There's been an exciting shift in scientific thinking about life beyond Earth recently. Extraterrestrial-seeking group SETI has started to recognize the importance of detecting alien artifacts along with radio signals. The most viable target for these searches is the Dyson sphere, a massive shell that envelops an entire star. But as Discovery's Ray Villard rightly points out, the difficulty of finding these near-invisible objects could be offset by searching for them in nearby galaxies where their presence would be much more obvious.
Finding a Dyson sphere is not easy — and it's not because we don't know how to do it. As we've discussed before, the trick is to distinguish the infrared signature of a Dyson sphere candidate from natural phenomenon. According to Villard, Dyson hunters have catalogued 17 "quasi-plausible" signatures — but they can't prove one way or another that they're not just, say, stars enshrouded in dust.
Consequently, it will be very difficult to prove that a Dyson sphere exists — at least one that's nearby.
But as the work of Fermilab's Richard Carrigan points out, there is another way for us to go about this. It's conceivable that advanced extraterrestrials don't just stop at one Dyson sphere, and instead embark upon interstellar colonization. This would initiate a colonization wave that would spread outward like a bubble from the originating star. Villard elaborates:
It's imaginable that a super-civilization would begin a wave of colonization that spread out to neighboring solar type stars from its home base. Each offshoot would "astro-form" the colonized planetary system by constructing a Dyson sphere around the host star.
Carrigan envisions seeing "Dyson bubbles" in nearby galaxies. These would be clusters of Dyson spheres that enclosed a grouping of stars colonized by a Type II Kardashev civilization. The logic is that after you've built a backyard fence you can start to conceptualize building the Great Wall of China and still hope to gain perspective on the process, Carrigan writes.
These would be detected as anomalous dark voids in a galaxy's disk. When these voids were observed in infrared light they would glow brightly with the heat radiation from the surfaces of Dyson spheres. This would show that they are not that simply voids where solar-type stars are conspicuously missing.
A good candidate for such a search would be the Andromeda Galaxy, which is only 2.5 million light years away. At most, we'd be glancing back a couple of million years into the past, which is not significant from a cosmological perspective. Moreover, we have the tools to conduct such a search, namely NASA's Hubble Space Telescope. As Villard points out, "This could make an excellent citizen science project, to scour the Andromeda fields for anomalous-looking regions."
That said, preliminary scans of the galaxy have revealed nothing that would indicate a colonization wave. It's clear that more work needs to be done to rule out each galaxy one-by-one, including gigantic elliptical galaxies which are completely devoid of light-blocking dust.
And discouragingly, all our searches may end up falling short. Villard explains:
An apparent lack of any evidence for large-scale artifacts in galaxies as old as ours would begin to set an upper limit on just how technologically advanced alien civilizations can evolve to become.
Kardeschev hypothesized about Type III civilizations that would harness the energy of an entire galaxy. The observational evidence of astro-engineering a complete galaxy is lacking, and so it's fair to say that Type III civilizations either don't exist at all, or at least not yet.
The universe has had 12 billion years to evolve Type II or Type III civilizations. If there's not obvious archeological evidence, then maybe intelligent beings don't evolve all that far beyond mega-engineering to the scale of a single planetary system.
Image via astrophoto.