Science-fiction fan Rick Robinson runs the numbers on laser weapons:
So an ideal diffraction-limited laser zapping in the near IR, with a wavelength of 1000 nanometers firing through a 2-meter telescope, has a spot size of 12.2 cm at a range of 100 km.If your laser has an average power output of 1 megawatt, each square centimeter is getting hit with about 8.5 kilowatts — about 850,000 times the intensity of sunlight at Earth’s surface. The target surface will get very hot, very quickly.
The most refractory material we currently know of, graphite, requires some 50 MJ to vaporize 1 kg — roughly the energy of 12 kg of TNT — and has a density of about 2.2 g/cm3. Cutting to the chase, our beam will burn through it at not quite a millimeter per second. Most metals are much less resistant to heat, so the laser will burn through metal hulls way faster. But if you substitute a 5 meter mirror — or a 400 nanometer beam, at the short end of the visible spectrum — you’ll burn a smaller hole at half a centimeter per second. Or it will have the same spot size and burn rate at 250 km.
Lesson: For a given beam power, the bigger the mirror and shorter the wavelength, the greater the effective range. And lasers cannons, at least in the classical IR-visible-UV band, probably won’t look much like guns, but perhaps more like a TV satellite dish.
The consequences:
If you can see it, you can zap it, and vice versa. As noted above, laser spot size is closely related to telescope resolution. If you can focus the beam to a couple of dozen centimeters, that is also the resolution your sensors can gain, simply by looking through the telescope between pulses. Which means that lasers of this precision don’t just score random hits like World War I battleships; they fire at specific points on the target surface. (If mechanical or thermal limits preclude this precision, you won’t get the penetrating burn-throughs described above, just scars burned along the hull surface.)Thus the objective won’t just be to blast an enemy ship but to mission kill it by zeroing in on critical systems — such as armament. In a laser battle, if you can hit the other guy effectively at all you can shoot the gun out his hand. But it gets better. What happens when two lasers are zapping each other? Their targeting optics are pointing straight at each other — so the optics concentrate the incoming beam right onto the laser itself. I have no idea what the effect is, but it could easily be dramatic. Laser engagements lend themselves to a mutual eyeball frying contest. Whoever zaps first, probably wins.
But there is another and even more curious implication of laser combat. So far I’ve been talking about beams concentrated down to blowtorch intensity, kilowatts or metawatts per square centimeter, able to burn right through refractory materials by heating the surface to thousands of degrees K. But what about mere scorch intensity? Say, the 50 watts/cm2 that causes primary thermal burns to humans and sets paper on fire. This won’t burn through armor, but it will likely burn out delicate components such as sensor elements, or at any rate saturate and ‘dazzle’ them.
Thus laser weapons can blind the enemy, temporarily or permanently, at much greater range than they can do serious physical damage to structures.
(Hat tip to Nyrath.)