on known physics - other than to note that they're an option, they aren't proven to be impossible, but you'll have to make up at least half of your explanation since no-one really has much of an idea of how it would actually work.

I feel obliged to mention one final non-science-based option, since it seems like a perfect fit: psychic weapons. If your world allows ESP, telepathy, or anything similar, then a purely mental attack will do exactly what you're looking for.

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    $\begingroup$ I'd add one refinement to your neutrino/anti-neutrino idea. My understanding is that neutrinos ordinarily move just under the speed of light, but there's no definite speed they have to move at. If you fired one pulse of neutrinos fired by a slightly faster pulse of anti-neutrinos, you could have them reach the same point at the same time inside the target. It'd be so fast that from the point of view of anyone other than the Flash, it'd be just one "zap," maybe consisting of several very precisely done neutrino / anti-neutrino pulses. That way you don't need two intersecting beams. $\endgroup$ – Darth Wedgius Mar 21 '15 at 16:59
  • $\begingroup$ This is a very cool idea, @DarthWedgius $\endgroup$ – Elliot Bonneville Mar 21 '15 at 19:22
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    $\begingroup$ I love the neutrino idea, it seems like the only one that meets the requirements of no damage to walls, and no collateral damage. Plus it has the advantage that nothing can shield against it, and it can be used against non-organic targets. $\endgroup$ – 2012rcampion Mar 23 '15 at 18:26
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It would depend on what your walls are made of.

Essentially, any type of radiation you produce will penetrate some materials, but not others. Something like x-ray radiation or IR will penetrate any substance that is transparent to that wavelength, and will be absorbed by (and excite) any substance that is opaque to that wavelength, causing heating and damage.

A wall which is mostly transparent to x-rays would be pierced by an x-ray gun, which could then injure a person standing on the other side, since substances like bone are opaque to that form of radiation and would absorb the energy. However, a wall made of something that absorbs x-rays would block the weapon.

You can think of it this way: if I make a gun that shoots radiation that is absorbed by humans, I can stop that gun with a wall of human corpses. There aren't any forms of radiation that will hurt a person that won't be absorbed by my corpse-wall, or a wall with corpse-wall like properties.

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    $\begingroup$ Makes me imagine a 'gun' that first pings the target with a low power burst of many wavelengths, then chooses the one that is most effective and delivers a lethal dose of that wavelength. As you say, still doesn't work against every possible obstruction, but minimizes the number of obstructions that it can't penetrate. $\endgroup$ – Bryon Mar 20 '15 at 19:46
  • $\begingroup$ This definitely deserves a +1 for the mention of using corpses as shielding (which I previously hadn't considered, but is a very good point). I've updated the question to include wall materials, perhaps you can update this answer to include a specific set of wavelengths? $\endgroup$ – Aify Mar 20 '15 at 22:28
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Most of the answers so far have focused on using some type of radiation to kill your target (since it's been established that projectile weapons are a non-starter). However, there are some problems:

  • Microwaves are stopped by even thin metal (e.g. your microwave oven doesn't roast you every time you use it). They also have a long wavelength (on the order of 10 cm), so you need a large emitter to create a focused beam; a handgun-sized microwave weapon would spray radiation indiscriminately.
  • Going to shorter wavelengths doesn't help. Terahertz radiation doesn't penetrate objects very well (the body scanners used by the TSA can see through normal clothes, but that's about it). Many answers suggested going even higher to x-rays. However, low-energy x-rays are stopped by things like pieces of paper and air. High-energy x-rays pass right through your body (which is why they're useful for medical imaging), so they violate the "no collats" requirement. There is no intermediate range where x-rays penetrate walls but not tissue. (This is because humans are made of low-Z elements: hydrogen, oxygen, carbon, and nitrogen.)

With projectiles and radiation pretty much discounted, that leaves only one weapon left: information.

There are two scenarios I have in mind.

Nanobots, Nanobots Everywhere

Typically I don't like to lean on the crutch of nanotechnology, but it fits pretty well here. Imagine a future where invisible nanomachines cover every surface, fill the air, and even our bodies. They do a lot of amazing things, like turn every surface into a microphone and touchscreen display, constantly analyze the air for contaminants and monitor weather patterns, and continuously repair our bodies.

However, one organization has turned the system against us. Whether it's the government building backdoors into the system, or a group of industrious blackhats, someone has a way to override the nanomachines' programming. They now have thousands of ways to kill you, from reversing the nanomachines that increase your blood oxygen and endurance so that they asphyixiate you, to instructing the brain-machine interface nanomachines to kill all your brain activity. They can even make it look like a natural death or an accident, and when the override is wiped from the nanomachines' memories, no one will be the wiser.

Mark of the Beast

And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand...

In a less technologically-advanced (but perhaps more dystopian) future, the government might require everyone to get 'chipped' with an ID tag. It's a great convenience, since you no longer have to carry around your ID, worry about getting it lost or stolen, or renewing it when it expires. Maybe it even functions as your credit cards, ID badge, passwords, and more; it might even be voluntary but ubiquitous, like cell phones.

Once again, someone has backdoor access to the system, and has found a way to kill you using your chip. This could be a secret feature placed there by the government, The Corporations, or a supervillain blackmailing the designer. It could also be a simple design flaw: when overloaded in a certain way, the fried chip releases a toxin into the bloodstream, or creates a destructive feedback loop in the nervous system.

The method of death can be as subtle or obvious as you like. It may be clear that the target was killed by their 'malfunctioning' ID implant, but without comprehensive logs determining who killed the target may be impossible.


In both of these scenarios, the 'gun' merely acts as a target designator, like the scariest point-and-click interface ever. The great scary thing about this system is that is scales perfectly. An aspiring neo-Nazi could use the system to kill anyone whose DNA he doesn't like, or a supervillain could hold the entire planet hostage without raising a weapon.


Of course, the most thought-provoking part of this question for me is, why would police need to be issued untraceable weapons? (!!!)

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    $\begingroup$ That last paragraph is a really good point. $\endgroup$ – Linkyu Mar 21 '15 at 8:07
  • $\begingroup$ What if the police in the future are up against very deadly terrorists, who could easily snipe off said policeman if he had used a traditional, traceable weapon? Would that not justify the need for an untraceable weapon? $\endgroup$ – Aify Mar 23 '15 at 17:17
  • $\begingroup$ @Aify Are you using "untraceable" to mean "undetectable," like a super-silenced weapon? Or untraceable as usually used in reference to weapons, meaning that even after forensic analysis the identity of the shooter cannot be determined? I would imagine that the former has plausible utility for special forces or SWAT (the kind of people who usually deal with terrorist threats), but no so much for beat cops (which is what "issued... to replace their [sidearms]" makes it sound like). Even in a totalitarian state, the latter would (I think) only be useful to criminals, assassins, and terrorists. $\endgroup$ – 2012rcampion Mar 23 '15 at 18:22
  • $\begingroup$ In the setting I have, the local pd act as soldiers as well as SWAT and SEAL, etc. Highly trained general practitioners. But yes, I do mean "undetectable" as well as "untraceable". $\endgroup$ – Aify Mar 23 '15 at 18:37
  • $\begingroup$ @Aify I was thinking about this for some time, but I still don't think untraceable weapons are justifiable for cops/military. Unless the criminals/terrorists have their own evidence-based justice system for determining if an officer is 'guilty' of a killing it wouldn't have any added utility. However, a stolen weapon would have great utility to a criminal. The biggest problem though is that such a weapon is too easy to abuse. A racist cop wants to kill some minority citizens? If he knows his gun is untraceable, and therefore he cannot be caught... $\endgroup$ – 2012rcampion Mar 25 '15 at 14:18
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Obviously no physical weaponry will be able to harmlessly pass through walls until it hits the target, so you are limited to some kind of energy weapon. Unfortunately the energy levels required to instantly kill a person are going to be so incredibly high that you are certain to have significant damage to everything in the line of fire for a considerable distance.

The only way of accomplishing this with realistic physics is to have a wide array of beams controlled by a central targeting computer. Each beam is by itself is weak enough to be rather harmless (no significant damage to anything between emitter and target or off the other side of the target), by all the beams focused on a single point could deliver a high total level of energy. Additionally, some of these beams would at times be blocked by intervening objects, so a single shot might not do it (nothing sees everything not human as perfectly transparent and humans perfectly opaque - we are not a special kind of matter different from everything else).

This will not be man-portable, but would likely require a large number of emitters in the area focused on the same point from different directions. The number of emitters you need would be inversely proportional to your aversion to collateral damage.

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  • $\begingroup$ What kind of beam would this be? Beams of light? Beams of... gamma particles? (Beams of light kinda make me think of a big magnifying glass.. lmao) This is a really interesting idea though, as I could potentially have a bunch of satellites in orbit, and the gun itself would just be a fancy targetting pointer lol $\endgroup$ – Aify Mar 20 '15 at 22:23
  • $\begingroup$ Actually, this was a 'thing' in DS9's 'Field of Fire'. The killer used a weapon fitted with a micro-sized transporter. Targeting was done via a scope that could see through bulkheads. $\endgroup$ – hrbrmstr Mar 21 '15 at 12:33
  • $\begingroup$ @hrbrmstr That is nothing like my suggestion whatsoever. You are talking about teleportation science fantasy - I'm taking about multiple beams of energy focused on a point to damage tissue (a real world application). $\endgroup$ – pluckedkiwi Mar 24 '15 at 19:29
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    $\begingroup$ @Aify You would probably use something in the hard x-ray range to penetrate most intervening materials and still impart significant energy to the target. You still need it from many angles to ensure not much is blocked, but enough material will stop anything. $\endgroup$ – pluckedkiwi Mar 24 '15 at 19:50
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There is already a weapon designed to kill living things without damaging walls: a neutron bomb. The reason it works is that living things really care about having tiny subcomponents damaged, whereas walls do not. The more refined our electronics are, the more they will care--in the 60s, electronics worked fine. Modern cellphones may not. Anyway, as neutrons are zero-charge, there is no good way to stop them except for lots and lots and lots of matter.

The problem with using a neutron weapon is that it's very hard (because neutrons are zero-charge) to point them where you want. Radioactive decay will spit out neutrons in random directions. Not so useful, unless you can deploy the bomb right next to the target.

But you can do it by accelerating charged atoms (usually Deuterium, which is one proton and one neutron) into a target (usually Tritium) and blocking all the resulting mess except for the neutron that continues flying in the same direction. These are called neutron generators.

They wouldn't literally leave no traces, and enough walls would eventually stop the neutrons, and if you happened to blast computers with them you might cause failures. But it's pretty close, otherwise, to a fry-humans-through-walls-without-traces weapon. If people didn't know about hand-held high-yield neutron sources, it would likely take them a very long time to figure out what was going on. (E.g. we do not currently have any sources like that--they're all vastly bigger and less portable than a gun, at least at the power needed to yield a quick death.)

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  • $\begingroup$ This is a good choice, but the question now is: would the neutrons not pass through the target and hit the person behind? $\endgroup$ – Aify Mar 21 '15 at 4:54
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    $\begingroup$ @Aify Neutrons will pass through tissue about as easily as they pass through walls. The neutron beam will probably also cause secondary radiation to be emitted from the objects it passes through. $\endgroup$ – 2012rcampion Mar 21 '15 at 5:07
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    $\begingroup$ The true reason why neutrons are so effective at killing without damaging most substances is that the mass of a proton is about the same as the mass of a neutron. For that reason, it will easily kick protons (hydrogen nuclei) out of bonds, while leaving heavier nuclei virtually unchanged (they are simply deflected). Unless, of course, they are absorbed; in that case they may "activate" the nucleus (make it radioactive). As it happens, biological systems have lots of hydrogen, including at crucial positions, so the neutrons will do lots of damage. Substances without hydrogen will be mostly … $\endgroup$ – celtschk Mar 21 '15 at 12:34
  • $\begingroup$ … unaffected. Also, the best material to protect from neutron radiation is water, because the neutrons will be effectively stopped (and the kicked protons will be stopped quickly since they turn into ionizing radiation). Ah, I forgot: Those kicked protons by themselves also do much damage in biological tissues. $\endgroup$ – celtschk Mar 21 '15 at 12:36
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    $\begingroup$ @Aify - They absolutely would, as would every other actual thing that meets your criteria (save for cheats like nanobots that tunnel through walls). To get around this, you would have to make sure no-one is behind. If you had an array of neutron beams that were all focused on the person, then the beam would be too diffuse pretty soon to cause any immediately noticeable damage, so you might only have to make sure that 15 feet or so was clear behind them; it all depends on the geometry of the arriving beam. $\endgroup$ – Rex Kerr Mar 21 '15 at 21:54
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I think a microwave weapon could roast someone's brains pretty fast. There are materials that would block it and I'm having a hard time finding out what building materials these are but you could actually cook their brain. I think the water in the body would also absorb enough of the energy that it wouldn't be as deadly to any standing behind the person.

Knocking it up to an X-ray gun would be putting a lot more energy out and would really have to look at what will be stopping the lethal blast on the other side. But having a weapon that can change frequencies to dial in a more useful wavelength depending on circumstances would be the best.

This is really just a specialized laser...

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  • $\begingroup$ Basically, a phaser. $\endgroup$ – Samuel Mar 20 '15 at 19:47
  • $\begingroup$ @Samuel :) yep, that would be another name for it! $\endgroup$ – bowlturner Mar 20 '15 at 19:50
  • $\begingroup$ @Samuel What about a laser-phaser-bobaser-bananafanafofaser? $\endgroup$ – KSmarts Mar 20 '15 at 19:53
  • $\begingroup$ @KSmarts It would take to long to warn someone. "He's got a laser-phaser-bo--" ZAP! $\endgroup$ – Samuel Mar 20 '15 at 20:03
  • $\begingroup$ Aren't phasers typically plasma weapons or particle weapons? I was under the impression that either of those types of weapons would damage said obstacle $\endgroup$ – Aify Mar 20 '15 at 22:18
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So these uh... kinda stretch what you mean by plausible. But here goes.

Wormholes

The gun first uses a laser/acoustic system to analyze the wall and determine the thickness. It then users as-yet-unknown tech to open a wormhole to the inside of the wall, and fires whatever energy payload you'd like at the target.

4th-Dimensional Gun

Imagine this same question gets asked in Flatland. One possible answer would be to fire a weapon over a wall, using the normally inaccessible 3rd-dimension. This would appear to go through walls and solid objects to the inhabitants of Flatland.

Presuming there might be some sort of 4th-dimensional equivalent for us, we could shoot a weapon using that 4th spacial dimension. This would avoid walls and objects for a short period, then come back and strike the target.

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  • $\begingroup$ The 4th dimensional weapon is a great idea - but then the targetting mechanism becomes a problem. $\endgroup$ – Aify Mar 20 '15 at 22:59
  • $\begingroup$ The wormhole answer reminds me of this episode of DS9, where someone uses transporter technology to make his bullet appear right next to the target. $\endgroup$ – 2012rcampion Mar 21 '15 at 1:25
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If this is an untracable assassination tool and not a regular sidearm, you can go with a very hard to aim but highly untracable weapon by firing two really small projectiles that don't react with the target, but each other. You'd need to know the exact distance to target, though.

Basically use a weapon that has two barrels; one fires a small number of uranium atoms (these might be large enough to damage a wall, but with enough speed will punch straight through it and leave a hole that requires at the least an electron microscope to spot; effectively untracable) The other barrel fires a stream of neutrons. These are too small to impact a wall (and probably fly straight through it, and the target).

If you aim really, really, really well and the uranium atoms go through the stream of neutrons, than at the point of impact, you'll get a miniature nuclear explosion. It'll be easily enough to blow up someone's vital organs, but might not even be visible outside of his body. Any bystander would just see the person suddenly collapse. There's no way to trace where the blast came from, and there's no reasonable defense against it.

(Of course, it's still a bit of a stretch because you have to be impossibly accurate to do this, but you might get away with it in a sci-fi setting)

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Basically you need as many x-ray guns as you can get (and deploy/undeploy stealthily).

  1. You'll need a whole network of these radiation 'guns'.
  2. Each individual 'gun' is too weak to damage the walls or the person it's targeted at.
  3. You fire all 'guns' simultaneously at the same target (say you've arrayed them in a hemicircle around the building.
  4. The amplitudes add up in a single convergence point. Your victim.

Boom. Dead. Incidentally, this is also how we treat cancer.

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I'm unsure how far off into pseudoscience this goes, the Vic Tandy wiki seems unsubstantiated yet plausible, leading me to believe there might be interesting and different ways sound can make you sick. Also that I've no idea how to make this work (if we did, wouldn't we have these already?) -And if I knew 'bioharmonics' was a thing I wouldn't have gone this far.


enter image description here

Basically my idea is that, like the technology behind LRAD's (Long Range Acoustic Devices), instead of directly producing the disabling sound, an algorithm decides at what Hertz it needs to play a sound to your side of the wall, in order for the necessary one to be produced on the other side (in conjunction with a real time, x-ray vision-like map of the surrounding topography). Theoretically go through as many walls as you want, the final one being the perp's skull, vibrating at 2.5 kHz. This is somewhat inspired by those speakers that are just drivers; whatever you attach them to becomes the 'woofer'.

Different forms of high intensity energy are easily dismissed, E.g., anything that could leave a radioactive signature is out. Being able to later detect what sound waves were present seems pretty high tech.


Excerpt from Guns and Energy, sometime in the not too distant future...

The iGun

State of the art infrasonic projection system. Disables perpetrators with a click of a button. Simply identify the target with the onscreen display or with your iSeeU reconnaissance head set (for use with non-visual targets), then press and hold the trigger while keeping the device steady (allows for tolerances of up to 30°!). With the nearly instantaneous AI v.7 operating system, reliable wall penetration has been achieved in up to 12" thick steel reinforced concrete.

This is due to the increase of the accuracy in the acoustic detection system, allowing for a more precise fix on the target's bioharmonic frequency for their given location. We are currently the in development of expanding its capabilities to engage multiple targets at once but as of yet, no prototypes have proven up to our exacting standards of absolutely negligible collateral damage.

This amazing technology has only recently been cleared for use by law enforcement. Unlike the LRAD's of old, which can be countered by simply using ear plugs, its revolutionary use of... [Content removed due to the joint proposed IFC, EPA regulation titled under heading, Unlawful Reproduction of Source Material Concerning Infrasonic Waves Inductive of Harmful Interference to Humans]... to cause multiple adjacent surfaces to resonate at just the right frequency so as to harmonize within the subjects cranial cavity; incapacitation happens almost instantly with the loss of motor control. That means when your partner opens the door, the suspect will be on the floor.

(nausea is to be expected and in almost all cases, fatal side effects did not occur)

Call today!

-Coming soon: militarily grade scrambler settings; no more worrying that they'll ever get back up.

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You would need a high energy source that allowed you to create a wormhole through another spacial dimension, and then send waves through the wormhole with a frequency and focus that would cause them to create a wormhole with the exactly inverse spacial effect (no more, no less) at the time when they have traveled a certain distance in normal 3D space. Then point the gun at the victim and use radar of some kind (electromagnetic, aural, space-time ripple, etc.) to pinpoint the distance till said target, use that info to determine the features of the attack ray mentioned at the beginning, and fire said ray.

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  • $\begingroup$ the spacial "hole" from the ray would theoretically cause matter to be removed through the wormhole, since all wormholes are 2-way streets, and that would cause either fatal or inconsequential wounds, from low focus to high focus (where changing the focus requires an inverse change in frequency to keep the ray traveling the same distance before intersecting with our plane $\endgroup$ – user2425429 Mar 20 '15 at 20:36
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With infinite energy I would suggest a neutrino gun. Neutrinos pass through EVERYTHING and they only very^1000 (yes very to the 1000th power) rarely interact with matter.

However if you do get enough of them they can be harmful, acting like most other high energy radiation when they do stop being lazy and act like high energy particles.

So like most things based on probability you just scale up the gun to neutrino-murder levels of energy (alot) and fire. Presumably everything in a line is in trouble though so we can bypass all the walls you want (even the entire planet) you just need to check allllllll the way down range before pulling the trigger.

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    $\begingroup$ But if Neutrinos pass through everything, my target won't die - it'd just pass through him. Also, this violates the rule of not piercing through the target. $\endgroup$ – Aify Mar 20 '15 at 22:14
  • $\begingroup$ Enough would be fatal, Xkcd did a bit on this very fact, (supernovas can produce lethal amounts of them for example, range not withstanding) I took piercing to mean "make a hole" neutrinos would do the damage the same way radiation would most of it would just pass through with no harm done (no holes) $\endgroup$ – Culyx Mar 20 '15 at 22:43
  • $\begingroup$ Yes, but it passes through the target whilst killing him, i presume, which means that any other person standing behind him will also die (which is exactly what i'm trying to avoid) $\endgroup$ – Aify Mar 20 '15 at 22:47
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Every human is implanted with unremovable implant at birth. Cop "weapons" are remote controls that make these implants to kill the host.

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I would go with a particle beam made of very short-lived particles (neutral mesons? heavy baryons?): The particles don't interact with matter, like neutrinos or neutrons, but after a very short time, they decay to high-energy photons (and possibly other particles) that do interact with matter. So in theory, you'd adjust the beam velocity so that the decay happens mostly at the desired target location. Unfortunately, these particles won't decay after an exact time span, but probabilistically. So there will be high levels of radiation before and after the target point along the ray, thus possibly giving away the location of the shooter...

Another idea would be to have a beam that interacts strongly with water, maybe because water molecules have exactly the same resonance frequency as this beam (think MRI): A drywall contains very little water, so the ray would pass through unharmed. But when the ray hits the water in the target's brain, the water starts to boil, killing her instantly.

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I see a lot of discussion here about radiation, but I think we all agree that it's way too hard to aim/make able to penetrate anything/not kill the people behind them. I feel like what you're really looking for is some kind of smart bullet.

Now hear me out -

Though this technically breaks the rule of not poking holes in the victim, some kind of bullet that can maneuver around corners, through small holes, and strike a predefined target almost instantaneously could certainly do it in such a way as the minimize any danger to a bystander standing behind the victim [i.e: strike the victim at an angle perpendicular to the line between the shooter and the target] which is what OP is ultimately worried about.

From an engineering standpoint I have no idea how something like this would work, but I really feel like we shouldn't rule out projectiles altogether. A futuristic smart bullet seems to fit the bill.

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  • $\begingroup$ Technically, i'm okay with the victim being dead due to a hole in his skull; the only restriction about the victim was that the the person behind the victim shouldn't get hurt; if the victim dies and the bullet is lodged in his brain, that's absolutely ok. The "untraceable" part doesn't pertain to "how did this guy die", just the "where did the shot come from" area $\endgroup$ – Aify Mar 21 '15 at 21:05
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Protons were mentioned before, but you might check out some of the new proton therapies for inspiration.

http://www.proton-therapy.org/howit.htm

https://www.mdanderson.org/patients-family/diagnosis-treatment/care-centers-clinics/proton-therapy-center/what-is-proton-therapy.html

The basic idea is that protons tend to dump most of their energy when their velocities are in a certain range. When they move too fast and they mostly pass through things, but once they slow down enough (because of passing through materials) the energy transfer is much more significant.

With proton therapy that means that the treatments can be 'tuned' to transfer energy mostly at the depth of the tumor and not before.

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Something sound based might work. Not sure if you could make it energetic enough for instant lethality, but the wall is most likely less sensitive to the vibrations than a human head - you can keep the energy levels low enough to avoid serious collateral damage.

High energy radiation can also potentially make it through a wall, but is probably too dangerous to use - anything capable of instantly killing is also capable of setting the wall on fire if it happens to hit the wrong material inside the wall.

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  • $\begingroup$ But sound bounces off walls. Also, what if you're in a soundproof room? $\endgroup$ – Aify Mar 20 '15 at 22:24
  • $\begingroup$ You'll be interested in overpressure from blast waves to improve this answer: en.wikipedia.org/wiki/Overpressure Let me know if you do. $\endgroup$ – PipperChip Mar 20 '15 at 22:30
  • $\begingroup$ @PipperChip doesn't that basically state that the moment you get to a level where fatalities occur, structural integrity of buildings is compromised? $\endgroup$ – Aify Mar 20 '15 at 22:58
  • $\begingroup$ @PipperChip isn't that essentially saying that an explosion in this room can kill someone in the next room? Not exactly a user-friendly weapon, and only avoids damage if the wall happened to be made of solid steel. $\endgroup$ – Quentin Clarkson Mar 20 '15 at 23:07
  • $\begingroup$ @Aify yes, soundproof would be equivalent to bulletproof. The weapon would be primarily intended for direct use, but when necessary you can work out how reflection or going through walls will change the output and adjust for that. $\endgroup$ – Quentin Clarkson Mar 20 '15 at 23:07
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You could have the weapon use some sort of scanner to determine where the target is, then open a worm hole to place a lethal dose of whatever you want (perhaps electricity) into the target.

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    $\begingroup$ The science-based tag indicates . . . well, exactly what it seems. Instantly opening a wormhole seems more sci-fi. It could very well happen, but for now it's a bit of a stretch. Could you perhaps elaborate a bit on the scanner method, and the weapon details? $\endgroup$ – HDE 226868 Mar 20 '15 at 23:41

protected by a CVn Mar 21 '15 at 18:40

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