As Brin notes, many would-be broadcasters come from an academic area where for decades the standard assumption has been that aliens are peaceful zero-population-growth no-nuke greens, since we all know that any other sort quickly destroy themselves. This seems to me an instructive example of how badly a supposed “deep theory” inside-view of the future can fail, relative to closest-related-track-record outside-view. As Brin says, the track record of contact between cultures, species, and biomes is not especially encouraging, and it is far too easy for far-view minds to overestimate the reliability of theoretical arguments to the contrary.
In fact, it’s a lot worse than that. As far as I can tell, nobody talking about interstellar contact has a model even vaguely close to a reasonable analysis of the situation. Short form: these discussions are the equivalent of the natives of a Polynesian island deciding who shall be allowed to wave as the galleons heave into view. Our own technology, today, is getting close to detecting Earth-like planets around other stars, for heaven’s sake. The galleons see the island, not the waving. Scientific elites declaring moratoria on SETI transmissions are about as important to the future of the human race as whether we call Pluto a planet or a dwarf planet. The discussions are entirely about political dominance among scientists, and nothing to do with reality.
Reality is that any alien race out there with whom we have any kind of physical contact at all is virtually certain to have (a) full-fledged nanotech, and (b) hyperhuman AI. Given these capabilities, if they want to find Earth-like planets anywhere in the area of space they would have the physical capability of travelling to, they will find them. Period. Doesn’t matter whether we are standing on the shore waving or not.
Of course, that assumes they are interested in Earth-like planets in the first place. Most commentators on the subject seem to be stuck in E. E. Smith’s universe, worrying about whether the aliens who notice us will be the (kindly, academic) Norlaminians or the (evil, rapacious) Fenachrone. The aliens, wearing bodies like ours (or at least some form of animal life) will have spaceships and spacesuits and takeoff and land on planets and basically act like people on ocean-going boats.
Star travel is expensive; it costs on the order of a ship’s own mass in equivalent energy to get it up to relativistic speeds. Any culture capable of that will be at least a Kardashev Type I civilization, and most likely a Type II. And the reason they’ll be doing star travel is to work their way up towards Type III. Any sentient creatures that actually get here will be nanotech-based robots, not water-based organisms. They won’t have spacecraft, they’ll be spacecraft. They will be unlikely interested in the carbon-poor mudballs of the inner solar system, but reap abundant carbon from the outer planets and carbonaceous asteroids to build Dyson-sphere-like structures around the orbit of Mercury.
We simply aren’t going to see less sophisticated visitors due to the starship paradox: send a starship out now with all Earth’s current technological resources behind it, and then wait and send one in 50 years with full nanotech. The second one gets there first.
We aren’t going to see any less ambitious visitors due to simple evolution: in a universe where the ultimate meaning of “carbon footprint” is the total mass of the superintelligent diamondoid robots you’ve built, spaceships burning cellulosic ethanol simply aren’t going to be anywhere near the fittest. Indeed, cultures that aren’t inherently aggressive and ambitious aren’t going to put the effort into sending out starships at all. The question is, what are they going to think of us, the thin layer of green slime coating an insignificant rock?
If I were an aggressive superintelligent nanotech robot, I would tend to place the boundary between “people” and “raw material” at the boundary of aggressive superintelligent nanotech robots and everything else. I might — just might — make a sentimental exception for intelligent organic species such as my ancestors. “Such as” in this case means intelligent organic species which are on a clear track to building aggressive superintelligent nanotech robots.
Or, of course, has already done so. If you really want them to show up as friendly neighbors, start working on that Dyson Sphere yourself.
If, on the other hand, you’re a culture that has elevated cowardice (“Precautionary Principle”) to be its highest virtue … you’re just dirt.