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[Sheb]

Sidearm IN SPACE!

[i2amroy]

Yes. Despite being massless, photons still have slight recoil. However, not enough to do anything unless the gun's made of extremely light materials.

Or you're firing longer bursts in a relatively frictionless environment (i.e. space).

And yeah, pulsed lasers are what you want in an atmosphere; the first pulse/s ionizes the air to get bloom out of the road, and the later (now invisible) pulse/s are the ones that actually do damage to the intended target.

It might also be worth mentioning here that (and I think I mentioned this in the futuristic weapons thread) the type of damage you are doing with a laser is thermal, not kinetic, meaning that in plenty of scenarios you could get by with a whole lot less force than would be needed to destroy something kinetically, or vice versa (needing more energy). A good example of a material that doesn't have similar strengths against thermal energy as it does to kinetic is aluminum; if you want to try to cut through aluminum with a laser you are going to need very large amounts of energy, because the aluminum both disperses heat quickly and has a very high reflectivity. However towards kinetic force aluminum is relatively weak compared to many other materials, and can be punched through with a very small amount of energy comparatively.

From this we can see that assuming that you need "an equal amount of energy" in your laser gun compared to your bullet is a bit of a red herring. In some cases you could get by with significantly less energy in a laser and still do more damage, while in others your laser would be virtually worthless until your energy levels were much higher than in a handgun.

[Shadowlord]

It does sound shiny.

[Culise]

At those energies, you start to bump heads with the restraints I mentioned on page one.  That's why I mentioned them. Basically, when you start emitting that much laser energy that you get bullet like recoil from the radiation pressure, your beam will self-refract and other nasty things. Basically, you cannot make a laser weapon that will do this.

Certainly not in a handheld form factor.

Here's a link to a pulp science article which provides both a link to the source research and give the derived hard limit on laser energy before spontaneous QED breakdown happens and the beam instantly becomes incoherent.

http://theastronomist.fieldofscience.com/2010/08/limits-on-lasers.html

According to the article, that hard limit is 10 to the 25 W/cm.

IIRC, the bloom limit in atmosphere is substantially lower, closer to 10MW. (there's a reason why LaWS is a 50KW pulsed laser!!)

In short, a laser pistol capable of producing recoil equal to a conventional pistol will cause the air in front of the gun to bloom with astounding energy, equal to a small explosion, and will probably cook/kill the soldier firing it. Only a tiny portion of the beam energy would make it to target.

For practical reasons, such a gun is not useful as a sidearm, and can be discounted from being reasonably considered.

Again, that was the point I was delicately trying to convey.

I'm fairly certain that is Ebbor's point as well, given his stated conclusion.  That is, he argued in a fairly straightforward manner that to get a recoil at all similar to some particular kinetic handgun chosen as an example, you need it to deliver a thoroughly impractical amount of power to the target, as well as a thoroughly unnecessary amount.  Specifically, in his example, this is a whole six orders of magnitude more - that is to say, recoil only becomes a problem in the case of a "thoroughly unreasonable weapon."  It would follow trivially by implication, therefore, is that to deliver an amount of power similar to that of the kinetic pistol, your recoil would similarly be lower by multiple orders of magnitude, and thus, to use his words, "recoil is not a problem." ^_^

[Stirk]

Interesting. I'm not sure why that would be.

I have no electrical background to even make a guess :/.

There is one thing to keep in mind with Laser weapons.

If you fire a bullet at someone, you essentially use the momentum of the bullet to locally destroy the tissue, while the overall energy of the bullet is not all that high and could only increase the temperature of the body part by some millikelvin.

In a Laser weapon however, the locally concentrated energy of the beam vaporizes a small part of the target, leading to a hopefully destructive explosion. The locally transferred energy is the important quantity, whereas the photon momentum is not.

And this is why a Laser weapon has no noticeable recoil.

While that would be a likely avenue for laser weapons, those currently in existence don't seem to work that way and do seem to put out enough energy that there could be recoil. In that video, while it did burn up, it didn't explode or vaporize to my knowledge. That is also a good point-the bullets *need* momentum to do their job while the lasers don't (directly). However, laser weapons would still need to use up a lot of energy, which would raise the momentum they need indirectly. Even if that energy is less than a thousand J, we are still looking at what could be a powerful impulse.

Its also important where the vaporization occurs... Say, in someones eyes:

https://en.wikipedia.org/wiki/ZM-87

Also that very likely is not the only infantry-portable product in use. Laser weapons have been in use for decades and no, even the heaviest dont have meaningful if even measurable recoil.

Boeing also has that cute anti-drone laser gun that fits in a trunk together with its tripod and batteries.

Great find! It looks more like a weaponized laser pointer more than anything . That is to say, there are actual laser pointers with that much power! At 15mW, we are looking at 0.015J/s. To meet the criteria of the .308 I have been using so far, it would have to fire 241133 seconds, or roughly 2 days and 19 hours. The power difference between this and the theoretical weapon are astronomical, I wouldn't expect something with that little energy and that much mass to have noticeable recoil, in the same way I wouldn't expect an 80lb .22 to experience noticeable recoil. But scaling down the weapon and scaling up the power could easily have a different result, correct?

Many awesome people have detailed the very good point about laser weaponry here (along with very good details on the answer). 'Recoil' as I understand it is the reverse force which occurs after launching a physical projectile--technically there 'wouldn't' be, because  considering what a rifle or pistol size to the body or position of firing, your tactile senses won't pick up on the force as something 'backwards' from the firing point.

But if you consider technicalities of a reverse force existing from the generation of a laser to create recoil: Technically nope. (Actually Yes, but in a 'people feeling it', technically nope see below)


~Edit~
To clarify: There's a threshold of 'feeling' thanks to this person, as in when you can feel a difference between objects or a force+object which provides the information between the 'lack of felt recoil' [ie Rifle/Pistol in hand or braced by the shoulder], and 'I can feel recoil'. When I said 'technically nope' up there, I'm assuming the energy being used by a laser rifle or pistol (though...I think I'm assuming a very different picture than most of what the thread says o_O {As in, I'm imagining those laser tag toys?}).
They do have recoil. It depends if it is felt by the amount of energy.

Recoil, as I understand it, is momentum mostly. This relates to force, in the same way a car crash does, but it isn't so much "reverse force" as it is "reverse momentum". While a laser weapon as described wouldn't have any push from a physical projectile, it would have "reverse momentum" from all the photons its shooting out. It is my argument that this momentum, at the low impulse of laser weaponry, would cause notable recoil.

That link gives us something to help classify "notable recoil" as, which will help if we can ever find out how many J it would put out .

I've already asked this in a separate thread (haven't checked earlier posts though) so here's what they said:

Yes. Despite being massless, photons still have slight recoil. However, not enough to do anything unless the gun's made of extremely light materials.

This is why I'm using a gattling laser for my armored transport (along with swiping chainblades).

Please go to the Theoretical Weapon's thread next time, thanks.

But I don't want to talk about Theoretical Weapons, or disrupt actual discussions on them. I just want to talk about one specific topic. While I appreciate the answer, I am still not particularly satisfied or convinced that it wouldn't be noticeable.

We can calculate recoil easily. Let's assume we want a laser gun with the same amount of recoil as a pistol. Picking a bullet at random, I find a weight of 8.04 grammes and a speed of 360 m/s.

The formula for the momentum of light is easy. P=e/c.

The result is 860 MJ of energy, 6 orders of magnitude more than that of the bullet. Suffice to say, recoil would not be a problem.

http://m.wolframalpha.com/input/?i=8.04g*%28360m%2Fs%29*speed+of+light&x=0&y=0

https://en.m.wikipedia.org/wiki/9×19mm_Parabellum

I actually had this math in the OP, before taking it down . The reasons I took it down:

1. While that is true, that is the momentum of a single photon.
2. While the momentum is low, recoil is essentially the force given off by impulse.

Impulse is F= delta p/delta t

Now lets say we use your numbers. How long does it put out this force? Lets say 30 nanoseconds (does that sound fair?)

860 MJ/30ns= 2.87*10^16 watts. That is a very large amount of recoil, if that math was correct.

The LAWS fire 50 kW in the infrared. That's 50000 J/s. Now, light does have momentum, because while its rest mass is zero, it does have energy so it has mass. E = mc², so m=E/c². Momentum (p) is mass time velocity, so p=mv = (E/c²)*c = E/c

So your LAWS laser create light with (5*10^4)/(3*10^8) kg*m/s of momentum per second. Since momentum is conserved, your LAWS' momentum (or the ship it is attached to) will have its momentum shifted in the other direction. Now, change of momentum over time is a force, so we can express it in Newton. When fired, your laser create recoil of 0.00016 Newton. That's force equivalent to the weight of 0,016 grams. (Which is a lot more than I expected).

So yeah, there is technically recoil. But I think it's more realistic to ignore it.

Edit: Kind of ninjaed by 10ebbor10.

Infrared! Thank you!

Now, as I said before, that doesn't take into account the impulse. In addition, that only is for a single photon. But now that I know the spectrum, I can (probably) find out the actual recoil!

**WARNING**
Dubious math ahead. Take with a grain of salt.

You see, while the combined energy is a ton, a single NIR proton has an energy of around 1.24eV. The energy of an infrared proton with that wavelength will always be the same.

1.24 eV =1.987*10^-19J

So the actual math for a single photon would be:

p=1.987*10^-19J/c=6.628*10^-28 N s

Which seems like it is a lot smaller. However, that is for a single photon still.

Bringing up the .308 again, to get the equivalent energy we would need 1.82*10^22 photons.

Now I am *pretty sure* we can't do this realistically, but if we multiple the momentum times the number of photons, we get .00001206 N s, which still seems small.

Does that seem valid so far? I'm pretty sure it doesn't. The LaWs weapon would take .07 seconds to produce that much energy (Which, if we put into an impulse equation, would give something like a 10^-4 N, but I am pretty sure we can't use that?)). I still have a question, what is the impulse time used for light in this situation?

We can calculate recoil easily. Let's assume we want a laser gun with the same amount of recoil as a pistol. Picking a bullet at random, I find a weight of 8.04 grammes and a speed of 360 m/s.

The formula for the momentum of light is easy. P=e/c.

The result is 860 MJ of energy, 6 orders of magnitude more than that of the bullet. Suffice to say, recoil would not be a problem.

http://m.wolframalpha.com/input/?i=8.04g*%28360m%2Fs%29*speed+of+light&x=0&y=0

https://en.m.wikipedia.org/wiki/9×19mm_Parabellum

At those energies, you start to bump heads with the restraints I mentioned on page one.  That's why I mentioned them. Basically, when you start emitting that much laser energy that you get bullet like recoil from the radiation pressure, your beam will self-refract and other nasty things. Basically, you cannot make a laser weapon that will do this.

Certainly not in a handheld form factor.

Here's a link to a pulp science article which provides both a link to the source research and give the derived hard limit on laser energy before spontaneous QED breakdown happens and the beam instantly becomes incoherent.

http://theastronomist.fieldofscience.com/2010/08/limits-on-lasers.html

According to the article, that hard limit is 10 to the 25 W/cm.

IIRC, the bloom limit in atmosphere is substantially lower, closer to 10MW. (there's a reason why LaWS is a 50KW pulsed laser!!)

In short, a laser pistol capable of producing recoil equal to a conventional pistol will cause the air in front of the gun to bloom with astounding energy, equal to a small explosion, and will probably cook/kill the soldier firing it. Only a tiny portion of the beam energy would make it to target.

For practical reasons, such a gun is not useful as a sidearm, and can be discounted from being reasonably considered.

Again, that was the point I was delicately trying to convey.

I agree with your point, however as I stated before I wish to disregard all non-recoil issues of laser weapons while focusing on a single issue. I am aware of blooming, refraction etc, and have been informed several times on this article why such a weapon would be impracticable. As I stated before, I am not attempting to design a practical laser weapon. I'm just trying to figure out if one existed, would it have recoil? It could be powered by magical fairy dust if it helps the problem. Or it could be shot in a void, making the light speed easier to calculate while removing the blooming/refraction thing, if you prefer a more practical solution in your thought experiments .

Yes. Despite being massless, photons still have slight recoil. However, not enough to do anything unless the gun's made of extremely light materials.

Or you're firing longer bursts in a relatively frictionless environment (i.e. space).

And yeah, pulsed lasers are what you want in an atmosphere; the first pulse/s ionizes the air to get bloom out of the road, and the later (now invisible) pulse/s are the ones that actually do damage to the intended target.

It might also be worth mentioning here that (and I think I mentioned this in the futuristic weapons thread) the type of damage you are doing with a laser is thermal, not kinetic, meaning that in plenty of scenarios you could get by with a whole lot less force than would be needed to destroy something kinetically, or vice versa (needing more energy). A good example of a material that doesn't have similar strengths against thermal energy as it does to kinetic is aluminum; if you want to try to cut through aluminum with a laser you are going to need very large amounts of energy, because the aluminum both disperses heat quickly and has a very high reflectivity. However towards kinetic force aluminum is relatively weak compared to many other materials, and can be punched through with a very small amount of energy comparatively.

From this we can see that assuming that you need "an equal amount of energy" in your laser gun compared to your bullet is a bit of a red herring. In some cases you could get by with significantly less energy in a laser and still do more damage, while in others your laser would be virtually worthless until your energy levels were much higher than in a handgun.

Someone mentioned that above too! I know it is a big issue with this thought experiment, but I couldn't really think of any other standard to use  . However, I believe that if I am incorrect and the laser weapons would not have any recoil (or that it would have almost no recoil), then it will not matter what (reasonable) amount of energy is used, as the recoil would always be insignificant. If I am correct and they would have significant recoil at the energy level of modern bullets, then they would still have *some* recoil even if they where only pumping out a few hundred joules. As such, I don't believe the current numbers invalidate any results we would get for them.

Folks, I think you're all missing the most important part of this thread:

I mean, it would be like "fixing" the recoil on the guns by having them throw enemies around like water from a sprinkler :X.

Why doesn't this Fallout mod exist yet? It shouldn't be too difficult; you'd need to attach a script to all firearms that would ragdoll and shove their wielder backwards upon firing, and also remove all melee weapons to force gun use.

Just imagine pitched battles between bandit clans, drifting like tumbleweeds across the wasteland...

.....Alright, that mod does sound awesome  . Get a minigun, stroll through the Commonwealth like a human tornado .

[wierd]

No. NO NO NO.

The 10 to the 25W/CM hard limit is the LIMIT IN VACUUM!!!

That is the limit at which the energy of the beam causes spacetime's electrodynamics to break down like an imploding building, and spontaneous pair formation happens.

There is no theory. YOU CANNOT MAKE A BEAM BIGGER THAN THAT. PERIOD.

The beam itself MAKES the stuff that scatters it!!!

I cant possibly make that any more clear! The weapon you are proposing simply cannot exist!

[Stirk]

No. NO NO NO.

The 10 to the 25W/CM hard limit is the LIMIT IN VACUUM!!!

That is the limit at which the energy of the beam causes spacetime's electrodynamics to break down like an imploding building, and spontaneous pair formation happens.

There is no theory. YOU CANNOT MAKE A BEAM BIGGER THAN THAT. PERIOD.

The beam itself MAKES the stuff that scatters it!!!

I cant possibly make that any more clear! The weapon you are proposing simply cannot exist!

Apparently I am the one who needs to make something more clear. I am not trying to make a realistic weapon. I am not "proposing" a weapon design. I know something like this would/could/should never exist. Again, this is why I am ignoring all the problems that could and would appear from this weapon existing. In addition, the "Vacuum" comment was more to atmospheric blooming (which seemed to be his main point) than anything.

But anyway, 10^25 W/cm is much, much more then what would be needed here, isn't it? Unless I am reading the units wrong. A Watt is just a J/s. That is 10^25 J/s/cm^2, and would be able to impact the energy of a .308 in 3.16*10^-22 seconds. Even if a cm^2 is far to big, a small, small fraction would still be easily capable of getting the energy comparable to a firearm. In addition, this thought experiment doesn't require the beam to be a certain size, we could simply make the beam as big as it would need to (the number seems to be more about beam density, correct?). Again, I am not trying to be particularly realistic here. But I really don't see how such far off limits are an issue here (...unless I am misunderstanding, in which case I apologize.)

[NullForceOmega]

not 10^25, 5x10^25, we are talking 50 septillonas W/cm^2  An amount of energy that no one is going to need for weapons grade lasers.

[ToastGoats]

I never expected a DF thread to blow my mind.

[wierd]

I dont understand what you are wanting, apparently, or you have moved the goal post.

1) The "impact energy" of the laser is different from the recoil.  The recoil happens ONLY from radiation pressure. How far it knocks back the target from the thermal explosion on contact is not a direct correlation.  The LaWS laser is 50kw. It has negligable recoil. You would not feel it. The energy delivered to target is converted to thermal energy, which creates a small explosion of high temp gas. That gas has mass, and gives the target a big push. Functional analogy-- rubber band gun shoots a stick of dynamite. Dynamite explodes at target. The recoil of the rubber band gun is significantly less than the kinetic energy delivered to target by the explosion.

2) you explicitly stated that you were discussing this radiation pressure when discussing recoil. (dont make me quote you.) At the energies required to make that happen, the gun would kill the operator from the flash bloom. A hand weapon capable of delivering similar energy as a handgun would not recoil in any meaningful way. (orders of magnitude lower.)

What am I missing here?!

[Sensei]

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.

[wierd]

Thank you.

[Stirk]

not 10^25, 5x10^25, we are talking 50 septillonas W/cm^2  An amount of energy that no one is going to need for weapons grade lasers.

The link suggested it was ~10^25 to ~10^29. In any case, even the 10MW wierd pointed out was the maximum for the atmosphere would be 1*10^7 J/s/cm^2, which would be enough for the necessary thought experiment.

I dont understand what you are wanting, apparently, or you have moved the goal post.

1) The "impact energy" of the laser is different from the recoil.  The recoil happens ONLY from radiation pressure. How far it knocks back the target from the thermal explosion on contact is not a direct correlation.  The LaWS laser is 50kw. It has negligable recoil. You would not feel it. The energy delivered to target is converted to thermal energy, which creates a small explosion of high temp gas. That gas has mass, and gives the target a big push. Functional analogy-- rubber band gun shoots a stick of dynamite. Dynamite explodes at target. The recoil of the rubber band gun is significantly less than the kinetic energy delivered to target by the explosion.

2) you explicitly stated that you were discussing this radiation pressure when discussing recoil. (dont make me quote you.) At the energies required to make that happen, the gun would kill the operator from the flash bloom. A hand weapon capable of delivering similar energy as a handgun would not recoil in any meaningful way. (orders of magnitude lower.)

What am I missing here?!

Moving the goalpost? I'm asking for more of a discussion than a debate . I am sorry if I am still unclear, I will repost my purpose for asking this question that I typed up earlier.

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?

I hope that helped clear things up! Sorry if my lack of clarity made you angry  .

Back on topic:

1) I acknowledged that, and showed this in my math above. The momentum (or as you say, radiation pressure) is related directly to the amount of energy of the photons, and (I think?) how many photons there are. As I said before, however, I don't have any real amount of Joules to compare it to. I do not know how much energy it would take to do an equivalent amount of damage. But, as I had just recently argued, that is not a huge problem. If you are correct, and the recoil would be very negligible, even multiplying it by a thousand times should still keep it low (with the numbers I have been using, a 3J laser would be around a thousandth of the energy). If I am correct and there is noticeable recoil on something with this high energy, we can then scale it back to find the exact point where there wouldn't be. While the LaWS is about the closest thing to think about right now, I can't find anything on its recoil .

2) Indeed I did! It was, of course only yesterday! Impulse and momentum are what cause recoil, in light or lead. And as for the second part, isn't that what we are discussing? You keep saying that is how it works, I keep saying that it might not be. I simply wish I had enough information on the subject to get pure numbers down, which would *hopefully* explain things more clearly.

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.

Nice numbers! Having the necessary energy for lethal lasers is something very useful to the current discussion.

I had actually come to some of the same math in my last megapost, though I still have some questions about it.

F = E/C

, shouldn't that be p=E/C? Or am I thinking of a different formula? If it is the same formula I am thinking of, wouldn't that be the momentum of a single photon, rather than the entire beam? I tried tackling this up there^, and will repost if I am on the right track.

The second issue:

Over 1/20 of a second,

I think this is the part that really confused me (and makes it so I can't complete the impulse equation properly ), wouldn't this be the amount of time that the laser stays on its target, rather than the gun? That is to say, while the entire beam is being spat out for 1/20 seconds, each photon is only imparting momentum/radiation pressure on the weapon for a very small fraction of that. It is a smaller deal with F=E/C then it is when I try to calculate impulse, if you where right it wouldn't be a problem. Also, why did you multiply your result by 20, if it was only done over 1/20 of a second? Where you trying to use the F=delta (F?)/delta t, the impulse reaction? If you where using impulse, we again go back to the same problems as before, in that the equation above (p or F=E/C) seem to be for a single photon, while I am still not sure if that is the correct time to put on the impulse.

[Sensei]

It not for a single photon! It is for the energy (total) of all photons. The equation is meant for black bodies emitting radiation (stars) with a large surface area, and it is P (pressure) = E_f (energy per square meter "energy flux") / C. However instead of calculation pressure (force per area) caused by E_f (energy per area) we can divide the area out of both sides, giving force per energy instead of pressure per energy flux.

I could probably even write a full unit conversion if I were on my computer, but that's the gist of it. If it lends any credence, I have worked with these equations in college courses, I'm not completely winging it.

[Stirk]

It not for a single photon! It is for the energy (total) of all photons. The equation is meant for black bodies emitting radiation (stars) with a large surface area, and it is P (pressure) = E_f (energy per square meter "energy flux") / C. However instead of calculation pressure (force per area) caused by E_f (energy per area) we can divide the area out of both sides, giving force per energy instead of pressure per energy flux.

I could probably even write a full unit conversion if I were on my computer, but that's the gist of it. If it lends any credence, I have worked with these equations in college courses, I'm not completely winging it.

Sorry if I sounded like I was accusing you, I was working with remarkably similar equations earlier  . Also, I haven't worked much with this, so your word is worth more then mine on the subject . Pulling out my physics textbook, I do have the same equation for p=I/c where I=Power/Area=Energy/Time/Area, assuming total absorption.

Alright, I am pretty much convinced. That was fun  . The only question I have is how did you go from 0.000017J to 0.00034J? I mean, it looks like you multiplied it by twenty, I just can't figure out why.


**EDIT**

Aaand one last question! Do you mind if I put your answer in the OP? While we had many wonderful answers, I really appreciate how you put numbers to everything!

Thanks you to everyone who answered! It was really interesting hearing everyone's ideas on the subject, a fun and educational experience.