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Thread: Would Laser Guns Have Recoil?

  1. #1
    Good thing I like my day job geardrops's Avatar
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    Would Laser Guns Have Recoil?

    http://thevirtuosi.blogspot.com/2010...tion-near.html - Equations and Maths done by people far smarter than I.

    Coworker and I debated whether or not saying a "laser bullet" has the same kinetic energy as a normal bullet is a reasonable assumption. (I voted yes simply because, as seen in film, the energy imparted on the victim appears to be the same as a bullet, and it's not like a laser can invent energy as it goes along...)

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    A Gentleman of a refined age... thothguard51's Avatar
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    The recoil on most firearms has nothing to do with the bullet but the explosion that takes place in the chamber.

    The impact of the bullet on the subject it hits is based on caliber, velocity of the moving object against a solid object, yada yada yada...

    I vote no impact from a laser hand gun or rifle because there is no explosion in the weapons chamber.
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    Cookie monster FreeWhistler's Avatar
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    Thoth is totally right about the explosion in the barrel causing the recoil. However, if there is recoil in a laser gun it's from photon pressure. The question is, though, do photons exert pressure?

    These guys explore the question in greater detail.

  4. #4
    I've been around the block. lachlan's Avatar
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    Lasers don't impart kinetic energy. They might heat the target, but that's all you'll be able to measure. Stick a hot dog under a heatlamp -- the hot dog will cook, but the heatlamp does not go flying upwards.

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    It's a doggy dog world benbradley's Avatar
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    The "equations and maths" may be good (I didn't look that closely), but I don't think it's proper to equate the kinetic energy of a bullet with the energy of a burst of light and then figure out the recoil of "something going at lightspeed."

    Yes, photons have something that acts like mass (insert "I didn't know they were Catholic" joke here) or momentum, as can be demonstrated with a lightsail, but it's very low and I'd think insignificant for a hand-held laser, unless it's so powerful the beam causes the air it shoots through to heat up so fast it burns and explodes, in which case it's got worse dangers than a little recoil.

    I'm not sure how to calculate the recoil, but I'm confident the method used is wrong. It assumes some small mass is accelerated to lightspeed, but it would actually take infinite force to do that. I'm not sure if the Lorenz transforms would be used here, or something else. I think I'd go to Wikipedia and look up lightsail and photon pressure (for the frequency of light the alleged laser pistol puts out), and calculate it from the pulse power (how many photons get emitted in a firing of the laser gun). This, as best as I can figure (by just thinking about it a minute), is unrelated to the kinetic energy of a bullet.

    Again, the laser gun WOULD have some small recoil, as the light comes out one end, and nothing out the other (in a "lasing" medium, light goes all directions, but gets amplified most along the longest dimension, and for pulse lasers there's usually a mirror at one end to make it all go out the other end, so it's going to push on the mirror just as it does on a lightsail), but not all "guns" have recoil. A bazooka is essentially a rocket launcher with a hole out the back, and all the expanding gases go out the back instead of building up in the barrel, so a bazooka has no recoil.

    As for the extremely short time of the recoil force, I don't think that's a "problem." A gun's recoil starts when the bullet is fired, then pretty much ends as it comes out of the barrel, and that happens in what, maybe a millisecond or two? It causes the gun to have a backward speed, and that pushes back on the hand or shoulder, depending on it being a pistol or rifle. The same recoil from a 30 nanosecond laser pulse wouldn't be noticably different from a 1 millisecond bullet shot. As far as human perception, the gun is suddenly, instantly moving backwards at a certain speed.
    Last edited by benbradley; 04-22-2010 at 05:42 AM. Reason: Tiny edit, buttressing my argument.
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    It's a doggy dog world benbradley's Avatar
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    Quote Originally Posted by FreeWhistler View Post
    Thoth is totally right about the explosion in the barrel causing the recoil. However, if there is recoil in a laser gun it's from photon pressure. The question is, though, do photons exert pressure?

    These guys explore the question in greater detail.
    I'm not sure, but I went to that link and I think I smell bolognium.
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    A Gentleman of a refined age... thothguard51's Avatar
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    Turn a high powered flashlight on...does your hand recoil? No.

    More than likely, any laser gun, (hand or rifle), will have gas escape ports to keep the gasses from building up in the chamber and perhaps exploding as the light heats the gases on follow up rounds... Just a theory of course...
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  8. #8
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    Short version: Yes, a laser creates pressure, no a laser gun has no recoil, hah the calculation in the link is hilariously misguided.

    Long version:
    Photons do exert a certain amount of pressure whether it be on a mirror as in a solar sail, or on a lasing medium when created. But that pressure is unbelievably small. For any laser gun, the resulting recoil is completely unnoticeable.
    The recoil of a gun is due to the law of nature that states that for every action, there's an equal an opposite reaction. Which means that the expanding gas inside the barrel does not actually push the bullet out one end, it pushes the gun and bullet apart from each other, imparting the same kinetic energy to both. Since the bullet has much less mass than the gun (and whatever the gun is attached to), the bullet gets accelerated to far higher speeds, so that it seems only the bullet is moving. A little like the opposite of when you fall, actually you and the earth both fall towards each other, just that you fall most of the way.
    The energy emitted by a laser is not kinetic, so comparing laser energy to bullet energy is completely meaningless for calculating recoil.
    As a side note, a weaponized laser doesn't shoot single "bolts" either, the penetration is achieved by a series of pulses with pauses in between. A single "shot" with a laser gun might consist of a dozen or more individual pulses over a millisecond. Optimal number of pulses and minimum time of pauses is determined by the material you're shooting at.
    Also, a laser gun wouldn't look like WWII Battleship turrets, as they do in Star Wars, but more like a big crystal ball. Handguns would probably look similar to current firearms, but only because aiming with something that's shaped like a flash-light doesn't work so well.

    Quote Originally Posted by thothguard51 View Post
    More than likely, any laser gun, (hand or rifle), will have gas escape ports to keep the gasses from building up in the chamber and perhaps exploding as the light heats the gases on follow up rounds... Just a theory of course...
    While you could use a gas laser to create a portable weapon, they're generally more suited to large and high-powered applications. For a useful hand-held weapon, you'd most likely need a solid state laser.
    Last edited by Lhun; 04-22-2010 at 06:45 AM.

  9. #9
    It's a doggy dog world benbradley's Avatar
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    So how much recoil does THIS laser have?
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  10. #10
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    Still less than the pilot spitting out of the window.
    This one's pretty cool as well: http://www.youtube.com/watch?v=LThD0FMvTFU

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    Hmmm, I'm not entirely sure I believe that first one. There's no sign of the laser in the vapor plume & smoke coming from the hotspot and the spot looks from what I could see to have too much charring around it and to be too irregular in shape. It looks more like a thermite charge placed underneath the hood to me.

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    Quote Originally Posted by benbradley View Post
    I'm not sure, but I went to that link and I think I smell bolognium.
    The Quackometer (as seen in this week's New Scientist, agrees with you.
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    practical experience, FTW lpetrich's Avatar
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    It's momentum that causes the recoil, not kinetic energy directly.

    Let's first calculate it for a bullet shot from a "slugthrower" gun. Bullets and their "muzzle velocities" vary widely, so I'll use some figures that I found for deer hunting. I'll depend on some hunting enthusiast to set me straight if need be.

    Hunter Mass: 200 lbs = 90 kg
    Gun Mass: 9 lbs = 4 kg
    Bullet Mass: 150 grains = 10 grams
    Velocity: 3000 fps = 900 m/s

    Momentum: 9 kg*m/s
    Gun recoil velocity: 2 m/s
    Hunter recoil velocity: 0.1 m/s

    I'm ignoring the combustion gases from the bullet cartridge's explosive powder, but that likely has a comparable effect.

    In general,
    (momentum) = (total mass-energy) * (velocity)
    where
    (total mass-energy) = (rest mass) + (kinetic energy)/c2,

    Yes, Einstein's famous formula E = m2

    This rifle bullet's kinetic energy increases its total mass-energy by about 5*10-12


    Now let's calculate for a laser. I'll use the kinetic energy of that bullet as a reference; it is about 4000 joules. The momentum of that energy of light is very small, because light travels very fast: 1.4*10[sup]-5/[sup] kg*m/s.

    This is about 1.5*10-6 times that bullet's momentum, so one will not feel any kick from a laser pulse with that energy.

    The next question to ask is whether that will be enough to do any significant damage. A laser will have to vaporize some material in its target, and the departure of the evaporated material will kick back.

    That energy is enough to vaporize about 1.8 grams of water, 0.37 grams of aluminum, or 0.64 grams of iron. The vaporized material would have a perpendicular velocity of about 500 m/s, giving 1, 0.2, and 0.3 kg*m/s momentum for each material -- much less than the momentum of that bullet.

    So one will likely need energies of about 200 kilojoules to produce a vaporization kick comparable to that bullet's momentum. But even that will produce less than about 10-4 of the bullet's recoil momentum at the laser itself.

  14. #14
    Who's going for a beer? waylander's Avatar
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    Laser guns just got a bit closer to reality
    http://www.bbc.co.uk/news/technology-10682693

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    Quote Originally Posted by lachlan View Post
    Lasers don't impart kinetic energy. They might heat the target, but that's all you'll be able to measure. Stick a hot dog under a heatlamp -- the hot dog will cook, but the heatlamp does not go flying upwards.
    That would make baseball games a lot of fun, though.

    The pitch. A HIGH FLY HEAT LAMP TO CENTER FIELD! ITS GOING! GOING!---bonk!
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    I come in peace from another genre with information to take to your leader:

    1. I have a laser pointer. No recoil.

    2. I have a laser night-sight on my personal protection handgun. When I engage the target in night practice, there's no recoil until I fire, at which time the forces propelling the bullet indeed causes recoil.

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    Lagrangian LOG's Avatar
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    How big/intense would a laser gun have to get before the light of the laser is visible to the naked eye, even when standing next to it?
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    Onwards, ever onwards ClareGreen's Avatar
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    Um. According to something I read the other day, most laser weapons are in a wavelength we can't see - and however potent the laser, some of the light gets reflected/refracted by things in the atmosphere, which is why we can often see that visible lasers are being used even when they aren't pointed at us.

    The answer is 'it depends on whether we can see it beforehand' and 'it depends on if it's being used in an atmosphere or other place with enough junk around to show'.
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    On Mac's double secret probation. Dommo's Avatar
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    Generally any powerful laser isn't going to be visible to the naked eye. The only exception is if there's A LOT of dust/smoke in the air. In a case like that a laser would start burning the stuff and it might show up. I've been around enough C02 industrial lasers to know that they're pretty much invisible.

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  21. #21
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    If the laser is powerful enough it can also become visible by heating the air up until it's red to white-hot. It's kind of what makes lightning visible. (although that also ionizes the air)
    It happens quicker if the laser wavelength gets absorbed by the gas. (i.e. and infrared laser and CO2)

  22. #22
    God, I love when discussions involve actual equations. It warms my little geeky heart

    I've written laser guns without recoil, though the shots are visible and do ionise the air.

  23. #23
    Comic guy Bartholomew's Avatar
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    Quote Originally Posted by ClareGreen View Post
    Um. According to something I read the other day, most laser weapons are in a wavelength we can't see - and however potent the laser, some of the light gets reflected/refracted by things in the atmosphere, which is why we can often see that visible lasers are being used even when they aren't pointed at us.

    The answer is 'it depends on whether we can see it beforehand' and 'it depends on if it's being used in an atmosphere or other place with enough junk around to show'.
    If there are ever civilian lasers, they'll probably, by law, have some sort of laser-pointer built in, just so that the military weapons will have an advantage. They'll probably also be significantly weaker.
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    Quote Originally Posted by Bartholomew View Post
    If there are ever civilian lasers, they'll probably, by law, have some sort of laser-pointer built in, just so that the military weapons will have an advantage. They'll probably also be significantly weaker.
    Why? Civilian guns don't have mandatory laserpointers.

  25. #25
    Today is your last day. FOTSGreg's Avatar
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    In regard to the calculations above, if a laser vaporizes 1.8 grams of human tissue, shouldn't there be significant "kick" to the victim from the expanding gases (it's my understanding that water vapor expands at something like 10,000 times the mass vaporized)?

    I'd expect some significant effects on the victim even for such a low energy laser strike.

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