The Search for Extraterrestrial Intelligence (SETI) has traditionally used radio telescopes to search nearby stars. Excellent. But I really loved a new study published last week by Griffeth, et. al, which differs from traditional radio searches in two ways. First, Griffeth and team did not look for direct signals sent by extraterrestrial intelligences (ETs). Instead they looked for excess heat produced as a waste byproduct of energy use at galactic scales. Second, they did not look nearby inside our own galaxy. Instead they looked at galaxies far, far away. 100,000 of those galaxies in fact. This approach may seem counterintuitive. But I think it’s one of the best ways to look for ETs.
The basic idea of my argument is straightforward and indeed has led other authors such as Fermi (10), Dyson (11), Hart (12), Simpson(6), and Kuper & Morris (13), to conclude that extraterrestrial intelligent beings do not exist: if they did exist and possessed the technology for interstellar communication they would also have developed interstellar travel and thus would be present in our solar system. Since they are not here (14,15), it follows that they do not exist. Although this argument has been expressed before, its force does not seem to have been appreciated.
The key to understanding the passage above is timescales. It takes billions of years for complex life to evolve, with Earth as our example. On the other hand replicating robotic space probes could fill the galaxy in millions of years. Three orders of magnitude less. So expansion is over in an eyeblink compared to evolution. The first to evolve floods the galaxy. Or more exactly the first to evolve that is capable of expanding, and then decides to do so won’t have any competition.
Not All Aliens
The most frequent counter to the Hart/Tipler argument is one I’ve started thinking of as the “Not All Aliens” rebuttal. Carl Sagan and William Newman have a version in their 1983 paper where they argue alien civilizations won’t colonize the galaxy:
The idea that there will be no new and more compelling challenges for such civilizations than relentless galactic colonization represents a serious failure of the imagination — although it is a natural enough notion were we to extrapolate carelessly from recent human history. But the colonization of our region of the Galaxy does not require that every advanced galactic civilization colonizes; only that one does. If there are abundant civilizations in the Galaxy, their absence on Earth requires a principle of universality so compelling as to admit essentially no exceptions.
And their favorite compelling no exception principle:
[T] here are a number of other candidate principles summarized in Paper I, one of which we wish to stress: the intrinsic instability of societies devoted to an aggressive galactic imperialism.
Sagan was of course brilliant, so he correctly sees it only takes one expander to spoil the party. I just don’t find his rationale for 100% universal non-expansion very compelling. After all, given the distances between stars, it’s seems highly unlikely there would be a centralized galactic imperial system at all. Which is why it reminds me of the Not All Men meme from last year. If you’ll recall, Not All Men spoofs men defending misogyny by saying, well, not all men do it. True enough, but that misses the point. One man is all it takes. Likewise just one ET expander is all it takes.
Not All Rabbits
Another way this argument is a bit counterintuitive is the expansion is exponential. Let’s consider a Not All Rabbits example. Rabbits were living in Australia starting in 1788. But they were kept caged. So Not All Rabbits expanded. Then in Barwon Park in 1859, twenty-four rabbits were released into the wild. These expanded. Local populations could double in six months. As we know from Moore’s law, exponential growth brings amazing change that can defy natural intuition. From those original 24 rabbits, the population reached 600 million by 1950 a century later. Note: those numbers are now down to about half that due to the introduction myxoma virus. But still plenty. See the expansion map below.
The analogy here of course is that Australia is our galaxy. And rabbits are aliens. Sure, Not All Rabbits (Aliens) will expand. But it’s very hard to simultaneously argue rabbits (ETs) are super common in the galaxy, plus also believe every single one of those common rabbit groups will not expand. Or at least explore. Of course, with so little data on the Fermi Paradox, we can’t be sure. But it’s certainly reasonable to argue the reason we don’t see any rabbits is because there aren’t any rabbits. Or at a bare minimum rabbits are very very rare. This means a much better place to look for rabbits is on other continents where they’ve already expanded. That is, look in other galaxies. And due to exponential replication, if an ET did expand, their replicated descendents could consume energy at a galactic scale. They’d breed like, well, rabbits. Ones with sharp, pointy teeth. In fact in this scenario the massive energy consumption of emitted starlight would skew the ratio of visible to mid-infrared light. Which is exactly what the Griffeth’s study looked at. That’s why this is my favorite way to look for ET. It accepts both the timescale and expansion argument, but doesn’t give up on ET being out there.
So what did they find? Unfortunately not too much. Quote:
No galaxies in our sample host an alien civilization reprocessing more than 85% of its starlight into the MIR, and only 50 galaxies, including Arp 220, have MIR luminosities consistent with >50% reprocessing. Ninety of these (likely) extragalactic sources have little literature presence; in most cases they are likely barely resolved galaxies or pairs of galaxies undergoing large amounts of star formation. Five are new to science and deserve further study.
What they are saying is the Wide-field Infrared Survey Explorer (WISE) space telescope used in the study was sensitive enough it could tell that none of the 100,000 galaxies they surveyed were converting 85% of visible starlight into waste heat. At the lower threshold of 50% conversion, they found 50 galaxies, but these may have had normal reasons for a lot of mid-infrared, like high star formation. So a great study asking a really interesting question, even if this result didn’t show anything conclusive. For more technical detail, I recommend the write up by Lee Billings here, which is quite good.
What’s next? George Dvorsky interviewed one of the study’s co-authors, Jason Wright. Wright said they believe they can improve their techniques and sensitivity so they could “search for civilizations using 10% or even 1% of the starlight in a galaxy.” If you think about it, harvesting 85% of all light in a galaxy is very rapacious. Getting detection thresholds down to 10% or 1% opens up the possibilities to finding ETs who are not so over the top. Though that does bring up an important point: if you are searching for aliens in a galaxy far, far away, it’s much easier to find galaxies filled with Death Stars than ones filled with Ewok villages. You’ve been warned.