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I was bumming around Reddit the other day when I came across a discussion talking about how laser guns in Warhammer 40,000 have appreciable recoil, enough that a laser sniper rifle was described as having a kick strong enough to leave a bruise on ones' shoulder. Now I know 40k plays fast and loose with the laws of physics even compared to most science-fiction series, but this seemed incredulous even to me. It was my understanding that laser weapons would have no recoil since photons do not have appreciable mass, only momentum, or the amount of force they would apply to the firer would be absolutely miniscule. I've also not heard much about real life lasers ever having significant recoil.

What's more surprising is that nobody in the actual threads seemed to be able to figure out whether laser weapons would have appreciable recoil. In both the discussion and another linked blogpost, people propose all sorts of explanations saying "yes it would have recoil", "no it would not have recoil", or "yes but the recoil would be slight", and it's hard to tell which are accurate to real life physics. The links are posted below if anyone is interested in the actual explanations given.

https://www.reddit.com/r/40kLore/comments/kc3ddl/why_do_lasguns_have_recoil/ http://thevirtuosi.blogspot.com/2010/04/today-id-like-to-approach-question-near.html?m=1

So my question here is would a laser weapon have significant recoil or not? And by this I mean an actual laser weapon, not a particle gun dressed up as a laser or misidentified as one like Star Wars blasters.

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    $\begingroup$ The recoil is due to the conservation of momentum. The momentum imparted on the projectiles (bullets, cannon balls, photons, doesn't matter) must equal the opposite momentum imparted on the gun. So, yes, a laser cannon will have some amount of recoil, which can be easily computed. But it's not worth mentionining it, unless we are speaking of a gun of huuuuuge power; so huuuuuge that it would make any recoil problems completely irrelevant. $\endgroup$
    – AlexP
    Dec 13 '20 at 21:11
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    $\begingroup$ I'll let someone else put the details into an answer, but if a ground-based laser beam can push a space ship (e.g. this Q), and if we assume Newton's third law is still valid, then yes, push enough photons out of your rifle and it'll knock you flat on your butt. If this isn't true, either Newton's 3rd law somehow doesn't apply or lasers can never be used to push space ships. $\endgroup$ Dec 13 '20 at 21:58
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    $\begingroup$ @JBH: Spacecraft can be pushed by ridiculously small forces. For example, the on-board ion thrusters of the Starlink satellites produce a maximum thrust of about 20 millinewtons. That's good enough to allow the satellites to raise their orbits from the parking orbit where they are delivered to the operational orbit (takes them about a month to accumulate a delta-v of about 58 meters/second), to keep station on the operational orbit, and to deorbit at the end of life. Even a tiny acceleration can have spectacular results if kept on for a long time. $\endgroup$
    – AlexP
    Dec 13 '20 at 22:50
  • $\begingroup$ @AlexP Satellites... Getting from Earth to Mars in a year is about 5,000 m/s. At 20 mN it would take about 7 years orbiting the earth to get to that velocity, and I'm pretty sure they can't enter Mars orbit with that little thrust. And it doesn't really change my point. If you can use light to produce thrust at all, then unless the 3rd law doesn't apply, it's only a matter of scale to produce recoil. $\endgroup$ Dec 13 '20 at 23:34
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    $\begingroup$ @vsz: And where did I say otherwise? I was speaking about the recoil, which can be easily computed from the momentum of the photons emitted by the laser. I said nothing whatsoever about the effects of the laser upon the target. $\endgroup$
    – AlexP
    Dec 14 '20 at 12:45
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No, while there would be recoil, it's not enough to be perceptible. For example, according to https://en.wikipedia.org/wiki/Radiation_pressure#Solar_radiation_pressure, the radiation pressure of the Sun over a square meter is 10 μN. So, a laser with 100,000 the power of the sun over a square meter beam or 1,000,000,000 times the power of the sun over a square centimeter beam would only generate 1 N of force, which is about the same force as holding 1 apple in your hand.

[EDIT] AlexP brings up good additional information in a comment. The energy of the Sun over 1 square meter is ~1,360 Watts. Plugging those into our hypothetical 1,000,000,000x power beam above, that's 136 MW. One megawatt is one megajoule per second and a megajoule is roughly equivalent to energy of a stick of dynamite. Therefore, keeping this hypothetical beam on for one second delivers 136 sticks of dynamite to the target every second for a peak recoil force of 1 N. That's far more powerful than a fictional "laser sniper rifle" would be.

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    $\begingroup$ One billion times the power of the Sun over 1 square centimeter is about 136 MW. (Which is useful to know, to get an idea about the power requirements, and on the effects.) Lasers with impulse powers on the order of 1 PW (7 orders of magnitude higher) do exist in real life. Fortunately, the impulses are very very short... $\endgroup$
    – AlexP
    Dec 13 '20 at 21:31
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    $\begingroup$ Hmmm... light pressure is negligible, but if it's a chemical laser then what about exhaust products from the reaction that generated the energy? I assume they would have to be vented in some direction, and "towards the enemy" is probably safest. (Acknowledge that real life "recoilless" anti-armour weapons have a danger area to the rear from the back blast.) $\endgroup$ Dec 13 '20 at 21:40
  • $\begingroup$ You describe in terms of the law of conservation of energy. But there is also the law of conservation of momentum. It would be nice to add about momentum. Just from Wiki: In technical terms, the recoil is a result of conservation of momentum. $\endgroup$
    – ADS
    Dec 14 '20 at 8:44
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    $\begingroup$ Could you please add data related firearms from (Wiki)[en.wikipedia.org/wiki/Recoil#Perception_of_recoil]? It would allow direct compare in terms of force and recoil energy. $\endgroup$
    – ADS
    Dec 14 '20 at 9:13
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    $\begingroup$ This hypothetical weapon would deliver such explosive amount of energy to the end of it's barrel and not the target. Assuming there is an atmosphere then after a brief moment the laser would turn the air in front of the barrel into an opaque plasma which absorbs most of the energy from the shot. I would assume, that 136 sticks of dynamite energy exploding at the end of your gun would result in some felt recoil :) The standard way to resolve this (other than multiple emitters) is to use a much shorter laser pulse that delivers the energy before plasma can form but would have sharper recoil. $\endgroup$
    – SilentAxe
    Dec 14 '20 at 10:44
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A laser would not have recoil enough to affect your aim. Depending on the needs of your story, there may be various secondary causes for recoil. Perhaps "imparted motion" would be a more accurate name.

  • The laser projector has spaces inside that are open to the air. As the laser passes through the air heats, expands, and wants to evacuate through whatever holes are available. Any moisture/humidity in the air will gasify and expand, essentially creating a steam jet. If the only opening is the "muzzle" then that's where the hot+expanded air will go, providing a force backward from the beam. Possibly a surprise for a protagonise who just crawled out of a river or rainstorm and now has water inside their weapon.

  • The laser has some kind of cooling system, perhaps liquid-based which means a pump to cycle the fluid through a radiator and back to the gun to cool it again. Its possible the pump has a rotational "kick" to it at startup in opposition to the direction it turns. A soft-start in the pump's motor would mitigate this.
    You could also have inertia in the flowing liquid or fans, which provide a precession effect when trying to turn the gun. Not quite recoil, but it could be a perceptible effect.

  • If you require some funky consumable, eg the Primary Beam being an overloaded ray-projector in Lensman and needing to be replaced like a camera flash bulb, then the moving mechanism to eject and load a new one can have recoil. This also lets you use tropes like being "out of ammo" more than "flat battery"

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    $\begingroup$ well... lasers, being the energy guzzlers they are, may well need batteries as ammo $\endgroup$
    – somebody
    Dec 14 '20 at 9:21
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    $\begingroup$ One more: the necessary amount of sudden, focused energy can only be created as a result of a small explosion. Think: real-life EMP devices (EPFCG). $\endgroup$
    – Vilx-
    Dec 14 '20 at 10:45
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    $\begingroup$ I also like the idea of recoil being a deliberate User Experience choice, because consumers didn't feel their weapon was powerful enough or had enough feedback when firing, so they kept the projectile-weapon experience, similar to how people now are making cars with fake engine noises $\endgroup$ Dec 15 '20 at 17:31
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    $\begingroup$ @SimonFraser that deserves to be an answer by itself. Upvotes in comments are irrelevant, only answers matter. $\endgroup$
    – Criggie
    Dec 15 '20 at 18:49
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It is a explosive pumped laser and the hot combustion products are vented mostly in the direction of fire after the shot. Some attempt is made to balance the venting and limit the perceived recoil by using differing variations of muzzle brakes, but some still remains.

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Recoil: YES. Measureable? NO

Thrust/recoil wise, your laser weapon is just a photon rocket.

There is a very simple rule for a photon rocket: 300 MW/N

This mean that a 300 Megawatt light photon weapon will impart a recoil of 1 Newton (3.6 ounces)

And that applies only while it is continuously firing.

How strong is 300MW continuous beam? An Industrial laser cutter runs at about 5000w, to cut through 1.3cm Stainless Steel. Your laser rifle is already 60 000 times as strong as that. And for that, it has 3.6 ounces of recoil.

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  • $\begingroup$ One should take into the account that only reason laser cutter works is because it is MUCH MUCH closer to it's target than the enemy would be to your rifle. And unless the fight was in hard vacuum, there would be very big losses in atmosphere. Also 300 MW continuous beam would need some pretty serious cooling systems, which might very well produce serious forces. $\endgroup$ Dec 14 '20 at 22:32
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Moving this to an answer as suggested by Criggie

One reason a laser weapon might have recoil is for the User Experience - capitalised, because it's a design choice.

We live in a world where cars have carefully engineered engine noises, vacuum cleaners are noisier than they need to be because people view the quiet ones as less effective, and people choose tactile user interfaces that give real feedback.

It's not a stretch to imagine that for people coming from projectile weapons, it doesn't feel right firing something with no recoil and no visible effect. If physical feedback is engineered in, it will make people think it's more effective - and so sells more.

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