# How to protect your ship against TW-range lasers?

Let's say you live in a war-torn universe, with sub-FTL intertellar travel, big ships, big railguns and big ship-mounted railguns. At some point, people (read engineers) realized that fast moving red-hot lumps of [basically anything] are pretty easy to detect and dodge, if not fired at point blank range (let's assume this to be less that 10s of travel time to target) or anything capable of changing it's trajectory, really. (That means you can still use your shiny gun as a fire and forget bombardment device against, say, planets!)

For more somewhat up-to-date info on this universe, feel free to check the following:

Time to use some lasers. They can output at least 50TW continuously, but can be pulsed if needs be. We assume that since we have capacitor and cooling (read a big buffer) we can "safely" fire those for at least a minute before catastrophic failure, and no need to worry about energy consumption (unless you need to throw a star in your fusion reactor to make it work)

Now I want some armor to protect my ships against those mean photons the enemy vessel is throwing my way. I already have a Whipple Shield to help survive space rocks and maybe hostile projectiles, but I can't figure out if that would help against lasers. Actually I have to idea how to protect against this kind of weapon (and correct me if I'm wrong, but a mirror sounds like a Stupid Idea™)

So, What is the best way to protect a ship against lasers? I'm looking for a suitable material, or a particular construction that's good at not castastrophically failing when fired upon with lasers, for a realtively light wheight (strapping 30m of [insert material here] onto the ship won't do for obvious reasons)

As previous questions, tech level is several centuries ahead of current tech, extrapolating known technology to the limits of physics is okay, and feel free to ask for precisions/clarifications if you need any.

Not a dupe of how effective is a free electron laser in space combat because I ask about ways to protect against lasers, and not about the effectiveness of said lasers (still not a dupe).

• Comments are not for extended discussion; this conversation has been moved to chat. – Monica Cellio Jun 24 '18 at 4:38

# By not being there

By far the best way to protect yourself from any direct line of sight firing weapon is to not be in line of sight. Options range from being behind a planet, asteroid or other large body, to not even being in the same solar system. This is where things like artillery, missiles and other "over the horizon" weapons systems come into play. All you have to do is make sure there's a horizon.

# Mass and passive heat sinking

Lasers work by heating a small area to failure. Sufficient mass and heat distribution around your vessel prevents the weapon from being directly effective. Of course in the long term if you don't have some way to vent this heat your ship turns into an oven and cooks everyone inside, but hopefully you've won the battle before that happens.

# Active heat distribution

If your ship is rotating, for gravity or just because you think it's cool, then the laser won't be hitting a point but a line. Ideally a line moving fast enough that the laser doesn't do much more than clean and polish the surface as it passes over. You could also actively fluid cool the surface, again distributing the heat around the vessel. Eventually this suffers the same fate as passive heat distribution though, slow baked crew.

Active fluid cooling systems could also force the heat to a specific heat sink location, this works until the heat sink exceeds tolerances and fails, at which point you're on you way to the oven again.

Avoid, Evade, Disperse, Deflect, Absorb, probably in that order of preference.

• Relevant: toughsf.blogspot.com/2018/05/… - Not originally for such ridiculously powerful lasers, but the principles still apply at greater range. – Eth Jun 19 '18 at 10:44
• Reflection is a great way to avoid laser, just coat your hull with a reflexive material (also your ships will look cool). Another way is just disperse a "smoke curtain". Spread some fluid (ok, in the void it anything will disperse maybe too fast) but you got the idea, the particles of the "smoke" will absorve, reflect and defract the light beam – jean Jun 19 '18 at 12:37
• @Eth And this is why the TV series The Expanse has 3 different weapon types, each with ideal ranges. Of course, their long-range weapon is "guided missiles" because they have fantasy drives that can produce 10G worth of acceleration (on a massive ship) for virtually no fuel. So the real-world problems with guided missiles goes away. youtube.com/watch?v=YS4vzoQm_xw But assuming you have decent enough space travel for space combat, you've got this a drive like this anyway... – Draco18s no longer trusts SE Jun 19 '18 at 17:14
• @jean 's 2nd idea about launching stuff at the beam can be quite useful - reactive armor which launches any over-heating plate away from the ship means that until that plate is destroyed, it is blocking the beam without conducting heat to the ship (and if that plate is revolving than it may survive even longer...), similarly - launching some kind of "smoke" or "chaff" to disperse the beam may help lowering the energy reaching the ship to more manageable levels... This won't work on its own, but it's another layer of defense combined with passive and active heat distribution. – G0BLiN Jun 20 '18 at 15:36
• Also, as another layer of defense - if the defending ship can present a small silhouette to the enemy (e.g. it is long and narrow, and when fired upon turns to face the beam) it becomes harder to hit, and a short strafe will take it completely out of the beam - navigation can be linked to heat sensors to automate such responses. – G0BLiN Jun 20 '18 at 15:42

mirror sounds like a Stupid Idea

Why?

You have an incoming energy flow due to the lasers being fired, then your only options are:

• Absorb it: this what would normally happen, turning your ships into a cloud of energized plasma
• Reflect it: make your ship really shiny, so that all the light is reflected back.

Mind that, even though you might have a reflectivity of 99.99%, you would be left with 0.001% of a TW, which is still a tens of MW.

At this point you need to be sure that you have pretty good cooling of your mirrors, if you want to tell your grandchildren some war stories.

Oh, if you have the technology to bend space-time like a large mass would do, you could locally distort the space time and bend the light away from your ship.

In this way there would be no interaction between the laser and your ship, preventing your from dealing with the heat management issues.

You could even turn the laser around by 180 degrees, firing back to your enemies. That would be a nice trick to play!

• Tiled orientable water cooled mirrors are a possibility then. But then we try to destroy the cooling/orientation part with conventional projectiles and then fire lasers again. damn you, counter measure to counter measure! – Alexcommil Jun 18 '18 at 9:23
• @L.Dutch considering they are able to fire a terawatt laser for minutes and dealing with the waste energy, a few MW energy in the armor doesnt seem a problem. I would be more concerned with how a burned mirror immediately loses its reflectiveness meaning that less than a second after the beam hits you are taking the full terawatts of energy. – Demigan Jun 18 '18 at 10:25
• Sorry, won't work. "Reflectivity of 99.99%" assumes you're in the linear domain, but that's not the case with TW beams. Reflection of electro-magnetic radiation works because the incoming EM radiation creates a surface electric current. This current then causes an outgoing EM field. Obviously this works as long as the current is below the maximum possible current in materials. (It also explains why good conductive materials like metals reflect so well). Cooling is no solution - the TW laser flash-evaporates the surface. It then turns the resulting vapor into a plasma cutting lance. Oops. – MSalters Jun 18 '18 at 11:16
• The non-linear bits are which turns the mirror into plasma. But even if we ignore that, photonic crystals still have to work by the same mechanism. Reflection is not magic; you need a surface current both to suppress the further travel of the incoming laser beam as well as generating the outgoing beam. – MSalters Jun 18 '18 at 11:32
• @Demigan Interestingly, this actually gives a valid reason for saying "shields at 90% and holding". – Gryphon Jun 18 '18 at 13:53

According to Atomic Rockets, just keep your distance: http://www.projectrho.com/public_html/rocket/spacegunconvent.php

Lasers need a focal point, and when speaking of space distances lasers have trouble keeping the beam coherent. You need increasingly large focussing arrays for larger lasers and longer distances. So staying at distance reduces the amount of energy per meter and will eventually have most of the laser miss. Its one of the reasons why railguns and the like are considered the long-range options and lasers CIWS against missiles and the like. But considering your laser isnt cooking the entire ship after a minute of firing you might also want these lasers to remain accurate.

Mirrors are reflective, but too much energy will burn the mirror and reduce its reflectiveness and subsequently burn it faster. Mirrored surfaces would decrease the range where a laser is dangerous though.

Theres only two other ways I can think off to protect the ship. First is "simple" ablative layers that bleed off the heat by transforming to different states of matter, and using particular metamaterials that do this effectively. Not sure what kind of materials those could be. The second is extreme heat conduction+cooling and radiating it away. Considering you can fire a multi-TW laser no problem for a minute your heat management is legendarily good. Use that to spread out and lose heat and reduce the effects on the armor. Possibly you could use a Graphene layer (can get a bit hotter than the surface of our Sun, great heat conduction, lightweight and strong enough for armor) for this and have an ablative layer underneath which you keep pusing up against the Graphene so it never reaches critical heat and shows no signs of heat-damage for ships to focus on. Ofcourse there would need to be some channels to get the ablated material out of the ship or the heat will keep rising anyway.

Stop the lazer before the ship by dumping dust where you expect the laser to be!

10TW heating your ship is bad, but 10 TW heating a cloud of dust (even if just meters away from your ship) can be safely ignored.

This of course means you have to survive the initial strike for a couple of seconds or so until you dumped enough dust in the direction of the laser. (Or you just preemtively cloud the area between you and the enemy EDIT: surviving the initial blast is pretty easy, thanks to nzaman's Ablative layer, which itself could already diffuse dust when heated, making the response immediately)

By having your lasers/sensors at the end of the ship (where you don't dump dust) you can still use your weaponry and detection, even if it means a small unprotected part of your ship.

Since the dust-cloud is pretty limited in size (and if the dust is magnetic you can dispel it easily) it won't mess with your sensors etc too much, and the energy requirement is close to zero.

Of course this would have consequences for prolonged or large battles where eventually both fleets are surrounded by an uncontrollable amount of dust, but that would be strategically acceptable imo.

[Bonus of this method that just occured to me: if you already have magnetic dust on board, this setup can also double as a measure to limit the enemy's sensor effectiveness (eg. make your ship appear massively bigger for the enemy or conceal where your ship is precisely, meaning that their non-laser weapons can only target your general location (= the dust cloud) instead of the ship itself, making them a little less reliable]

• Could you explain how this method is superior to having solids inbetween you and the laser? When a TW laser hits a solid it vaporises it and makes it expand, causing effectively an explosion. The expansion makes the created dust fly in all directions. A cloud would also expand and clear out of the beam but faster as theres less cohesion and density of matter in the way. Re-attracting it towards the ship would mean the heat of the dust has more time to transfer into the superstructure and Cook the inhabitants. I dont think dustclouds would be useful, ablative armor seems an upgrade in every way – Demigan Jun 18 '18 at 15:14
• he probably means something like magnetic refractive particles moved around the ship like a coat to the point where the lazer hits – beppe9000 Jun 18 '18 at 18:34
• @Demigan It's basically an Ablative shield, just cheaper. The Advantage is that you dont need to cladyour whole ship in armor. Also, the Enemy cant destroy a single point of your "armor" since its not a fix point. Also it can (/has to be) be combined with ablative Armor, but allows to reinforce spots where the laser has burnt through. – Hobbamok Jun 19 '18 at 16:32
• @beppe9000 : no, that was OP's original Idea, discarded for being too much of a hinderance to be worth it. [In another related Post of his] Being able to Manipulate the Dustcloud would greatly improve the performance though. – Hobbamok Jun 19 '18 at 16:37
• +1 I like this answer because I already make a similar comment. This can function similar to Reactive Armour – jean Jun 20 '18 at 16:31

Clouds

Lasers aren't great through atmosphere. Armour your ship with big tanks of water** - when punctured, a massive cloud envelops your ship and the laser is diffused over a harmless area.

** There may be a better liquid than water for this, but hey, water is also useful for drinking.

• You need an extremely large tank of water. 1 ton would heat up at the rate of about 1 000 000 degrees kelvin per second. Meaning it would quickly become steam, which would quickly expand, leaving you with less water between you and the laser. wolframalpha.com/input/?i=1TW%2F((4.18+J%2F(g+K))*1000kg) – Taemyr Jun 18 '18 at 12:10
• Corrected link since stackexchange does not like the star : wolframalpha.com/input/?i=1TW%2F(1000kg%2A(4.18+J%2F(g+K))) – Taemyr Jun 18 '18 at 12:47
• @Taemyr But how fast would the water actually disperse in 0g? I mean, if the laser cooks the water to plasma, that plasma is still there, absorbing the laser. – Angelo Fuchs Jun 18 '18 at 18:03
• @AngeloFuchs You'd better hope it disperses quickly. If the water does not disperse the pressure will increase linearly with temperature. That means that after 1 second you have about 3000 atmospheres of pressure. Coincidently the same pressure as a water jet cutter. – Taemyr Jun 19 '18 at 7:48
• @Taemyr That doesn't feel like much, a water jet cutter cuts metals only on very short distances and only because of added abrasive sand. Also, as we ejected the water towards the laser, as long as it doesn't disperse too quickly it would need a lot of energy to push it back at us. So, if my guessculations are about right you would need a constant laser of several seconds to form a plasma tube in the waterblob (diameter ~70cm, about a ton of water) if said laser is very close. Longer if the distance is several kilometers (as would be expected in a space fight). – Angelo Fuchs Jun 19 '18 at 9:55

If you are hit by a 50 TW laser, it's like being hit by a megaton bomb, ten times a second. Dodge until you can get a planet or large asteroid between you and it.

On the other hand, if the attacker's laser is 50% efficient, he has a megaton bomb going off ten times a second inside his ship. Use whatever cooling system the enemy uses, only on the outside of your ship instead of the inside.

# Many distant layers of whatever

No matter what you use, it'll inevitably evaporate. You may use mirrors hoping to reflect 99% of the energy, but they'll evaporate immediately. Even with 99.99% reflection rate, you'll be left with tens of MW, which is still too much to cool.

So let's let the shield evaporate. The vapors will absorb some energy and let some energy through. This is where the next layer comes into play. It must not be too close to the first layer so it doesn't get destroyed immediately by the heat of the vapors from the first layer. It'll be hit by a fraction of the laser energy and evaporate. The game continues.

For the last layer, you may want to use a cooled mirror, as it gets hit only by the $n$ times weakened beam and can survive. Or maybe not as two or three additional layers may be cheaper and equally effective.

So my design would be something like hundreds of tiny lightweight shields. Maybe aluminium foil layers spaced one meter from the next layer. They take quite some space (which is plentiful in space, isn't it?), but they're cheap and lightweight.

## The Glass Ship

Nanomaterials are awesome! Build your ship entirely out of materials transparent to the wavelength of the TW-range lasers.

Works best for missions where you don't need to accommodate cargo or a carbon-based crew.

• That could be a good idea for drones, thanks! (But manned vessels would see their crew bursting into rapidly expanding coulds of plasma instantly, so a no-go here) – Alexcommil Jun 19 '18 at 12:20

Deflect it, in time honoured tradition.
Multiple layers of shielding with slightly different refractive index gradually bend the light till it is discharged away from the ship's body.
The problem here is twofold: first the heat generated by the light will warp the semitransparent layers, changing both the shape of the surface and the refractive index of the material. Also of note is that a hit perpendicular to the surface goes straight through. Secondly, the refractive index changes with the frequency of the incident light, so all the enemy really have to do is change the colour of their laser.
Just to be safe, you'd want an ablative layer underneath the refractive layers just to make sure nothing gets through in under a minute.

You could equip the ship with sheets of tiny dielectric laser mirrors with high reflectivity indexes mounted on the sides of the ship to deflect the photons (think sequins on a dress). I presume the technology for the fabrication of these has advanced enough to make them small and light enough, similar to sequins sown onto clothing. If the mirror can handle the inside of the laser, it should handle what comes out the business end as well. If you're worried about heat damage, stack the sheets to turn it into an ablative defense. If passive is not enough, vibrate the sheets to constantly change the position of the mirrors. In theory, if the mirrors are small enough, the pressure of the photons on the mirror might even be sufficient for passive deflection.

• As I pointed out in my answer, even the flimsy reflectivity of dielectric mirrors poses significant challenges regarding the absorbed light... – L.Dutch - Reinstate Monica Jun 18 '18 at 12:47
• You're not thinking about this with a military mindset. There's nothing in the question stating the defense mechanism cannot be disposable. That is the definition of ablative armor. The idea of such a defense mechanism is not to indefinitely survive the attack, but rather to buy the crew time to respond to the attack and either neutralize the source or take evasive actions. – Juan Jimenez Jun 19 '18 at 10:02
• I extrapolated that from OP's "strapping 30m of [insert material here] onto the ship won't do" – L.Dutch - Reinstate Monica Jun 19 '18 at 10:05
• When it comes to lasers, a dieletric mirror <> Inert material. :) – Juan Jimenez Jun 19 '18 at 10:08
• Insert, not inert... – L.Dutch - Reinstate Monica Jun 19 '18 at 10:09

In addition to building reflective layers and dumping particle screens (water, dust, whatever), if possible keep your ship rotating about an axis perpendicular to the beam. Damage is nonlinearly dependent on dwell time, so by rotating you reduce the duty cycle on any given spot. (technique is actually proposed for current-year Terran missiles to defend against DEWs)

• So basically a magnetic bottle full of particles spinning and distributing in all directions. Sacraficing themselves sure but lot more where that came from. Same bottle would deflect the plasma. If the stuff spins fast enough... – joojaa Jun 18 '18 at 20:16

Use a lot of very small retroreflectors on your hull.

A retroreflector is a special type of construct that enables incoming light to be sent back to the overall direction of the source with minimal scaterring.

If your enemy fires a laser into your direction and it hits a retroreflector, he is firing back into his own laser cannon - he will do some damage to you in the process, sure, but in the end his weapons will be taken out of comission in a very explosive way almost at the same instant they fire it.

This has the advantage of instant retribution - hopefully, his lasers won't be able to fire long enough to damage your hull but will fire long enough to destroy themselves. Lasers are delicate things, and probably won't withstand the extra energy being pumped back at them in this sudden manner.

• Now this idea is brilliant! Someone above pointed out that even a 99.9% effective mirror would rapidly evaporate, but in this solution the enemy wouldn't dare to fire in the first place because everything else would come straight back at them. Genius. – Whelkaholism Jun 19 '18 at 15:06
• This is simply not feasible in a realistic environment. At any likely engagement distance the time of arrival is going to be on the order of magnitude of > 1 second. As long as the velocity of the firing ship is greater than its length per second, any reflected laser beam will miss the ship entirely. Having your own lasers and detecting the enemy first is a much better use of resources. – abestrange Jun 19 '18 at 20:02
• @abestrange I doubt those ships will be more than one light-second away from each other. Also, retroreflectors are cheap - way cheaper than expensive lasers and, as any optics, work for free! – T. Sar Jun 20 '18 at 12:05

As noted. there are many problems lasers should have, that said:

Reflective / Refractive Ice

It does not need to be water made. It should have high reflection. It is cheap and can be used for many other problems. As an additional benefit, when the laser hit, it will create gas that will further absorb/distort the laser and hide you.

You might want to have multiple layers for different wavelengths / a way to adjust depending on the wavelength used.

As an additional benefit, your ship would end up looking like a comet when hit ;)

• Welcome to worldbuilding.SE. If you have not already done so, please take our tour and visit our help center to learn more about us. Mirrors are only valuable when the heat-generating capacity of the light source does not exceed the heat-absorbing capability of the "silvering" (the material that makes something reflective, whether polished chrome or silvered glass, etc.) because no mirror reflects 100%. Terrawatts are a whole lot of energy, and you can expect them to melt the backing material, unless that material can handle the heat. What about your backing makes the mirror practical? – JBH Jun 19 '18 at 18:47

Along with my answer here: In a future where lasers are the weapon of choice, why not wear mirrors?, you will want to consider the wavelength of the lasers, as different materials react to different wavelengths differently.

Mirrors can be reflective to just visible light, just UV, or probably a variety of other ranges of light. The material of the reflective portion of a mirror is also critical. When using my 80 watt CO2 laser cutter, I can't etch the reflective side of a mirror, but I can etch the back of the mirror to remove the reflective material, but only for some types of mirrors.

At the bottom of my linked answer, I did some testing of potential "armor" materials and posted the video to YouTube: https://youtu.be/WkOQffTjsC8.

I would suggest Googling "laser safe materials", then depending on what type of laser you're facing, don't use those materials. Often times, a list of laser safe materials will list materials that aren't safe, too. This unsafe list might also tell you why it's unsafe. For a CO2 laser, metals are unsafe up to a certain wattage of laser due to reflection. PVC, on the other hand, is unsafe because it releases chlorine gas at any wattage able to vaporize the material.

You might even have different laser systems depending on the material of the opponent ship. This could have the effect of your captain asking a science officer what the material makeup of the ship is before firing on them.

The makerspace I use the laser cutter at has their own list: http://wiki.qccolab.com/index.php?title=LC6090#Approved_Materials. This is specific to CO2 laser cutters. If you're using solid state or other lasers, you'll need different lists.

In my linked answer, I also have a list for different protection methods that could be adapted to "space lasers of TW" power. It includes all kind of safety goggles, windows, and other documentation.

http://www.lasersafetyindustries.com/Selecting_Laser_Safety_Glasses_Goggles_and_Protection_s/55.htm

As far as mass and heat sinks are concerned, industrial lasers of the KW range can cut 1" (25.4mm) thick steel and thicker with relative ease. Rotating, changing direction, or anything so "the laser won't be hitting a point but a line" will effectively be cutting your hull, or even your ship, into multiple pieces. A few punctures are much easier to deal with than gouges or a massive amount of punctures. Just ask the Titanic crew and designer.

Solar panels only work with certain waves lengths, only convert a fraction of the light to electricity, and have a maximum amount of light they can deal with.

https://en.wikipedia.org/wiki/Solar_cell_efficiency

As far as the "glass ship" is concerned, what about the organics within? They'll get the full blast of the laser instead of your ship, so will any wiring and anything else that can't be made transparent.

Using water as clouds, coolant, or other materials for the same all take mass that a ship would have to accelerate, which there's a point at which this becomes detrimental to your space fight. What is more important, maneuverability or attempting to deal with the laser directly?

A "space laser" would attempt to be a perfectly collimated beam, so that it would have the same effect at 1 AU as it is at 0.1 AU, as well as 10 AU. Current technology doesn't allow that to happen, but future tech could bring that much closer to ideal. This means that simple distance may not be an option.

https://en.wikipedia.org/wiki/Collimated_light

As for "not be there", it's pretty hard to avoid something that travels at the speed of light. You have no way to detect a laser in time, then move out of it's way unless they miss and you see dust vaporizing in it's path or you have precognition. Even if this happens, the sniper/bombardier/targeting specialist/computer/whatever will be able to re-aim the laser faster than your ship can get out of the way.

Getting behind a planet is a decent idea, but how long is it going to take to do that? And is your opponent going to be following you there? How will you fire your laser back?

What might work is a lens to de-focus the laser. A laser cutter works with a collimated beam of laser, then passes it through a lens to focus it at the surface of the material you are trying to cut. If you do the reverse, it'll spread the laser light to be much less effective over a much larger area. You can move a "small" lens of 12" (304.8mm) (or whatever size you need) and it's framework faster than you can a 1 million ton spaceship.

In a laser cutter, you have to clean the lens on occasion. I've accidentally put the lens in backwards, and then it has next to no cutting power. Even the collimated beam has more cutting power than the light going "backwards" through the focusing lens.

Also, a lens that bends the light to a different angle, like a prism, would help, if you can get it to bend far enough to avoid your hull entirely. Something like fiber optic cable, at a massively enlarged scale, might work. Optic cable has a minimum bending radius, but if you keep that in mind, you could shoot the laser right back, without using mirrors.

As a reminder, this question has a "science-based" tag, not "hard-science", so please remember this when making comments. I do not pretend to have all the answers, just the ones I wrote down. Unfortunately, with all the answers I wrote down, it would be pretty easy to think I am pretending to have all the answers. ;-)

It's possible to cancel out light waves using other waves 180 degrees out of phase. How about a defensive laser or light turret that calibrates to match incoming laser fire with 180 degree phase difference to cancel out the attack? It wouldn't be perfect and would take a moment to calibrate, which adds a bit of drama and means lasers would still be dangerous.

• Welcome to Worldbuilding! I'm inclined to believe that doing this in reality would be... Hard. Very hard, considering that you'd have to detect that they're firing a laser before you get hit. Nevertheless, it's a great potential way of going about solving this! – Mithrandir24601 Jun 19 '18 at 20:05

## SCS

Solar Panels do a "good enough" job of turning photons into electricity to be used at an industrial level on earth. Assuming we have improvements in solar capability by the time star wars happens we can capture a lot of the excess power from the lasers. This leads to the development of SCS or Solar Cell Shielding.

This excess power can be used return fire at the enemy at a fraction of the cost or could be used to power expensive defense (read electronic heat sink). Whatever you use the power for, whoever fires the first shot is at the disadvantage in this fight.

The next big improvement is changing the wavelength for the laser being fired, which could mean decreased electric conversions for the solar cell shielding resulting in faster critical levels of heat. Making adjustable shielding is then key to accompany the adjustable photon frequencies being sent at ships.

This adjustable shielding would then make good invisibility cloaks outside of combat as they could absorb or reflect background electromagnetic radiation based on it's surroundings.

I think we should consider refractive meta materials, these are entirely manmade materials that refract light around the object without absorption, current designs include similar materials like a teflon substrate with ceramic cylinders, but this can only be designed to defend against one frequency at a time. In the future we may have a material that could refract all or most frequencies.

# Being far away

It is not possible to get a perfectly focused laser beam. Even the most advanced laser technology will have extremely minor deviations, multiplied over the vast distances of space combat. At close rangers, this un-focusing of the beams can be ignored, but if the distance between the ships is sufficient, the beam may be fully disintegrated.

Verdict: Depends on how good the laser is.

# Being really far away

If your ships are sufficiently far apart (i.e. a few light-hours, a stone's throw by cosmic standards), then it'll be very hard to hit a ship, especially if its size is not on the same order of magnitude as that of a large planet.

Assuming your ships are just one light-hour apart, it will take an hour for the laser to hit. And on the enemy ship, while they are targeting the victim, the "picture" of the ship (so to speak) will be an hour old, and by making minuscule (but sporadic, hard-to-predict) adjustments in course, it will make hitting the target quite hard.

Although it is hard to avoid something coming at you at the speed of light, it's harder to hit something really really far away that's moving around, especially when your beam's direction cannot be changed after firing. What might work, however, is intentionally defocusing your beam based on the range of the ship such that you have a large beam with lower energy. As stated in this answer, a 1 MW laser can melt 2kg of steel per second. You have 50 TW, orders of magnitude more than the aformentioned steel-melting laser. You can spread those 50 TW into a larger beam, decreasing your required accuracy a lot.

Verdict: Depends on how far away you are and how advanced targeting technologies are.

# Mirrors

Mirrors are good at reflecting light. That's a bad thing, because if your weapon is reflected, it doesn't cause damage. The good thing is that mirrors are not 100% effective, and even assuming the enemy has their entire hull coated in 99% reflective material, 1% of 50 TW is still a hefty load (capable of causing quite a lot of damage). In addition, by tuning the laser to other frequencies, the mirrors will be rendered useless.

Verdict: Only works if we're dealing with visible lasers/what your mirror reflects.

# What might actually work

The laser needs to be focused. That is its undoing: By installing lenses to de-focus the laser, the energy of the beam can be quickly dissipated. While the lens must be positioned exactly where the laser beam strikes (which is nearly impossible to determine beforehand), coupled with other defense systems (like the mirrors), enough time can be bought to get the lens in place. The only issue is that the lens must be correctly aligned and capable of dissipating the beam enough that the ship isn't damaged.

# Spin around

Lasers work by heating a single region to the point of failure. To counter this, you can spin the ship, meaning that there is very little time for each spot to actually heat up and fail before the ship turns around. The faster you spin, the less the damage is.

# Jinking, Fairy Dust and Plasma Mirrors

Terawatt lasers requires presence of mind, but even this cannot beat the absence of body. Never be whenever the shooter is aiming at. You do this by taking care never to come too close to potential threat sources, and never following a straight line. Granted, this means you're spending a lot of reaction mass. On the other hand, the ship could be equipped with a secondary tethered ballast ship, and travel in sync with that. By rotating around a common center of gravity and paying out and reeling in at random the connecting tether, you can achieve efficient repositioning and save something on artificial gravity.

Releasing small amounts of fairy dust whenever the local density of cosmic dust is not sufficient allows to easily and safely detect laser beams through Tyndall refraction (those beams that don't hit; those that hit are self-detecting).

Finally, the surface of the ship could be covered with a layer of low-temperature melting metal (gallium or Wood's metal or...). When heated, the metal melts and forms a highly reflective surface; it also vaporizes, creating a local plasma shield that disrupts the incoming beam.

For better performances you can add external electrodes and create a plasma mirror around the ship; this would be ruinously expensive unless you get energy for free, but if you do, it can be made thick enough and dense enough to reflect even higher-powered lasers. Actually, plasma mirrors are being studied for that very purpose - to contain the energies required to ignite nuclear fusion in frozen fuel pellets (1018 W is one million terawatt).