[citation][nom]groveborn[/nom]Also, gravity has minimal effect, so the laser will always travel in a single dimension (straight). Horizon would prevent it from going much more than 15 miles directly across the land (which will not be an issue as it's pointed UP). If fired in a vacuum (like space) the range would be significantly increased.[/citation]
Ok here's a little physics lesson....lasers are focused beams of photons (packets of light) which have ZERO mass. Gravity doesn't have a "minimal" effect...it has NO effect. Light traveling in a vaccuum or on Earth isn't affected at all by gravity.
And before someone corrects me....yes, light traveling to earth from thousands of lightyears away can be "bent" around certain massive objects like black holes etc. This is called gravitational lensing and is how we can see certain objects that are "behind" black holes and other super-massive objects. Technically the space around these objects is bent by gravity and not the light itself so NO gravity doesn't EVER directly affect light.
Also, these lasers are still moving through three dimensions. It takes a lot of computation for these systems to determine the X, Y, Z of where to shoot. The fact they go in a straight line doesn't mean they move in one dimension.
The horizon isn't much of a factor since it's aiming at objects in the air not on land.
The two main things that limit the range are attenuation (weakening of the beam) and from atmospheric distortions. The latter of the two can be compensated for by adaptive optics which has already been proven with the Air Force's Airborne Laser (ABL another great acronym).
Adaptive optics use one laser to measure interference within the atmosphere between the weapon and the target. It then "distorts" the laser beam in such a way that after passing through the atmosphere it arrives at the target perfectly focused again.
I am all for these energy weapons as the risk of missing or collateral damage is far less than with projectiles. Also, since they move at the speed of light, it doesn't get any better as far as ways of intercepting incoming missiles, planes or anything else. Also, take into account that the types of ships that will initially carry these lasers are most likely nuclear powered which means virtually unlimited "ammo".
Next step? X-Ray lasers in the megawatt range