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.
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:
- Complexity and size
- Focus spot
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.
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.)
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:
- Tiny batteries with a capacity of a hospital backup generator
- Space tech lenses
- Space tech mechanical parts and electronics
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:
- The color of the gas itself
- 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?
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. :)