Quote Originally Posted by sycld View Post
Our best lasers don't stay collimated for an infinite amount of distance, and they certainly wouldn't stay collimated for astronomical distances.
Hrmm, maybe you know something I don't, but I was given to understand that collimation means the rays are close to parallel and thus the light doesn't spread out quickly as it travels. I guess the point at which light can be called "collimated" thus somewhat arbitrary? Anyhow, as I understand it, collimated light, whether it comes from a laser or anything else, will stay collimated forever unless the rays start hitting things that change the direction they're traveling. This doesn't mean it doesn't spread as it travels, but it remains collimated (i.e. having a small angle of divergence between individual rays) as it does so. My degree isn't a scientific one and I'm getting my understanding of this from layman-type sources though, so I could easily be wrong.

Anyhow, I wasn't saying that it's collimation which makes laser light uniqutely desirable for long-distance communication; both lasers and non-laser light sources can produce collimated light (and some lasers produce uncollimated light). What makes lasers uniquely desirable is that laser light is not subject to the inverse-square law, which non-laser light is subject to even if quite well collimated (I think).

Quote Originally Posted by sycld
Also, I have never heard of a "RASER." You, however, must be familiar with one of them, so please, tell us about it and its operation.
Sure you have heard of them, they just aren't usually called "rasers". They are usually just called "RF masers". Many masers emit on radio wavelengths. Although I have read (can't recall where) that lasers for interstellar communication might actually be visible-light.