Would a realistic laser weapon be incredibly painful for a human level of pain sensitivity? I'm making a story where a very high tech AI will come into conflict with a group with earth like technology levels and I want to limit the AI. Would it be reasonable to assume that some kind of galactic weapons treaty has banned laser weapons being used against biological life if they could not kill quickly? If it would be painful how powerful would a laser have to be to instantly or near instantly kill let's say a humanoid of 4 to 9 feet in stature?
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I'm using the Atomic Rockets site as a reference: http://www.projectrho.com/public_html/rocket/sidearmenergy.php
You don't need to boil the water of the subject, all you need to do is vaporize/plasmize (that a word?) the surface of your target, wait for the ineviteable explosion as the vapor/plasma expands rapidly and then fire the next beam. A pulsed laser that shoots many shots in a short timespan, say 0,01 seconds or so (for all pulses to have happened), will be capable of "exploding" it's way into a body. The advantage of a pulsed beam this way over delivering your energy in a single massive jolt of energy is that the material you turned to plasma will not obstruct the beam and shield the target as much. The timing of the pulses can change how deep you burn into your target. Potentially you can make this more efficient by assuming the target is armored. The first row of pulses is designed to better cut through armor and the later pulses of the same shot are designed to better cut through soft tissue.
According to the site, something on the lines of 1KJ of minimum energy per shot (divided into 1000 pulses) spread out over a 1mm spot on the target would be enough to cause a 4cm broad hole 30cm into the soft tissue of a human body, more than enough to kill if you hit some organs or large arteries and the hole won't be cauterized due to the plasma scraping and ripping the surrounding tissue away before it can well and truly transfer a lot of heat into it. Subsequently, the wound would collapse and expand several times, causing some shockwaves into the body which some tissue like the lung tissue won't take kindly to. So even if the wound was cauterized, the shockwave afterwards would rip the tough cauterization apart. So similar to bullets, ofcourse it hurts! You are creating holes in people! And shockwaves! And bloodloss! The fact that conventional warfare is deemed "humane" in any way is a surprise in the first place!
The big problem with such energy weapons isn't just the energy requirement, but also backscatter of the beam. Reflective surfaces would instantly be destroyed by the laser, but not before a portion of the pulse is scattered around. The backscatter isn't enough to cause any real harm to most of your body, but if it hits your eye or sensor equipment it's going to fry. This is a problem as you get a lot of innocent bystanders that could be blinded by these effects, additionally if everyone is armed with a reflective coating to increase the chances of the one's shooting you getting blinded by accident the want to use a laser weapon decreases a lot. You would be better off trying for railgun/coilgun type weapons that accelerate projectiles to hypervelocity with the amount of energy you normally use for the laser.
You've got to remember that a laser is nothing but a very specialized, single color flashlight. And (intense) light is hot (which is how lasers cut). But we're 70% water, so to kill a human, you've got to boil enough body water to kill the person.
Naturally, that's painful.
Boiling a hole through a person's heart would instantly kill him, as would boiling through his head. Those are also painful, so you'd want to do it fast, like a 10th of a second.
For the sake of argument and simplicity (since there's also bone involved), let's say there's 1kg of water which has to be boiled.
Raising 1 gram of water by 1°C requires 4.18 Joules of energy.
Q = L×m Q = heat supplied L= latent heat of vaporisation ( boiling water) m = mass of substance (water) Q= 2.2572×10^9kJ/kg × 1kg Q= 2.2572x10^9**kJ* == 2.2572x10^11J
Since a Watt is 1 joule/second, to burn a hole through someone's heart in 1/10th of a second would require a 2.25 terawatt laser.