Built on Facts

Laser rifle recoil?

There’s a really snazzy physics blog run out of Cornell called The Virtuosi. Sort of like this blog, they’re big fans of looking at interesting scenarios through the equations of physics. Not long ago, they had an interesting post looking at whether or not laser guns would have recoil.

I’m not going to duplicate the steps of the problem here, as you should go read it on their excellent site. I’ll just state the conclusions: if the laser pulse has energy roughly comparable to a bullet, the momentum imparted to the gun will be tiny – on the order of 10^6 times smaller than that even of the gently-recoiling .22 LR. But the emission of the light happens over a very short period of time, perhaps a few nanoseconds in the case of the “typical” movie laser with a beam of visibly short length.* Force is change in momentum divided by time (say, a few nanoseconds), so carrying out this calculation gives a force of around 15 N, though only for those few nanoseconds. This is macroscopic, and perhaps comparable to the force on the butt end of a .22 rifle. But on the other hand, the rifle is applying this force for a few hundred milliseconds, not for the factor of a million shorter time that the laser is firing. Can this still be felt?

There’s some discussion in the comments on this. I suspect the answer is no, and that the total impulse is the parameter of interest in a case of very short application of a force – not least of which is that the body of a pistol may not be able to propagate a sound wave of ~30ns duration. GHz ultrasound just doesn’t propagate well; the propagating pressure wave carrying momentum from the laser cavity to the shoulder stock is likely going to widen in temporal duration into something much longer, meaning we’re back to a small force over a longer time.

How good are humans at detecting this sort of high-force, low-duration impulse? One rough comparison might be our response to sound. Sound is after all a sequence of short-duration pressure waves. Normally we can’t feel it, but high-intensity sounds such as thundering bass at a rock concert can sometimes be felt in the skin. Let’s try to draw a rough comparison between the recoil and an equivalent sound wave.

We know from The Virtuosi’s work that the recoil impulse is about 4.5e-7 kg m/s. Let’s say the laser gun has a mass of 5 kilograms. Dividing the impulse by the mass of the gun gives a recoil velocity of 9e-8 m/s. Since kinetic energy is equal to:

i-d4bb9fe09d6b78eb832985821d8a307f-1.png

We have a recoil energy of about 2e-14 J. This is pretty tiny, but then again human ears if not human skin can detect some pretty tiny sound energies. Now in a context like this the pressure of a sound wave is given by

i-88516ca505d84f969b7ab15a965c3fb3-2.png

Where I is the intensity in watts per square meter and v is the speed of sound in our material. We don’t know what the intensity is, but we can estimate it by saying that it’s equal to the energy (2e-14 J) divided by the time of recoil (probably milliseconds, not micro/nanoseconds because of the difficulty of propagating very short acoustic pulses in bulk media – let’s say 1ms to be generous), divided by the area of the shoulder stock (say, 25 cm^2). Taking the speed of sound in the shoulder stock to be the 3300 m/s value typical of wood gives us a sound pressure level of about 2.4e-12 pascals. The threshold of hearing is about 20 micropascals, which is much higher. Even assuming the sound pulse really is only 1ns long doesn’t quire reach the threshold of hearing. If the pulse couldn’t even be heard it probably can’t be felt.

Now this is a pretty rough calculation with a lot of estimates and assumptions. Nonetheless I’d expect the recoil of a bullet-energy pulsed laser would not be detectable to the person firing.

*Of course the fact that you can see it moving means it’s not a light pulse, but leave that aside.

Comments

  1. #1 Whomever1
    April 21, 2010

    Well, do you think you could feel it if the beam caused an explosion of super-heated air in front of the muzzle?

  2. #2 Matt Springer
    April 21, 2010

    Could be. Fun fact: some guns have so-called ported barrels which redirect the gas backwards to compensate for the recoil of the bullet. The disadvantage (and reason they’re not common) is that it greatly increases the noise and muzzle blast. It is singularly unpleasant, and feels like being hit in the face every time you fire. But for very powerfully recoiling cartridges sometimes it’s a worthwhile trade.

  3. #3 Jesse
    April 23, 2010

    Matt, I like your analysis. Of course, it all depends on the size of the laser. In a similar vein to the laser rifle, but in honor of earth day, I calculated how powerful the death star laser was. And then I calculated how much the death star would recoil when it fired that laser. Let’s just say that, well, the answer is a lot.

  4. #4 rob
    April 23, 2010

    I know what you’re thinking — “Did he fuse six deuterium-tritium pellets or only five?” Well, to tell you the truth, in all this excitement, I’ve kinda lost track myself. But, being as this is the National Ignition Facility, the 192 most powerful lasers in the world and would blow your head clean off, you’ve got to ask yourself one question: “Do I like physics?” Well, do ya, punk?

  5. #5 Uncle Al
    April 23, 2010

    There might not be any emitter recoil as such. On 31 January 31 2008 the US Navy fired a railgun shell with 10.64 MJ, a muzzle velocity of 2520 m/sec. Project goal is over 5800 m/sec. The barrel does not whomp backwards. The recoil occurs in the EM field.

  6. #6 ball valves
    March 27, 2011

    The solid truth about this article is, that it spoke to me greatly. Thank you for sharing your thoughts and concerns.

  7. #7 ball valves
    March 27, 2011

    The unveiling truth about this blog post is, that it spoke to me deeply. Thank you for sharing your thoughts and concerns.

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