This is probably an impossible question to ask, but I feel bound to give it a try. Assuming a level of technological know-how that has enabled human beings to journey beyond the heliopause - if not to the nearest stars - just how far can it be reasonably supposed for message-bearing laser beams to travel in space before they finally become incoherent? Again, this is like trying to second-guess the future, but is there a fundamental limit to laser technology in this respect? For example: if I posit two or three thousand AU, will everyone fall about laughing? :( PS. I've tried to get a handle of this via google, but apart from wading through the occasional PhD thesis on laser technology - which are mathematically way above my head - nothing else out there seems to come anywhere near to addressing this query of mine.