Sandcasters, Plausibility?

I've been reading an article on battle-starship design, and they bring up the interesting point that lasers can impart kinetic force. So when a laser hits a sandcaster cloud, wouldn't you now be faced with dozens of tiny particles moving relativistic speeds towards your ship?

No.

Sandcasters sux

http://www.projectrho.com/rocket/enginelist.php#Photon

It does impart kinetic force. However....

Which is the same site where I got the sandcaster argument, so unless there's some internal inconsistency, I can see what they're saying. Basically that all the particles, to dampen the laser's effects, have to interact with it in a way which would in turn have the laser acting on them like a bunch of tiny solar sails. I think.

As a different defense against lasers, what about a layer of armor that functions as an optical metamaterial?

Metamaterials sux.

Seriously though metamaterials can only work over a very narrow band of frequencies which is built into the structure. In space you can hit by a laser of almost any frequency. With a free electron laser you can tune your laser weapons whenever you want.

Well, velocity=acceleration * time. Acceleration = force / mass. Force (in our case) = power / C. Momentum = Mass * Velocity.


So the change in momentum of the particle is (power*time)/ C

C is O(10^8)
time is O(1) unless you plan on shooting the thing for like 10 seconds
Power ~ K*S*T
...... where K is some constant (somewhere between 0 and 1, not inclusive)
.................S is the cross sectional area of the particle (really small, 10^-6 m^2 for cubic sand grains 1mm to a side)
.................T is the total output of your big space laser



What you then get is:

delta P = O(10^-6) * T * O(1) * O(10^-8)

Which can be further reduced to

delta P = T / O(10^-14) plus or minus an order of magnitude or two.



Long story short, for any appreciable change in momentum, you need a LOT of laser output for a little momentum change.

And even if you did make the particles fly at C they are still just individual particles. Sort of like throwing a hand full of sand into someone's face.

Even if you did manage to throw 1kg of sand at him, the impact would be quite different than that of the equivalent mass of rock (as in a solid object). And the same should go for this.

The energy is there, but the mass of the particles is just to small to do anything.

Furthermore, and I might be wrong here. But this is for a FT setting with FTL or at least near light travel right?
If so your ship is going to have to be armored enough to resist high speed particle impacts anyway since it will be impacting cosmic dust, micro meteors and stuff all the time anyway. (especially if the ship has armor to protect it from enemy fire)

I can't really see it doing any real damage as a weapon.

Note though that the utility of a "sandcaster" against beam weapons may be overrated, as it can be easily burnt through. Once the sand is evaporated - and a weapon laser should be designed to evaporate tons of armor, not just kilograms of sand - there's nothing standing in between. The only way a defense like that can work is if you are delivering it with immense flow rate, velocity and precision. And even then it's just a stand-in for active ablative armor.

Now, against a missile flying at c-fraction velocities, it may be different. Although, since at these velocities steering is impossible, a c-fraction missile would be nothing more than an alternate form of a mass driver.