By Hill, via Wikimedia Commons

In a previous post I addressed the issue of why we might have lots of extraterrestrial civilizations nearby and never see them. It was a stab at addressing the Fermi Paradox, which is basically asking, if there are aliens out there, why don’t we see any evidence of them?

In that one I was speaking about civilizations at roughly our level of technology. By that I mean that they use radio, that they haven’t got anything we’d necessarily describe as “magic” and they haven’t got some way around the speed of light limit.

But let’s think about civilizations that are way ahead of us. Imagine a species with a million- or billion-year head start at having a technical civilization. We literally can’t imagine the kinds of thing they could do, anymore than a member of Homo erectus could figure out how an iPad works. We wouldn’t even have the language for their technologies.

With capabilities that impressive, those hypothetical beings ought to be able to do things that are pretty spectacular. Like re-engineer galaxies, or at least, build Dyson spheres – huge structures that surround a parent star and create living space on the whole thing, making use of all of the star’s energy. So why don’t we see a bunch of weird-looking stars that are obviously engineered?

It turns out that there’s a number of big limitations on building structures like Dyson spheres. That doesn’t mean it’s absolutely impossible (in the physical sense) to overcome them. What we find though, is that if you have the ability to build a Dyson sphere in the first place, you likely as not don’t need it anymore.

So, what’s a Dyson sphere? Back in 1960 Freeman Dyson, a theoretical physicist and mathematician, first proposed the idea of using a sphere of satellites surrounding a star to capture all of the star’s energy for use by whatever civilization built it. He wasn’t proposing as solid shell — such a structure is not mechanically possible, no matter what the technology.

A solid shell has two major problems: First there is drift. A Dyson sphere is still subject to the local sun’s gravity and is being pulled inward. But there’s no net force on the star in the center because there’s an equal amount of matter on all sides of it. That means that the star in its center will drift, and eventually punch a hole in the sphere unless the sphere has either some way of propelling itself or repelling the star inside. The other problem is stress. A shell of anything that was completely solid would act like two attached domes. At any point the weight of the domes would compress the material. Imagine a dome that is 93 million miles high, under the weight of the gravity one would feel from the sun at that distance. That’s a lot of force when applied to a structure that big. There isn’t any material anyone knows about that could handle that.

A Dyson sphere is also pretty useless as living space. Standing on the inner surface of a Dyson sphere would result in you falling “up” towards the sun in the center of it (if very slowly at first). The only way to prevent that would be to rotate the sphere, but then the “gravity” inside wouldn’t be constant. It would be greater towards the equator and less towards the poles (eventually becoming zero at the pole itself). So your useful living space would be limited to a band around the equator.

All this is to note that the idea of looking for the signature of solid Dyson spheres doesn’t make a lot of sense. If you can replicate and gather enough material to make one, there’s a lot of other stuff you can do that’s more useful.

Far better is the Dyson ring configuration and its variants. This involves building a huge ring around your local sun and having a swarm of satellites in many, concentric but differently-oriented rings. This still presents pretty big dynamical problems – you’d need some way to keep the satellite formation stable — but at least it’s more feasible and sensible if you want to catch as much energy from a star as possible and provide living space to go with it.

The thing is, at interstellar distances you won’t necessarily see them. A ring won’t affect the light from a star at all unless it is right in your line of sight, and the odds of that are very, very tiny. A warm of satellites wouldn’t affect the light from a star that much either. From a great distance the star’s light might look dimmer, but it would be hard to distinguish the effect from that of interstellar dust and a big swarm of asteroids (dust clouds and the like are pretty common).

That said, a sensitive enough search would show sun-like stars with an anomaly: the amount of infrared radiation would be larger than expected if a Dyson swarm were absorbing and re-radiating energy. Thus far nobody has seen anything like that, but unless your swarm were very, very closely packed it wouldn’t be easy to pick up.

Again we have the same issue as with radio emissions: there might be a Dyson ring-capable civilization close by. But we’d never notice because their ring just happens to be oriented the wrong way.

What about the really big project, like galacto-engineering? That’s still something of a problem because of the time constraint on life. That is, there’s only so far back in the history of the universe you can go before you can’t have life at all, because the relevant chemicals were not formed yet in the cores of stars.

This is important because any galactic engineering project is going to take millions of years to do – the speed of light limit ensures that. If you were re-engineering a cluster, it would be several million, possibly a billion, years before the new shape or color of the galaxies involved was visible.

The earliest dates for life in the universe are under some debate. The earliest estimates for life to emerge would be a mere 100 million years after the Big Bang, some 14 billion years ago. Avi Loeb, an astrophysicist at Harvard, thinks that because the cosmic background was at that point a comfortable 70 degrees or so, you wouldn’t even need stars to shine for life to form on planets out in the universe, and there might have been enough elements heavier than lithium to form them. It seems unlikely, though, that you could get intelligent life that way, even if he is correct.

Given that it seems to take billions of years to get to intelligent beings, that would point to the earliest civilizations appearing at least a few billion years after the Big Bang, On Earth 4.5 billion years went by before tool-using creatures appeared. A planet that formed in the earlier universe might have taken less time – or longer. But let’s be optimistic and assume it took only a couple of billion years to have the first smart beings.

That means they would have civilizations 10 billion years old by now. We can see most galaxies out at least that far. Many galaxies are an order of magnitude closer than 10 billion light years. If these beings were engaged in some galactic re-shaping project, we should see the effects on galaxies like Andromeda, which is only 2 million light years away.

The thing is, we don’t know what to look for. I mentioned earlier that we might not have the language to even express what these super-beings are doing. It is also far from clear we’d be able to see their effects on other galaxies for the same reason that you can’t see the largest human structures on Earth without a close look. Humans have wreaked profound changes in the Earth’s atmosphere and biosphere, but an observer looking at us from afar wouldn’t be able to pick up those changes.

In a similar vein, it’s possible the aliens did some pretty major work on Andromeda. But since they didn’t do anything obvious to us, visible on a scale a hundred thousand light years across, there’s no way to tell. Maybe they re-engineered every single planet in their galaxy to have a great big “Hi we’re here” sign — we would never see it. Perhaps they are drawing energy from Andromeda’s magnetic fields — again, there would be few ways for us to tell.

All this sounds pretty bleak for finding aliens. But Ethan Siegel, a former professor at Lewis & Clark College in Oregon and a theoretical cosmologist. He said looking for signatures of artificial elements might be one sure fire method of detecting aliens. Technetium is one such element. It has no way to form naturally, so if you find it in the emission line of a planetary atmosphere, you’ve found your aliens. Loeb suggests something similar – look for chemicals in planetary atmospheres that are artificial, or largely so. Essentially, see if the aliens are polluting.

Another method Siegel suggests is to look at the planets directly. Build a telescope big enough and you could theoretically pick up artificial lights from the nigh side of a planet. We are just approaching the ability to do that now, so in the near future we’ll get a better idea of how rare or common we are.

In a future post I’ll talk about the real meat of the paradox: why aliens aren’t approaching in spaceships – or don’t seem to be.