I couldn't find it, but I recall an old thread here where someone gave a very good explanation of why light stops being visible at a certain distance. The explanation involved holding a piece of paper in front of a flashlight beam so that the light appeared on the paper as a circle, then increasing the distance so that eventually the circle was greater than the size of the paper, so the paper was receiving less and less of the light. What about a laser beam? Does a laser beam have a constant circumference at any distance? If so, with no dispersion, what would determine the distance at which it could not be seen?