On the Fermi paradox
Speculation in the most rarefied air
Let me start by saying that when Robin Hanson titles a paper using the term “grabby1”, your first instinct should be to call the police. Anyway, I’ve gotten a couple of questions about their particular approach to the Fermi paradox, so I thought I’d give my view of their analysis. Thankfully for my nausea, the paper title and abstract use the term “loud” (but “grabby” appears in the text).
First off, none of these people appear to be exobiologists or astrophysicists. One author (Daniel Martin), as best I can tell, was an undergrad — possibly a math major? One is from a hedge fund. Thus the analysis appears to be more of a mathematical experiment, divorced from physical reality. It’s speculation in rarefied air — a vacuum, if you will.
Their paper attempts to solve two problems — or I guess propose the solution to one problem as a solution to the other. The question they ask is: Why does our species seem to have appeared so early in the evolution of the universe? If we have appeared “early” then that can also solve the Fermi paradox: Why haven’t we seen aliens?
They propose that intelligent aliens basically expand and take over all suitable planets. We don’t see other aliens because they would have taken over our planet if they were available for us to see. In order to get the math to work out “sensibly” they have to make two silly assumptions:
Species expand through the galaxy at ~ c/3
Red dwarf stars are capable of supporting intelligent life
Regarding the first point, and in addition to the plain silliness of an average rate of expansion of ~ c/3, the paper does not make any reference to the fact that you would have to accelerate at a survivable rate and then decelerate at a survivable rate. That would mean the peak velocity2 would be greater than c/3 in order to average to c/3. To date, we have not even hit c/100.
Regarding the second point, red dwarf stars are problematic. They have a very close habitable zone, which generally means planets will be tidally locked (one super hot side, one freezing side). Additionally, sunspots on red dwarfs can be like 30% of the visible disk; their energy output would vary by several percent for years. Neither of these are very suitable for life — which means most astrophysicists and exobiologists do not think red dwarfs would harbor much life much less intelligent life. This red dwarf assumption underlies a great deal of the analysis — providing both more as well as longer-lived stars suitable for life.
But then there is the question of whether we even are early on the scene. They claim:
Only ∼8% of interstellar gas has yet been turned into stars, and if stars of all masses were equally habitable [ed. red dwarf assumption], most habitable years should lie in planets at the far-longer-lived small mass stars [ed. red dwarf assumption]. After all, 95% of stars last longer than our sun, and some last roughly two thousand times longer [ed. red dwarf assumption]
Why is the conversion of gas completely into stars the relevant scale here? It’s just kind of assumed. A maximum entropy production argument3 shows that the infrared radiation of “starlight shining on dust” is one of the biggest contributions to entropy production in the universe. Therefore, the MaxEnt assumption should allow for a large amount of material that isn’t incorporated into stars for the stars that do exist to shine on.
All of this points to playing with parameters in a particular model built from particular assumptions to get the result you want — a very “economic” approach in more ways than one. There is no “physical” approach here. We don’t include every effect at some scale we working at. The things we add to the model and the things we leave out have no organizing principle behind them besides getting an answer we want.
And since this is a sci fi blog let me also add: this isn’t even a novel solution to the Fermi paradox. It’s not too far from the “dark forest” of the Three Body series — everyone is quiet because if you’re loud you get destroyed before you get too powerful4. Liu Cixin’s approach to loud aliens has be benefit of being more interesting from a story perspective. Although there definitely are holes in the delivery, my short story Wigner and I at least does something I’ve never seen before5. It proposes we don’t see intelligent aliens because consciousness collapses the wavefunction and therefore every conscious species in the universe lives on a different fork in Everett’s many worlds — unless you build a pocket of elsewhere big enough to fit a person!
Those are my two cents on “grabby6” aliens.
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And that doesn’t even take into account time dilation. The time in, say, the rest frame of the Milky Way would be much longer than the time for those traveling at such a large fraction of the speed of light.
Bousso and Harnik’s paper is actually an argument that the particular parameters we observe out of the myriad possibilities of the string theory landscape are sensible — an entropic principle instead of an anthropic principle. However, entropy is additive so we can ignore the other (interesting!) contributions to the MaxEnt picture.
In Liu Cixin’s universe, scientific advancement over the 100s or 1000s of years it takes to travel between stars means you have to crush any civilization you notice.
Would love to read a similar story if it exists! Leave a comment or send me a message here or via DM at newqueuelure.bsky.social.
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