Robin Hanson joins the Intelligent Voluntary Cooperation group to discuss his Simple Model of Grabby Aliens. His claim: we have the data to make informed predictions around how common “loud” (expansionary, fast-growing and environment-changing) extraterrestrial civilizations may be, where they are and when we may come into contact with them.

This meeting is part of the Intelligent Cooperation Group and accompanying book draft.


Presentation: A Simple Model of Grabby Aliens

  • Most of us have heard of what Robin calls “quiet” aliens, or isolated alien civilizations that don’t much expand past their home planets or systems, run their course and eventually end.
  • The density of these sort of civilations are estimated by the Drake equation, but we know very little to nothing else: after all, they are quiet!
  • What Robin discusses here: LOUD aliens! Loud aliens don’t die off: they grow and expand out into the universe. They make an impact. Even if there are relatively small numbers of these groups, they would be noticed eventually.
  • So how can a model for the expansion of these civilizations be made?
  • First, show and tell :)
  • Three parameters determine the model: the spawn rate of such grabby populations, the expansion rate of these groups once they start spreading out into the universe, and the power law governing the chance that any particular group evolves to start spreading in the first place.
  • The spawn rate is based on the assumption that human civilization becomes “grabby” in the next 10 million years, meaning our origin is a random sample of these spawns.
  • The power law is determined by “hard steps” theory of evolution. This theory states that in the history of life on Earth, there was a series of “hard steps” various forms of life passed through to get us where we are today.
  • Robin explains this power law parameter using lockpicking metaphor of evolution:
    • Imagine you’re faced with a task: many locks need picking in a certain time frame. Each lock has a certain average time it takes to pick, and a lock is called “hard” if this average time is longer than the total amount of time you have to pick all of the locks.
    • Now imagine you’re faced with picking a series of “hard” locks in a certain timeframe, and you get lucky and pick them all with time to spare.
    • The statistics of power laws shows that in this situation, when each lock is hard and you pick them all, the times it took to pick each lock and the time remaining until the deadline after the final lock is picked are drawn from the same distribution. AND the chance you had of succeeding is a power law of the time you had to do it raised to the number of locks picked.
  • The key idea here is that evolution is like picking locks: many hard steps needed to be taken, and Earth has a deadline rooted in it’s time in the habitable zone. Based on this, we actually have some data on those lock-picking durations.
  • The first lock, from the time Earth became first habitable to the first basic life was about 0.4 billion years. The final duration, between now and when the Earth will leave the habitable zone (at which point we better have started spreading) is about 1.2 billions years.
  • Due to the math of power laws, these two should have been drawn from the same distribution, and so plugging them into the model, this shows that 3-12 “hard steps” have happened on Earth (assuming that no other steps were taken we don’t know about before Earth)
  • This number of steps determines the power law curve of civilizations spawing per the chart below:

  • Based on the chart, you can see that the model predicts that the universe is mostly empty for while then civilizations are popping faster and faster toward the “end” until space runs out.
  • When we look up at the sky, we don’t see any galactic structures or evidence of other civilizations, and if they were there they’d be so big that we would (like, bigger than the full moon big).
  • Either they are invisible somehow, perhaps by excising the portion of their backward light cone that would intersect ours, or they’re not there yet.
  • So… are we early?
  • A lot has to happen to lead to the rise of grabby aliens. Stars appeared, then planets formed around them that are habitable, evolution occurs on these planets, and the life there makes it through all the hard steps from fungus to spaceflight.

  • Based on our best “Galactic Habitable Zone” knowledge, the earliest habitable planets were still much too volatile for complex life like us. Habitable planets for “fragile life” like us, began peaking around 12 billion years ago.
  • Using this info, the power law information from above, the age of our star and planet, and the assumption that we will become one of the grabby groups we’re interested in, we derive the equation below:

  • It seems from this model that humanity is early in the history of the universe… why’s that? If we were not early, and in fact these sorts of “grabby aliens” exist already, then we would be seeing evidence of them.
  • That means that ANY civilization that’s gonna lay claim to any of the universe will have to be pretty early in the game: the universe gets gobbled up pretty fast later on.


In 1982, I attended a full day session on finding intelligent life. Frank Drake finished with a statement: “you’re arguing about the slope of the straight line through your data point.”

  • I’m fitting three parameters to data, so it’s actually data driven
    • 1. Our origin date.
    • 2. The fact that we don’t see aliens out there.
    • 3. The history of life on Earth (this is structured data)


Critiques from that day: maybe life can’t survive in space

  • Then why are we so early?


Another argument: the galaxy fills in 100million years, not billions

  • The power law means that there is a long period of not filling up and then a short period of filling up


Your charts showed civilizations getting arbitrarily large. Isn’t there a speed of light communication delay that actually limits the reach of any civilization?

  • “Civilization” in our model doesn’t make any assumptions about coherence or governance, just “starts from place and spreads”


I have been loking at the Fermi questions for some time. I have a paper out recently on the “hard steps” model of evolution. I am only annoyed by the full moon claim. You should Steven J Olsen papers about rapidly expanding civilisations When you try to look at the angular size, you need to do a full relativistic treatment. When the civilization is coming towards you, you only see the edge, so it’s very small.

  • This is a point that is elaborated in the paper. In our simple cosmology, things show up at points and expand at constant speeds from that point. But the real universe is expanding. So really our model holds in conformal time and comoving space, in which the model directly applies, in that things show up in positions and stay in positions. The actual expansion process is probably via outposts being constantly established and expanded. What we expect to see is a constant speed relative to comoving stuff.


I call into question the obviousness of the full moon point… what are these civilizations actually like? Just spheres of light? And is our civilization actually an average grabby civilization? Finally, do all galaxies meet the criteria that shape the curve of when habitable planets show up? Can early be earlier than us?

  • When we started, I was hoping to take the actual spatial formations of the universe, but the spatial scales we are using in the model are larger than the scale at which different galaxies have interesting differences. The literature on the “galactic habitable zone” is still pretty spotty… we’re estimating here.


What if…. THE SKY IS FAKE!?

  • “once in a while, it’s worth just analyzing ‘what if things are what they seem?”’


In your model we are us and not grabby?

  • In our model we may become grabby soon!


An alternative theory for a resolution of the Fermi paradox similar to yours: In your model, the reason we don’t see grabby civilizations is that they excise a large fraction of their backward light cone. Another theory suggests that when civilizations get sufficiently advanced, they run stupid science experiments that destabilize space-time and create vacuum bubbles that excise the light cone just like your model.

  • Yes, this would have similar results as our grabby model, in that there are still these large volumes of the universe that get “taken up” and therefore our earliness is necessary to claim some space before they are. Our model is a bit more optimistic than “advanced civilizations reliably destroy the universe”, so a mixed model could certainly be called for in which not EVERY civilization destroys the universe (and hopefully none do!)


In the Dark Forest, the third book in the Three Body Problem series, the “civilizations destroy local physics” scenario is discussed directly! It also suggests that advanced civilizations have a strategic reason to limit their light leakage and therefore their visibility to others without needing to destroy themselves to accomplish it.


I understand in your model when civilization bubbles meet each other there is just sort of a wall: they stop expanding and continue through time. But this doesn’t happen in nature! What you’re dealing with in this model is an ecosystem, made of ecosystems that will have predators and prey and ecological health that determines all sorts of qualities about them.

  • In our model we’re dealing with civilizations controlling millions of galaxies, so we are abstracting away all the internal details of what might be going on in there. The key point is “civilization X takes up this volume and will prevent other civilizations from occupying its space.”


Does the fact that matter is not very uniformly distributed effect this?

  • Initially we thought that this would make a difference and we thought to bring in those details: filaments and pancakes and whatnot. But we found that the scales we are dealing with in the model are just too vast for these structural details to matter. For a much more detailed model, these may become important again.


If it turns out that interstellar space is too hostile for us to exist in, then we transform into machines and we build whatever machines necessary to travel. It’s not really us, it’s the whole ecosystem that supports us that must travel with us.

  • We’re talking about what we might be capable of in 10million years, so it’s really not rooted in what we are capable of now.


We have a strong history of competing and war… You think we could possibly get to a point where we’re so coordinated we can expand like you propose out into the universe?

  • The most likely scenario is a lack of central coordination propelling the spread, as opposed to a centrally coordinated civilization spreading.


One of the traditional answers to the Fermi question is “We’re the first!” but that doesn’t seem very likely. You’ve shown something more compelling backed by some data. Really what you’ve shown may be the most extreme reasoning for Manifest Destiny ever!

  • The high-order bit: We SEEM early relative to a universe in which there is no deadline for expansion.


A theory: advanced civilizations upload their existence to servers and find that way better than expanding into the universe and they somehow power all this without needing to expand for resources.

  • “You know, as always, one has to say that any analysis is going to be based on our current understanding of something. And it’s always possible we completely misunderstand something in our purview of analysis such that we go completely wrong and that’s true of every analysis we ever do. And it should be a footnote on every analysis, but no more than a footnote on every analysis because it’s a tedious dimension that every analysis could be very wrong if you are very wrong about something.”



Seminar notes by James Risberg.