Argthtjtr

Don’t block the shot: block where they’re aiming.

Basically: you can’t block the laser once it’s been fired, as the moment you know about the laser is when you’re hit by it. What you can do is monitor the emissions/light bouncing off of your enemy prior to the laser being fired.

For this to work you need two things on your ship and one further thing to be true:

1: You need stupidly good sensor packages that can track nearby threats and their emissions/any light bouncing off them. Using LIDAR (like radar but with lasers) to actively paint potential threats would be useful, but kinda defeats the point here...

2: You need stupidly good threat analysis software that can identify ‘they are pointing their guns at me’ and ‘their gun is about to fire x kind of laser’, and bring up the appropriate shielding faster than they can aim and fire at you.

3: The enemy weapons need to be visible from your ship so you know where the laser will hit you. The exactness of these measurements will depend on how precise your lasers/shields need to be. More precision means better info on laser gun position is needed, but if your shields cover whole arcs of the ship then you can get away with knowing ‘my enemy is that way’.

Once you have those things you can bring up shields in the instant you get the ‘ they're Aiming at me and firing’ em radiation from your enemy, thus blocking the laser that will arrive shortly thereafter.

This method can be blocked/jammed/messed with in any number of wonderful plot-hooky ways, from your own shielding temporarily blinding you to your enemy deploying drones that spoof active laser signatures so your defences can’t be focussed, but if you can see your enemy you can use this as a pre-emptive defence method.

A laser beam is made of photons, and photons travel at the speed of light. Therefore whatever they emit will reach the target together with the laser, and there is no much use for it.

The only way is to detect the fingerprint of the device used to emit the laser, which will be necessarily working before the emission of the laser.

If the X-ray laser has a different fingerprint than the IR laser then the target can adjust the shield accordingly. Mind that the fingerprint can precede the shot of just few millisecond or even less, therefore the shield has to be able to quickly adjust itself.

However I also assume that the attacker will also try to shield or alter the fingerprints, so that the target is lured into using the wrong shield.

If you don't detect enemy ships and their laser-capabilities you just do not have any time for detecting the laser-shots as they go at the speed of light. For your ship moment of you see the shot is the same moment it hits you.

But there are ways to combat lasers.

1. 
   

   Every ship would want to have good detection capabilities and some measure of stealth to be able to detect enemy ships first or push range from where you can be sniped at far enough . Long distance would allow to employ an evasive patterns in the movement of your ship. For example, at the distance of Earth-Moon enemy ship would need to "guess" where your ship will be in 1.3 seconds. They try to predict you or get closer and both ships detect each other.

   
   
   

   Lasers may look like they are a lines, but they are more cones, so extra distance spreads it's energy over more area.

   
   



2. Passive defense. Your hull can be covered in layers of reflective surfaces with a gap with a metal foils, that when hit by a high-energy laser beam evaporate and produce plasma clouds, that absorb even more of the energy from the lasers.




3. Hybrid one. Use floating sheet drones around your ship, that cover up the ship if there is a danger of laser assault or any other methods that will give you time to detect enemy ships and return fire.

For more effective shield you may use them in layers. Each layer is optimal for different type of laser. After analysis of attack or scan of detected attacker you convert most of your layers to optimal version. Methods above may be used to give you more time for the preparations.

There are two potential ways to have your shields adjust. One from remote sensing, one from direct sensing.

Remote Sensing:
For a light-speed weapon, if the first time it is fired in a battle is against you and if it hits on that shot then the first notice (outside having a "shell" of FTL sensing somethings) you will get of that is of it actually hitting you. See direct sensing for that case. However, if it is ever fired other times prior to the shot that hits you (against other targets, or against you and missing) then you have a chance to determine its frequency. Lasers have one very telling property that makes them relatively easy to discern, and that is they are extremely monochromatic. This is what makes it relatively easy to make laser "radar detectors" (really lidar detectors) for cars on earth. The laser of a police lidar is scattered somewhat by dust and by whatever else it hits and that scattering combined with the fact it is extremely monochromatic makes it relatively easy to distinguish from other EM sources. For space-born laser weapons, well, even in space there are some particles. The Earth's observable exosphere extends to at least 10,000km. For some planets this could conceivably extend much, much further. Even far away from a planet with an atmosphere, there is always some dust, some hydrogen - in some areas much more than others. And, presumably, in an area where there is a space battle, there could be a lot of "stuff" around that might cause some scattering of a laser weapon. Gas and particulate from engines and thrusters, debris, vented ship atmosphere, and the hulls of other ships being hit with the same weapons. And with laser weapons, which are presumably quite powerful, it wouldn't take much to create enough scatter for another ship to detect it. In fact, there may be enough scatter just from the laser's own focusing lens, since no lens is perfect. Ships' combat and target threat management systems and operators can be set up to look for that monochromatic scatter and trace it back to the firing ship in order to determine the best shield parameters to use for that ship's weapons. While the firing ship may have more than one laser weapon, it's unlikely that any one weapon will be able to fire many different frequencies. Lasers are something, at least with technology we can reasonably foresee today, that you can't really make to be frequency agile.

Direct Sensing
As noted above, in a scenario where there is a surprise shot, where an enemy ship fires for the first time on you and that shot hits, then the first notice you will have is that shot hitting you. Outside of FTL somethings. However, that doesn't mean the game is over. People tend to think of a laser as either hitting or not hitting. In reality it is, of course, more complicated. The amount of energy actually transmitted by a laser weapon will depend on the power output of the laser (in watts) and the length of time it dwells on target: E = P × t which means that a large factor in your shields protecting you against a laser they are not currently configured for will be their reaction time. The better welding masks, that protect your eyes from the extremely high intensity light of arc welding at close distances, are LCD masks. They are, essentially, just a large single cell of liquid crystal between two panes of glass, connected to a light sensor. When the light sensor goes off, the LCD goes dark and blocks the light. But this means some of that light has already passed the filter by the time it reacts. The key is, was there enough to do any damage to the retina? The answer is no - these type of masks are proven safe and are regulated by many different national standards agencies. So, the lower the length of time, t, that the laser is on you, the less power is actually transmitted to the ship. With a sufficiently rapid shield reaction, there would be essentially no damage. No matter the power, P, of the laser, as t approaches zero, so does the amount of energy transmitted to the ship. Of course, the more powerful the weapon, the faster the shields must react to prevent damage. This might infer an "arms race" trying to make lasers that produce very high energy pulses, and shields that react more quickly.

You look at what weapon is pointed at you.

Lasers are actually not very good as ranged weapons in space combat. The laser will spread out too much and lose the energy it has over toi much surface area to be effective. Also firing a laser generates a lot of heat on the ship itself, which is very hard to get rid off on a space ship. This means lasers are mostly thr CIWS of future space ships. Great for point defense against fighters and incoming missiles but useless against long-range targets.

Now imagine that because you have shields, your fights happen at much closer range. Now that laser is effective right? Unfortunately you have a shield that can deflect laaers. This means you need to alter your shield so that your lasers have as little effect on your own shield or you would lose both firepower and desabilize your own shield a little. But since your enemy can simply measure which wavelengths penetrate through your shield they can simply make their shield opaque to those wavelengths, neutralizing the effect of those lasers.

Ergo: you can react to something that moves at the speed of light by knowing what is necessary to fire something at the speed of light.

You've got it backwards

Stopping solid projectiles with a trapped plasma is much, much harder than you seem to imagine. As an example, consider a re-entry vehicle: that stuff heating up the nose is a plasma. After a few hundred kilomtetres of that, all that is vaporised is the ablation shield.

(As a small aside, the plasma isn't trapped between magnetic fields; it's trapped within a magnetic field, between zones of high Z field gradient, which tend to reflect ions.) In contrast, any plasma dense enough to vaporise a projectile during the milliseconds it takes to pass, is going to be pretty much opaque to all EM waves.

There are also lots of other issues with beam weapons. Writers who have thought about the engineering issues seem to assume that space combat will consider 100 km to be "point blank range", and most engagements will be at several thousand kilometres. At these ranges, simply focussing the beam on the target tightly enough to do damage is extremely difficult. It becomes practically impossible if the target starts dodging randomly; a plasma shield just ups the required power threshold for damage even further.

So it seems practically mandatory that weapons have terminal guidance. Once you have that, even fairly simple missiles can destroy all but the most extremely armoured spacecraft. (To complicate evasion, they should, of course, be stealthed, with skin chilled with liquid helium and course corrections by low signature engines such as cold gas, rail guns, or ion thrusters.)

Furthermore, vaporising the projectile isn't enough:

1. If it's a kinetic kill weapon, it still has the same mass, and is hitting at the same hypervelocity impact speed. The exact penetration depth will change a little, but above a certain velocity the hardness of the projectile doesn't make much difference.


2. If -- as is more likely -- it's a missile with a warhead, it just detonates when its proximity sensor detects your plasma is close enough. Given the scale of space combat, they will most likely be nuclear. Against most targets, sinply detonating near by will be enough -- plasma shield be damned. For very heavy armour, such as the classic "hollowed out asteroid battleship", you can use the x-ray hohlraum of a fusion device as the compression driver of a Munroe effect charge to project a few tonnes of tungsten plasma forward at several hundred kilometres per second. Due to extreme pressures achieved in this sort of device, the tungsten plasma density is several times higher than the solid metal density! Try trapping that with your magnetic mirror.

BTW, issues of the physics of space combat were explored in quite some detail by the space opera RPG "Traveler" way back in the 1970s

Having the laser touch the ship would not defeat the purpose of a shield.
Laser weapons generally work by heating anything they hit. Assuming you're not thinking about insta-kill death rays, it probably takes a while for the laser to do significant damage. Your ship could get hit for a very short while, and then adjust the shields - assuming this process is quick, you'd probably take minimal, damage, if any. Just like when you touch something hot, you immediately take your hand away and suffer minimal burns (or none, if you're lucky) - if you didn't, your hand would end up roasted.