But that assumption leads to known problems in an infinite universe e.g. it implies a strong form of Doomsday argument, where only a tiny fraction of civilisations survive and become space-colonizing.
What is the problem with this argument, besides an unpleasant conclusion? We haven’t actually seen any space-colonizing civilizations, and we would expect to see at least some unless we were either one of the first civilizations in our light cone or if most civilizations weren’t spacefaring (possibly because they don’t survive that long).
Well the main problem is the sheer severity of the Doomsday effect.
Suppose space-colonizing civilizations have an average population a billion times that of “doomed” civilizations, then the mediocrity argument implies that fewer than 1 in a billion civilizations become space-colonizing. If the population ratio is a trillion, then fewer than 1 in a trillion become space-colonizing.
But there are something like 10^22 stars in the observable universe, and a space-colonizing civilization could reach a very large portion of them; further, it would tend to do so, if there is no real competition from other colonizing civilizations (the competition would instead be arising at the edge of the expansion wave, causing travel speeds to increase and approach the speed of light). So the most likely population increase factor is something like a billion trillion or more, implying a chance of civilization survival of 1 in a billion trillion or less. That does seem unreasonably pessimistic.
What is the problem with this argument, besides an unpleasant conclusion? We haven’t actually seen any space-colonizing civilizations, and we would expect to see at least some unless we were either one of the first civilizations in our light cone or if most civilizations weren’t spacefaring (possibly because they don’t survive that long).
Well the main problem is the sheer severity of the Doomsday effect.
Suppose space-colonizing civilizations have an average population a billion times that of “doomed” civilizations, then the mediocrity argument implies that fewer than 1 in a billion civilizations become space-colonizing. If the population ratio is a trillion, then fewer than 1 in a trillion become space-colonizing.
But there are something like 10^22 stars in the observable universe, and a space-colonizing civilization could reach a very large portion of them; further, it would tend to do so, if there is no real competition from other colonizing civilizations (the competition would instead be arising at the edge of the expansion wave, causing travel speeds to increase and approach the speed of light). So the most likely population increase factor is something like a billion trillion or more, implying a chance of civilization survival of 1 in a billion trillion or less. That does seem unreasonably pessimistic.
Fair enough, and on reflection I agree that those kind of survival odds are unreasonably pessimistic given the information we currently have.