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Take, if you will, a sci-fi setting in which power is cheap and handheld energy weapons (blasters, lasers, plasma rifles, that sorta thing) are common. In this setting is an isolated community basically dedicated to stealth and assassination (think "ninjas in space").

Ostensibly, my space-ninjas favor traditional ninja weapons (shuriken, swords and the like) and ninja tricks over the energy weapons which are not only significantly more powerful but far easier to obtain, because of the stealth aspect: Any energy weapon which can generate and use enough power to be…well…an effective weapon, would basically be detectable by pretty much anyone with a mind to detect them. It would be like trying to sneak around with a giant neon sign flashing "← Here is a ninja".

They may be many things, but space-ninjas are not stupid. If they could still do their job and sneak around while using a blaster, they'd totally do it. Right now, I'm sorta handwaving this away as being logistically impossible: sure, we could make you a blaster that can't be detected, but it would require so much shielding that it would either (a) lose all portability or (b) end up prohibitively expensive. And even then, it wouldn't help you when you decide to actually fire the thing.

But I'm not sure how realistic this is. It seems to me that due to being portable yet practical hand weapons in the first place, heavy shielding would already be a built-in necessity just to prevent the device from arcing electricity everywhere or sticking to every metal surface around when it's turned on. However, my knowledge of electromagnetism is pretty much limited to high school physics class and having watched a lot of Star Trek, which isn't really as helpful as it might seem.

I'm still researching exactly how the weapons would work, technology-wise, and since all worldbuilding to this point has focussed on the ninjas who plain don't (or shouldn't) use them it's not been a huge concern yet. I could easily just say that the plasma rifles are of necessity run off of a handwavium power cell — instantly detectable by any easily-produced handwavometer — and get the desired results (i.e. ninjas avoiding tech), but I would prefer to keep my handwavium within reasonable limits. For the purposes of this question, I'm less interested in the detectability of the hypothetical sci-fi power source and more about the detectability of any electrical device designed to draw and handle that much energy.

Assuming everything is basically powered by electricity, how viable would it be to have — and use — that much power portably without it producing enough of a distinguishable effect to instantly set off an alarm from any system designed to look for it?

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    $\begingroup$ Having problems answering this without a better idea of what the weapons entail. What exactly is generating the power in the weapon?...current technology wouldn't be capable of creating a weaponized plasma burst solely through what can be stored in a portable battery. $\endgroup$ – Twelfth Nov 28 '14 at 0:09
  • $\begingroup$ @Twelfth: Just a guess but from his mention of fields and sticking to things I guess he's thinking of superconducting ring storage. $\endgroup$ – Zan Lynx Nov 28 '14 at 2:03
  • $\begingroup$ I like your 3rd paragraph in showing the balance your world strikes between its ability to conceal and the power you give up in order to be concealed. That balance is really helpful for giving answers! $\endgroup$ – Cort Ammon Nov 28 '14 at 2:09
  • $\begingroup$ @Twelfth I'm still researching exactly how the weapons themselves will work, all focus to-date has been on the ninjas who just don't use them so it's not been a major issue. For the purposes of this question, I'm less concerned about how detectable the power-source itself is (assuming there's not something fundamentally detectable about having that much KJ/cm3 regardless of source) as I am about any electrical system that's designed to handle that much power all at once. $\endgroup$ – goldPseudo Nov 28 '14 at 23:20
  • $\begingroup$ At that point it just sounds like a metal detection issue as much as anything else. Without a current flowing through it, it won't produce an abnormal magnetic field anymore than carrying around a flag pole would, no? Then the question is how would you make these weapons avoid metal detection and the shielding needed there. Toby's answer seems good to that extent. $\endgroup$ – Twelfth Nov 29 '14 at 0:08
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Uhm... I'd just like to mention some things from the humble perspective of a physicist.

"energy" weapons would be among the stealthiest weapons EVER (if not THE stealthiest). Why? Well, first let's draw some parallels between our world and a world where "energy" weapons would even exist.

I am assuming by "energy" weapons you are visialising fancy lasers and plasmas. If that is so - you are missing on cool stuff like sound and other sort of light.

I will first overview the problems with having such weapons today and what would be the derivative of being ABLE to have such weapons at all.

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Lasers

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Currently we, present generation people of Earth, are unable to use lasers for anything more than things like scientific studies and carefully planned surgery because of several limiting factors:

  1. Power
  2. Complexity and size
  3. Lenses
  4. Focus spot

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Power

The amount of power needed to make a laser that can literally burn whatever it is aimed at is not that great - a few small AA batteries should be able to do the trick - the laser in your DVD is actually powerful enough to do something like this, since as you might know DVD-writing devices actually use a laser to drill tiny holes in the disk. The problem is that when I say "literally burn" i mean just that - it would take seconds for a person to feel pain from that and if you want a laser that can kill people better than a gun - you'd need a LOT more power, which is not un-doable, but you either need powerlines next to you or a battery, bigger than yourself.

Complexity and size

Lasers that can do you any good are much more complex than your average hand-held laser-pointer, I should know - until recently I worked on a project about laser ablation. Lasers used in medicine and science are actually complex rigs, consisting of many moving parts, mirrors, lenses and etc. The size of the whole thing is at minimum half a human, and this is only half the reason you can forget carrying anything like it around - the other half is there rigs are stationery for a reason - moving them erratically would certainly break them.

Lenses

Lasers are THE most concentrated, monochromatic and straight ray of light you will ever see (or actually you won't but more on that later) lasers emit light only in the complete unerrorfulness and unfailingness of a straight line... until a certain distance. Like everything even lasers dissipate after a while, their range depends on your rig and lenses used, your focusing lens needs to be of high quality (which is no trouble, you can get high quality lenses for a good price even today) but its size also depends on your rig and the range you need the laser to be effective at, so like the size issue, today you would be unable to get a reasonably sized lens for a "killing" laser

(mind you, let me mention there is sort of a lens/power relation -> to boost the power output of a laser, any laser, you can either use a better lens or more power. Yes, if you hook up your laser pointer to a power station the beam it would output in the first 0.0000000001 nanoseconds would be enough to kill an elephant... after which the laser would either melt or explode. On top of that the elephant won't die because this short time would be insufficient to transfer enough energy to its cells if the beam even stays focused, in layman terms.)

Focus spot

Lasers are lasers because they focus light onto a teeny-tiny spot, which is so teeny and tiny you would need to aim at vital organs to do ANY damage and even then it would rarely be lethal. Increase the focus spot, you say? Sure. You would just need several TIMES more power, like doubling the focus spot (and remember - your starting focus spot is about the size of a needle's tip) would require power^2, i hope you can work it out from there on.

So in closing - all you need for lasers to be at least as effective as guns is:

  1. Tiny batteries with a capacity of a hospital backup generator
  2. Space tech lenses
  3. Space tech mechanical parts and electronics

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Plasma

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No, just... no. I would guess you are all imagining plasma weaponry as some gun-formed... gun that shoots reddish blobs of something and that something really hurts. I say that because movies, tv shows and etc. have always shown us "plasma" as something that does not look like plasma at all. Plasma is just ionized gas (layman terms), meaning you could imagine it as a puff of cigarette smoke and you would not be wrong. It's exact color depends on two things:

  1. The color of the gas itself
  2. The temperature of the plasma

You can have plasma at room temperature, actually fluorescent lights use gas that gets jolted by electricity to become a sort of plasma. That particular plasma emits light in that particular way, because of the nature of the gas and what happens to it when it is exited by electricity. Other gases can be ionized without them lighting up like a christmas tree, some can be made into plasma and you would never notice the difference. But for plasma that you would like to hurt people - you'd need something else.

Meet the Argon Plasma Coagulator this is very much a real-world device used in medicine today. It basically consists of a power source, a probe and a grounding electrode. The basic idea is that the probe is cylinder-shaped and inside it are several cathodes. Gas flows through the cylinder and electricity is applied to the cathodes. The electricity then flows through the gas to the body of the patient and through it to the grounding electrode. By flowing through the gas the electricity ionizes it and creates plasma then either the electricity itself or the created plasma treats the patient, that depends on setting, intended treatment and manufacturer.

The major problem with this device is that it requires a grounding electrode, this means that the electricity really flows through the patient to the electrode for the apparatus to function. That is why if the grounding electrode is not positioned properly the patient can suffer up to 3rd degree electrical burns. The reason I am mentioning this is because without a second electrode you don't get an electricity flow and you don't get any plasma, because as you may know if you point electricity to a point in the air and tell it "Go there!" it's gonna say "I don' want to. :( ". Basically to have plasma you need to have two electrodes - one at the starting point and one at the end point.

How do we solve this problem? We don't. Some time ago I worked with a colleague on a "jet stream" plasma propagation version of the APC. The idea was to use a similar setup of a cylinder with electrodes with gas flowing through and with controlled electricity flow to create a sort of "plasma bullet" that would propagate on itself to the patient. The benefit of this setup is that it eliminates the need for a grounding electrode and thus the danger to the patient. There are articles on the jet stream propagation of plasma - i.e. the exact thing you probably want to do, the reason it is hard to do is because it is very much like trying to control the movement of a feather in a windy day just by blowing. Plasma bullets are really pig-headed and either don't get created at all or don't last long or don't go where you want them to. On top of that they are not nearly lethal enough, both because they are not that hot and because they cannot propagate more than a few centimeters, after that they just go "puff". To have a gun that can fire plasma that can kill people, even if you can supply the power, you'd have to come up with a way to ionize a puff of air and later making that puff of air travel in a straight line a long distance (good luck with breaking physics).

But considering you can do all that, and if your world is space-age - why not, here are the reasons these weapons would be undetectable.

Lasers emit NO SOUND.

Nada. Zilch. Nichego. Zero. The only thing, relating to a laser, that can emit sound is either the target as it sizzles or the power supply. The power supply could emit sound if it has thousand upon thousands of volts going through coils in it - that is why transformers hum (not the Michal Bay kind, the real-world kind) but if your power source is humming the least of your problems is you would get discovered and killed, therefore if it is safe enough to be handled by people - it would be made in a way that would not make it be several thousand ampers of power waiting to burst. So no sound.

No sound? But they can see it, right? Nope. Well, they can see the target melting, but you can never, ever, ever, ever, ever see lasers, unless one is directed at your eye, in which case - it would be one of the last things you ever see, even the modern day kind. The human eye only "sees" anything because light from somewhere has bounced off from said object into the eye. To see we need light to deflect from something, lasers shoot straight, there is no dispersion at their effective range, so you can only see the spot as the beam hits the target, that is why you only see a tiny spot from the laser pointers and no laser beam - forget all the movies you have seen. So to recap. Lasers - no sound, no light. Any real ninja would tell you that's the best weapon ever. I'll even throw in a bonus - you can't definitely say it was a laser that killed someone. What lasers do is actually 5 different effects, depending on power and exposition time, but let's just focus on the thermal effect, because you probably won't want to make people literally disperse because of photon wind. So lasers would burn people, the victim would be charred. Just like if you would use an open flame, a flamethrower, a molotov, etc. So while further investigation could determine it was a laser that killed the victim it can NOT be simplistically done at first sight. Also, there are no sensors for this sort of thing, unless you want there to be magical sensors, there is no way someone can tell you there was electromagnetic radiation here even a second ago, this thing does not leave any traces (besides the burns). It's like asking someone if they can tell, given all the equipment in the world, if someone has flashed a flashlight here yesterday.

Plasma is pretty much the same, the only difference is that the plasma bullet, itself, would make noise due to the heat and friction and fluid turbulence effect and etc. but not a deafening blast, but more a buzz or hum.

In closing, let me just say I absolutely deplore the lack of a "new line" formatting on this site. :)

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Faraday Cages Are Your Friends

Faraday Cages are cages which block specific frequencies of light/electricity. They don't require any active concealment, because the concealment happens by virtue of the materials of the cage. In fact, your microwave has one, which is why you cook the food inside the microwave and not the person looking to see if the food is done. A similar setup would work for your energy-weapons.

It's actually a low-tech solution, but it's taking advantage of physics to achieve your goals. People love doing that.

Solenoids Are Good, Too.

Solenoids have the nifty property of making a very strong magnetic shield inside the loops but a very, very low magnetic field outside the loops. If you use these fields to direct, say, plasma, then this would be a good way of not only directing them but also concealing the weapon from any sensors.

You should also note that solenoids make a practically uniform magnetic field inside its coils. This is good for accelerating things. People take advantage of this in locks and other linear actuators.

Other Considerations

Your weapons can be considered stealthy for a few reasons. Range, size of the weapon, how loud the weapon is, and how detectable they are are all issues.

Consider arming your ninjas with long range weapons, like a type of sniper rifle. They're stealthy because of their range and because snipers catch enemies at unexpected times.

Perhaps you should consider your space-ninjas using simply unusual weapons, like the actual ninja. It is well-established by many sources that historical ninjas uses gardening tools as weapons, which most people would be (understandably) confused at seeing. ("You killed Bob with a plastic trowel! IT'S PLASTIC!")

Above all, when choosing or designing weapons, you must bear in mind that you don't need the craziest or most effective weapon, you just need to kill. A rock to the head can kill someone just as dead as a stream of high energy plasma.

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  • $\begingroup$ Faraday cages were among the first things I considered, but I'm not sure how distinguishable they themselves would be: While detecting the energy signature of a weapon would raise all sorts of red flags, detecting something that would hide the energy signature of a weapon wouldn't be much better unless it's indistinguishable from the benign faraday cages people happen to be walking around with. $\endgroup$ – goldPseudo Nov 29 '14 at 0:40
  • $\begingroup$ @goldPseudo Just because it says Faraday "Cage" doesn't mean it's actually a cage. It could be as simple as a wire mesh used inside the gun (very unobtrusive). $\endgroup$ – PipperChip Nov 30 '14 at 20:57
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There are probably alternative reasons for ninjas to not use blasters

First off, to answer your question about energy storage we'll need 2 parts. There is a lot of hand waving between present energy storage and science fiction blasters. Lets handle them separately.

"Naked" energy, without sufficient containment, is an issue even today. We usually have to dress it up in chemicals or magnetic fields to keep it from getting in trouble. There is usually a tradeoff between energy density and how fast you can pull that energy out. The most "naked" energy sources are things like capacitors that are willing to let lose all of the energy in a flash (literally). More contained are supercapacitors, which are capable of storing much more per cm^3, but generally have more limits as to how fast that energy can be withdrawn. This makes them good stores of energy, but the energy has to be unspooled into normal capacitors before it's fast enough to work well in a blaster.

More constrained than that are the chemical stores like batteries. They are even slower than supercapacitors, but currently have better storage density. They are also cheaper, and very safe (a supercapacitor discharging across your finger will still hurt!)

The military has an interesting solution which gives a very good balance of energy density and power: thermal batteries. Some batteries, such as the molten salt batteries, take advantage of the fact that the reactions which deliver energy occur faster at high temperatures. In many missiles and similar one-shot devices, the batteries consist of a salt electrolyte which is melted by a thermal charge before the missile is fired. These can generate incredible power (energy per time), and have great storage density for their size!

When you go into the science fiction realm, and start worrying about blasters, a little more hand waving is required. Obviously they have an energy source which is vastly superior in both power and energy density, and you suggested this means they must be shielded. You are correct, but there is a strong difference between "shielded so that it is safe to hold and operate" and "shielded so that ELINT teams cannot detect the eddies from the power source." The former only has to shield the hazardous classes of radiation, like ionizing radiation. The latter requires actively trying to disguise the signature of a weapon. Given how much money the military spends on SIGINT and ELINT, it is clear this is not an easy task... which means its not cheap. Most people won't care if anyone knows the blaster; they're wearing it on their hip anyway. The mass market will only support development of safe-to-fire shields. ELINT shielding would have to be developed by special markets

However, there is another reason why your ninjas might choose to avoid the blaster. Consider Obi Wan's comment about the blaster, an inelegant weapon for a less civilized time. That sounds like Jedi prejudice, but you also note the pragmatic Sith hardly use blasters as well. It seems all those who learned the lightsaber tend to forgo the blaster when given the chance.

When learning to be a ninja, you learn to turn your body into a weapon, fully under your control. As you approach this point, there are tradeoffs you have to make. It is simply not possible to be a master with the blaster and the lightsaber because you only have so many neurons in your mind. A ninja would learn a set of weapons which turn them into the ultimate stealth asassin. If that suite of weapons is sufficiently effective, and the blaster training is too coarse to fit in with the rest of their skills, it actually becomes a hindrance to learn it!

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I'm going to split this into two parts, since the energy signature of the weapon while it's sitting quietly in your holster is going to be drastically different from the signature when you pull the trigger.

Detecting a holstered weapon

This might be a lot easier than you're expecting, depending on exactly how your weapons function internally. I can see two key areas where you might get energy leakage boosting the signature: the power source, and if the weapon needs to charge something before firing.

Powering the weapon is the big question mark here. If you're running off batteries and don't have to keep any internal force-fields/plasma reservoirs/whatever charged, then the electromagnetic signature should be pretty close to zero. If you're not actually using power from a chemical battery, then it's not going to be giving off any electromagnetic radiation. There are still ways you could detect the battery itself (I'd suggest something like an MRI scanner), but they're likely to be short-range; good for a metal-detector type installation on a doorway, not so much for aerial surveillance.

At the other end of the scale, if you're running off a micro-fusion reactor you're going to light up every sensor from here to the North Pole. A 'quiet' energy source like a chemical battery might also cause the same problem if you need to run internal electromagnetic containment fields for some other reason (storing plasma, perhaps...). Compressing that down to a portable size will take a huge, constant power drain - and huge, constant power drains are not at all stealth-friendly.

The middle ground between those two would be if the weapon needs to spool-up somehow in order to be ready to fire. Perhaps it does use strong internal magnetic fields, but doesn't need to run them permanently - thirty seconds warning is enough to let you charge it up and get ready to fire. In that case, the 'powered-down' state would be stealthy but the 'charged' state wouldn't (and the 'charging state is potentially less stealthy again...)

It's worth noting that any constant-signature weapon can be turned into a spool-up weapon in theory: all you need to do is design it so it can be turned on and off in the field. Whether that's possible in practice is entirely up to the world designer - maybe there aren't any man-portable batteries with enough juice to kickstart the internal generator once its shut down, or maybe cooling and reheating the internal plasma store can't be done in the field, or maybe it's just too risky carrying a weapon that takes a minute to get ready to fire. (Similar arguments can turn any spool-up weapon into a constantly-hot weapon, in turn.)

Running through the standard sci-fi weapons (and ignoring the generator question for now), I'd expect lasers to have a low-level constant power drain (with the drain getting higher the faster it can recharge and fire a second shot); plasma-based blasters and particle beams to have a high signature due to plasma containment fields (although they could potentially be designed to spool-up rather than run hot constantly); and railguns/coilguns to be a high-signature spool-up weapon.

Detecting a firing weapon

This is where stealth is most likely to go out the window. Anything using electromagnetic fields to fire is going to show up on sensors - that means all rail/coilguns and 95% of plamsa weapons. There's essentially no way to shield this, either, since you'd need a completely enclosed shield... and that leaves nowhere for the shot to get out.

A neutral-plasma weapon could potentially be a 'silent' energy weapon, but would be short-ranged and very hard to design. (It wouldn't need to include any electromagnetic fields, since the plasma has no electric charge... but that means you can't use electromagnetic fields to fire it either. And there are very few other effective methods for handling weapons-grade plasma.) A neutron-based particle beam would have similar advantages and disadvantages, but trades potentially better range for being even harder to build.

Lasers are potentially the most stealthy energy weapon to actually fire. Virtually all of their electromagnetic discharge is contained in the beam - that's the defining feature of a laser - so detecting one directly would require you to be right in the line of fire. (At which point you don't need fancy sensors, because you just got shot) Atmospheric interactions and reflected light/heat from the target would still be potentially detectable, of course, but a well-designed laser will still likely be significantly more stealthy while firing than any other energy weapon.

In summary, you can make energy weapons as stealthy as you need them to be. Most realistic energy weapons are probably going to be fairly easy to detect, but there are ways around that if you want to use them.

Overall, the single stealthiest energy weapon available is probably a single-shot laser.

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    $\begingroup$ I like your answer...but Neutral Plasma is a foriegn concept to be that I didn't think was possible. New research topic for me. $\endgroup$ – Twelfth Nov 29 '14 at 0:18
  • $\begingroup$ @Twelfth A plasma is ionised by definition, but one with equal numbers of free electrons to positive ions is electrostatically neutral from any distance greater than the Debye length - that's what I was referring to. After double-checking (I was working from memory when I wrote that) it turns out I made one mistake there: you can contain it with a magnetic field, but still wouldn't be able to launch it with one. $\endgroup$ – Toby Y. Nov 29 '14 at 5:02
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The most undetectable would be lasers operating outside the visible spectrum. The most common ones available today are infra-red and ultraviolet lasers.

While lasers themselves are highly undetectable due to being highly directional, dust particles in the path can and do deflect some of the light and make the laser beam visible. One can imagine specialized smoke grenades being used to detect laser fire.

Once you make them invisible however, detection via particulate matter in the air would be rendered useless. Note that while the human eye can't detect infra-red, some animals such as bees can. More importantly, any cheap digital camera/smart phone detects infra-red (try it yourself with your smart phone and TV remote). So IR laser can be detected. UV can also be detected by glow in the dark pigments. So one can imagine special smoke grenades that would make UV laser beam glow in the air.

That leaves us with more exotic parts of the light spectrum. One of the more interesting ones is the x-ray laser. It was developed to knock out ICBM's. It's effective against hard targets like steel or concrete or bone but doesn't work against soft targets like cloth or jelly or flesh. Another interesting frequency to use that I can think of would be microwave which would be useless against hard targets such as metal but highly effective against targets containing a lot of water such as human flesh.

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How exactly are you detecting energy weapons? It's less an issue of making the weapon undetectable and more about tricking the detector that there's nothing to see. And counter intuitively, the "smarter" the heuristics, the easier they are to fool. Compare raise the alarm if energy density> 3kJ/sq.m.

vs.

raise the alarm if energy density> 3kJ/sq.m. IF there are no vehicles operating there, or IF no tools are recorded being used there or IF there are no more than 3 people there.

Register the use of power tools in the area and the system will ignore your energy weapon's discharge.

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