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  #1  
Old 04-29-2007, 10:04 PM
Ranchoth Ranchoth is offline
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Would a laser pistol have noticable recoil?

Okay, for the purposes of the question, say I have a laser pistol. It's the general size and weight of an M1911 Colt .45. (It operates at a wavelength of...eh, 475 nm.) Now, say, the beam is *roughly* powerful enough to equal a the damage caused by a .45 ACP (so, I mean, I can't shoot down satellites with it, but I don't have to hold the beam on target for two minutes to burn through a phone book. You know what I mean.).

Now, my question is...if I pick up and fire my laser gun, do I feel any noticable recoil?

(If any further technical details are needed to make the question answerable, I can probably pull them out ofI mean, figure them out, as needed.)

So...anyone?
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  #2  
Old 04-29-2007, 10:07 PM
drachillix drachillix is offline
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Originally Posted by Ranchoth
Okay, for the purposes of the question, say I have a laser pistol. It's the general size and weight of an M1911 Colt .45. (It operates at a wavelength of...eh, 475 nm.) Now, say, the beam is *roughly* powerful enough to equal a the damage caused by a .45 ACP (so, I mean, I can't shoot down satellites with it, but I don't have to hold the beam on target for two minutes to burn through a phone book. You know what I mean.).

Now, my question is...if I pick up and fire my laser gun, do I feel any noticable recoil?

(If any further technical details are needed to make the question answerable, I can probably pull them out ofI mean, figure them out, as needed.)

So...anyone?
In a nutshell no, a lasers damage is caused by heat, not kinetic energy.
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Old 04-29-2007, 11:16 PM
MikeS MikeS is offline
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Well, let's see.
  1. The muzzle velocity for a 200-grain bullet (about 13 grams) coming out of a .45 ACP is approximately 300 m/s.
  2. Therefore, the momentum imparted to the bullet, and which will therefore be imparted to whatever's holding the gun as "recoil" is about 3.9 kg m/s.
  3. The kinetic energy of a bullet coming out of a .45 ACP is, according to the link you gave, approximately 500 J.
  4. If we assume that our laser pistol imparts this same amount of energy to its beam, and we recall that the momentum of a photon is related to its energy by E = p c, where c is the speed of light, the total momentum of the beam will be about 1.7 x 10-6 kg m/s.
So under these assumptions, the recoil of a laser pistol would be negligible.
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  #4  
Old 04-29-2007, 11:19 PM
bouv bouv is offline
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Quote:
Originally Posted by MikeS
So under these assumptions, the recoil of a laser pistol would be negligible.
Heh, this reminds me of the Rifts role-playing game. In the section describing laser pistols and rifles, it mentions that a true laser pistol has no sound or recoil, but due to years of popular culture showing them emit sound and force, manufacturers eventually made the guns have noises and sometimes a fake recoil because the people buying them wanted those features.
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  #5  
Old 04-29-2007, 11:26 PM
Squink Squink is offline
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Originally Posted by MikeS
under these assumptions, the recoil of a laser pistol would be negligible.
Your laser pistol might still jump some when you pump 500+ watt seconds through whatever coils, flashlamps or circuitry you use to create the beam. That wouldn't strictly count as recoil, but it could be enough to mess with your aim.
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  #6  
Old 04-29-2007, 11:28 PM
danceswithcats danceswithcats is offline
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  #7  
Old 04-29-2007, 11:44 PM
Bobotheoptimist Bobotheoptimist is offline
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Remember that blasters are a different story altogether, firing as they do an extremely high-energy gas compressed into a beam of intense energy particles that are propelled by or fused with (or somedamnthing) a beam of light.

No recoil with lasers, however
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  #8  
Old 04-29-2007, 11:52 PM
Valgard Valgard is offline
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Quote:
Originally Posted by MikeS
Well, let's see.
  1. The muzzle velocity for a 200-grain bullet (about 13 grams) coming out of a .45 ACP is approximately 300 m/s.
  2. Therefore, the momentum imparted to the bullet, and which will therefore be imparted to whatever's holding the gun as "recoil" is about 3.9 kg m/s.
  3. The kinetic energy of a bullet coming out of a .45 ACP is, according to the link you gave, approximately 500 J.
  4. If we assume that our laser pistol imparts this same amount of energy to its beam, and we recall that the momentum of a photon is related to its energy by E = p c, where c is the speed of light, the total momentum of the beam will be about 1.7 x 10-6 kg m/s.
So under these assumptions, the recoil of a laser pistol would be negligible.
Way back in high school I did a physics report on the whole "Star Wars" thing (as in SDI, not the movie). One bit I remember reading was that a laser would need to pump closer to 20,000J into a human target to vaporize a hole that would do the same kind of damage as a .45 bullet does by plowing through tissue. Much less efficient.

Not that it'd really change your answer - even 100x that tiny recoil is still...tiny.
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  #9  
Old 04-30-2007, 02:39 AM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by Valgard
Way back in high school I did a physics report on the whole "Star Wars" thing (as in SDI, not the movie). One bit I remember reading was that a laser would need to pump closer to 20,000J into a human target to vaporize a hole that would do the same kind of damage as a .45 bullet does by plowing through tissue. Much less efficient.
Using a laser as a weapon would almost certainly rely on the thermal shock effect, i.e. you heat the affected area so rapidly that it explodes into a cloud of superheated steam, which then proceeds to do the majority of the damage. You would presumably do this with a pulsed laser, firing repeatedly at a rate of once every few tens of microseconds to increase penetration. This would still likely be more energy than the ~500 joules in a .230 grain 45 ACP, but less than that required strictly to evacuate a .451" hole.

However, this is all dependent upon a compact power source that could deliver sufficient energy to the lasing medium to power such a device, and of course a handgun-sized lasing apparatus that could generate sufficient throughput. Both of these are well and beyond any current technology. For the time being, we'll have to stick with projectile weapons to cause serious damage to one another at range.

In any case, the amount of recoil would be negligable; in order to generate any measurable amount of recoil would require terawatts of power, vastly more than could conceivably be generated by any extant power source.

Stranger
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  #10  
Old 04-30-2007, 03:48 AM
drachillix drachillix is offline
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Quote:
Originally Posted by Squink
Your laser pistol might still jump some when you pump 500+ watt seconds through whatever coils, flashlamps or circuitry you use to create the beam. That wouldn't strictly count as recoil, but it could be enough to mess with your aim.
How? What is the beam pushing against to exert force that would cause any noticable motion of the gun in a 1G 1 ATM environment.
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  #11  
Old 04-30-2007, 04:54 AM
eshen_perch eshen_perch is offline
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Originally Posted by Stranger On A Train

However, this is all dependent upon a compact power source that could deliver sufficient energy to the lasing medium to power such a device, and of course a handgun-sized lasing apparatus that could generate sufficient throughput.

Stranger
How about a current-tech supercapacitor for a few 2,000 joule pulses at a time, pumped by a compact diesel generator. You could fit that in a shopping cart. It would be like the minigun in Predator, where a user would actually have to drag around a car battery to power the rotary barrels, and a big box of ammo. The latest weapon tech semiconductor lasers can deliver about 20 kw continuously. Dunno how big they are but that could go in the cart too, with the beam directed down a hand-held optical fibre.
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  #12  
Old 04-30-2007, 05:23 AM
chrisk chrisk is offline
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Quote:
Originally Posted by Bobotheoptimist
Remember that blasters are a different story altogether, firing as they do an extremely high-energy gas compressed into a beam of intense energy particles that are propelled by or fused with (or somedamnthing) a beam of light.

No recoil with lasers, however
Blasters do no such thing. Blasters are nothing more than 'lasers' of a variety that emit in the microwave spectrum, thus causing aqueous-fluid-filled bodies that they strike to burst apart.

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  #13  
Old 04-30-2007, 06:56 AM
Squink Squink is offline
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Originally Posted by drachillix
How? What is the beam pushing against to exert force that would cause any noticable motion of the gun in a 1G 1 ATM environment.
The beam's not pushing against anything. The gun contains a fatass pulsed power supply of some sort. When that discharges it'll generate a transient magnetic field. That field will exert force on the conductor and any nearby metal: Why Do Transformers Hum?
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  #14  
Old 04-30-2007, 08:07 AM
WhyNot WhyNot is online now
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Quote:
Originally Posted by bouv
Heh, this reminds me of the Rifts role-playing game. In the section describing laser pistols and rifles, it mentions that a true laser pistol has no sound or recoil, but due to years of popular culture showing them emit sound and force, manufacturers eventually made the guns have noises and sometimes a fake recoil because the people buying them wanted those features.
I wouldn't be surprised. There's no reason for my digital camera to make a shutter clicking sound, either, but it's an option.
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  #15  
Old 04-30-2007, 08:47 AM
Mr. Slant Mr.  Slant is offline
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Originally Posted by WhyNot
I wouldn't be surprised. There's no reason for my digital camera to make a shutter clicking sound, either, but it's an option.
I thought that was to keep you from using it for upskirt shots on the stairwell at work!
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  #16  
Old 04-30-2007, 09:01 AM
WhyNot WhyNot is online now
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Originally Posted by Mr. Slant
I thought that was to keep you from using it for upskirt shots on the stairwell at work!
Since I can turn it off, it seems that it's a customer expectation thing, not a safety feature. However, it's pretty old, and I do think I read something about newer models needing to "click" or beep to alert others when it's taking pics. But that wasn't the initial reason digitals could make a wholly artificial digital click and whir noise. It was originally added because we expected it, and it's an almost unconscious signal that the picture was indeed taken. (How many times have you stood there while your Aunt Edna peers into the wrong end of a camera saying, "Did it take? I don't know? One more, just to be sure!")

Likewise, one might find that experienced shooters unconsciously correct for recoil, and so a recoilless pistol might be less accurate in the hands of experienced shooters than in those only accustomed to "shooting" the remote at the television. It could be practiced, of course, but people who want to add a laser pistol to a collection of firearms might simply prefer an artificial recoil built in to make things more consistent.
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  #17  
Old 05-04-2007, 06:49 PM
drachillix drachillix is offline
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Originally Posted by WhyNot
Likewise, one might find that experienced shooters unconsciously correct for recoil, and so a recoilless pistol might be less accurate in the hands of experienced shooters than in those only accustomed to "shooting" the remote at the television.
Thats a recoil anticipation flinch and occurs with actual shooters as well, making them tend to shoot low.

Quote:
It could be practiced, of course, but people who want to add a laser pistol to a collection of firearms might simply prefer an artificial recoil built in to make things more consistent.
As someone who used to compete in handgun matches I would take zero recoil any day of the week.
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  #18  
Old 05-04-2007, 07:03 PM
Xema Xema is offline
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Originally Posted by WhyNot
Likewise, one might find that experienced shooters unconsciously correct for recoil, and so a recoilless pistol might be less accurate
But 99% of the recoil effect happens after the bullet is on its way. Any experienced shooter would prefer the no-recoil option.
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  #19  
Old 05-04-2007, 08:40 PM
Gary Robson Gary Robson is offline
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Quote:
Originally Posted by Bobotheoptimist
Remember that blasters are a different story altogether, firing as they do an extremely high-energy gas compressed into a beam of intense energy particles that are propelled by or fused with (or somedamnthing) a beam of light.
What we really need is something that isn't so clumsy or random...
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  #20  
Old 05-04-2007, 09:48 PM
ExTank ExTank is offline
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Could thermal bloom cause some sort of recoil-like effect?
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  #21  
Old 05-04-2007, 10:12 PM
Elendil's Heir Elendil's Heir is offline
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Greedo and I were in the Cantina the other day, talking about this very topic....
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  #22  
Old 05-04-2007, 10:20 PM
Chronos Chronos is offline
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Quote:
Could thermal bloom cause some sort of recoil-like effect?
Not unless you're really, really close to your target (in which case, you'd be better off using a knife). Thermal bloom happens inside the target, not inside the gun, and the effects will reach you only by travelling through the air, so they'll fall off and dissipate very quickly with distance.
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  #23  
Old 05-05-2007, 03:13 AM
Tuckerfan Tuckerfan is offline
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Originally Posted by Chronos
Not unless you're really, really close to your target (in which case, you'd be better off using a knife). Thermal bloom happens inside the target, not inside the gun, and the effects will reach you only by travelling through the air, so they'll fall off and dissipate very quickly with distance.
Don't forget that the laser has to burn a hole through the air. I've been told that high powered lasers make a great deal of noise when fired.
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Old 05-05-2007, 06:44 AM
Xema Xema is offline
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Originally Posted by Tuckerfan
Don't forget that the laser has to burn a hole through the air.
I'm puzzled as to why this should be necessary. If the air contains particles, these could absorb photons - but the air itself should do very little of this.
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Old 05-05-2007, 07:06 AM
Tuckerfan Tuckerfan is offline
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Originally Posted by Xema
I'm puzzled as to why this should be necessary. If the air contains particles, these could absorb photons - but the air itself should do very little of this.
Air is particles, if you stop and think about it.
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  #26  
Old 05-05-2007, 08:16 AM
Bosda Di'Chi of Tricor Bosda Di'Chi of Tricor is offline
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Blasters develop out of Tractor Beam Tech.

Imagine a Tractor Beam, forcused on a 1 Square Centimeter area, with a 2 kilogram pull.
Then, reverse this to a 2kg push.

Repeat, 5,000 times a second.

The target will blast apart quite nicely.
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Old 05-05-2007, 01:23 PM
ExTank ExTank is offline
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Quote:
Originally Posted by Chronos
Not unless you're really, really close to your target (in which case, you'd be better off using a knife). Thermal bloom happens inside the target, not inside the gun, and the effects will reach you only by travelling through the air, so they'll fall off and dissipate very quickly with distance.
I was thinking of the air that is superheated around the muzzle as the laser beam exits the barrel.

Edit: I should've read some more: kind of what Tuckerfan is saying.

Last edited by ExTank; 05-05-2007 at 01:25 PM..
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  #28  
Old 05-05-2007, 03:05 PM
Xema Xema is offline
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Quote:
Originally Posted by Tuckerfan
Air is particles, if you stop and think about it.
Right - molecules. But unless there are some larger particles hanging around, its ability to absorb photons is distinctly limited. I can see no real reason why a laser should have to "burn a hole through the air."
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  #29  
Old 05-05-2007, 04:29 PM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by Xema
Right - molecules. But unless there are some larger particles hanging around, its ability to absorb photons is distinctly limited. I can see no real reason why a laser should have to "burn a hole through the air."
Thermal blooming is a very real problem for high power (>100kW) lasers. The abstract of this paper gives a basic explanation of the phenomenon:
The distortion of the laser beam is the result of heating of the path by absorption of a small fraction of the laser beam power by the medium which changes the index of refraction and therefore distorts the beam. Thermal blooming effects can limit the laser powers which can be effectively propagated through the atmosphere, or in media which absorb laser power such as industrial or laboratory environments, liuquid or gas cells, or even laser active media themselves.
In short, a small fraction of the energy is absorbed by the volume of atmosphere it travels through, the heating caues this mass of air to become turbulent, which increases its refractiveness and amplifies absorption, ad nausum. This is a significant problem with high energy lasers, particularly those in optical or near-optical spectra, and for all the talk about adaptive optics it remains a stumbling block in strategic laser defense applications.

But none of this is going to cause a laser weapon to recoil. Recoil in firearms is due to conservation of momentum from a reaction to the impulse imparted in ejecting a bullet and gases exit out the barrel. The amount of impulse in even a high power laser is insignficant compared to the mass of the lasing cavity and media, not to mention anything small enough to be wielded by hand.

Recoil has virtually no direct effect upon the accuracy of a weapon, since the largest impulse comes immediately after the bullet leaves the barrel. However, particularly with handguns, flincing in anticipation of recoil is a major cause of inaccuracy, and of course higher recoil limits how fast a shooter can accurately aim and fire a second shot. There would be no conceivable benefit to simulating recoil on a hypothetical pistol-sized laser.

Stranger
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  #30  
Old 05-05-2007, 05:08 PM
Happy Fun Ball Happy Fun Ball is online now
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Quote:
Originally Posted by Xema
Right - molecules. But unless there are some larger particles hanging around, its ability to absorb photons is distinctly limited. I can see no real reason why a laser should have to "burn a hole through the air."
Stranger's right (as always)... Look on wikipedia for the Kerr effect, or optical Kerr effect. Basically, all materials will have a change in thier index of refraction proportional to the intensity in the beam. For a gaussian beam, this produces what is in effect a grin lens which focusses the beam until catastrophic failure occurs. Even in air. I worked (briefly) in an ultrafast laser lab and played around with chirped pulse amplification to get ultra high peak power laser pulses. Amazingly, wikipedia has an entry on CPA too...
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  #31  
Old 05-05-2007, 06:00 PM
Xema Xema is offline
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I'll concede the point about thermal blooming while continuing to believe that it's a bit misleading to describle this as burning a hole through the air.
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Old 05-06-2007, 12:24 PM
MrDibble MrDibble is offline
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According to the laser trooper cheat in Age of Empires, lasers make a kinda crackling noise. Don't know about recoil, though.
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  #33  
Old 05-06-2007, 12:47 PM
Kevbo Kevbo is offline
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Originally Posted by Xema
I'll concede the point about thermal blooming while continuing to believe that it's a bit misleading to describle this as burning a hole through the air.
When the light intensity is high enough, the E field intensity is sufficient to ionize the air. Given that a flame is ionized gas, it is not a discription that falls very far from reality. This happens with lasers of less than ten watts if the beam is focused to a point, and is one of the limiting factor when attempting to drive single mode fibre.
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Old 05-06-2007, 02:28 PM
Chronos Chronos is offline
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I had neglected the atmospheric effects, on the notion that air should be almost completely transparent to a laser beam. But I suppose that, for a weapon-power laser, that probably would be a consideration.

Then again, it sounds like, before the bloom would get to a point where it would cause a detectable recoil, it would make it ineffective against the target, as well. So any practical laser weapon which is eventually developed would have to have some workaround to this problem (don't ask me what it would be). So maybe it is justified to neglect it?
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