# What body armor protects against a laser?

I've been working on a mostly-realistic story set, let's say, 500 years in the future. Humans have colonized the solar system and no longer live on Earth. Semi-automatic weapons with frangible rounds will be commonplace for combat onboard a spaceship, but these are severely lacking in penetration against armor; there are slow-firing laser sidearms for that (see my earlier post: How can I explain a one-shot, slow-to-reload laser sidearm?).

I have looked into aerosol sprays and proper vision protection, but I would also like to have some limited body armor that lessens the damage of a laser. If a victim can walk away with their life and moderate non-life-threatening injuries, it's perfect.

The closer to current-day technology and the less hand-waving, the better. It doesn't have to look great and doesn't have to cover the whole body, and can be disposable. Please don't suggest mirrors.

• Also check out ablative armor. – JBH Sep 14 '17 at 22:45
• Depends overwhelmingly on the laser. – zxq9 Sep 15 '17 at 3:34
• – Paula_plus_plus Sep 15 '17 at 11:49
• @PaperBirdMaster Here's a link to aluminium foil usage example, also related: microwave gun and this. I would use microwave gun against a guy-with-a-slowly-reloading-laser, and I would not do this without this suit – Sanya_Zol Sep 15 '17 at 17:41
• Just a comment on the bullets. Why not use armor piercing bullets? I bet that in 500 years they could have an embedded chip and an explosive charge. If they miss, the embedded computer makes them explode before hitting the hull. If they hit even better, then it explodes inside the target. – Zan Lynx Sep 15 '17 at 19:59

# Thin films, mirrored surfaces, and destructive interference.

Thin films are exactly what they sound like: thin layers of stuff on different stuff. Since lasers shoot specific wavelengths of light, you need to find a material which is highly reflective in that wavelength, and deposit it on a person's clothes or armor. It may not be highly reflective in the visible spectrum, so it need not look like a shiny suit.

Reflective materials do not absorb the light, and the laser will be rendered mostly harmless to the target behind the film. Even if the mirror does heat up, the majority of the laser's energy can be redirected elsewhere. (For real mirrors, this can be >95% of the energy, making a laser a poor choice of weapon.)

Additionally, controlling the depth of an additional thin film can allow you to cancel out the waves reflecting off of the surface in a process called interference, so you protect yourself and neutralize the threat to others. It should be noted that this cancelling effect depends on the angle the laser hits, but will still reduce the overall intensity of a reflected beam!

For those who don't remember or know optical physics: interference does not violate any of the laws of thermodynamics, and is commonly taught in US high school physics courses and at least tangentially studied by anyone who has seen a soap bubble. Interference has been studied in great detail for a very long time: at least since the mid 19th century with wave theory and even by Newton in 1717. As you may well guess by now, it is very well established science, and we use interference in many modern applications. If you still have questions, ask any high-school physics teacher, a physics book, or a physicist. Alternatively, you can learn the basics of interference from this informative youtube video or this, or this and this article.

To summarize: this would be a anti-reflective layer providing the wave-cancellation effects with a highly reflective layer underneath.

These thin films can be chemically deposited on clothes and armor, and need not even be large plates. This could be sprayed on like any other cloth treatment, or be special layers added to the surface of the fabric.

• Cancelling out the waves means not reflecting them. Not reflecting them means absorbing them. Absorbing them means heating. That's not what you want. The best you can do for protecting self and others is dispersing the light into all directions. – maaartinus Sep 14 '17 at 23:25
• @maaartinus Read closer, or get a physics textbook! You can ask the physics stack exchange about the difference between "interference" (or "wave cancellation") and absorption: these are very different things! Try this link as well: physics.stackexchange.com/questions/214852/anti-glare-question or: en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation) and en.wikipedia.org/wiki/Thin-film_interference . – PipperChip Sep 15 '17 at 0:49
• My physics is a bit rusty, but what I'm using is pretty basic. It's even called "the first law of thermodynamics". So just one question: What happens to the energy when the waves get cancelled? – maaartinus Sep 15 '17 at 4:03
• @maaartinus If you measure the energy of the reflected and refracted waves, you'll find that the laws of thermodynamics are satisfied: energy in is equal to the energy out. There are still waves there: their effects are just "cancelled out," and the formal term for this is interference. I'm done trying to convince you of very basic and well established physics. Try this video: youtu.be/-ob7foUzXaY or this book: amazon.com/Optical-Physics-Babies-Baby-University/dp/1492656216/… – PipperChip Sep 15 '17 at 5:37
• Nope. Thin films are not magic. Like all optical systems, they can reflect, transmit or absorb. Thin films are tuned to transmit with very low reflectance at specific wavelengths. If you put them in front of a mirror, they are just going to transmit that wavelength really well on its way in and on its way out. – Dancrumb Sep 15 '17 at 12:48

Reflective armour will generally not avail you: weapon-grade lasers generally work in pulsed mode, where (say) a 1 KJ shot will be divided into 1,000 1 J pulses, 5 µs apart from one another. The first pulse, though low-energy, will be high-powered (because it's very short, on the order of tens of nanoseconds, it causes the skin of the target to erupt in a small explosion which generates a bit of plasma going perpendicular to the surface, that is, in the direction of the laser beam.

The point of pulsing, then, is to wait for that cloud of plasma to disperse so the second pulse isn't absorbed by it, and it instead generates a second explosion on the bottom of the crater created by the first, and so on for all subsequent pulses.

Now, regarding mirrored surfaces, the problem is that they will not be perfectly reflective (say, pretend the efficiency $\eta = 99.5\%$), and that will cause it to absorb part of the pulses' energy and eventually (after $1 \over 100\% - \eta$ pulses) it will have absorbed as much energy as it would have absorbed from the first pulse, had it been completely nonreflective. For $\eta = 99.5\%$, that's $200$ pulses. At that point — actually, probably earlier, since it might need less than the power of a single pulse to do it — it blows up with a small explosion and creates a crater. The cratered suface, however, is no longer reflective (because it needs to be smooth in order to reflect specularly) and thus the other $800$ pulses will hit the target as if the armor weren't reflective.

At these time scales, any currently-known material cannot conduct enough heat away to matter for these calculations, so you're better off trying to get a material which carries the energy away via plasma — probably some sort of carbon composite like fullerenes.

(A big thanks to Winchell Chung and his great Atomic Rockets site, where much of this information comes from)

• What if the mirror moves or rotates with high speed? Each subsequent impulse will hit another part. – Vi. Sep 16 '17 at 2:09
• This will make such mirrored armor quite similar to modern kevlar. Kevlar doesn't make you completely immune to bullets, especially not to higher caliber ones. You will still get seriously hurt, but it might improve your chances of survival by a small margin. Similarly, such mirrored armor won't make you immune to laser weapons, but some people would still wear them. Maybe if the shot comes from far away and is diffused by the atmosphere, or if the aim wobbles a little and the target moves, the pulses might not all hit the exact same spot. ... – vsz Sep 17 '17 at 12:52
• ... Maybe energy sources are bulky, so smaller, concealable laser weapons are less powerful, and reflective armor would be more effective against them. The point is, wearing the armor would send you to the hospital, not wearing it would send you to the grave. With such a setup, armor would still have its uses, but won't make laser weapons obsolete. Exactly as kevlar vests didn't make guns obsolete in the real world, not even all soldiers wear it today. – vsz Sep 17 '17 at 12:55
• @Vi. The armour must be moving/rotating at a speed superior to the spot size (typically 1 mm diameter) every pulse (about 5 µs), so that means 200 m/s (720 km/h, ~450 mph, Mach 0,58) — that doesn't look like a safe speed for what is supposed to be hard material millimetres away from one's body. – Wtrmute Sep 19 '17 at 13:29
• @vsz The main point here is that the relevant time scales are so short that everyone can be considered stationary. So the pulses will hit the same spot because neither laser nor armour can get out of the way in time (the plasma barely can). By the time anything can wobble, the pulse train has long ended. – Wtrmute Sep 19 '17 at 13:32

Smoke.

As the laser burns the first layer of the armor it makes a lot of dense smoke very quickly, this smoke ruins the optics of the laser making much less of the energy continue to reach the target.

Conductors.

Lasers hurt you by making a small place very hot. If the heat can easily move around the armor the area that gets hot is larger so not as hot.

• I mentioned 'aerosol spray' as a laser dispersal agent, but I do like the idea of making it into armor... I might add that in somewhere into my story! – Adam Rilatt Sep 14 '17 at 21:54
• It doesn't even have to be that dense. Just enough to diffuse the beam. Any kind of easily vapourised outer-layer would do. It's really just ablative armour. – bp. Sep 15 '17 at 9:10
• You could even have smoke that only scatters the particular wavelength of the laser, so that you can still see through it. This is of course only relevant if you get large enough smoke clouds. – Dining Philosopher Sep 16 '17 at 10:42
• Can you cite some proof that smoke would be effective? It'd have to be able to absorb and disperse a pretty significant amount of energy. I've seen enough lasers of pointer-strength cut through dense fog and smoke without much dispersal at all. My intuition leads me to believe the density and volume required for this to be effective would make it impractical in combat for a number other reasons. – talrnu Sep 17 '17 at 6:33

In addition to PipperChip's thin films and destructive interference, I'd suggest exploiting the wave nature of light to your advantage, coupled with high-energy-absorption materials.

Light is reflected, refracted, diffracted, or absorbed and re-radiated when it encounters a solid material. Translucent or transparent armor with impurities of differing refractive indexes or even tiny apertures would cause the focus of the laser to spread as it passes through and encounters materials with different refractive indexes.

Behind that, you might have a layer of some material that can absorb large amounts of energy without heating up. Water is known for its high specific heat (~4200 J/kgC), but hydrogen has a tremendously higher specific heat (~14300 J/kgC). Pressurized hydrogen sealed within armor plates of refractive/diffractive materials should negate enough energy transfer to mitigate the effects of laser fire. Helium also has higher specific heat than water if you're not keen on walking around with something so volatile protecting you from laser fire.

The downside (or maybe upside) would be that such materials would be pretty susceptible to conventional ammunition.

• I believe an armor with pressurized hydrogen will be prone to catching fire if it is penetrated. – Vylix Sep 14 '17 at 21:34
• @Vylix I did offer helium as an alternative if sudden, rapid oxidation is a concern. You could also always just use water. If the armor is penetrated and you rapidly heat the water, you could create steam. That might be enough to ruin the laser focus for long enough to escape. – Chris M. Sep 14 '17 at 21:41
• @Chris M. This is pretty clever! I had thought about armor made of gel packs or heat-resistant foam, but your solution is more... dignified, and it definitely has a higher specific heat capacity than gel or foam. Also, thank you for staying away from explicitly suggesting mirrors. – Adam Rilatt Sep 14 '17 at 21:58
• The problems with the light gases is they are light. By mass they are efficient, but with armor you probably care more about effective, and to achieve a certain degree/J would require a lot more volume with a gas than water or metal. And if you assume it is a thin layer the pressure container can't be neglected. There are a number of dense hydrogen storage technologies being thought about, but really high pressure gas isn't a front runner. – user25818 Sep 14 '17 at 22:08
• The problem with pressurised hydrogen/helium is the weight of the container. Light gases are leakier and you therefore need thicker walls – nzaman Sep 15 '17 at 5:15

How about a retroreflective surface, like car or bike rear reflectors and many street signs.
A considerable amount of the radiation would return towards the attacker, not accurately but it might be enough to make you stop firing!

This approach is maybe less practical than using an ablative material that creates smoke- carbon-filled poly-ethelene maybe. The surface hit would rapidly heat up, but in evaporating remove the energy, whilst simultaneously providing lots of smoke to diffuse the incoming beam.

Some ablative materials are also "intumescent" which means they foam up when heated. That would mean the armour might bulk up and insulate from the heat well- albeit moving would become harder.

Lasers are destructive based on the absorption of the laser energy from the target, but very powerful beams (the ones which are used for weapons) would actually that the target material to a plasma. This is actually useful since the plasma is generally opaque to any further inputs of laser energy, so makes a small cloud which absorbs the energy and rapidly expands, dispersing the energy away from the target. Combat uniforms might resemble quilted jackets with pockets of an easily ionized foam to create the plasma when hit.

At high enough energies, the laser will actually "ignite" the air into an ionized plasma, which actually travels back "up" the beam towards the emitter. If the crew of the boarded ship has sufficient time, they might "spike" the air with some easily ionized gas to induce this effect against enemy lasers. This would also make it far easier to counterattack with more conventional kinetic energy weapons like assault rifles, shotguns, grenade launchers and so on. Being struck with a beanbag round or similar in a confined setting might be enough to knock down or disorient the attacker, and being shot in the faceplate of a spacesuit is never a good thing.

I wonder if you need any armor actually...

I'm gonna go with a portable riot shield, so read on.

Let's suppose the target is a human being, so mostly water. The energy to heat up one gram of victim's meat from 37°C to 100°C is 263 J, then 2.2kJ is required to vaporize 1 gram of water, so a 1kJ shot would at most vaporize 0.4 grams of meat, if delivered fast enough, via high power pulses.

Most likely it would remove less meat, as some of the high-power pulse would heat the meat to much higher temperatures (incl. into plasma) so this energy is not available for vaporization.

So, the target now has a tiny dent in their skin, with a cloud of superheated steam/plasma exploding around it at enormous pressure.

I guess the damage mechanism to a human would have a lot more to do with the resulting pressure and explosion than the burn. The 1kJ energy here is equivalent to about 0.2 grams of TNT. Not a lot, but you can already shoot a pretty deadly bullet with that. It's also the kinetic energy of a brick fast enough to turn a skull into a watermelon. Of course only a small part of this would damage the target, but it would definitely result in a bloody mess!

Now, we could use a more powerful laser to really make their head explode. Say 100 kJ instead. But then you have a problem: if you miss the bad guy and hit, say, the nice window overlooking into space instead, then the resulting explosion will be equivalent to 20 grams of TNT, and everyone dies. In fact, it would be a lot safer to use a shotgun loaded with buckshot, which tends to do a lot more damage to people compared to bulletproof glass. Also it would ricochet on bulletproof glass, whereas the laser will make it explode no matter the angle it hits...

Another fun thing with lasers is that if you hit, say, a nice sci-fi space craft wall made of shiny brushed aluminium with the 100kJ laser, then it will vaporize about 10 grams of it (handwaving 100% efficiency)... so now you have a nice cloud of superheated aluminium vapor...

At this point, it occurs to me (ant to everyone else on board) that aluminium nanoparticles are added into thermobaric bombs because they don't just burn in oxygen, they BUUUURN. The 10 grams of Al vapor yield about 300kJ through the fuel-air explosion.

So we have 100kJ lazzer blast plus 300kJ secondary explosion, which gives about 100 grams of TNT, so the spaceship goes boom and everyone is dead.

Hmm....

Okay, so if the purpose of the laser is safety, better use a low power one and don't aim at windows.

Now, the armor. Well, if it's made of kevlar (or space kevlar, diamond, UHMWPE, etc) then all this stuff is pretty much hydrocarbon-based, so the vapor will be very combustible too... So there will be a secondary boom. Same with metals (worse, in fact). If it's ceramics, well, watch out for the chemical composition, as pretty much everything will burn in oxygen if hot enough (boron, silicon...) and as for the answer above who suggested hydrogen of all things...

Water would be different, though: it would turn into a plasma of H and O atoms which would then recombine into water (ie, burn), but this would not bring extra energy to the blast, since the energy required to break down the molecules was borrowed from the laser, it is released again when the hydrogen burns.

So, here's my suggestion: you need a material which will probably be combustible once vaporized, while being able to soak up lots of water relative to its own weight, so that most of the laser energy creates non-combustible water vapor. This can be one of the super-absorbent polymers or hydrogels.

If you're in a real hurry, you can cut up a riot shield-sized piece out of a foam mattress, soak it up in the shower, and then use it as a laser-proof shield to charge at the enemy. You'll never forget the looks on their faces! If there's enough water in it, or you duct-tape it to one of these solid stainless steel kitchen pot covers, it will also (probably) stop frangible bullets.

Also, it is a riot shield, not in direct contact with your body, so the laser-caused vapor explosion will kick it back, but it won't shock your internal organs. All you need is earplugs, and as always when using lasers, safety glasses!

If you're in less of a hurry, you should optimize the water content, because this thing is gonna be quite leaky. If you want a readily available source of hydrogel polymers, remember it is a commonly used hydroponic substrate, and raid the hydroponics bay:

(I mix some of the stuff in powder form with the soil in my flower pots, it increases water retention).

Remember it is also used in baby diapers and, obviously, and perhaps ideally for plot reasons, menstrual pads.

You can also use "personal lubricant" products, or the slimy mucus your space alien buddies produce in copious amounts (no offense, guys).

Or even gelatin from the kitchen.

Basically anything that turns water into a gel and attaches it to a shield, or allows a bit of foam to be soaked and not leak, will work.

I don't suggest plywood as a backing material, it would spall when the laser explodes a bit of water. But any metal would work fine. Plus, as I said, if the water-gel layer is thick enough, it will also stop frangible bullets.

If you can get a large pan of Jell-O from the kitchen, it's a readymade laser-proof shield. Also you can eat it after the battle.

This would be quite heavy though, so you'll need to repurpose any kind of wheeled vehicle like a kitchen cart, or maybe a wheelchair.

If their lasers take 30 seconds to reload, you can probably kill them all with Jell-O and shivs.

Also, obviously, you can counterattack by making the air combustible.

Getting hold of a propane tank on a spaceship should be quite difficult, but you can just lob a kitchen bag of flour or corn starch over the enemy ranks, then hit it with your laser (try to aim carefully!). It will explode and disperse into the air as a very combustible dust cloud.

Flour (or sugar, etc) contains 4x more energy by weight than TNT. However it won't be mixed optimally with air, most of it will end up in the floor, but still...

Now, thanks to youtube, when you want a video of someone doing something very stupid with explosives, the question isn't if you gonna find it, but how many you're gonna have to watch before finding one that looks good!

Here.

How about 500 years of tech improvement to solar cells? Absorb the laser, charge super capacitors, and use the energy in your own weapon? This ends up being very similar to the actual book 'Starship Troopers' vs the very loose movie tech.

As people have mentioned, with a military grade laser, what it hits will burn even if it is reflective. With a pulsed laser, even if the first pulse is reflected, the reflective surface will be marred and the next pulse will have full effect (or near full).

What you want is a black friable surface. It absorbs the heat and explodes outward, taking the heat with it. Aside from making the armor thick enough to take several pulses to the same spot, the material thrown off from the first pulse will absorb or diffract some of the energy from the next pulses.

That armor would probably be built with overlapping plates so that plates can be replaced after a battle without having to replace the whole armor.

This armor doesn't have to be very heavy but it will likely be bulky.

It would also have an odd effect of knocking the wearer back from a hit since a portion of the heat will be converted to kinetic. Most of the heat should be held in the pieces that are being blown off but the same force given to blowing off the material will be felt by the wearer.

Prism sand

You could reach the same effect of a mirror using armor that use manufacture sand; with each grain being a small prism.

When the laser reachs this sand, each prism would refract and disperse the light around the other prisms, reducing his speed and increasing the area of effect enough to avoid any real damage to the person wearing the armor.

All a "laser" does is heat a target object to extremes. The more powerful the laser the faster it can achieve some destructive result.

So really your armor can be a bunch of things:

• mirror based to reflect the light
• heat shielding based, just resist the heat like tiles on the space shuttle
• transference, there are materials capable of routing received light to a collection point or back out to another direction.
• simply thick enough to protect the user long enough to react to evade.

Reflection is the obvious approach. Indeed, highly polished mirrorlike medieval plate armor would be very effective. (I wouldn't want to be standing next to the target.) A densely packed army of plate armored soldiers would produce a disco ball effect.

• There are two problems with this. 1) You don't control where the reflected laser light goes and 2) metal may not actually be the right thing to reflect a high-powered laser. But yes, reflection and optical physics is the obvious route here. – PipperChip Sep 15 '17 at 0:54

So what about metamaterial cloaking? It uses cartesian mesh and other metamaterials to change the coordinates of a magnetic field bending light around a target, making lasers ineffective in the first place, but im assuming this system would be very expensive and vulnerable to kinetic penetrators, and it cannot exist on all parts of the vessel or person because you couldnt use sensors.

Here is the source:

• This sounds like technobabble to me. Can you provide some sources or a clearer explanation of meta-material cloaking? – sphennings Nov 24 '17 at 15:20

# Option 1: Ice

I'm assuming you can arrange either some kind of power armor or low shipboard gravity? These will make even quite heavy armor easy to manage.

You want high albedo ice on top of your normal armor. A 5 cm layer or more. The high albedo reflects most of the laser energy harmlessly, and what's left has to deal with the high heat capacity of ice and the handy latent energy of fusion. How much latent energy? 334 kJ/kg. But you're reflecting a lot of energy before that starts. Sea ice can have an albedo of up to 0.7, I'm not sure how much higher a designer ice could get, but now you've got armor that's guaranteed to absorb over 1 MJ/kj, and without getting up to dangerous temperatures. Every degree you can chill the ice below freezing adds 7 kJ/kg capacity. And the melt product? Water? Well, if you're in a micro-gee environment... it will form droplet which will automatically mess with their optics and de-focus their lasers.

Best of all, your ship can just make more whenever you want.

# Option 2: Thin, layered, high albedo panels.

For this option, you're counting a lot more on reflection than absorption. We can get pretty high albedo designer materials (I think the top is something like 99.7%, but I can't find a good source at the moment), but that will eventually break down. But you don't actually need it to withstand any more than it takes to degrade the reflectance in any given layer, because once it's no longer highly reflecting then that layer has provided it's contribution - and it can burn out and expose the next, fresh, cool, once again super high albedo layer just 0.5 mm down. This means this armor is always super light, and can be as thick as is practical for mobility reasons. Chest plates and helmets could be quite thick indeed. This option can be trickier than the first, as ice has a very broad spectrum albedo, and your designer material might have to sacrifice some reflectance for spectrum coverage. Since it's light, you can also stock lots of riot shields made of the stuff, and they won't be much hindrance to wield.

May require specialized fabrication, and so your ship may have to settle for carrying extra panels rather than fabricate more en route.

Note that it is considerably harder to protect a ship against lasers, because the enemy ship can mount much bigger lasers and has much much more power available than enemy boarding troops.