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After reading up on space warfare for a while now, I'd like show how I imagine a realistic depiction of lasers in space combat. This question is more about the general perception of lasers the characters and audience will get than their physics. Please correct me where I'm wrong, but do not change my basic assumptions unless they are fundamentally flawed. Should some things sound like they are rather on the optimistic side of technical possibilities, that is because my setting depicts the far future.

Laser Basics

Two basic types of lasers are used. Continuous and Pulsed beam. Continuous beam lasers are pretty weak unless they use vacuum frequencies (some UV, X-ray, and gamma), you have a large waste-heat management advantage (shooting from an ocean planet at an incoming fleet) or the laser is ridiculously powerful (a free-electron laser installation meant to push interstellar spacecraft up to several percent of light-speed). Pulsed beams generally give you better penetration and allow you to use longer wavelengths. They are preferred over continuous beam variants as weapon systems. Range and spot-size are improved with a shorter frequency, but shorter frequencies reduce laser efficiency leading to more waste-heat. Spot-sizes range from the centimeter to the meter range. The heat economy of lasers in the setting is generally worse than that of kinetics, resulting in large radiators laser-equipped vessels have to carry.

Laser Defense

Lasers ruled the battlefields of space for centuries until near perfect wide spectrum mirrors capable of reflecting anything from IR to UV with high efficiency were invented. (This is a black swan technology no-one really expected and is an integral part of the setting.) These mirrors aren't magi-tech, but they are very good. Bottled light and laser defense are possible but hardly perfect. This reduced engagement ranges and made missiles and kinetics important in space warfare.

Battle craft design is still dictated by the need for laser defense. Plasma shields (plasma windows) (the closest thing the setting has to energy shields) are a first, mobile layer of defence. They are projected towards the enemy when laser fire is expected, but block all vision of the enemy and can be overwhelmed by lasers. Kinetics don't really care about plasma shields and are used to counter them. The hulls of the ships are covered in layers of the near-perfect mirrors which are cooled to reduce the efficiency loss and act as a whipple shield as well. The armor is sloped to increase the spot-size any hitting laser-beam will have and rotates quickly in order to always present unused armor to the next laser strike or follow up pulses. The last kind of defense is a mass or rather an ablative shield. It is a big chunk of ice and carbon materials, which is mobile and positioned where ever it is needed to block enemy fire.

Attacking lasers attempt to degrade the mirrors by heating and vaporization. Cooling and automated repair mechanisms attempt to counteract this. The lasers usually win, but battles can last a long time.

Laser Visuals

One doesn't see a laser in space except if you are just a few degrees off the firing axis. In that case, an observer will see the beam as a "muzzle flash" coming from the direction of the firing spacecraft. The other case where one might see a laser beam in space is when it travels through a cloud of gas. This could be leaking atmosphere or propellant or plasma shooting into space from a laser hit. Laser hits will create a bright fountain of ejecta.

EDIT1: Clarification after an answer pointed it out: You will obviously not see anything unless the laser is within a wavelength you can perceive (in the far future that doesn't necessarily mean visible light) or something is heated to emitting photons at these wavelengths.

I'm not sure how a laser hit will sound inside a hit spacecraft which wasn't destroyed by it. I imagine it will be a loud bang as the vaporized material will expand expensively.

Are my assumptions, especially about the visuals and the sound right?

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    $\begingroup$ "Wide spectrum mirrors" won't save you unless they are made from something that isn't conventional matter. You can't reflect x-rays specularly (your electrons are too bumpy), and they'll still toast you good. For longer-wavelength light, non-linear optical effects with powerful laser beams will make all your electrons depart for fresh woods and pastures new, and the remaining nuclei will undergo a coulomb explosion. For similar reasons, thermal superconductors can't save you either. Honestly, you may as well say "energy shield" and stop pretending ;-) $\endgroup$ – Starfish Prime Oct 11 at 11:53
  • $\begingroup$ (to clarify, do you want everything there reality checked, or just the sound and visuals?) $\endgroup$ – Starfish Prime Oct 11 at 12:19
  • $\begingroup$ @StarfishPrime 1. Wide spectrum doesn't mean full spectrum. Ionising and long wave is out; I edited the question to make it clearer. X-ray lasers are kept down by how inefficient they are. 2. Will the mirror have at least some effect? Assume that they reflect about more than 99.9999% of the light hitting them instead of 95% as classical mirrors do. They'll to be thin and have hundreds of layers on top of each other and I'll ditch the cooling. $\endgroup$ – TheDyingOfLight Oct 11 at 12:21
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    $\begingroup$ You don't need mirrors, just really white paint. The reason you want a mirror is when you want to keep the beam coherent after it bounces off. Since that doesn't help you in any way here, you don't need a mirror. You just need as much of the energy as possible to bounce off. There's a reason we paint flat roofs with white elastomeric paint rather than trying to put a bunch of mirrors up there! $\endgroup$ – Cort Ammon - Reinstate Monica Oct 12 at 5:44
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    $\begingroup$ @StarfishPrime You just need really expensive paint.. You need the vantablack of whites. If we're hand-waving in 4 sigma worth of mirror improvments, I feel we can handwave in some improvements to diffuse scattering materials. My point is that we can relax the requirement for mirrors. They do extra work we don't need. $\endgroup$ – Cort Ammon - Reinstate Monica Oct 12 at 16:06
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X-ray lasers are kept down by how inefficient they are

Bomb-pumped gamma and x-ray lasers are also affected by very low efficiencies, but making a bigger warhead to pump them is relatively straightforward once you've solved the problem of targetting and shooting straight. There's no issue of waste heat, certainly.

near perfect wide spectrum mirrors

That's a little bit handwavey, but it can pass. It isn't energy-shield levels of science fantasy, at least. What is a lot less plausible is using the stuff as armour. It won't work against vacuum frequencies, because you won't be efficiently reflecting those any time soon... they'll blow off your electrons and your mirrors will be toast. It won't work against high intensity visible light lasers, as non-linear optical effects will similarly destroy the mirrors. Once the outer layer has been blasted away, it will generate fragments and fumes that will ruin the surface finish of the underlying layers. Similarly, deliberately discharging lasers into clouds of gas, dust or plasma surrounding the ship will drive energetic particles into the mirrors, ruining them. Contact with diffuse debris clouds or particle beams will similarly ruin the mirror finish. Buttering up your target with clouds of hypervelocity grit delivered via coilgun or missile sounds like a good first strike... the grit cloud can be wide (and hence hard to dodge) and is somewhat impractical to intercept with point defence. It would mean the first strike always has to be sublight though, which is an important change over purely laser-based combat.

Consider also that high-speed travel through interplanetary space, or spending large amounts of time in busy orbital space (eg. LEO) may also cause abrasion of your mirrors

If the mirrors are cheap to make, you'd use them anyway because every little bit helps and against low-quality, poorly-focussed or extreme range laser beams they will indeed be highly effective. Tactics to remove their advantages are likely to evolve quickly, however.

Plasma shields (...) are projected towards the enemy

You'd need to project them window frame and all, along with a power supply to maintain them. A cloud of unconfined plasma will expand very rapidly, even "cold" plasma. I would suggest that keeping overlapping cold plasma shield frames mounted to your warships would be a better use of these, and might end up being a better laser armour than your mirrors. They aren't perfect scifi shields (they have vulnerable frames, after all) but they do glow in a pleasingly awesome way, and are depleted by being shot at. You can decide which bits of the hull to project. All power to front shields is a reasonable thing to yell in the heat of battle. And indeed they're more deserving of the term "shield" because they're physical objects you have to hold between you and the enemy. "Scales" might also work. There's an interesting dragon-like aesthetic to be found there, I suspect...

The armor is sloped to increase the spot-size any hitting laser-beam will have and rotates quickly

You don't need to flank much to remove the sloped armour advantage. Firing out laser mirrors to deploy in space to get banking shots or missile busses to deploy warheads into flanking shots is straightforward. You may as well slope your armour anyway though... dumb projectile weapon fire will usually be coming from the front, after all.

Remember that rotating armour that takes a big hit is a potential liability. If the belt isn't massive, then it will make useless armour and won't protect you well enough. If it is massive, then blowing a chunk out of it will result in a big wobbly off-centre mass that's gonna be hard to stop and will ruin your aiming (no point defence!) and navigation (no dodging or running away!). One-shot heavy-hitting weapons like bomb-pumped lasers, casaba howitzers and large dumb projectiles will do that sort of damage, as will powerful laser pulse trains or clouds of KEWs.

The last kind of defence is a mass or rather an ablative shield.

All armour is ablative armour. There's not really any kind of "non-ablative" stuff you could usefully use against space warship-sized weaponry. But mass is always in fashion for hiding behind, so it's a good thing to have.

One doesn't see a laser in space except if you are just a few degrees off the firing axis. In that case, an observer will see the beam as a "muzzle flash"

Unless you are a very, very long way away, the "muzzle flash" is going to be powerful enough to blind you and burn sensors, because it is merely the edge of a massive weapon-grade beam profile

The other case where one might see a laser beam in space is when it travels through a cloud of gas.

It'd have to be a very dense cloud of gas, which would be a surprising thing to find in space for long. You might get a brief, faint flicker. Where you will reliably see scattered laser light is in low orbits above planets with atmospheres where misses and overpenetrating hits can intersect the top of the atmosphere.

I'm not sure how a laser hit will sound inside a hit spacecraft that wasn't destroyed by it. I imagine it will be a loud bang as the vaporised material will expand expensively.

I like the "expand expensively" finish, because it certainly will! The noise will depend slightly on the type of laser... bomb-pumped ones and anything powerful enough to drill through faster than the speed of sound (eg. x-ray lasers) will go bang once, but most of the time you'll have a pulse train that will produce overlapping bangs. I'm not entirely certain what this will sound like, other than very, very bad news.

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  • $\begingroup$ «Unless you are a very, very long way away, the "muzzle flash"» - even a laser beam's intensity decrease quadratically with the distance. See diffraction limit, because the Huygens–Fresnel principle still holds. $\endgroup$ – Adrian Colomitchi Oct 12 at 12:46
  • $\begingroup$ @AdrianColomitchi uh, yes? you'll still want to be a long way away. A warship-sized laser weapon is going to have an effective killing range of thousands of kilometres, minimum, and the OP implies that his setting has had laser weapons for centuries, so they'll be a lot more long ranged than that. Perhaps you disagree that needing to be hundreds of thousands of kilometres or more away from a huge gun shooting at you is a very very long way? $\endgroup$ – Starfish Prime Oct 12 at 13:17
  • $\begingroup$ hundred of thousands of kilometers may be reasonable for interplanetary space travel/warfare, but is ridiculous small for speeds one needs to achieve for interstellar travel - at 0.1c, if my armor lasts 3 secs, I'll be at hundred of thousands of kilometers. Besides, you'd need heck of a collimation, because if you concentrate 418MW for 1 second on one square meter, you'll just manage to boil 1 cubic meter of water from 0C. If you work with a pulsed laser, I reckon you'll need something over TW of power in your laser. $\endgroup$ – Adrian Colomitchi Oct 12 at 13:49
  • $\begingroup$ @AdrianColomitchi the OP says nothing about interstellar travel, nor high sublight speeds. If you feel it should, maybe you could take it up with the author? These comments are not relevant to my answer. $\endgroup$ – Starfish Prime Oct 12 at 13:52
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You're correct about the situations where you won't see a laser beam 'in flight', but remember also that you still won't see a 'muzzle flash' from a laser with the naked eye unless the wavelength is within the visible spectrum. Equally a laser passing through a medium will only be visible if it causes the medium to emit visible light, either by direct stimulated emission or simply by heating to a sufficient black-body temperature.

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  • $\begingroup$ Thanks for pointing that out, I edited the question. $\endgroup$ – TheDyingOfLight Oct 11 at 12:04
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1) Continuous and Pulsed beams

Actual power of pulsed laser is by order or two less than continuous one. You see, while pulse laser peak power is incredably small, the duration of the pulse is incredably small eather. So total energy per pulse is very small, and time between pulses is long enough to laser to cool down (by several orders longer than pulse itself), and thus for target to cooldown also.

So if really want to heat something up you'd better use continuous laser. Pulse laser is great for ionising and disabling optics and exposed electronics. (I am talking about long range now, not those lasercutters)

2) Defence

  • Near absolute mirrors has one great disadvantge - they do not dissipate heat through radiation. So you just cook youself without help of enemy lasers. If you would leave spots for radiating heat (like have only one side covered in mirrors or having special radiators) - that spots where enemy would strike (as it is with tanks now - frontal armor is meters of equvalent steel, top armor can be penatrated with AK with AP rounds in some places). As I mentioned before, consider using directly the opposite: material with high melting point and completely black in all spectrum: this material would radiate all the heat from incoming lasers as blackbody radiation. With such an armor laser can heat up this armor for all the eternity without doing any dammage. You can even utilize energy that laser brings, say with Stirling engine to power you (cooling) systems.
  • Plasma is good, but why not just to use say CO2 snow? It has high albedo, can be easely generated in high quantity (just open the valve) and doubles as a fuel for your orientation systems. Plasma is perfect aginst consentrated EMPs, but this are not lasers. but if enemy could use both - why not?

3) Laser hit

Since both pulse and continuous lasers need time to heatup your craft, first thing you hear would be cracking. Exactly like old pot put on fire (and exactly for same resons). You will smell hot metal and burning plastics (you may even die from poison gases this plastic emits). With not-so-laser-proof construction of you spaceship uneven heating may case permanet banding of difeerirnt part of it. It would be feelt as strange movement and tremors of walls around you.

With pulse laser you would also experiens all that static electicity staff: sparks, ozone, dust sticks to surfaces, hairs sticking up and so on.

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    $\begingroup$ Pulse lasers are necessary for anything that can reduce a portion of the target to plasma. That plasma will absorb any further laser energy until it dissipates. Therefore, in order not to waste your energy, you pulse your laser based on the properties of the target. The energy per pulse is slightly less interesting than the power of the pulse. They do damage by exploding chunks of the target, not gently heating it up and melting it. $\endgroup$ – Starfish Prime Oct 11 at 13:33
  • $\begingroup$ Also, spraying out CO2 snow is less useful than having that snow (or the equivalent mass of something more useful) used as armour in the first place, because a fair amount of it will be wasted, and some of it will be blasted back onto your ship. Forget sandcasters, they're pointless. $\endgroup$ – Starfish Prime Oct 11 at 13:35
  • $\begingroup$ @StarfishPrime, its energy (impulse change), that kills, not the power. What damage from those hundred atoms turned into plasma, if my armor has billion billions of them? What damage from explosion of milligramms of TNT per m^3 per second? $\endgroup$ – ksbes Oct 11 at 13:36
  • $\begingroup$ Delivering a million joules over an hour will do almost nothing. Delivering it in a millisecond will be Quite Bad. Nonlinear optical effects come into play at very high power levels, and you'll need to leverage those effects if you want a laser weapon that's actually worth of the name. $\endgroup$ – Starfish Prime Oct 11 at 13:38
  • $\begingroup$ But my point is that pulse laser can't bring million joules per second: its avarage power is 10-100 times smaller (at best case) than of a continuouse one (of same caliber). $\endgroup$ – ksbes Oct 11 at 13:42

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