Why do I get the feeling that you will tease me about the discrepancy between my probability estimates on aliens in the milky way contra the rest of the universe?
That’s actually probably my favorite single question: when I first took the survey I went ‘universe: dunno, maybe 60%, Milky Way: eh, 30%.....wait a second aren’t there more than 2 galaxies‽’
There’s only something wrong with holding both “Pr(intelligent life in Milky Way) non-negligible” and “Pr(intelligent life in observable universe) not-almost-1“ if the events “life in galaxy 1”, “life in galaxy 2”, etc., are independent or approximately so. So if you assign substantial probability to propositions like “intelligent life basically can’t actually emerge naturally at all, but we were put here by a god” or “subtle variations in the laws of physics across the universe mean that our galaxy is suitable for intelligent life but most others aren’t” then you can consistently give such answers.
I can’t think of any reason for the right sort of intergalactic correlation that’s likely to be thought probable by many LWers, though.
That’s actually probably my favorite single question: when I first took the survey I went ‘universe: dunno, maybe 60%, Milky Way: eh, 30%.....wait a second aren’t there more than 2 galaxies‽’
In which case your probability estimates would seem to suggest exogenesis.
you need to make some pretty strong assumptions to overcome a 1:170,000,000,000 difference.
Actually you don’t. Consider the following highly simplified toy model.
You’re not sure where the great filter is but you think there is a 50% chance it’s before evolving intelligence (scenario A), and 50% that it’s afterward (scenario B).
In scenario A each galaxy only has a 0.1% chance of having intelligent life. (Note that nevertheless the observable universe will still have life somewhere since 0.1% is a lot more than 1⁄170,000,000,000.)
In scenario B each galaxy has (multiple) planets with intelligent life in it.
Combining these two scenarios gives 100% for life in the universe and 50.1% for life in the galaxy.
By changing these numbers and adding more scenarios you can get different but similar results. You should try this yourself, it’s a good way to get an intuition for how Bayesian probabilities work. For example, try adding a scenario C where intelligent life is extremely rare and we exist only due to the anthropic principal. What happens when you assign scenario C 40% and keep scenarios A and B equally likely?
Any answer is consistent as long as P(aliens in universe) > P(aliens in milky way), but the ratio implicitly depends on how densely populated you think the universe is conditional on aliens existing at all, assuming aliens are distributed randomly among galaxies.
Why do I get the feeling that you will tease me about the discrepancy between my probability estimates on aliens in the milky way contra the rest of the universe?
That’s actually probably my favorite single question: when I first took the survey I went ‘universe: dunno, maybe 60%, Milky Way: eh, 30%.....wait a second aren’t there more than 2 galaxies‽’
(Nice interrobang.)
There’s only something wrong with holding both “Pr(intelligent life in Milky Way) non-negligible” and “Pr(intelligent life in observable universe) not-almost-1“ if the events “life in galaxy 1”, “life in galaxy 2”, etc., are independent or approximately so. So if you assign substantial probability to propositions like “intelligent life basically can’t actually emerge naturally at all, but we were put here by a god” or “subtle variations in the laws of physics across the universe mean that our galaxy is suitable for intelligent life but most others aren’t” then you can consistently give such answers.
I can’t think of any reason for the right sort of intergalactic correlation that’s likely to be thought probable by many LWers, though.
See my comment here.
In which case your probability estimates would seem to suggest exogenesis.
See my comment here.
I was pretty sure there were about 100 billion galaxies so I just divided by that factor.
That assumes aliens arising in each galaxy is an independent event when conditioning on your uncertainty.
...why yes, yes it does. Dur.
As I pointed out here that logic doesn’t work.
There’s >170 billion galaxies in the observable universe; you need to make some pretty strong assumptions to overcome a 1:170,000,000,000 difference.
Actually you don’t. Consider the following highly simplified toy model.
You’re not sure where the great filter is but you think there is a 50% chance it’s before evolving intelligence (scenario A), and 50% that it’s afterward (scenario B).
In scenario A each galaxy only has a 0.1% chance of having intelligent life. (Note that nevertheless the observable universe will still have life somewhere since 0.1% is a lot more than 1⁄170,000,000,000.)
In scenario B each galaxy has (multiple) planets with intelligent life in it.
Combining these two scenarios gives 100% for life in the universe and 50.1% for life in the galaxy.
By changing these numbers and adding more scenarios you can get different but similar results. You should try this yourself, it’s a good way to get an intuition for how Bayesian probabilities work. For example, try adding a scenario C where intelligent life is extremely rare and we exist only due to the anthropic principal. What happens when you assign scenario C 40% and keep scenarios A and B equally likely?
I’m mentally tired from banging my head against R and can’t think through this, so I’m dropping it here.
Feel free to try tomorrow.
I found that section SO hard to answer without wishful thinking getting in the way. So I just left them all blank. I WANT smart alien friends! =/
There are good justifications for putting down very similar probabilities, to the point that they round to the same number.
Any answer is consistent as long as P(aliens in universe) > P(aliens in milky way), but the ratio implicitly depends on how densely populated you think the universe is conditional on aliens existing at all, assuming aliens are distributed randomly among galaxies.