# How to explain why the humans in my setting limit laser weapons to large anti-tank armaments instead of miniaturizing them

So, in my world, the human’s UEN military mainly uses conventional firearms as their primary infantry weapons. However, they do have one non-ballistic weapon: the heavy, shoulder mounted M/DEW-96C Laser Cannon- this device can fire devastating pulses of high energy laser beams at targets, and is mainly used to destroy enemy tanks, vehicles, aircraft, and even buildings.

Here’s something I thought of though: since the M96 is so effective and extremely powerful, readers might wonder “Why doesn’t the UEN simply miniaturize these devices and deploy laser weapons everywhere?”. The technology for handheld laser rifles/guns exists in my setting, so my question is, how do I explain why humans don’t use it?

• "The answer to any question starting, 'Why don't they—?' is almost always, 'Money'." —Robert A. Heinlein – GrumpyYoungMan May 18 at 4:24
• In reality, large laser weapons are almost possible, but (even if they did get them working properly) can't be made any smaller because huge power supplies are required. Why did you decide to say that small ones are possible in your story in the first place? Do you want aliens to have them? – A. B. May 18 at 5:15
• @A. B.: Yes, aliens have them – DT Cooper May 18 at 9:24
• That makes sense then. – A. B. May 18 at 9:29
• It's worth pointing out what kind of battles the infantrymen will find themselves doing. – Daron May 18 at 17:47

Technological Limitations:

Though theoretically creating one of these big guns is possible there are various technical challenges that block the way, one of the main problems is Energy, These lasers require a whole lot of energy, which is easily available on a tank, but carrying 2 huge battery packs along with a big gun for a single shot in the battlefield is just not feasible. Another challenge is heat dissipation, These lasers generate a lot of heat, if you try to put it on someone's shoulder and fire it, the user himself will get a burn from its heat, even on a tank they are used in a remote control fashion from inside the safety of the tank.

Money: These lasers are just so damn expensive to provide them to individual soldiers.

Moral Ground: These weapons are just inhumane, savage, and ruthless. A soldier who is tasked to protect his country from enemies, may not want to become a monster and lose his mind over it after using a handheld version of this thing. Using it from a tank with a remote control and firing it from your own shoulder is quite different.

• @ZizyArcher, sure. Once or twice. Assuming we can miniaturize these lasers is... exactly that. An assumption, and a horribly flawed one at that. The technology may simply have a minimum viable size, below which, the effectiveness falls off much more steeply than the cost (in size, mass, actual money to make it, etc.). – Matthew May 18 at 13:46
• Also, fire rate may not scale; a LAW is effective at its intended purpose, but a miniature version with the same rate of fire would not make an effective anti-personnel weapon. Likewise, a laser that can only fire once every few minutes may be fine for taking out vehicles, and horrible for taking out people. – Matthew May 18 at 13:46
• Most modern laser weapons are actually chemically pumped. Firing them for a single shot requires a truckload of toxic/volatile liquids, which are spent. Some of the newer stuff is solid state, but given the energy requirements requires some small powerplany to power them... there were talks of including a 100kW laser on the JSF, supposing they could miniaturize it enough and spare enough of the power budget for it. I think those were mostly pipe dreams though. These just aren't battery-friendly toys. – John O May 18 at 19:32
• "These lasers are just so damn expensive to provide them to individual soldiers" - and they're so careless! Why, just last week, this guy was running with one and he dropped it! Heck, I was HALF A MILE AWAY and the explosion BLEW MY HAT OFF and it got ALL DIRTY! The NERVE of that guy!! – Bob Jarvis - Reinstate Monica May 18 at 20:01
• Yep. Energy and heat dissipation. Let's say a useful laser output is 50KW (that may be low). Laser efficiency is often around 30%. So now we need 160KW input to fire the laser. We are now talking two electric car sized battery packs. So, we sent 50KW down range. What happened to the other 110KW? Heat. Gets a bit toasty. Like, flash-fry a person toasty. How you solve these two problems will determine your ammo and fire rate. – UrQuan3 May 18 at 21:49

The Law of Non-Linearity.

Suppose a soldier can carry a 20kg laser cannon that can explode an enemy tank.

Since we have the technology for laser rifles, we can produce a smaller 2kg weapon with (all other features being the same) only 10% the power of the cannon (still much more powerful than bullets) and equip our footsoldiers with those, for shooting other footsoldiers. Right?

Wrong. There is a temptation to assume everything scales linearly. But in practice it is equally believable this doesn't happen. For example the smaller weapon might only be 1% as powerful as the cannon, or suffers from lower firing rate, heavier ammunition, overheating et cetera. This requires no justification, and leads to small laser weapons being strictly worse than conventional ones.

For some real world examples consider (a) the Javelin Missile:

This is a big heavy handheld weapon that can be used to destroy tanks. However you will find that soldiers do not carry scaled-down javelin missiles for anti-personnel. The technology simply doesn't scale down well.

(b) Nuclear bombs. Nuclear bombs make big explosions relative to the missile size. However the technology does not scale down to create a nuclear grenade the size of a tic-tac.

• "The technology simply doesn't scale down well." The gyrojet says otherwise, IIRC. It just doesn't offer any significant advantage over proven designs. – nick012000 May 20 at 4:44
• @nick012000 imho the gyrojet is a perfect example even. It's totally doable and the result does do some damage, but is absolutely inferior to the regular alternatives. Same could go for the laser. Maybe it does work perfectly well, but each shot requires a 500g battery pack, making it barely suitable for snipers going for high-value individual targets and thats absolutely it. Or similar stuff – Hobbamok May 20 at 8:28
• @nick012000 I have never heard of the gyrojet -- coincidence? – Daron May 20 at 14:45
• en.m.wikipedia.org/wiki/Gyrojet – nick012000 May 21 at 2:49
• @nick012000 The gyrojet is my favorite go-to weapon when you need something with no recoil. The bullets are rockets, so they're accelerating after they leave the barrel. Great for questions about drones, space weapons, etc. Also great for futuristic smart weapons. – DWKraus May 21 at 3:24

Firing lasers is easy. Firing powerful lasers is difficult. To get a laser usable on the battlefield you need a lot of energy and (miniature) technology. The smaller versions simply don't have the impact you want. Heat is likely to spread on the target, not doing enough damage. Lasers can have a physical impact with some interesting mechanics, but also this is impractical on smaller sizes. It still needs insane amounts of energy, can lose potency in an atmosphere depending on the kind of laser and might not fire fast enough (you don't take a slow firing weapon into many engagements), or require longer exposure times on the target, to do the right amount of damage. A gun with propellant is then much more effective, easy to control and can be used for longer durations.

Further miniaturisation is ineffective

Further problems of miniaturisation is that it only helps so far. Making things smaller helps for many electronics and such to reduce the amount of energy required. The laser however has a fixed amount of energy you might want to send downrange. The energy output would plummet if used in smaller sizes, making them not as effective. If you need to charge or can only fire intermittently, normal guns will again be much more effective.

Covered in mud test: Can your laser rifle still fire after being rained on for hours, dropped in the mud, then used to bash open a door? A regular rifle is fine. This is fine for an anti-tank laser, because it won't be abused in the same manner.

Ammo cost test: Does your laser rifle cost actual money to fire? A rifle bullet costs cents. It's fine if an anti-tank laser costs actual money to fire.

Ammo quantity test: Can your laser rifle hold 30 rounds in a device the size and weight of a magazine? It's fine if an anti-tank laser can only carry a single shot.

Weight test: Is your laser weapon lighter than a rifle? It's fine if an anti-tank laser needs two people to carry.

Rate of fire test: Can your laser weapon continuously fire over minutes? One of the reasons caseless ammo never got off the ground is that the ammo case carries a significant amount of heat away from the gun. It's fine if an anti-tank laser can only fire once every 30 seconds.

Combined, the prototype laser weapon system didn't like being covered in mud, cost $100 per shot because of the disposable batteries, could only hold 12 shots, needed recalibration every few shots, melted if you fired it too fast, weighed 30 pounds, and one of the lenses cracked when it was dropped. In the end, a rifle is a better rifle than a laser is. (It might still be a pretty good sniper rifle, though. This is also a specialty device where a laser has some important benefits: silent firing, lack of interaction with wind, and pure range.) • Regarding the cost-to-fire: a lot of people think that this will be cheap af for lasers, but this is absolutely not the case. A canceled-due-to-cost laser based CRAM ran about 1000$ per shot due to rather expensive coolants etc. being used up each time. en.wikipedia.org/wiki/Skyguard_(area_defense_system) or longer en.wikipedia.org/wiki/Tactical_High_Energy_Laser – Hobbamok May 20 at 8:33
• the covered in mud problem cannot be overstated. Try keeping any optics perfectly clean in a combat situation. If a laser of this power's emitter is not perfectly clean when you fire it it just destroys itself. a single shot weapon can be kept sealed until use. Also one downside of a laser sniper, there is a perfect line pointed directly at you when you fire that any decent battlefield Hud cameras should be able to see. – John May 20 at 19:11
• Laser weapons are not silent. The air they pass through generally becomes superheated, and it would sound like a thunderbolt. – DWKraus May 21 at 3:29
• @DWKraus You just made laser weapons twice as awesome. – Tom May 22 at 4:33

# The power source is too large:

The power source for your laser weapon needs to output some ginormous amount of energy, in a very short pulse.
The humans do not have any form of power storage that is good enough for it, instead what they use is a very tiny fusion pulse generator. One charge, throwaway. Must be used instantaneously, cannot be stored or distributed. And "tiny" being "not less than a cubic foot, and massing 20kg"

If you are going to be lugging a suitcase sized power supply around at minimum, you might as well couple it with a laser that actually uses that much power in one shot, hence the rather huge and overpowering scale of the M96.
(Dropping buildings with a shoulder-fired weapon? that's Bad Ass )

The aliens have the tech to store power compactly, thus they can use much smaller, sensible infantry-scale laser weaponry.

• A tank's shell's muzzle energy is 12MJ. If you laser delivers the same energy in one second, it needs 12MW power source. So, yes, it's gonna be pretty big gamesaelectric.com/… – Pete Kirkham May 18 at 23:12
• @PeteKirkham tank does not rely on muzzle energy for destruction it used HE in the round. And, laser would not fire for 1 second, if it "blows up building". Try 1 ms, with a 12 Gw pulse. The powerpack requirements would be ludicrous. – PcMan May 18 at 23:44
• Not just the power imagine the waste heat coming off of it, one shot and the laser-gun is too hot to handle. – John May 18 at 23:52
• @PcMan If I wanted to do a full analysis, then I'd write my own answer rather than just upvoting yours with an illustration of what a power plant size would be. But a lot of tank-anti-tank rounds, such as the question is about, are not HE but kinetic penetrators, so muzzle energy is a measure of a lower limit of the amount of energy you have to put into it. – Pete Kirkham May 19 at 0:11
• @PeteKirkham yeah, for tank-killer what matters is not so much total energy, but having it concentrated enough. But the whole blowing-up-buidings thing the OP also wants is pretty much just delivering enough energy into the target, in a short enough timespan for it to be explosive. That's not really something a laser is good at, and really ups the power needs of the device a lot. – PcMan May 19 at 6:20

It isn’t a laser, it is a long range induction heater

Your weapon produces a rapidly alternating electric current linearly ahead of the laser. This lets you heat metal to near melting point. This will break vehicles, computers, and planes, but if shot at a soldier it will only disable the gun and maybe burn the soldier holding the gun. A person shot with the weapon would be unaffected since the induction heating only affects metals. Even if the weapon were smaller it would have to be aimed directly at the enemy gun, and wearing gloves would save users from burns from the weapon. Normal guns would be more effective and actually be lethal. While a miniaturized version might exist for police to disarm people, these weapons would be aimed by sharp shooters and only in certain situations.

## Lasers Brightness vs Armor Effectiveness

The US military has abandoned a number of research projects over the years into the use of plasma and laser based small arms because of things like power sources and maximum energy outputs, but in recent years, those problems have mostly been solved here in the real world. So why are we not seeing our armed forces equipped with laser weapons yet? Well one of the biggest remaining hurdles is that they reflect and scatter so much light that they blind anyone standing near by. So, if you were to arm a squad with laser rifles, every time you shoot someone, everyone in the area would be blinded... this is especially bad for the people using the laser rifles because most of the people on the battlefield looking at the flash of light when it happens would be friendly forces leaving your whole squad vulnerable every time you go to shoot someone.

If you think of it like this, the alibeto of your average military uniform is about 0.3 meaning that 30% of the light that hits it is reflected. The lowest power laser theoretically capable of burning a person faster than they can get out of the way is ~1kw. This means that you are looking at a spontaneous flash of light equivalent to a 300w flash bulb. To put this in perspective, the flash bulbs used by most professional photography cameras are about 70-400w. So, at the very low end of anti-personnel lasers, the flashback would be momentarily blinding, shooting up an enemy position would be like having a line of paparazzi flashing cameras back in your face... but this is just for weapons of the power level required mow down unarmed civilians.

If the development of laser technology stopped here, then you could just have your soldiers put on some goggles that filter out the wavelength of the lasers and then they could go wreak some havoc by blinding and disorienting their enemies while they shoot them up.

The problem comes in when armor and weapon developers start trying to one up each other.

Whenever you introduce a ubiquitous weapon technology into a setting you need to assume that the opposition is already aware of it and would try to find a way to counter it. This brings me to what anti-laser armor would look like. Through the use of dielectric mirror based armor, enemy tanks, aircraft, and soldiers could adapt by using anti-laser armor that reflects at ~99.9999% of a laser, as long as they know the wavelength the laser is operating at. This means that laser designers would have to design their weapons around firing a lot of converging lasers of different frequencies (which they already should be doing anyway to reduce thermal blooming).

That said, dielectric mirror are no good against multi-spectral attacks. The most reflective known broadspectrum materials can only reflect 99.9% of light, but as comments point out, even this is much more ideal than you can expect under battlefield conditions. In terms of practical reflective armors, about 80-90% is probably the best you can expect under combat conditions... but this is still enough to be a problem. This means that if enemy soldiers simply start to use bright white uniforms, they could make themselves pretty hard to kill at just 1kw. So, HEL designers would need to scale up to 3.5-7kw to burn heat resistant white clothing. but in doing so, you increase the total reflected light by somewhere between 933-2100%. At these intensities, the reflected light would be enough to risk long term damage to the eyes or anyone not wearing proper protective gear including innocent bystanders.

But reflective armor is just one way to increase resistance to lasers. There are other technologies like thermal ablative insulation which can make it incredibly hard to burn through something with a laser. The more you layer known defensive technologies, the more powerful lasers will need to become to cut through them. By the time laser armor starts to mature, you could be looking at needing laser rifles hundreds of times as powerful as those basic 1kw lasers if you want to actually do any damage. If we pretend that portable laser technology could be escalated to meet the challenge, you will now be looking at amounts of reflected light equivalent to thousands of high end camera flashes going off in your face across a broad spectrum of light frequencies. As nick012000 pointed out in comments, this is VERY bad for any innocent civilians anywhere near the battlefield who happens to not be wearing some very good protective eye gear. Furthermore, because it is a broad spectrum reflection, you can no longer use simple filtered visors anymore. Your only real choices here are blackout goggles that momentarily shut out all light when a laser hits something (blinding you but not permanently), or to have a closed faced helmet that uses a camera and internal display that does not get bright enough to ever hurt your eyes, even in white out situations.

Any way you add it up, using HELs able to deal with armored opponents would simply light up the battlefield too much to actually have a battle, and the collateral to civilians would make the weapons pointless for 95% of operations where ground troops would otherwise be the preferred tactic of choice.

So why use M/DEW-96C Laser Cannons?

For the M/DEW-96C Laser Cannon, this is less of an issue. Although it is a higher energy laser (probably in the 10s-100s megawatt range if you want to kill laser armored vehicles), how you use it is different. Whenever you need to blow up a tank or a building, your whole squad other than the person attacking already has to take cover so they don't get taken out by debris. So everyone in your squad also having to shield their eyes as the one guy takes the shot is a reasonable additional action. And for the person attacking, they only need to make one shot; so, they could have some kind of blackout goggles so that your Laser Cannon Troopers eyes don't get seared out of his face, and not be left at a huge disadvantage not being able to make an immediate follow up shot like you would expect to want form an anti-personal gun. Also, when you need to blow up a tank, innocent bystanders are generally already much less of a concern than when performing more surgical infantry operations.

• There are specialized laser goggles that protect people's eyes by blocking the specific wavelengths of the laser. Presumably, any military squad equipped with them will also have appropriately-designed protective goggles. The problem with laser weapons causing blinding to bystanders isn't with worrying about blinding all of your soldiers - the problem is with permanently blinding all the nearby civilians. And yes, this is a real problem - any laser capable of causing physical damage will cause permanent blindness upon seeing the "laser dot" reflection of the beam for even a few moments. – nick012000 May 20 at 4:40
• @nick012000 You bring up a good point which reminded me to consider what all of the incidental technologies would be that would crop up around ubiquitous laser weapons. If laser weapons become common, you'd not just see an escalation in protective eye gear, but armor and armor penetrating lasers too which means the battlefield would become a lot brighter than I first assumed. See revised answer. – Nosajimiki May 20 at 14:22
• Mirror armor doesn't work on weaponized lasers because one it gets damaged (e.g. because it's not 100% efficient and it got hit by a military grade laser that's melted a portion of its surface), it stops reflecting the laser light, at which point it's no longer protecting its wearer. That said, even without mirrors, seeing the impact point of a military laser without any mirrors involved can cause permanent blindness because of ambient reflection of the laser light. – nick012000 May 21 at 2:42
• @nick012000 That is mostly true of normal household mirrors (which typically only reflect ~85-95% of light). My point is not to use a "mirror", but to use the best available material for reflecting the energy of the laser instead of absorbing it to become heat. With modern materials that can reflect 99.9% of broad spectrum light, it means that there exists the potential for armor that takes 20-60 times as much light to heat up as a normal mirror and 700 times as much light as a person. That makes heating it enough to melt it very difficult. – Nosajimiki May 21 at 13:19
• And when it gets scuffed, dropped, and covered in dust, because it's being used as military armor and it'll have to endure harsh environmental treatment rather than being handled carefully in a laboratory? You can't coat it in a transparent lacquer, because those aren't 100% efficient either, and once they start to heat up, their transparency will drop and defeat the point of having a mirror anyway. – nick012000 May 21 at 14:35

They are not better than normal guns

Ever watch Star Wars and think that a fully automatic weapon or a grenade could easily overcome those Laser-deflecting Jedi?

The point is that while a large Laser weapon would be great for defeating large machines (maybe they ignite the fuel or destroy the electrical systems or something) without having to lob a big clumsy explosive, a small one is really no better (or maybe even worse) than a simple AK-47 against enemy people. It's been tried, soldiers haven't liked it and complained, and the better, easier, cheaper solution turned out to be normal machine guns.

• The Jedi have a simple answer against bullets: The force – Jake Fuller May 24 at 21:46

Just make them worse than ordinary guns. Why would you have laser guns?

• Powerful "rocket" laser: big and heavy with slow rate of fire and wrecks a tank/plane/bunker. Great weapon.
• Ordinary gun: small and nimble and shoots fast and kills a person. Great weapon.
• Anti-vehicle gun: semi-heavy, slow rate of fire, destroys a car. Reasonable weapon - way less use of that.
• Laser rifle: anti-vehicle gun that gives your position to everyone. Poor weapon.

## Risk.

If you fire a laser weapon and hit a shiny doorknob that doorknob will almost instantly burn and lose its shininess. Almost instantly. The laser beam/pulse will spread around the room in an instant, this might not damage bodyarmor enough but it will blind anyone hit by the beam in the eyes. The problem is that there are a lot of shiny objects to be found in the human world and an enemy will likely try to use it by placing tons of shiny things everywhere. Its just too risky.

## Treaties and conventions.

Some weapons and ammo types are banned because they cause unecessary damage and suffering when killing (or failing to kill). This is why incendiary small-arms ammo isnt really used by most of the world anymore. Your lasers are banned from small-arms weapons for this reason.

## Heat generation.

Your infantry carried anti-tank lasers are used much more sparingly. You keep having to move, find a good position, fire a few rounds and reposition. This gives the weapon a lot of time to bleed off its heat generation. A small-arms would be used much more often and continuous. This heat would quickly start burning the users.

## Capacitor charging takes time.

The capacitors take time to charge after each shot. This isn't a problem for a large anti-tank weapon that you won't be firing on full-auto all the time, but for a small-arms that drop in fire rate is a problem.

• Your first two points seem to be one point: lasers used against troops have a too-high chance of causing permanent vision loss (to the targets) but not killing. They're banned as a mutilation weapon. I could see that with some hand-waving how they're not the lasers we know today -- it's some new, better tech that acts like lasers, with a few small differences. – Owen Reynolds May 18 at 16:28
• @OwenReynolds the first point is actually about the risk of allied troops getting permanent vision loss (although likely in one eye). When a laserbeam hits an uneven reflective surface (for it's wavelength) it will scatter up, down, left, right, backwards but not forwards. This puts anyone looking in the direction you are firing at risk but minimal risk to enemies being fired at (to be hit by the scatter). If we assume current warfare statistics for ammo consumption per hit then even a small platoon might blind a significant amount of it's own members before hitting enemies. – Demigan May 18 at 17:05
• That would naturally be exacerbated by the enemy spreading purpose-build reflective surfaces when they become engaged. Compared to zero risk of scatter when firing non-laser weapons it's a good reason to limit small-arms laser technology. – Demigan May 18 at 17:07
• @Demigan Any squad equipped with laser guns would presumably also have laser goggles tuned to block out the wavelengths used by their own lasers. The problem isn't blinding your own troops, it's blinding civilians who are fortunate enough to be in line-of-sight to the battle. – nick012000 May 20 at 4:46
• @Nick012000 I don't think it's as simple as "just wear the right goggles", you might simply not have the material to protect eyes well enough without impairing vision. Besides that a single hit will burn the glass and force you to take them off, saving you from one hit but not two. That is still a risk you do not have using non-laser weapons. – Demigan May 20 at 4:59

# Treaties

We can create biological and chemical weapons, and neutron bombs, all of which are likely to be far more devastating than any flavor of "big gun" (except space-based mass drivers). Why don't we?

Because it's universally acknowledged1 that to do so is extremely not-cool. And the last time the most-powerful countries on the planet got together to draw up international rules-of-the-road, they were trying to be morally enlightened, and so they either banned those things immediately or created the legal framework that would inevitably do so.

So, the reason militaries don't miniaturize the mega-laser is because it is illegal to even talk about having once imagined what it would be like to see the blueprints for a hand-held version. The ban is enforced partially through restricting & monitoring the supply chain. Just as with nuclear weapons, there is probably an international agency whose mission it is to detect and prevent proliferation.

And the mega-laser is beyond the technical abilities of everyone except the world's leading experts to even design, so there's no worry about non-state actors creating their own... yet.

1 Acknowledged by everyone who is not evil. Everyone who does not is evil, trivially by definition, because a person who refuses to put away doomsday weapons because he intends to use them himself is evil.

• I don't find your way plausible - everything else was banned because it was too destructive. A smaller laser isn't more destructive than the big one, so why would you succeed in banning them? Human is as dead by an ordinary bullet or by a laser bullet. – Zizy Archer May 18 at 12:56
• chemical weapons aren't too destructive. chemical weapons are inhumane, and are less effective than simply firing explosives (if you compare the amount of money/effort and amount of shells required). People did use it in trench warfare as late as Iran-Iraq war, but for modern movement based armies, normal munitions is simply more effective. Also gasing your enemy means your have to wait before you can take the enemies positions, not good for mobile armies. Compare this to antipersonell mines, which some countries refuse to sign the banning treaty. – Benjamin May 18 at 14:33
• That's an idea, but needs work. Landmines are likely to harm civilians 10 years later. Chemical weapons cause ghastly lifetime injury (lungs, eyes, seizures), plus can also harm civilians if the wind changes. But lasers don't seem any more horrible than bullets. Laser burns would have to have some long-lasting effect worse than bullet holes. – Owen Reynolds May 18 at 16:21

Special weapons are often kept for special use, because the popularization of them has or would create effective counter measures.

I chuck down a grenade that creates a cloud of reflective chaff.

Your laser weapons are now useless, b/c the chaff would scatter the beams everywhere. This could create collateral damage as the beams split and diffuse everywhere, possibly injuring innocent bystanders or friendlies.

Altnernatively... I create mirror-like armor that reflects & refracts the laser light, making it harmless. Just like a fresnel lens can consolidate light into a focused beam, a person could come up with a material that does the opposite, taking direct laser light and scattering it until it's harmless. Or, reflecting it, which, again, would possibly cause collateral damage.

Standard fire arms can punch right through this stuff. Hence, standard firearms are still used.

The big bad shoulder-mounted laser would be called in as a special-purpose weapon in situations where they know there's no counter-measures.

• As a side note: mirrors don't necessarily render a laser harmless because some light is always absorbed. A simple white sheet of paper actually reflects more light than most mirrors because how much light you reflect is more important than what pattern it is reflected in when talking about energy absorption. It is also very hard to make something that reflects all wavelengths well. A dielectric mirror might reflect a 99.999% of a particular wavelength making you immune to one laser frequency, but barely reflect anything in another frequency making universal mirror armor hard to do – Nosajimiki May 19 at 20:48

## Know-how

... or rather, don't-know-how.

You say "humans" don't use it, implying that others do. The three reasons for this are that you can't, you don't want to, or you're still building it.

It can be done, there's a reason they'd want to ... so they're still building a safe handheld version. The miniaturised power supply might be toxic, the reflected light might be harmful if you're outside an armoured shell, or there might be the modern problem of not knowing how to build a miniaturised energy store.

/the human’s UEN military/ Which implies there are other than humans out there.

The laser cannon is not a human weapon.

These laser cannons are different from anything else in the human arsenal. The humans are happy to have acquired some. Supporting and maintaining these weapons is very much an art form. The "manual" (if it is a manual) has been translated in a few different ways and people subscribe to their favorite version. It is a fine weapon but it is not clear that its makers intended this thing to be a weapon. It can clearly do more than blow up tanks. The humans who use this thing often glue caps and put tape on top of some areas of the laser cannon, to avoid inadvertently activating these other functions.

The nonhumans who devised these laser cannon are not very approachable as regards proposed improvements, or even questions about the device. Reverse engineering the cannon has not been successful either, at least as regards duplicating the machine. Fortunately the ones they have are very durable. The space marines understand the importance of bringing these laser cannons back from any expedition.

Lack of stealth

If you fire a ballistics weapon it is relatively hard to pinpoint from where are you shooting. You would need to precisely track the bullet and then back-propagate this path to the shouter. If you want to track a laser gun user you just have to take a photo with laser-activated camera. When you fire a laser you makes yourself a easy target for every single artillery, drone, smart mine, automatic turret, sniper or loitering munition is the area. Such a risk is acceptable if you want to destroy a tank, not for a infantryman main weapon.

Long startup period.

Before you fire a M/DEW-96C Laser Cannon you need to start the charge up, then wait a 5 second, then fire in 10 second before you run out of the cooling fluid. This is acceptable when you shot at an airplane ( many MANPADS have similar limitations. ) or a tank. Having to wait even a second before you can shout someone or running out of cooling flood after a 10 minutes of combat is unacceptable for a rifle.

• Nice first post Piotr, welcome to worldbuilding. We invite you to take our tour and refer t the help center for guidance. Enjoy the site. – A Rogue Ant. Jun 9 at 23:19

It's a painter, not a laser.

Strong lasers are tough technology...that's why they are not employed in large scales from current militaries.

What you have on the tanks is not a laser that can actually destroy stuff. It is just a laser that is visible from space. And after that....

a multi-gigawatt orbital ion cannon fires on the target, neutralizing any combatants in the vicinity

Taking the tank's laser and scaling down will result in a laser that just isn't viewable from the orbital weapon. I guess this answer is just an implementation of the one by @Daron .

One big reason not to deploy handheld lasers is aiming.

Stand about 3 steps away from a wall and hold a laser pointer at arm's length. Hold the laser's dot steady on a specific spot on the wall. Pretty easy, right? Now, repeat the experiment but aim at the wall at the far end of a long hallway. I don't care how strong or coordinated you are, that dot will be wobbling all over the place with even the slightest movement of your arm. Just imagine how bad it would be when aiming at a target 200 meters away. There's no practical way you could risk using a weapon like that in combat. There's far too much risk of hitting unintentional targets. Plus, as your laser dot moves around, the energy it delivers is spread out instead of burning your target. Lasers are precise, but they're far too difficult to fire accurately by hand.

A bulky, shoulder-mounted laser is something completely different. When the operator kneels to fire it, their body forms a stable platform that minimizes movement. The unit itself has vibration dampeners and gyroscopes to keep the laser emitter still even when the operator moves slightly. It also has a computerized fire control system that will only allow the laser to energize when movement is minimal and the sensor optics are confident that the shot will hit the location under the operator's crosshairs (the machines used in laser eye surgery already do this).

All that technology is needed to make these lasers controllable, but there's no way you could cram all that in a hand-held form factor. Vibration dampening typically involves adding mass/counterweights and shock absorbers of some sort, which will necessarily take up more space than you could comfortably hold in your hand (consider the Steadicam mount for cameras, for example). Your shoulder-mounted unit is simply as small as you are able to get while maintaining a usable level of accuracy.

Note that traditional weapons don't have this problem because they fire discrete rounds, each gyroscopically stabilized to stay on course after leaving the weapon. Accuracy must only be maintained for as long as it takes to pull the trigger. A laser delivers energy over time, thus must maintain its aim throughout the entire duration of the laser pulse (which might be several seconds long).

• As opposed to missing with a gun, which is far more destructive and harder to aim. – John May 20 at 19:04
• @John A gun is much easier to aim since aim doesn't have to be maintained over time (milliseconds, as opposed to seconds for a laser). And this question implicitly assumes that the laser is more destructive than a gun, otherwise the whole thing is moot. – bta May 20 at 19:19
• then the risk of unintentional targets lower not higher. either it is harder to actually damage something or there is a high risk of collateral death it can't be both. – John May 21 at 3:56
• @John How well you aim determines the likelihood of collateral damage. The strength of the weapon determines the magnitude. Those are completely independent of each other. The goal here is to aim well enough that collateral damage (of any magnitude) doesn't happen in the first place. That's not reasonable with a hand-operated laser, but a large shoulder-mounted unit like OP mentioned would be quite capable of it. – bta May 21 at 23:41

## Control of Potential WMD:

The laser weapons are wonderful, but there's a catch. The lasers are reconfigurable to induce a nuclear reaction in relatively low-grade fissionable materials. The government doesn't advertise this fact, and it would require a fair amount of know-how. But keeping track of thousands of laser cannons is challenging already. Now give every potential soldier a laser weapon, and watch the weapons slip into the wrong hands.

The laser cannons are too useful to discard, but the risk gives military planners fits. Despite the huge potential for using laser rifles in battle, the powers-that-be simply can't run the risk of someone being a little too clever.

Some unobtainium is needed

Aliens have successfully been able to miniaturize their lasers because in their home-planet there is abundance of some very particular radioactive isotope. Maybe their homeworld is far younger than Earth, so that there is still enough concentration of that isotope in nature (while on Earth and other worlds controlled by humans it has already decayed to almost zero).

Without this isotope it is impossible to miniaturize the laser more than the size actually used by humans.
Maybe it is possible to synthetize it by enrichment (like U-235), but in this case it would be too expensive to produce in the quantity needed to equip with laser weapons the human infantry.

While the large laser tanks on the tanks... can produce and do a substantial amount of damage the tanks also offer the advantage of radiation shielding, allowing for it to be fired without doing substantial damage to the humans using the tanks.

However, when the laser weapons scale down, on both sides such radiation protection is no longer available. The aliens who fire the handheld laser weapons don't take an issue with this as they are far more resistant to ionizing radiation, as are the flora and fauna from the planet they originate which means while they don't mind the laser weapons and neither does their flora and fauna of which they've transplanted into the environment, when used on humans it causes a bunch of nasty side effects and further causes extreme ecological damage to both earth-based biology and the host of biology on the planets they're fighting on. This is understandable,a thing which humans in the UEN regard as a bad thing, while the aliens they're fighting against don't seem to care much. Or maybe the human doesn't care either but doesn't like their highly trained soldiers getting sick, and they're highly valued and expensive mech's getting damaged.

Go the the Wiki page for "Laser Weapons", and scroll down to "Problems".

The terms "Thermal Blooming", "Phased Array Issues" look promising. It also talks about size to power ratio as well as Guidence Issues.

I would BS it all with words like Thermal Blooming and problems with the Phase-conjugate mirror which at a micro level causes overheating issues that lead to a massive reliability factor. It also might not be a power issue, but a mirror issue. You can't minaturize the mirror yet that is needed to make the guidence system work. Or on a minute level, the primary amplifier simply can't generate enough power to focus laser down big targets. Humans sure, but a bla bla bla reinfoced tank with bla bl bla armor. Forget about it!

Or, it's not that you can't, but that it is unreliable and not worth the failure rate.

Maybe they do miniturize it, and most fail, until your story needs it not to fail at at the all is lost moment.

Maybe they can minaturize them down, but now it takes multiple lasers focused at just the right point to aplify the lower power output on a single target. (Could be a plot point)

• Welcome to WorldBuilding.SE! Can you explain these issues in more detail? "Look up XYZ on Wikipedia" isn't a very good answer because Wikipedia is ever-changing and there's no guarantee that the information will still be there when OP goes to look. – F1Krazy May 24 at 9:52