If you’re looking for a guide to this series, click here.
Last time I tried to wrap up circumstellar solutions, but before moving on to intrinsic variations, it’s clear I need to address the megastruture in the room.
Hypothesis 9) Alien Megastructures
Background on this hypothesis is here, and here:
Now, I can’t give this hypothesis the treatment I’ve given the others, because, even compared to the vague, general families of solutions in the previous hypotheses, this one has very little to hang any physics on. Ancient alien civilizations could be arbitrarily advanced, and so it’s not clear what physics we’re allowed to assume. We don’t know why they would create megastructures (though energy collection seems like a good guess to me) so we have no good reason to expect any particular shape or size for them.
That said, we can build up a straw man model, and see how well it holds up.
But first, let’s dispense with some unnecessarily complicated ideas. Lintott & Simmons made me smile with their article in the Journal of Brief Ideas in which they used the timescale of secular dimming to estimate the construction time for a Dyson sphere. Similarly, Villaroel et al. skeptically mentioned this possibility in their paper.
But we don’t have to imagine building a gigantic sphere in a century to hypothesize how megastructures could explain the dimming.
Imagine that it is advantageous to collect solar energy to be used for some purpose, and that there is enough material to do so, but not an infinite supply. Imagine that the panels have a range of sizes and orbit the star in a range of orbital periods. In this case, the cost (in time, energy, and mass) to construct a panel is balanced by the benefit (the energy intercepted, which favors close-in panels, times the efficiency of the panels, which favors far-out panels). If too many such panels are created, the close-in ones will shadow the farther-out ones, reducing their efficiency.
In this toy model, one expects (in a rough order-of-magnitude sense) to end up with a swarm of panels that have an optical depth near 1. That is, they should not capture, say 99.999% of the photons, because the marginal efficiency of the next panel is reduced by a factor of 100,000 compared to the first one. And they shouldn’t intercept only 0.001% of the light, because 99.999% of the light still free to take with virtually zero efficiency hit. You’d expect somewhere between, say, 10% and 90% of the light to get absorbed.
The smaller panels will appear as a translucent fluid around the star, constantly blocking some fraction between 10-90% of the light, roughly speaking. This fraction will vary as denser parts of the swarm come into and out of view, and as chance alignments of parts of the swarms at different orbital distances align. We might see variations in brightness on scales from hours to centuries. Particularly large panels—even bigger, perhaps, than the star itself—will cause large, discrete dips as they transit, with profiles according to their shape. The timescale for crossing may not be a good indicator of their distance from the star, because they might be so thin that radiation pressure is important (see the appendix here).
This is actually right in line with the observations of Boyajian’s Star: we see a constant dimming of the star, one that erratically has increased by about 15% in the past 115 years, and occasional dips.
OK, so how does the toy model hold up against long-wavelength observations? Well, not great. The balance of efficiency with orbital distance is trickier, and interacts with the shadowing argument, but you might expect there to be a typical, characteristic temperature that is around 150K (95% maximum efficiency for a star like the Sun—we argued all of this in more detail in our paper here).
The long-wavelength constraints apply almost as well to megastructures as they do to dust. We argued in that paper that it is reasonable to expect nearly all of the energy collected to be reradiated as waste heat. As we saw from the long-wavelength limits, it just can’t be that 15% of the starlight is being reprocessed at 150K, or at any other temperature:
The black line is our toy model swarm blocking 15% of the light, and it is strongly inconsistent with the red upper limits from WISE and Thompson et al.
Now, there are a couple of ways out. First, the aliens might be doing some non-dissipative work with this starlight. Perhaps they are launching interstellar probes. Perhaps they are sending out powerful laser or radio transmissions. Anything that puts all of that stellar luminosity into energy that leaves the system in a low-entropy way would reduce their waste heat luminosity. Can that do it?
Well, if the work is all done at 65K, then the maximum (Carnot) efficiency allowed by thermodynamics is about 99%. This means that they could reduce their long-wavelength luminosity down from 15% of the star’s, to only 0.15%. The upper limit at this temperature, as you can see in the figure above, is 0.2% (the purple line).
So… not completely ruled out, yet, but I’d have to say we’re close enough that I’d put this one down as very unlikely, even after normalizing for the unknown chance that there are megastructures there in the first place.
Another way out is that it’s not a spherical swarm. Make it an edge-on ring, and you can further reduce the waste heat. Make them radiate towards their ecliptic poles, and you can reduce it further. Turn the energy into low entropy radiation at high efficiency and do both of the above, and they’d be nigh undetectable.
So you see, the hypothesis is sufficiently flexible that it can fit almost any observations, but I do think we can say this about the vanilla, simplest, strawman scenarios:
- IR/mm observations rule out a spherical swarm doing only dissipative work (like humanity does) being responsible for the long-term dimming
- We can further rule out a spherical swarm sending the energy off in another way at maximum efficiency with temperatures above ~90K
- We can lower this cut-off temperature with more sensitive measurements
So, subjective verdict: unclear, with an extra multiplier of very unlikely for the simplest spherical swarm strawman.
The megastructure hypothesis would find support if Gaia shows us that the star is actually much more extinguished than the reddening suggests (meaning there is a significant optical depth of geometric absorbers: the swarm of solar panels). Or if, you know, we detect those radio waves that all those panels are producing!
OK, next time: back to the natural explanations, this time looking at intrinsic variability.