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In a space-faring, war-torn world with mostly-realistic technology, the engineers have been developing laser weaponry and defenses. All sides of the war have roughly equal technology. This question How to protect your ship against TW-range lasers? provides a good Q&A regarding different approaches to defend against a laser that is able to fire 50TW of energy over the course of a minute (without self-destructing).

For laser weapons of that power we have to assume that the attackers have the capability to focus a large amount of energy through a lens, and are able to dissipate a massive amount of energy. Using that same technology wouldn't defenders be able to refract any laser attack away using lenses, and/or dissipate the heat of the attack? Or is there some way that laser technology gives an advantage to the attacker?

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    $\begingroup$ A lens / mirror in a laser weapon is designed to do only one thing (be reflective / refractive) at a specific frequency over a specific small location. A ship's hull serves multiple purposes, including defending against damage from multiple types of sources (e.g. projectile / micrometeoroid impact) or laser weapons across many possible frequencies over a very large area. So clearly no. $\endgroup$ Feb 26, 2021 at 16:49
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    $\begingroup$ The best defense against a laser is a very big baddaboom guided kinetic projectile weapon. It is very, very difficult to NOT make a laser weapon its own targeting system, painting a very big bullseye on it. $\endgroup$ Feb 27, 2021 at 15:49

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wouldn't defenders be able to refract any laser attack away using lenses, and/or dissipate the heat of the attack?

This is a common misconception (here and elsewhere) in which people really don't appreciate the power levels of a practical laser weapon. That's fair enough; it isn't intuitive.

First, note that your lasers probably won't be using refractive elements, because they'll be hard to cool. Mirrors are more convenient in that regard, as you can put coolant channels on the non-business side. They also give you the ability to have deformable mirrors and so on.

Next note that the energy density at the mirror is going to be much, much lower than the energy density at the target: this should be obvious, because you don't want to incinerate your own optical path, and you don't want the enemy to be able to trivially defend themselves by simply fitting mirror armour.

As the intensity of light incident upon the target goes up, you start getting non-linear effects. One such effect is multiphoton ionisation, whereby highly intense light of longer wavelengths (say, visible) can blast the electrons off matter it interacts with. Once this starts happening to the target, no amount of magical heat-sinking will help because free electrons floating around will do bad, bad things to the chemical and crystal structure of the material. This also applies to lasers emitting ionising radiation, such as UV or X-rays, though these will be harder to engineer than longer wavelength devices.

Finally note that even for purely thermal effects, the thermal conductivity of your armour plate and structural materials is limited. Even if you have more that enough cooling capacity to keep your ship lovely and cold even in the face of a terawatt of heat, the point at which the laser hits will likely heat up faster than you can cool it down so you'll still take damage.

Some scifi settings introduce "thermal superconductors" to handle the latter, but remember that they won't save you from ionising effects!


There may be plausible ways to defend yourself effectively from laser attack, but they are even more science-fictional than practical laser weapons.

It may be possible to produce a cloud of dense cold plasma and hold it in a magnetic field on the outside of your ship. It would clearly look awesome: it glows! It could also be opaque to lasers, which would have to burn off the shield cloud first before reaching the hull, and the shield cloud could be refreshed from within the ship.

Getting the cloud dense enough, and holding it in place under strong external heating is extremely hard, but it might work. It certainly isn't the same technology needed to build your massive laser cannon... different branch of physics and engineering entirely, and a lot more advanced.

But you can say "all power to the forward shields!", and really, that would make it all worthwhile.

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Using that same technology wouldn't defenders be able to refract any laser attack away using lenses, and/or dissipate the heat of the attack? Or is there some way that laser technology gives an advantage to the attacker?

Neither. For the former: we can focus frankly absurd amounts of energy in small areas quite easily (we smelt tungsten pretty easily), but that doesn't imply that we could make something to survive that environment. There are two major issues here:

  1. The attackers know precisely where all of their energy is going, and can keep everything perfectly aligned so that everything stays under material limits, whereas the defender has to deal with it coming in at any point from any direction, which is far harder.
  2. The attackers can focus energy from multiple emitters on one point, whereas the defenders have to deal with all of the energy at the point it's arriving.

For the latter: the defences against lasers are much lower-tech than lasers: fundamentally, you just need to either put a bunch of stuff in the way, preferably reflective/shiny stuff, or move fast enough/unpredictably enough that the enemy can't keep their lasers pointed at you well enough to get through whatever stuff you have in the way. Neither of these require any particular technology.

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Pax Armada

As user3482749 said, it is very complicated to use the same laser technology to deflect/defract/block incoming lasers. However, that is a technicality.

If you have a fleet with lasers, your defense against lasering from others is the threat of retaliation. If they shoot one of your ships down, you will shoot back. As long as the cost of conflict remains much higher than the cost of peace, things will be chill.

Don't forget to make alliances with more people who also have lasers. The more the merrier.

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