1. Originally Posted by vk3ukf
I can't find anything that specifically (I may have missed it) mentions in an atmosphere, vacuum, or both regardless.
It's hard to quote, because I found it as a Google book sample, but "Optical communications rules of thumb" By John Lester Miller, Edward Friedman, offers the similar theta = lamda/d as above, and: "This rule is widely used to estimate the size of a laser beam that has propogated through a vacuum and is also used as a first estimate even in atmospheric applications. [...] This rules provides quick estimations of minimum beam divergence."

I take that one can reduce the divergence as much as one wants by enlarging the beam diameter, but to reach zero divergence requires... an infinite budget (to produce an infinite diameter beam).

2. Laser beams diverge because of diffraction. The amount of divergence is dependent on the wavelength of the light, and the diameter (or radius) of the emitter.
I'll quote from OA, if I may,
A laser's range is limited by its ability to focus at long distances. Even a perfectly focused beam spreads, because of diffraction. This spread depends on the ratio of the initial width of the beam to the wavelength of the beam. The smallest possible spot size to which a beam can be focused can be calculated; if the initial beam width is D, the wavelength of the light is L, and the distance to the target is R, the smallest spot size (S) is given by

S = 1.2 R L / D.

This means that a green light laser (with a wavelength of 0.5 microns) emitted through a lens ten metres in diameter can be focused into a spot 6 millimetres in diameter 100 kilometers away. As a consequence, long range laser weapons will have large apertures for focusing, use short wavelengths, or both.

3. Originally Posted by cjameshuff
Laser light's easier to collimate...
Perhaps 'overkill' was the wrong word to use. I realize a laser would work, but... they would have to know we are here, unless they are just flashing likely targets.

4. Originally Posted by Rift
Perhaps 'overkill' was the wrong word to use. I realize a laser would work, but... they would have to know we are here, unless they are just flashing likely targets.
The same goes for radio, unless they're just targeting a nearby system and we happen to be in the beam. Broadcasting a signal intelligible at light years of distance would require a *lot* of power. Whether it's radio or laser, beams directed at likely prospects would be more practical than broadcasts that send the majority of their energy into empty sky.

And yeah, beams diverge, even in vacuum, because of wave effects from a finite source. A beam that doesn't diverge would need to be infinite in extent.

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see:http://en.wikipedia.org/wiki/Wow!_signal

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