do lasers have recoil? Options

[otomik]

People say they don't but then people also say crap about gauss weapons having no recoil. seems like their might be some recoil involved, photons have some mass (think of solar sails and stuff like that).

The military is talking about 25 and 100 kilowatt lasers. I'm pretty sure there's got to be a mathematical operation for converting that to joules or foot pounds.

[Nylan]

Damn...My physics notes are in another city currently! If that weren't the case I could give you a nice conversion. Finally, a practical application for that stuff: determining the recoil of a futuristic laser weapon for a role playing game...

My gut reaction is that they wouldn't have much appreciable recoil, even with high wattage like that. Solar sails are HUGE, and specially designed to be pushed by photons. If I recall from class, even a mole of photons has very little mass, and the kick shouldn't be very much, even with them travelling at 3.00 x 10^8 m/s. By the way, where did you get those numbers from? And for what type of application? That seems like quite a lot, and would take quite a bit of juice to work.

Again, this is just my not-too informed opinion on the matter. I eagerly await anyone with any real knowledge or numbers.

[Edward]

Given what (little) I know about the mechanism for photon release from exited atom I would say no recoil.

However any weapon grade laser is very loud and can not be fitted with any kind of sound dampening device. Even the stealth and silence spells wont help because it is not the weapon making the noise but the super heated air along the firing path.

Edward

[Madda_Gaska]

I'm inclined to agree with the negligible recoil opinion.

However, I do have another thought to add on this-
Depending on what sort of laser it is (visible or IR being the most likely ones unless I'm mistaken?), there's one of two major problems for observers.
If it's visible then I'm pretty sure it's meant to cause severe damage to your eyes even if you aren't looking directly at the beam. If it's not then the sound isn't really going to help someone pinpoint the firer anyway (though any heat haze remaining might).

Still, when you've broken out the weapons-grade lasers you probably aren't in the realm of covert operations any more.

[otomik]

[Quix]

The way you convert the kiilowatts to joules would be to multiply watts by the length of time for the pulse.

1 Watt=1 Joule/sec

Unfortunately I can't find a table giving equivalent energies for any useful reactions, like TNT or gasoline, much less fire arms.
Sorry


EDIT:
found gasoline: 1.32*10^8 joules per gallon

In order to get the same energy out of a 100 kilowatt laser. The laser would need to be on for 1320 seconds, or 22 minutes to deliver the same kind of energy.

[Austere Emancipa...]

7.62x51mm 150 grains @ 2900fps = 2802 ft-lbs = 3799 J. That means a 100kW laser would only need to be on for ~0.04 seconds to generate the same amount of energy as the 7.62x51mm bullet has at the muzzle.

But that's totally meaningless because the 100kW certainly doesn't all go towards launching particles in one direction. What we need is some physicist telling us exactly how much kinetic energy light has. I Googled for "kinetic energy of light" but I haven't got a clue what all those articles are about.

[RangerJoe]

Back of the envelope calculations:

E = mc^2

A 100 kW laser, pulsed for 1 second delivers 100x10^3 J of energy as radiation (100 kW = 100 x 10^3 J/s)

Equivalent mass delivered in a 1 second pulse (it's a wave! It's a particle!) = E/c^2.

E/c^2 = 1.11111111x10^-12 kg (using mks units)

Conservation of momentum dictates that mv is conserved. That is, m(phot)v(phot) = m('gun')v('gun'), with v's going off in opposite directions (this is where recoil comes from)

Assuming a 100 kg LASER OF DOOM (those boys at Lockheed are getting good at miniaturization):

(1.111111x10^-12 kg)(3 x 10^8 m/s) = (100 kg)(v 'gun')

v('gun') = 3.3333 x 10-6 m/s. Yup, that's a micrometer per second. Lasers are effectively recoilless (and that's with a 1 seocnd pulse!). Blaze with impunity.

Now it's time for coffee....

[Arethusa]

Seriously, light pressure is well beyond negligable here.

[edit]

Joe beat me to numbers. Still, point stands.

This post has been edited by Arethusa: Aug 24 2004, 01:51 PM

[RangerJoe]

Now, it might be cool to have quad lasers shake and shimmy, simulating recoil, a la Star Wars, but that would just look cool.

*wants his smuggler PC to be able to carry a SR version of the heavy blaster pistol*

[Nylan]

Thanks for the numbers Ranger Joe...if only I had my physics stuff here. And, just to be the biggest nerd ever, I'd like to point out that the blasters in Star Wars fire an energized gas and are not actually 'lasers.' But yes, it would be incredibly bad-ass to have guys toting laser pistols (I've been waiting for this for a long time, Solo...).

[mfb]

who's this, that thinks gauss weapons wouldn't have recoil? they must be found and punched, so that they might learn.

[Kagetenshi]

If sufficiently long, they could have negligible impulse.

~J

[mfb]

you mean reducing felt recoil by using a slower rate of acceleration? true, but then you're looking at a much longer firing time--in the scale at which felt recoil is a problem (hand weapons), you need to be able to fire much more quickly than that. larger weapons don't need to worry about felt recoil as much, since they're generally more firmly braced against the ground. i suppose you might be able to use that technique in order to make gauss guns more viable for mecha, or something.

[Arethusa]

I think you guys missed the bigger problem: negligable impulse or not, what the hell are gauss weapons being discussed for in a thread about lasers recoiling from light pressure?

Also, side note: recoil is definitely an appreciable problem with artillery. Simply bracing it on the ground doesn't fix everything; there's a reason artillery isn't simply a barrel and a breach at the end.

[otomik]

I'd like to expand this topic to cover recoil in sci-fi weaponry

I don't know where you get your info on Blasters but my understanding was that Tibana gas is only used in the focusing process to somehow enhance the energy bolt that comes out of the blaster. to me this sounds like a gas dynamic laser. What you described sounds more like the Ion Cannons which might operate on the same principal as Ion Drives, which are also common in the star wars universe. real life ion drives even have the familiar blue glow: http://science.nasa.gov/headlines/y2000/ast15jun_1.htm. It might also be that Tibana Gas has properties that make it better than the current day Xenon Gas (or that Tibana Gas is simply the name for Xenon Gas a long time ago in a Galaxy far far away).

One should be hesitant to look at Star Wars this way, explainations could be painful as Midichlorians. Star Wars was created with Kurosawa's Hidden Fortress and Joseph Campbell's The Hero with a Thousand Faces. If Lucas were ever a hard sci-fi guy he's been in hollywood doing so much coke and for so long... (remember Ewoks movies?)

[GrinderTheTroll]

Detailed physics aside (nice job you number crunchers!) a bullet that is fired generates recoil because of Newton's 2nd Law of Mothion stating "equal but opposite reaction", so the exploding powder pushing the bullet out of the gun one way, would need to push the opposite direction (your arm, shoulder, whatever) for the bullet to go anywhere.

A laser "pulse" would be generated differently by forcing the release of photons from some souce (this really depends on how you do it). More than likely, energy levels of some type of material is excited and phonts get realease (yeah I know it's not 100% the way it happens), but the main point is there is no "explosion" as compared to it's bullet counterpart.

As it's been stated and shown, the recoil would be negligible.

[Austere Emancipa...]

[GrinderTheTroll]

[Zeel De Mort]

Wow how did a thread questioning the recoil of lasers start? As has been shown already, the recoil is TINY - as you'd expect. But more importantly than that, the rules say laser weapons produce no recoil. :D

Gauss rifles would still push back on you when you fired them. Depending how long the acceleration chamber is would dictate how long the slug was accelerating for and thus how sharply the kick would be felt, but there would still be a repulsion in the other direction to that which the slug was fired in. Think about when you hold two positively charged magnets close together. They're not touching, there's no explosion or gas being moved etc, but they repel each other.

[GrinderTheTroll]

[Kagetenshi]

[RangerJoe]

I assume a "Gauss" weapon is a "rail gun" type deal? The idea being that current moving through a conductive slug, hung between two conductive rails, will accelerate when a current is run through the system, right?

If this is the case, then a Gauss rifle will produce recoil the same as any other slug thrower. The slug doesn't care (from a Newtonian perspective) whether it accelerates away from the user, or the firearm itself accelerates towards the user. It's all the same thing. The reason that he slug can accelerate away from the user is because the rifle itself is accelerating towards the user. Ye Olde 3rd Law (Newton) is not rendered obsolete just because the rifle uses electromagnetic fields to accelerate the slug and not expanding gas.

It's not magentism at work, just conservation of momentum.

[littlesean]

But for the same recoil you would get a higher slug velocity, since the gun is not also being pushed back by expanding gas.

[Kagetenshi]

The slug is accelerated through a coil rather than two rails, which has the advantage of the slug not actually making contact with the rails and thus not causing the kind of wear and tear caused by a railgun. In principle, however, they’re identical; the implementation is the only difference.

~J