It takes 3,624,433 kilowatts (3.6 gigawatts) of light to generate 5 pounds of thrust on a solar sail. I'm not going to lose sleep over the lack of recoil compensation in anything smaller than an 18 gigawatt laser.

No. If you shove mass forward, some mass will go backward.

"There is no escaping. The bullet is enormous. Jumping is useless."

well there are 3 options, either the gun moves and the bullet stands still or the bullet moves and the gun stays still or they both moves according to mass. ie gun moves a small bit as more energy is needed to move the mass while the bullet goes flying as the same amount of energy work on less mass.

while i belive that a gauss gun would have recoil it would not act in the same way as the recoil of a normal gun. rember that the force of an explotion pushes in all directions at the same time while a gauss gun works on the prinsiple of pulling and pushing at set times +-+-+-+-+-+ and so on. therefor the energy all works in one direction, the way the bullet is supposed to travel instead of the direction of least resistance (that happens to be the direction of the barrel in the case of a gun).

there is allso the fact that in a normal handgun there is physical mass pushing while in a gauss setup its a electromagnetic effect so there isnt realy any physical object between the back of the gun and the bullet. except air that is (vacume would lead to insane amounts of air pressure acting on the bullet so there have to be ports so that air can be sucked in as the bullet travels)...

there is allso the effect of the reload system on most propellant based guns to take into account as they work by useing some of the force to move parts of the gun. as those parts hit the end of thier movement the force will transfer to the gun proper and allso the fact that some of the mass of the gun have now moved into a diffrent posision, ever so slightly changeing the center of gravity of the gun (unless your fireing a revolver that is). this isnt that big a thing in a rifle but in a handgun that can play its part in the effects of recoil.

so while recoil can be distilled down to newtons law i belive its not the whole picture.

The gases aren't compressed - they're just burned in a reactor not unlike a rocket engine. Selective redirection of the exhaust gases should cancel the issue.

And SR lasers do not seem to produce clouds of toxic chlorine oxide (or whatever), so if they use a chemical approach, they don't vent at all and thus produce no (effective) recoil.

As for "sealed lasers," multi-kilowatt welding lasers are off-the-shelf items, and the Navy has been playing around with 10kW free electron (Sealed) lasers.

No, there are not 3 options. If the gun moves as a result of reacting against the bullet, the bullet will move, too. And vice versa. Basically, there's only the third option: both recoil.

The multi-directional push of a gunpowder explosion is moot: most directions are canceled by equal and opposite pressure on the opposite side of the barrel.

What ends up happening is that, like the gauss rifle, all that matters is that a mass (bullet+propellant gases) leave one end of the barrel, while the gun recoils in the opposite direction. The equations (see: conservation of momentum) are exactly the same in both situations, the difference being the velocity and mass spat out the barrel.

That some gauss rifles might use an on/off means of propelling the projectile is also irrelevant to the total felt recoil. The triggering of the electromagnetics occurs so rapidly that even a wired human won't notice it - it'll be one shove, just like with a single-charge gunpowder weapon.

Irrelevant. The magnets push forward on the bullet while pushing back on the barrel. The net forces are basically the same: one mass goes forward, the other goes back. Trying to focus on little details like where the exchange of momentum occurs will just get you bogged down - as you are - in misleading details.

No, it's still M1V1 = M2V2. One mass goes one way, the other mass goes the other way.

Details like how many masses are involved (separate recoil mechanisms, gun weights, projectile weights, etc.) and velocities are just details to be plugged into the same mechanism. Newton's laws do cover everything - you just need to plug them in correctly.

SR lasers do not use up the gas, thus they cannot be that type of laser. They must be sealed or recycled. The gasses are not "burned," they work like a florescent light bulb and are excited by either RF or Electricity. The gas does not escape the laser and thus causes some kind of rocking motion as the gas shoots though the resonator chamber.

The first is not effective against organic matter, as SR lasers clearly are. I haven't heard of the second.

The gun uses magnets or coils with electrical current running through them which basically function just as magnets at that moment. and magnetic force always works both ways - on the object you try to pull/push and on the magnet itself. So absolutely no difference to force-projection through solid levers or compressed gases or whatever - "actio = reactio" still stays in effect, has something to do with being a basic law of nature.

@littlesean:

microwave lasers are called Maser not only in SF settings but in RL as well.

If it is a laser with a 10 kW input per second it has an output of 10 kJ per second; assuming perfect efficiency. I do not know of a laser that is 100% efficient, so I approximate.

My apologies. Let me re-run the numbers for you.

10 kJ output per second, divided by c^2 = 1.113E-13 kg of mass
1.113E-13 kilogram mass = 1.113E-1 picograms, or 11.13 femtograms of mass.

I still garantee you will not feel the recoil from that.

What I'm really interested in, though, is whether all that 10kJ per second is kinetic energy.

Slightly more complicated question. The answer is: Sorta. Photons have energy and momentum, but no mass.

See this link for a more in-depth answer.
link

And this link for a definition of kinetic energy:
link

I was trying to guess what you meant by "compressed gases," since I wasn't familiar with the term in connection with lasers, and was thus led to think of COILs, which do burn gases. Do you mean a gas laser, like a carbon dioxide laser? Those gases are not highly compressed.

Welding lasers may not deliver energy fast enough to be effective battlefield lasers, but saying that they are not effective against organic material incorrectly implies injuries would not occur. A 3kW welding laser delivers three times the energy output of a microwave oven focused on a small spot.

http://ehs.uky.edu/radiation/laser_fs.html

"Do not place your hand or any other body part into the class IV laser beam. The pain and smell of burned flesh will let you know if this happens. "

I need that for my new sig, it is just too succinct! I love it!

I fall into the "gobbets from the steam explosion" camp.

One of the things I retained from my undergrad thermodynamics course (besides post traumatic stress syndrome) is how materials respond to varying levels of heating. When a liquid is exposed to a surface more than 100C above its boiling point, contact with the surface is not actually made - a sheath of vapor is formed.

This is why you can dip your hand in liquid nitrogen and not get flash-frozen: the LN2 simply flashes to vapor. (Of course, if you're stupid enough to dip your hand in liquid nitrogen, do so quickly. Boiling requires a lot of heat input, which means your hand will be cool rapidly.)

Likewise, battlefield lasers will pump so much heat into a small spot on a target that there's no time for the heat to disperse through the target. Ground zero for the laser (armor, clothing, upper layers of skin) will basically explode (convert to a gaseous state rapidly) because heat keeps piling in, but not leaving. Nearby flesh will suffer from that pinpoint explosion.

Put 10 kilojoules into a millimeter spot. That's about twice the energy of 7.62mm NATO ball ammunition in a much smaller area. How would such a pinpoint explosion manifest? It would seem like you could get a very penetrating effect, if the focus was tight enough and energy delivery period fast enough.

Thank you for making my first morning back at work after Gencon so entertaining.

That's the description of many real world lasers. Many lasers typically use a flash lamp (or LEDs, now) to generate laser photons from a gas or ruby rod. The lamps do not operate continuously, and the lasing gas/rod would just get hot (rather than emit laser photons) if it was illuminated continuously, so the lamps are strobed, resulting in laser pulses.

Of course, medical lasers are also trying to minimize and contain damage to a predictable area, aren't they?

Actually, they can be. Instead of using a turbine or vacuum, the military decided using highly compressed gas would work better (just like going to a turbine from a vacuum increased power), and it did give higher output power.

A stove can cause major injury, that still doesn't make it an effective weapon. Far be it from me to imply otherwise. I can verify that a 3KW Class IV laser (Mitsubishi 1212HC) does indeed cause damage to the skin in a very painful way, resulting in a fire even when striking a chicken bone... which brings me to the next point. I don't think lasers transfer energy very well into flesh. I've heard you and others say it will flash boil and whatever else, but never seen proof. I think perhaps it will explode a small area and cauterize the outside because you will never have a clean focus in the real world; the edges of the beam will be ragged.

Okay, you and I obviously have different definitions of "highly compressed." If it ain't 1000psi or higher, it ain't highly compressed to me.

Can you name this "highly compressed" gas laser? Or the gas used?

So you've only seen 3000 joules per second going into flesh, which is much slower than a bullet delivers its kinetic energy. Okay.

Have you seen 10000 joules - or more - going into flesh in a millisecond? A 10 megawatt laser?

Gasdynamic Lasers CO2.

No, have you? Has anyone? I've seen a 1mW shatter a clay bowl on TV, but my 3kW did that to a stone as well...

What's the operating pressure of those lasers?

The Ares MP3 delivers armor defeating shots. Do you know of any way to achieve that except a tight focus, several kilojoules, and millisecond delivery of the pulse?

Varies. Whatever level required to create a supersonic flow into the resonator, which changes based on the nozzle size, number of nozzles and so on. The white paper on that nuclear powered laser we read way back when used this method and generated 90lbs of recoil.

No, but blasting through plastic and steel usually results in lots of hot molten material, fire, even surface explosions with steel due to igniting the liquid metal. I'm not saying that 20+kW lasers won't damage a person, in fact you've moved me towards very painful flesh wounds since last the discussion. I just don't see very much bleeding or very deep holes in people. Then again, if you severed an artery, the blood pressure would easily overcome the light cauterization, as might running around or stressing the injury...

Was that from the light emitted or venting gases?

Sounds like slow energy delivery, with enough time to melt and flow rather than flash evaporate.

How much energy per unit time do you need to generate armor penetrating effects? Or, to rephrase that, how much energy per unit time do you need to pump into plastic and ceramic armor to generate such a surface explosion that the flesh below is rended? Is a megawatt enough?

Absolutely. Lasers and railguns, despite sci fi nerds' well known proclivities, will absolutely not be viable small arms any time remotely soon. Expect traditional cased weaponry to develop over the next hundred years, but do not expect it to go away.

That is not to say, of course, that there aren't places where these technologies are viable. Consider, for example, shipboard weaponry and missile defense.

Proof? I've heard it said but never seen anything like proof. I've never seen any transfer of light energy to flesh in a speed fast enough to cause it.

Plastic will need the exact same levels as flesh, the two share much in common for purposes of lasers. Ceramic... I don't know. Lasers put the energy at the surface, which with metals is ok due to the explosion and fire, but with ceramic that doesn't happen. We use ceramic blocks to absorb the beam when we have to cut something thin that requires a heavy beam power. When we've finished the job we go over and wipe the black scars off the surface, which are totally undamaged. However, we also run water through the blocks (which would not stop the beam from blasting metals that absorbed it).

From the gas hitting the resonator at supersonic speeds as I understood it. The article talked about jetting a secondary gas to counter the motion, IIRC.

Actually, the link form BGMFH makes a lot of the points. The multi-frequency beams seem like a pipe dream to me, you're already wasting 80% of the input energy, any frequency shifting will just increase that.