The Absolute, Definitive Truth About Alien Megastructures

The title of this post is a joke, or taken literally, an outright lie. The only definitive truth is that no one knows if anyone has ever built a megastructure, or even if they would if they could. I have persistent doubts if such things exist anywhere in the universe, but I can't yet tell you if such doubts are reasonable.

Update 8 December 2016: I left out one type of motivation for building a megastructure - planetary climate control. Although these "Dyson Dots" would be relatively small, they might be detectable for transiting planets. I need to run the numbers...

The Usual Disclaimer

So, we're going to be speculative here yet again, and very probably wrong. I won't be able to cite many facts, so if that is the sort of thing you like to read, perhaps now would be a good time to hit the back button.

But I'm Completely Serious

We are interested in the conjectured alien megastructures because we might have a chance to observe them with technology we have or could well have in the near future. These structures would be bigger than planets (my definition), and since we can observe planets about other stars, we might well be able to observe these things, and so looking for them is a kind of SETI. I've written before about why I think SETI is worthwhile.

The Null Hypothesis

Jason Wright at Penn State has led a group that has gone looking for megastructures from their waste heat, called Glimpsing Heat from Alien Technologies, or GHAT. They have mined data from infrared surveys like 2MASS and WISE looking for those telltale techno-signatures. So far, no joy for GHAT, but the instruments we have weren't designed to look for such things in other galaxies unless the signal is really strong.

GHAT found no definitive evidence of any civilization in about 100,000 galaxies that uses at least 85% of the energy of that galaxy. However, 85% of a galaxy is a tremendous amount of energy, and indicates an extended, focused and sustained period of exponential growth, apparently for its own sake. And, after you are sponging up all the energy in your galaxy and using it for purposes us adorably curious apes can not hope to guess at, what do They do then? The galaxies are very far apart, so the growth craters and they enter a deep recession during which demand for custom built planets is likely to be very low.

In his forthcoming book, Earth in Human Hands, author David Grinspoon points out that unlimited growth will quickly run out of steam - he calls this the Inevitable Expansion Fallacy. Perhaps, be puts to us, really advanced civilizations don't want or need to continue to expand mindlessly, but instead choose to treat their environs - however far flung into the cosmos - more like a garden than a power plant. Perhaps, the lack of evidence from GHAT is good news - very old civilizations, if they exist, are wise and mindful.

A Stab at a Taxonomy

Knowing that galaxy scale public works projects are rare is a helpful constraint, but it doesn't mean that there can't be planet-sized structures that advanced ETs might build for specific purposes. Reminding ourselves that epistemic humility is no excuse for epistemic timidity, let's plunge in and make a list of some of the broad categories of purposes of such megastructures. Here I'm relying a bit on the Benfords, who listed some reasons (not meant to be exhaustive) why ET might build radio beacons. Megastructures, due to their presumably great cost and scale, would likely have different purposes. So, let's make an (probably inaccurate and certainly incomplete) assault on a provisional taxonomy:

1. Astronomical megastructures - a massive phased array of optical or radio or some other elements that would permit detection and detail beyond a terrestrial astronomer's fondest dreams. I would expect that these would be built around cooler, dimmer stars - K or M dwarfs. I think this is a pretty reasonable thing to expect an advanced technological civilization to do.
2. Monument - a civilization has moved on to other things, or is perhaps long gone, and this is a record they have left visible from far away in the galaxy as it eclipses a central star, and is perhaps self maintaining. Think of it as a billion year pyramid. A bright main sequence star would be best, but not the relatively short lived O or B stars.
3. Energy harvesting and concentration- in addition to the well-known idea of a Dyson swarm, in which the energy is used locally, the energy might well be directed far out into space by a massive array of mirrors, where it is used for perhaps for propulsion, or even a weapon. A Dyson swarm would exhibit an excess of radiated energy in the mid infrared, but highly efficient mirrors may not radiate much IR excess. We would expect to find such structures preferentially around bright main sequence stars, which are not as numerous as the red dwarfs but bang out much more energy. in the same volume.
4. Moving the star, or stellar engineering. If you are very patient, you can use the star's own light to move it somewhere else (this is called a Shkadov thruster). I am not sure why you would want to do that, but perhaps it is the natural extension of the worldship concept. Just move your whole world, including its host star. I don't think that's likely to be the best way to travel, but what do I know? (don't answer that)  For such an extended journey, I would thing it would be long-lived main sequence stars that would be favored - class G and cooler. As for stellar engineering, we are really in the dark, but it's probably something quite nefarious.
5. Purposes humans wouldn't (or can't) think of. I conjecture that this is the largest category, but I don't know what the observables are.

Nothing Lasts Forever, Of That I'm Sure

We should note that any of these structures may be abandoned in any stage of their construction, and even if self-repairing, will eventually become derelict, or will be salvaged for other purposes, so what we would observe could well be the ruins of such a structure. Bits of it might collide and create clouds of smaller bits. Depending on how long you believe such civilizations can survive (remember it's all about L), this could easily be the most prevalent case. 

What Would we Observe?

I expect that scenario 1  - a giant astronomical instrument - is the most common type of megastructure, but it may take quite a bit of luck to see it from a distance.  The mirror should have a very narrow beamwidth, and unless it's staring right at us (wouldn't that be cool?) we might not see anything. Even if we are luck enough to see some of the mirror transit, we might not see big dips in the host star's brightness as the elements might be fairly diffuse. So, we might have looked right at any number of these and had no idea.

The megamonuments in scenario 2 should naturally be more conspicuous, but also require more maintenance. They would be designed in multiple orbit planes at different distances from the the host star so that transits were more likely.  If you were positioned fortuitously, you might see more than one group transit. 

For the Dyson Swarm, we already know what we think we should see - a big infrared excess. You'd see a peak in the visible spectrum that would be the host star's light, then another peak in the mid infrared.

For the concentrator, what we might see would be very different. Assuming the concentrator was pointing nowhere near us, we might see a large transit effect. since the mirror would be blocking a significant fraction of the host star's light.

If the concentrator was pointing in a direction near us, then the star might appear to be much brighter than it's spectral type tells us it should be, and perhaps this will be reflected in the parallax data from the Tycho catalog or in upcoming Gaia data releases. Occasional adjustments or glitches in the pointing of the concentrator elements could result in dips in brightness.

For Scenario 4, we might see large dips in brightness as the concentrating mirrors transit occasionally. If the purpose is to move the star, there might have already been some progress, so we might see an unusually high Doppler shift, or proper motion, or both. For stellar engineering, I expect we'd see some very odd stuff in the spectral data.

For Scenario 5, we ape descendants are out of luck for now. We're scratching our fur and looking out into the blackness of the void hoping, that a little seed of concept will go viral in our brains. Probably not any time soon.

Has This Got Anything to do with Boyajian's Star?

Maybe. We don't know that what causes the unusual light curve in KIC 8462852 is an alien megastructure. From the evidence we have so far, I would say scenarios 2 and 3 are the best fits, but scenario 3 has some problems, namely the lack of any observed infrared excess and the fact that the star does not appear to be farther away than its spectral type would indicate, based upon the first Gaia data release. Also, the spectrum looks normal and the motion of the star with respect to Earth is unremarkable, so 4 looks a little shaky, unless they're almost done with moving it and are coming into their new orbital slot.

So, is Tabby's Star a kind of monument? Or maybe it's just ostentatious signaling, like spending way too much money on your daughter's wedding or buying an ugly McMansion on a tiny lot?

We may know more soon, but probably what we will learn is how wrong we were....