Science News has a short article discussing a calculation someone has done showing how small the volume of space examined by SETI (Search for Extraterrestrial Intelligence) is relative the overall size of the galaxy.
With no luck so far in a six-decade search for signals from aliens, you’d be forgiven for thinking, “Where is everyone?”
A new calculation shows that if space is an ocean, we’ve barely dipped in a toe. The volume of observable space combed so far for E.T. is comparable to searching the volume of a large hot tub for evidence of fish in Earth’s oceans, astronomer Jason Wright at Penn State and colleagues say in a paper posted online September 19 at arXiv.org.
“If you looked at a random hot tub’s worth of water in the ocean, you wouldn’t always expect a fish,” Wright says.
I have no doubt that the amount of stars SETI has examined so far is a minuscule slice of the population of the Milky Way galaxy. And if SETI’s chief assumptions are correct, it’s entirely right to say that we shouldn’t be discouraged by the lack of results so far.
But it’s worth noting what one of those chief assumptions are, that interstellar travel is impossible, or so monstrously difficult that no one bothers. If true, then we wouldn’t expect the Earth to have ever been visited or colonized. This fits with the utter lack of evidence for anything like that. (And there is no evidence, despite what shows like Ancient Aliens or UFO conspiracy theorists claim.)
But to me, the conclusion that interstellar travel is impossible, even for a robotic intelligence, seems excessively pessimistic. Ronald Bracewell pointed out decades ago that, even if it is only possible to travel at 1% of the speed of light, a fleet of self replicating robot probes (Bracewell probes) could establish a presence in every solar system in the Milky Way within about 100 million years. That may sound like a long time, but compared to the age of the universe, it’s a fairly brief period. Earth by itself has existed 45 times longer.
People sometimes respond that the Earth may be in some type of backwater. The problem here is, if you know about where the Earth is in the Milky Way, in the Orion Spur off the Sagittarius Arm, about halfway between the center and rim of the galaxy, you’ll know that we’re not really in a backwater. The backwater theory might be plausible if we were thousands of light years off the galactic plane, beyond the rim, or in a cluster far removed from the main galaxy, but we’re not. Even then, the nature of the self replicating probe propagation is pretty relentless and would still eventually reach backwater stars.
Of course, if there is only one or a few other intelligent species in the galaxy, then it’s entirely possible that their Bracewell probe is here, just lying low, observing us, possibly waiting for us to achieve some level of development before it makes contact. (Or maybe it has been making contact 2001: A Space Odyssey style.)
But if the number of civilization is in the thousands, as is often predicted by people speculatively playing with the numbers in the Drake equation, then we should have hundreds of those probes lying around. Given their diverse origins, we shouldn’t expect them to behave with unanimity. Even if one probe, or coalition of probes, bullied the others, the idea that such an arrangement would endure across billions of years seems implausible.
And the Earth has been sitting here for billions of years, with an interesting biosphere for most of that time. The idea that none of these self replicating probes would have set up some kind of presence on the planet, a presence we should now be able to find in the geological record, again seems implausible. Indeed, if they existed, we should expect to have at least some of them in front of us now.
Now, maybe they are in front of us, and we’re just not intelligent enough to realize what we’re seeing. Monkeys, after all, likely have no understanding of the significance of the buildings and machinery they climb over. It seems like something we have to keep in mind, but historically it’s never been productive to just assume we can’t understand something, and taking this principle too much to heart seems like it would make it impossible to ever dismiss any dubious notion.
So SETI largely depends on interstellar travel being infeasible. This is actually the conclusion a lot of radio astronomers have reached. Could they be right? I don’t think we know enough to categorically rule out the possibility. If they are right, then SETI will be our best chance to someday make contact with those other civilizations, even if it’s only composed of messages across centuries or millenia.
As I’ve written here before, my own conclusion is that some form of interstellar exploration is possible, and that life is probably pervasive in the universe, although most of it is microscopic. Complex life is probably far rarer, although I wouldn’t be surprised if there aren’t thousands of biospheres, or more, in our galaxy that have it.
But intelligent life capable of symbolic thought and building a civilization? The data seems to be telling us that this is profoundly rare, so rare that the nearest other intelligent species is probably cosmically distant. If we’re lucky, they might be close enough that we can encounter them before the expansion of the universe separates us forever. If we’re not lucky, we’ll never have a chance for that encounter.
Unless of course, I’m missing something?