8
$\begingroup$

Inspired by this question, I have a feeling that my presumptions about laser weapons are all wrong.

Let's assume the following sci-fi laser weapon:

  • Form Factor : Rifle
  • Energy Delivered to Target : about 1 $MJ \over m^2$
  • Dwell Time : Negligible (soldiers just taught to snap fire, like we see in most sci-fi shows)
  • Wavelength : Anywhere where ambient air isn't absorbing too much (300 nm to 1,000 nm)
  • Power Supply : Handwavium (disregard; assume standard sci-fi trope of infinite easily portable supply)
  • Focus Technique : Handwavium (disregard; assume whatever it is, it delivers the specified energy to the target)

What does it feel like to fire this gun?

What does it feel like to be hit by a shot from this gun?

$\endgroup$
1
  • 1
    $\begingroup$ Normally I'd enthusiatically write an answer to this (because I like lasers and I have Opinions about what is required of a laser weapon) but alas I have no time. If there aren't a bajillion answers in a day or so, maybe I'll take a look. In the meantime, your "energy delivered to target" figure seems a bit... low. Only 1 joule per square millimetre? That's not a laser weapon. That's a high power LIDAR. Wrong units, perhaps? $\endgroup$ Dec 7, 2019 at 22:50

4 Answers 4

4
$\begingroup$

TL;DR: there's no recoil, there might be no noise, you'll see visible light beams, the damage will look a bit like drilling through hard substances and exploding soft substances. The effect of the shot on the target will be loud and bright. Do not look at laser victim with remaining eye. Wear futuristic carbon nanotube armour. Anything else is pretty poor against lasers.


I'm a big fan of Luke Campbell's work on scifi laser guns. He's got a (lamentably unfinished) chunk of his website on the subject here, and there's some discussion of a specific worked example of his on Project Rho here. I'll basically be following his stuff; its pretty good.

Assumptions

  • Not sure exactly what your megajoule per square metre is supposed to be, but a megajoule is far too high an energy for an antipersonnel laser, and a megajoule per square metre is far too low an intensity for a laser weapon. My example laser will be delivering 2500J to the target, on a spot 2mm across. That's an intensity of about 400MJ/m2. For comparison, the muzzle energy of a 5.56 NATO round is about 1800J, and a 7.62mm NATO round is about 3500J.
  • This is a pulse laser. The "shot" will be delivered in the form of 50 pulses of 1μs duration 50 joules energy each, with 4μs between each pulse for a total shot time of 250μs. Seems snappy enough. The reasons for this are complicated, but drilling through a target with a train of pulses is a good way to maximise damage without wasting energy in making hot gas or wide craters or risking atmospheric breakdown. Luke Campbell calls this style of laser a blaster
  • You said not to worry about focus, but remember that ranging the target and adjusting the optics will take a non-zero amount of time. Remember that when you say "snap fire"... shooting at an unknown range with an unfocussed laser might blind them, if you're luck, but probably won't do anything useful. The depth of field with a plausible laser gun for use by humans is going to be limited.

Shooting

The firing of the gun will be, most likely, silent. Peak power levels here are high (100MW when the laser is active) and noises mean wastage leading to mechanical movement or arcing and that sort of thing at those power levels will cook your gun. What won't be silent is the cooling system, because that will probably need to be active before shooting, as you don't want the coolant around the laser itself to get too hot. It'll get noisier after firing... you'll be wanting forced-air cooling in an atmosphere, so there will be fan noises, and they'll continue for a bit after the shot. The image stabilisation for the optics might make a quiet noise prior to shooting, too.

There will be no atmospheric breakdown with subsequent generation of plasma, noise and light. This is because any sensible laser that does that will be absolutely useless at the job of actually zapping a target, because all its beam energy will have been used up. Thick smoke and dust will force you to use lower pulse powers. The gun might be smart enough to handle that for you. It might warn you and tell you to do it yourself, but in the heat of the moment you'll probably not do it because you're a fallible meatbag. The peak powers achieved by this laser combined by the narrow focus of the beam close to the target reach an intensity that's perilously close to the breakdown intensity of clean air. As things get ground to dust, get vapourised or catch fire, you'll start getting plasma formation along the beam, starting from the target and reaching back towards the shooter as the air gets dirtier. That's a sign to reduce power or focus, or pause for a moment.

A visible light laser will be clearly visible in air. For a 500nm green laser, scattering will be about about 33W per metre which will be very clearly visible even in daylight (it will be brighter than a fluorescent strip light), showing everyone looking a clear line between shooter and target. A 1000nm near IR laser with the same power will scatter more like 2W per metre, but it will be invisible to the naked eye. It'll show up brightly in any sort of "night-vision" type gear, though.

The shooter will almost certainly not directly see the beam of the gun they are firing, because they do not want to be blinded by it. They'll wear filtering glasses, goggles or contacts (or camera lenses) that completely block the appropriate wavelength. You might still see hostile lasers which operate at different wavelengths than those you were prepared for, but if you knew which wavelengths your opponents were using you'd block those, too. Various kinds of electronic display will show you the beams of otherwise invisible lasers including your own so you can see where everyone is shooting. Such displays may also show you near-IR laser beams which will be invisible to the naked eye.

Being shot

Where the beam is incident on the target you'll get a very bright flash where material has been vapourised and partially ionised. This'll be basically broad-band high-colour-temperature light regardless of what colour the laser beam is. The pulses are too short and too close together for anyone other than a machine to distinguish them. It'll make a loud crack, or bang, probably not unlike a gun going off.

What happens to the target will depend on what it is made of. If it is protected with super futuristic carbon armour made out of nanotubes or other very strong and refractory materials, relatively little. It'll cut a hole a few mm deep. This weapon isn't for killing your equivalent of Space Marines. Against steel though, it'll do a wonderful job of melting and blasting a deep hole, a little wider than the spot and nearly 8cm deep. The hole will not be neat, but messy, caused by escaping hot gasses rather than merely melting. Steel is a terrible armour for use against lasers. Concrete is also a poor choice for cover, as the shot can punch through over 25cm of the stuff. It won't kill you after doing so, but a second accurate shot (rather than snap firing) certainly could.

Against an unarmoured human target, it'll blow a hole clean through them. The string of superheated steam explosions will expand with much more force than a bullet impact, and the damage will be consequently greater. If the beam hits a limb, it'll come off. If the beam hits the target in the torso, the chances of them surviving are minimal. The damage is largely mechanical, caused by rapidly expanding superheated gas (eg. an explosion), not by melting and burning. The wounds won't be neat or cauterised. It'll be messy and graphic.

The flash of the shot hitting the target will bebad for the eyesight of anyone with unprotected vision who was watching. Any laser light scattered by the gas and debris cloud will be bright enough to blind, as will any reflections. Using this weapon anywhere there might be bystanders you don't want to blind would be risky. If you use a wavelength that's absorbed very well by the cornea might cause superficial cataract-like blinding that is correctable, so you might be able to patch up the wounded afterwards. Civilian and police weapons might make use of this fact. The 1000nm wavelength is probably good for this purpose.

$\endgroup$
2
  • $\begingroup$ 2500 J, per 2 mm is about : 795 MJ, per square meter? I don't mind looking at some alternative designs. I picked 1 MJ per meter squared because I believe that's where the air turning to plasma starts to become a problem - and I wonder if it's an insurmountable upper limit to man-portable lasers. $\endgroup$ Dec 29, 2019 at 13:08
  • $\begingroup$ @JamesMcLellan the worked example above does not cause breakdown of clean air. $\endgroup$ Dec 29, 2019 at 15:00
6
$\begingroup$

It Makes a Sound

If the energy density were a little higher, there would be a popping sound as blooming causes some of the air along the path-of-fire to momentarily turn into plasma (an effect a little like lightning).

There's an even bigger sound when it hits the target (more on that in a bit)

It Has Recoil

I had never thought about this in sci-fi settings, but photons have momentum. It's the principle light sails are built on.

The momentum of a single photon is $ {{planksConstant \times frequency} \over {speedOfLight}} $. The frequency can be found from the wavelength and the speed of light $ frequency = {speed \over wavelength }$

For the sake of easy math, I'm picking an operating wavelength of 300 nm (slightly ultra-violet). That provides a frequency of $ {{3 \times 10^8} \over {300 \times 10^{-9} }} = {{3 \times 10^8} \over {3 \times 10^{-7} }} = {1 \times 10^{15}} $

The momentum of a single photon of this frequency is $ {{6 \times 10^{-34} } \times {1 \times 10^{15}}} \over {3 \times 10^8} $ = ${{6 \times 10^{-19} } \over {3 \times 10^8}}$ = $ 2 \times 10^{-27}$ kg m/s.

The energy of that same photon is equal to Planck's constant times the frequency, $ {6 \times 10^{-19} } $ Joules

A .50 caliber weapon has a muzzle area of 0.0014 square meters.

If the weapon is dealing just under 1 megaJoule per square centimeter (0.0001 square meters per square centimeter), the energy per shot at an instant on the target is $ 0.0014 \over 0.0001 $ = 14 square centimeters. Multiplied by 1 MJ per square centimeter, the total energy is 14 MJ.

Divide the 14 MJ (14 million Joules) total energy by the energy per photon $ {6 \times 10^{-19} } $ Joules to get the total number of photons $ {1.4 \times 10^{7} } \over {6 \times 10^{-19} } $ = $ { 0.23 \times 10^{26} } $ photons.

Now that we know the total number of photons we can compute the recoil : $ { 0.23 \times 10^{26} } \times { 2 \times 10^{-27} } = { 0.46 \times 10^{-1} }$ kg m/s = 0.046 kg m/s

How Much Recoil is That?

A .22 caliber hunting rifle fires at about 1,200 feet per second ($\approx 330$ m/s) a 40 grain ($\approx 2.5$ gram = 0.0025 kilogram) projectile has a recoil of $330 \times 0.0025 = 0.825$ kg m/s

The laser rifle, then, has about $1 \over 20$th (5%) the recoil kick of a gunpowder weapon in the same form factor. It may still require training to use the gun effectively.

I imagine someone trained and acclimated to laser weapons would think, if he or she ever fired a gunpowder weapon, gunpowder weapon technology useless. With it's 20x greater kick, someone acclimated to laser weapons would wonder how it was even possible to use such an unwieldy thing in combat.

What's It Feel Like to Get Hit With This Thing?

With the high energy and low dwell time, this weapon is not designed to cut a hole through you.

It's designed, instead, to hit you with enough energy that part of your own body turns into exploding gasses.

How much energy you actually absorb depends on a lot of complicating factors, but assuming you took a bad hit of most of the 14 megaJoules of energy, how bad is that?

1 kilogram equivalent of TNT energy is 4.1 MJ. A blast caused by a laser rifle deliver 14 megaJoules to your body, or nearby, then is $14 \over 4.1$ = 3.4 kilograms

How much is that? According to Wikipedia the typical modern scalable offensive grenade has 340 grams ~ 0.34 kilograms of blast force. Therefore, being in the room where this rifle strikes home with a single shot is like receiving 10 offensive grenades planted right on you, and detonated simultaneously.

Offensive Grenades, Pack of 6

There's a Blast

As we might have seen in some sci-fi films (but generally thought they were the cheesy ones) this laser creates a fireball. But it's more of a WW2 bombing run blast of expanding gas, than a lingering Hiroshima ball of burning air.

This would be true even in space, because the expanding gas was formerly the surface material of the target.

There's a LOT of Collateral Damage

In addition to the blast at the target location, tiny volumes of this beam have been getting reflected by the atmosphere in every possible direction during the trip this energy took from the weapon muzzle to the target.

In even low-powered laser labs, students are asked to wear glasses against the accident of laser energy reflected back at them. And in some instances, that small reflected bit can be enough to damage clothes - even in very low power settings.

Scaled up to 10 offensive hand grenades of delivery, this wandering energy is now injuring everything in front of - and behind - you when it's fired.

This isn't a weapon of limited warfare. Nor is it a weapon of precision. Random people who aren't wearing adequate protection might get bad burns.

Moving this into space, the situation remains, and gets worse. Most reflections will happen when a target is hit, deflecting part of the energy away. But there's no atmosphere to patiently dissipate the energy.

It seems like there is a small likelihood of craft even several light seconds from the space battle might take a stray shot. And, because space isn't dissipating the energy, it might not be unheard-of for the occasional random craft through history, even though several light hours away, to have the unlucky fortune of being struck by ricochet or missed shots.

$\endgroup$
8
  • $\begingroup$ You're now saying megajoules per square centimetre, whilst your question says metre. It'd be great if you could make them match. $\endgroup$ Dec 8, 2019 at 16:09
  • $\begingroup$ Oops. Let me check and fix that $\endgroup$ Dec 8, 2019 at 16:51
  • $\begingroup$ The choice of a energy value was based off table 1 here - ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150010986.pdf - looking for thermal blooming, and inferring the relationship between the radiuses. Looks like the question is right and I've made a mistake here. I'll have to go back through this all. Thanks for the catch! $\endgroup$ Dec 8, 2019 at 19:28
  • $\begingroup$ LOOK AT THAT MATH :D Good Job! $\endgroup$
    – PipperChip
    Dec 8, 2019 at 22:21
  • $\begingroup$ I mean, technically yes there's recoil, but I'm dubious that you'd even be aware of it under combat conditions. Trained snipers would probably notice that level of force being applied to their weapon, but I don't think a regular rifleman would. $\endgroup$ Dec 8, 2019 at 22:31
5
$\begingroup$

It would deliver built in tactile and auditory feedback.

It was disconcerting to have a gun that fired but you did not know it for sure. There is minimal kick. You cannot always see the laser. You cannot be sure it has worked. Soldiers were used to regular firearms that shoot projectiles. These offer tactile and auditory feedback so you know you have actually pulled the trigger and the gun worked. After laser rifles were rolled out many nearby items were shot with lasers at close range when soldiers tried to ascertain if their weapons were functioning; wasteful and dangerous.

Feedback methods were built in to later models. Various sounds were tried using onboard sound generation. "Pew Pew" was an early favorite, for societohistorical reasons but some officers found it undignified. Also, amid the cacophany of multiple identical "pew pew" noises soldiers could not be sure which of many firing guns was making the noise. Objects were still often shot at close range.

Ultimately, the specific tactile feedback offered by the trigger and auditory feedback delivered with a successful firing was left up to individual soldiers, who customized their gun sounds and feel as they do their ringtones.

Many excellent laser rifer sounds were then devised and implemented. Soldiers often try to synchronize and time their shots, to combine their various laser rifle sounds into electronic beat tracks. Rap battles ensue. It is still considered undignified by some (older) officers but it is great for morale, and so tolerated.

$\endgroup$
4
  • 1
    $\begingroup$ You need a sentence at the beginning: “Firing a laser rifle initially felt like nothing happened.” That would set the stage for the rest of your answer (and answer the original question). $\endgroup$
    – SRM
    Dec 8, 2019 at 6:35
  • $\begingroup$ @SRM-ReinstateMonica - good editing; thank you. I have switched the first and second part of that paragraph to serve as your suggested intro.. $\endgroup$
    – Willk
    Dec 8, 2019 at 14:31
  • 1
    $\begingroup$ Why sound? Sound is pretty much the last thing you want. During combat it's possible that you are too stressed to notice if the sound you heard was your own gun firing or that of someone else. In the Musket days they sometimes found guns that had been loaded multiple times because the owner didn't realize his gun wasn't firing over the sound and stress of battle. Sounds might also reveal your position. Tactile information is where it's at, It needs to happen only when the gun actually fired it's laser, say a panel moves under your hand when it's radiating excess heat after each shot. $\endgroup$
    – Demigan
    Dec 8, 2019 at 19:27
  • 3
    $\begingroup$ There needs to be a "funny and enjoyable to read, but not realistic" as an option for this board. Because it isn't quite as serious as, say, stackoverflow. The idea that a military would artificially add sound to their guns seems unlikely, as Demigan points out, but the images you've come up with make this worth the read for sure. $\endgroup$
    – Zwuwdz
    Dec 8, 2019 at 22:44
4
$\begingroup$

I think that firing the weapon would feel like playing with a child's toy rifle. It wouldn't make any noise unless some internal mechanism made noise. And, it wouldn't have any recoil to speak of since the mass of photons is very nearly non-existent. The weapon itself might exhibit some kick if there was a significant current flow generating a Lorentz force.

In an atmosphere, the laser pulse would heat the atmosphere causing the path of the pulse to form a partial vacuum. After the pulse passed, the atmosphere would rush back in and generate a snap or thunderclap -- I think it would be a snap like static electricity.

Getting hit by such a weapon would usually be painless since nerve tissue would be vaporized or cooked and rendered inert. If this is a steady-state beam, the energy transfer would ablate tissue and bone, boil blood. This conversion of the body into a plume of vapor would form a gaseous barrier as the superheated body tissues expand away from the body and would absorb some of the beam's power. For this reason, and others, the laser weapon might be pulsed so the target plume could dissipate.

Some of this plume of superheated vapor would move in the direction of the body, this would be another part of the engineered energy transfer. The superheated vapor would vaporize more tissue to a lesser degree. This pattern would continue until the energy was dissipated entirely.

Since the velocities of the superheated vapor would be far from relativistic there wouldn't be any of the things seen in movies and the tv where someone is shot and thrown back. The target would experience a pressure wave caused by the expanding gases and ablation of tissue. If they were in zero-g, it would act like thrust and push them backward. But on a planet, I don't know if it would be enough force to throw them off their feet.

Any damaged cells that remained in the body would be broken down in end up in the blood to be filtered out by the kidneys. And, just like a lightning strike, there could be a high risk of kidney failure and death as it gets plugged up. And, any boiled blood that remained in the circulatory system would no longer be able to transport oxygen or carbon dioxide, and breathing could be compromised. And, there would be some possibility of an embolism if gasified of tissues forced themselves into the circulatory system and made their way to the heart. I am told this is much overplayed in medical shows and isn't necessarily instantly lethal but would require medical intervention to avoid injury or being incapacitated.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .