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“Where is everybody?” That is the famous question about alien life posed by physicist Enrico Fermi (see: the Fermi Paradox): why have we not found evidence of extraterrestrial civilizations, given the likelihood of their existence, based on the fact that there are billions of stars in our own galaxy?

Over the years, a number of scientific papers have tried to partially answer that question, by calculating the actual time period required for civilizations to populate the galaxy (thus estimating whether E.T. simply might not have had time to reach us yet). However, most of those were fairly simplistic in nature: they used a 2D model of the galaxy in which the stars are all fixed relative to each other, and simply calculated the distances that spaceships would need to travel to traverse all of the stars.

However, the ‘Cool Worlds‘ channel on YouTube recently posted a video about galactic colonization that addresses this issue, and it’s well worth a watch. In the video, Jason Wright – Professor of Astronomy and Astrophysics at Penn State – discusses a new paper he has co-authored that attempts to model galactic colonization (“The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady States”).

But this time, the paper is based on simulations using a 3D set of stars that were actually moving relative to each others, as they do in reality.

Here’s the entire video – I discuss some of the more fascinating topics of discussion below.

The most interesting thing that Wright and his co-authors found was that, rather than having to traverse the galaxy by traveling long distances to stars, the more likely strategy would be to let the stars come to you (as much as possible), and then ‘hitch a ride’.

As Wright notes:

The truth is, moving near the speed of light just seems really really hard even for fairly advanced technologies…. It seems likely that if there are interstellar ships they move pretty slowly, and they take ten thousand or a hundred thousand years to get to their target.

What that means is: once you set up a settlement your ships aren’t much faster than the stars around you; and that settlement will move through the galaxy and constantly encounter fresh stars that haven’t been settled, and that this helps settle the galaxy.

“You don’t need to do it with the ships because the stars are moving,” Wright explains. “The stars are your ships.”

Additionally, stars closer to the centre of the galaxy move faster, and those further out slower. There are also a lot of stars in the galactic halo that don’t move in a disk – they don’t swirl like the rest of the stars: they go in towards the centre, they go out, they go every which way, they counter-rotate in the disk. What this means is that settlements can spread to different areas quicker than you might imagine.

As Wright says, “it’s sort of like you know stirring the jam in the porridge – you’ll just see the settlements eventually cover the whole thing, even if the individual ships aren’t very fast.” That is, the churning of the stars themselves within the galaxy would help civilizations spread far and wide faster than you might think.

“It’s not like they’re crossing the whole galaxy themselves – they’re just getting carried around by the stars,” Wright explains.

So the motions of the stars make settling the galaxy much, much easier, and that – as they say – sharpens the Fermi Paradox. It means it’s much easier to populate the whole galaxy than some of the more naive earlier models.

That’s not to say that the researchers assumed that every star system in the galaxy offered a potential place to settle. For a start, obviously, there are a number of systems that would not offer any planets that were compatible with human life.

But the authors also thought outside the box, suggesting that another reason galactic colonisers might avoid some star systems is because they are already inhabited. Wright and his co-authors called this the ‘Aurora effect’, naming it after the novel by Kim Stanley Robinson.

I really like this idea because it runs contrary to a lot of science fiction narratives that argue that [aliens] would want to come to the Earth exactly because there’s life here, and we have resources that they would want. But that assumes it’s a very natural tendency to see something someone else has and take it from them. And who knows maybe that’s completely contrary to the way that any any spacefaring civilization would behave.

The fraction of stars in the galaxy that are worth settling was just one of the many variables the researchers used to create the colonization modeling. “What that means, if that’s very low then you have to wait a long time before one comes into range that you can settle,” Wright points out, “and that slows down the propagation of the settlement front considerably”. Other variables included how long civilizations might survive, how quickly they might reach the spacefaring stage of development, and how far their ships might be able to travel, among others.

But they didn’t just make their own decision as to what the likely values were for each of the variables. Instead they “tried lots of different assumptions and then we mapped out the results of those assumptions”.

What this showed was that the results varied widely depending on what the original values plugged into the model were.

“What this research did for me,” Wright says, “is it helped highlight the assumptions that underlie people’s beliefs, [whether] they think there’s nothing out there, or that there’s a lot of things out there.”

So now when people talk about the Fermi paradox and say everyone should be out there, we can say ‘okay everyone should be out there, all the stars should have settlements if you believe settlements would last this long’ – and you can tell them exactly what it is that they’re assuming. I think that helps quantify the assumptions behind SETI and help inform what maybe we should believe.