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[Stirk]

Hai! Stirk here! Most of you probably don't know me, I mostly stick around the FG&RPG. A few of you may have played that old mod I made.

Anyway, I once came across a debate and am now unreasonably bothered by it. That debate would, of course, be the question question in the title: Would laser weapons have recoil?

I say, "Yes!" heartily, but when investigating the topic I found it quite frankly stunning and baffling how many people think this answer is "No!" for some reason. So this thread will be a place for me to either be proven wrong and joining the chorus of No! Or proven right, spreading Truth and Justice throughout the world as a HERO FOR SCIENCE!

~~Opening Argument~~

The main cause of recoil is momentum. The Wikipedia article goes over this in lots more detail! Light, despite having no mass, still has momentum. For a laser weapon to do the equivalent damage of a modern kinetic firearm, it would still need significant energy.
Its been a while since I took physics class, and I am currently hurrying because this has been *super* distracting me from two tests I have tomorrow, so take this with a grain of salt . What are your thoughts on the issue?

***The Answer***

Well, we have come to the point I am satisfied with the Answer! Thanks to everyone who answered, it was fun and informative! The following is Sensei's arguments, where he puts numbers to everything.

I think we've answered the question of whether a laser gun would have recoil that would affect aim. In case anyone's still doubting though, perhaps we should calculate the actual recoil for comparison with guns?

Edit: Warning, typed on phone, uncorrected selling mistakes ahead!

First we need to estimate the wattage of the laser.

Let's assume we want our laser to be a effective as a firearm- so it should be able to inflict a lethal wound in, oh, 1/20 of a second. This means we might need a lot more wattage compared to the LaWS, which takes seconds to kill, but on the other hand we'll assume we are aiming at flesh, not metal.

To create the smoking hole we see in movies, I think we want to sublimate (turn instantly to gas) muscle. The specific heat of muscle is about 3600 j/kg/c. I couldn't find a good number for how hot muscle needs to be to turn to vapor (now why could that be?)  But let's say 250 C. This might be a low estimate, but I touched a baking tray that hot once and the skin there was gone, and I'm sure it didn't actually reach 250C itself. Now, the area of, oh, a laser pointer is (guessing again) about 0.2 square centimeters. Multiply that by a 30cm thick person and we get 6 cubic centimeters of muscle. At a specific mass of 1.06kg/L this is about 0.006kg. To heat that much muscle from 37C to 250C (so by 223C) would take 223*3600*0.006 = 4816 joules. Let's just call it 5kJ.

To deliver that in 1/20 of a second would require 100kW. This is bad news and good news. It falls in between the LaWS and that one laser that took up a whole jet for shooting down missiles. On the upside, rayleigh scattering should make the beam visible like in fallout.

Oh one other thing, that's energy delivered to the target. Solid state lasers are about 25% electrically efficient, so we're probably looking at 400kW of electrical power. Let's assume it can only fire once a second while capacitors recharge, and our sci-fi power source only needs to generate 20kW, the equivalent of a modern diesel generator about the size of a fridge. Oh, also the capacitor bank would be the size of another fridge or two, and the laser itself would be the size of a fourth fridge. But let's assume that our laser is close to 100% efficient (or you'd be wasting 15kJ as heat every time you shoot, heating a 3kg steel gun to 100C after ten shots) and your Micro Fusion Reactor is capable of putting out the 100kW it would still need while fitting in a handheld gun, the same amount of power as a 2015 Fiat 500.

So, 100kW for 1/20 of a second, to make a laser pulse that is similar in lethality to a gunshot, or more if the target is armored.

So how much force does that 100kW laser make, in newtons?
I think we can treat our laser gun as a black body emitter. From the wiki article on radiation pressure, we can cancel out some stuff about area (because it's pressure) and we F = E/C. Our energy was 5kJ, and C is 3x10⁸. The physical energy transferred back on the gun (and the shooter's shoulder) is 0.000017J. Over 1/20 of a second, this is 0.00034 Newtons.

Is that a lot?

No. A winchester .308 rifle weighing 4kg pushes back at 2.2m/s (I don't know the exact acceleration/force because I don't know the duration for which gas leaves the barrel, but if it was fired while dangling on a rope it would swing back at that speed). Our laser gun with the same mass would be accelerated to 0.0000042 m/s. So, it's about two ten-millionths of the recoil of a .308 rifle.

Over a 10cm² rifle butt, this pressure would be literally imperceptible. Just how small is this force? A common number 12 sewing needle has 0.074mm of diameter at its tip, or 4.07x10^-9 square meters of area. If the thr gun was fired while hanging from a rope, and it's butt was replaced with that sewing needle, it would have a force of 70000 pascals, or 70kPa. This is about 10PSI.

This means that you you pressed a needle against yourself by firing the laser gun, you could just feel it (unless the gun's inertia prevented even that from happening in the 1/20 second firing time), but it wouldn't draw blood. Don't try that trick with the winchester .308 rifle!

In summary:
I hope thus has helped put the radiation pressure from a hogh-powered laser in real terms. It exists, but only barely, and you would need delicate instruments to observe it. It would be so gentle, it could press a sharp needle against your bare skin without hurting you. It would absolutely, definitely, not affect your aim, even if the laser were more powerful than the very biggest lasers that have currently even been built or planned. It wouldn't affect t your sim even if my math was wrong and the laser needs to be 1000 times more powerful.

[mainiac]

I'm confused by the units or lack thereof.  How much light energy or how many photons are you talking about shooting?

[wierd]

Photons are massless. While they have energy, and have light pressure forces, they do not convey kinetic inertia the way a physical projectile does.

As a consequence, the only recoil the device will have is from light pressure and blackbody emission. Not even enough to feel, even with a really huge weapon. (maybe a starship weapon would have some alteration of vehicle vector of motion from fire, but not a big one.)

The big problem with laser weapons is atmospheric blooming. When the energy of the laser reaches a certain threshold, it causes the air it passes through to become an opaque plasma which glows brightly. The "bloom".  When that happens, laser light no longer passes through the column of plasma, and the laser energy never reaches the target.  That is why pulsed laser weapons exist-- they allow a pulse of laser energy to go through the air, for the air to ionize, then disperse-- before the next pulse.

The second problem is energy requirements for laser light generation. While this has improved tremendously over the past 2 decades with the improvements in semiconductor laser emitters, this is still a very inefficient form of light production. Producing a weapons grade laser in a compact form factor with a long battery life is like asking for a floating castle in the sky.

[Stirk]

I'm confused by the units or lack thereof.  How much light energy or how many photons are you talking about shooting?

Yes, I just noticed that issue (single photon thing  ). The math was a complete mess looking back at it, and now that I think about it is more confusing than anything. I'll run through it *right* later. It was mostly wrong anyway, I did some very stupid stuff with it :p. I'm really, really sorry for that, honest  , this is what happens when you rush things folks.

Photons are massless. While they have energy, and have light pressure forces, they do not convey kinetic inertia the way a physical projectile does.

As a consequence, the only recoil the device will have is from light pressure and blackbody emission. Not even enough to feel, even with a really huge weapon. (maybe a starship weapon would have some alteration of vehicle vector of motion from fire, but not a big one.)

The big problem with laser weapons is atmospheric blooming. When the energy of the laser reaches a certain threshold, it causes the air it passes through to become an opaque plasma which glows brightly. The "bloom".  When that happens, laser light no longer passes through the column of plasma, and the laser energy never reaches the target.  That is why pulsed laser weapons exist-- they allow a pulse of laser energy to go through the air, for the air to ionize, then disperse-- before the next pulse.

The second problem is energy requirements for laser light generation. While this has improved tremendously over the past 2 decades with the improvements in semiconductor laser emitters, this is still a very inefficient form of light production. Producing a weapons grade laser in a compact form factor with a long battery life is like asking for a floating castle in the sky.

While photons are mass less, they do have momentum. As they do have momentum, the momentum needs to be conserved under newtons law, in pf+pp=0. Do you disagree? Momentum is what causes recoil, as discussed in the Recoil article on Wikipedia in great detail. Having mass, or not having mass, doesn't matter as far as the momentum is concerned.

And I do realize that laser weapons are unrealistic and impractical-I am very much a kinetic weapon fanboy- I merely wanted to discuss a single topic of if they would have recoil or not.

[wierd]

Photons having momentum is conserved through a feature/phenomenon known as "light pressure."

https://en.wikipedia.org/wiki/Radiation_pressure

Which I mentioned. It will be so small that you cannot feel it.

[Amperzand]

Laser weapons are quite practical, and DARPA is using them in various places. Not infantry lasers, they're for battleships and fortresses, but hey.

They do, technically, have recoil, AFAIK. It's been explained to me a few times, and I can rack my brains for the physics if you want, but photons do give the emitter an impulse. It's just a really, really, really small one, since the things have almost no kinetic energy, and were not accelerated.

[Morrigi]

Yeah, lasers have recoil but it's negligible for nearly all practical purposes, including handheld weapons.

[Amperzand]

Especially handheld weapons, since you couldn't feasibly carry enough batteries to run that powerful of a laser.

[Stirk]

Photons having momentum is conserved through a feature/phenomenon known as "light pressure."

https://en.wikipedia.org/wiki/Radiation_pressure

Which I mentioned. It will be so small that you cannot feel it.

Yes, I have read about radiation pressure and am talking about it. These are under normal circumstances, most light waves don't hit with enough force to do what a bullet can do. As shown in your link, higher energy photons have momentum, in E=pc. The speed of light doesn't change, so the momentum has to. Once that energy gets into the range of modern firearms, the momentum can become (comparatively) significant. While it would still appear small on its own, the force it causes (F= delta p/ delta t) would grow significantly from even small increases in momentum.

Laser weapons are quite practical, and DARPA is using them in various places. Not infantry lasers, they're for battleships and fortresses, but hey.

They do, technically, have recoil, AFAIK. It's been explained to me a few times, and I can rack my brains for the physics if you want, but photons do give the emitter an impulse. It's just a really, really, really small one, since the things have almost no kinetic energy, and were not accelerated.

I didn't mean to insult laser defenses  . For our purposes, the "Laser Weapons" only refer to "Laser Rifles and Pistols", as the title states. Saying both is simply too long :0.

Do you know what the energy is of those lasers? I'm mostly trying to compare it to modern infantry weaponry.

[wierd]

There are practical considerations to take into mind when considering what you are proposing.

1) There are only two ways to increase the energy of the laser beam. A) Increase photon density per unit of spacetime. B) Increase photon frequency.

2) If you increase photon density, you will run into some experimentally derived limits, even in vacuum. The function does not extend into infinity. Essentially, the energy represented by the photons will be of sufficient quanta that spontaneous production of electron-positron pairs becomes a significant factor, and those pairs then scatter the beam energy through absorption and re-emission.

3) If you increase photon frequency, you end up with hard gamma rays. Those interact with matter only weakly-- While devistating, for sure-- since they cannot be blocked easily-- due to the afore mentioned weak interaction-- the vast majority of the energy you release from your gun will go right out the back of your target without interacting with them in any meaningful way.

When you plot all of these factors out, you end up with a local maxima on just how much energy a laser beam can realistically impart to a target.

[Amperzand]

Wikipedia says the LaWS prototype has an intended energy between 20 and 50 kilowatts, and tested around 33kW. The power can be varied widely if they want it less powerful, and other test weapons can apparently reach 100kw.

Non-weaponized laser arrays, such as those used for fusion testing, can apparently reach the terawatt range with many impractically high energy sub-beams.

[Amperzand]

Well, hope the target's denser. That's all lead is, a denser arrangement of atoms that are more likely to get in the way.

[Stirk]

There are practical considerations to take into mind when considering what you are proposing.

1) There are only two ways to increase the energy of the laser beam. A) Increase photon density per unit of spacetime. B) Increase photon frequency.

2) If you increase photon density, you will run into some experimentally derived limits, even in vacuum. The function does not extend into infinity. Essentially, the energy represented by the photons will be of sufficient quanta that spontaneous production of electron-positron pairs becomes a significant factor, and those pairs then scatter the beam energy through absorption and re-emission.

3) If you increase photon frequency, you end up with hard gamma rays. Those interact with matter only weakly-- While devistating, for sure-- since they cannot be blocked easily-- due to the afore mentioned weak interaction-- the vast majority of the energy you release from your gun will go right out the back of your target without interacting with them in any meaningful way.

When you plot all of these factors out, you end up with a local maxima on just how much energy a laser beam can realistically impart to a target.

That is a very good point, again pointing to the impracticality of laser weapons. However, the weapon described above^ has 50kW. That is 50,000 J/s, by my count, making the energy necessary fairly plausible (it would give the equivalent energy to the .308 in .07 seconds). The energy necessary for this isn't implausibly high for light to impart.

Unfortunately, I can't find anything on the LaWS's recoil, and should *really really* get back to studying relevant things :p.

***EDIT***

Yeah, I get distracted too easily  . What wavelength of light does the LaWS use?

[Shadowlord]

Bear in mind that the navy's LaWS isn't firing a single burst, but turning on for a duration and then turning off once the target is no longer a threat.

I don't know how you'd even make a handheld laser rifle - we don't have either sufficiently dense power storage, or dense enough and powerful enough power generators, do we? Maybe you should investigate that first, to find out what the maximum power output of a handheld laser rifle could be with current technology. (Bear in mind that power storage and generation is not subject to moore's law)

[Stirk]

I guess I should clarify something-I'm not trying to design a feasible handheld laser weapon or anything like that . The scenario goes something like this:

Spoiler (click to show/hide)

I was discussing Fallout 4 when someone brought up a mod that was supposed to make it more "realistic" by removing laser weapon's recoil. On top of seeming to declare that moving parts where the actual cause of recoil (?) in weapons, it argued that they would "realistically" have no recoil. Seeing as how the mod has 10k downloads and a connected video has 900k, I got annoyed and started researching whether this was true.

So in short, I guess what I am really saying is "What is more realistic: A laser rifle/pistol that has recoil, or one without it?". This is mostly why I am ignoring the other problems with laser weaponry (such as energy requirements, atmospheric blooming, etc). For the sake of the problem, we can assume that the weapon has the same mass and muzzle energy as a conventional weapon, if necessary, as recoil is the only factor I really wanted to discuss. Basically, if the fictional weapon had existed, would it realistically have recoil?

Bear in mind that the navy's LaWS isn't firing a single burst, but turning on for a duration and then turning off once the target is no longer a threat.

I don't know how you'd even make a handheld laser rifle - we don't have either sufficiently dense power storage, or dense enough and powerful enough power generators, do we? Maybe you should investigate that first, to find out what the maximum power output of a handheld laser rifle could be with current technology. (Bear in mind that power storage and generation is not subject to moore's law)

Interesting-how long is the burst? The Wikipedia page suggests that it is at least several seconds long, which would be enough to be the equivalent of the example .308 several times over (by Stirk's math).