I’ve been studying & replicating the argument in the paper [& hopefully be sharing results in the next few weeks]
The argument implicitly uses the self-sampling assumption (SSA) with reference class of observers in civilizations that are not yet grabby (and may or may not become grabby).
Their argument is similar in structure to the Doomsday argument:
If there are no grabby aliens (and longer lived planets are habitable) then there will be many civilizations that appear far in the future, making us highly atypical (in particular, ‘early’ in the distribution of arrival times).
If there are sufficiently many grabby aliens (but not too many) they set a deadline (after the current time) by when all civilizations must appear if they appear at all. This makes civilizations/observers like us/ours that appear at ~13.8Gy more typical in the reference class of all civilizations/observers that are not yet grabby.
Throughout we’re assuming the number of observers per pre-grabby civilization is roughly constant. This lets us be loose with the the civilization - observer distinction.
I don’t think the reference class is a great choice. A more natural choice would be the maximal reference class (which includes observers in grabby alien civilization) or the minimal reference class (containing only observers subjectively indistinguishable from you).
Maybe I’m misunderstanding SSA, but wouldn’t “SSA with reference class of observers in civilizations that are not yet grabby” require that we weight by the relevant populations?
For example, if Civilization A has 10× more citizens (before becoming grabby or going extinct) than does Civilization B, wouldn’t our prior be that we’re 10× likelier to find ourselves in Civilization A than B?
We could further condition on something like “observing that computers were invented ~X years ago” (or something similar that distinguishes observers like) such that the (eventual) population of civilizations doesn’t matter. This conditioning means we don’t have to consider that longer-lived planets will have greater populations.
If we’re allowed to “observe” that computers were invented 80 years ago, why can’t we just “observe” that the universe is 13.8 billion years old, and thus throw the whole Grabby Aliens analysis in the garbage? :-P (Sorry if that sounds snarky, it’s an honest question and I’m open-minded to there being a good answer.)
Hanson et al. are conditioning on the observation that the universe is 13.8 billion years old. On page 18 they write
Note that by assuming a uniform distribution over our origin rank r (i.e., that we are equally likely to be any percentile rank in the GC origin time distribution), we can convert distributions over model times τ (e.g., an F(τ ) over GC model origin times) into distributions over clock times t. This in effect uses our current date of 13.8Gyr to estimate a distribution over the model timescale constant k. If instead of the distribution F(τ ) we use the distribution F0(τ ), which considers only those GCs who do not see any aliens at their origin date, we can also apply the information that we humans do not now see aliens.
Formally (and I think spelling it out helps) with SSA with the above reference class, our likelihood ratio is the ratio of [number of observers in pre-grabby civiliations that observe Y] to [number of observers in pre-grabby civilizations] where Y is our observation that the universe is 13.8 billion years old, we are on a planet that has been habitable for ~4.5Gy and has total habitability of ~5.5Gy, we don’t observe any grabby civilizations, etc
You originally wrote “We could further condition on something like “observing that computers were invented [80] years ago” … This conditioning means we don’t have to consider that longer-lived planets will have greater populations.”
I interpreted this comment as you saying “We could restrict our SSA reference class to only include observers for whom computers were invented 80 years ago”. (Is that right?)
And then I was trying to respond to that by saying “Well if we can do that, why can’t we equally well restrict our SSA reference class to only include observers for whom the universe is 13.8 billion years old? And then “humanity is early” stops being true.”
I interpreted this comment as you saying “We could restrict our SSA reference class to only include observers for whom computers were invented 80 years ago”. (Is that right?)
What I wanted to say was: keep the reference class the same, but restrict the types of observers we are we saying we are contained in(the numerator in the SSA ratio) to be only those who (amongst other things) observe the invention of the computer 80 years ago.
And then I was trying to respond to that by saying “Well if we can do that, why can’t we equally well restrict our SSA reference class to only include observers for whom the universe is 13.8 billion years old? And then “humanity is early” stops being true.”
Yep, one can do this. We might still be atypical if we think longer-lived planets are habitable (since life has more time to appear there) but could also restrict the reference class further. Eventually we end up at minimal reference class SSA
If there are no grabby aliens, then our civilization is highly atypical. But if there are grabby aliens, then we as individuals are highly atypical, living before the space expansion which controls orders of magnitude more resources, and therefore can supports orders of magnitude more sentient observers.
A possible solution would be, if the grabby aliens have to sacrifice their sentience in return for greater expansion speed. A global race to the bottom, where those who do not reduce themselves to the most efficient replicators get outcompeted by those who do. If replicators without sentience are 1% more efficient at replication than replicators with sentience, in the long run this is all that matters.
*
(Actually, this also seems to get the math wrong. Even if grabby aliens gradually lose sentience and become pure replicators, as long as they don’t lose the sentience immediately, there should still be orders of magnitude more sentient observers in the early phase of expansion than before the expansion. So our situation before the expansion remains highly atypical.)
I’ve been studying & replicating the argument in the paper [& hopefully be sharing results in the next few weeks]
The argument implicitly uses the self-sampling assumption (SSA) with reference class of observers in civilizations that are not yet grabby (and may or may not become grabby).
Their argument is similar in structure to the Doomsday argument:
If there are no grabby aliens (and longer lived planets are habitable) then there will be many civilizations that appear far in the future, making us highly atypical (in particular, ‘early’ in the distribution of arrival times).
If there are sufficiently many grabby aliens (but not too many) they set a deadline (after the current time) by when all civilizations must appear if they appear at all. This makes civilizations/observers like us/ours that appear at ~13.8Gy more typical in the reference class of all civilizations/observers that are not yet grabby.
Throughout we’re assuming the number of observers per pre-grabby civilization is roughly constant. This lets us be loose with the the civilization - observer distinction.
I don’t think the reference class is a great choice. A more natural choice would be the maximal reference class (which includes observers in grabby alien civilization) or the minimal reference class (containing only observers subjectively indistinguishable from you).
It’s best, in my judgement, to not use reference classes at all when doing anthropics. Explained more in this sequence: https://www.lesswrong.com/s/HFyami76kSs4vEHqy
Thanks!
Maybe I’m misunderstanding SSA, but wouldn’t “SSA with reference class of observers in civilizations that are not yet grabby” require that we weight by the relevant populations?
For example, if Civilization A has 10× more citizens (before becoming grabby or going extinct) than does Civilization B, wouldn’t our prior be that we’re 10× likelier to find ourselves in Civilization A than B?
Yep, you’re exactly right.
We could further condition on something like “observing that computers were invented ~X years ago” (or something similar that distinguishes observers like) such that the (eventual) population of civilizations doesn’t matter. This conditioning means we don’t have to consider that longer-lived planets will have greater populations.
If we’re allowed to “observe” that computers were invented 80 years ago, why can’t we just “observe” that the universe is 13.8 billion years old, and thus throw the whole Grabby Aliens analysis in the garbage? :-P (Sorry if that sounds snarky, it’s an honest question and I’m open-minded to there being a good answer.)
Doesn’t sound snarky at all :-)
Hanson et al. are conditioning on the observation that the universe is 13.8 billion years old. On page 18 they write
Formally (and I think spelling it out helps) with SSA with the above reference class, our likelihood ratio is the ratio of [number of observers in pre-grabby civiliations that observe Y] to [number of observers in pre-grabby civilizations] where Y is our observation that the universe is 13.8 billion years old, we are on a planet that has been habitable for ~4.5Gy and has total habitability of ~5.5Gy, we don’t observe any grabby civilizations, etc
Oh, I think I phrased my last comment poorly.
You originally wrote “We could further condition on something like “observing that computers were invented [80] years ago” … This conditioning means we don’t have to consider that longer-lived planets will have greater populations.”
I interpreted this comment as you saying “We could restrict our SSA reference class to only include observers for whom computers were invented 80 years ago”. (Is that right?)
And then I was trying to respond to that by saying “Well if we can do that, why can’t we equally well restrict our SSA reference class to only include observers for whom the universe is 13.8 billion years old? And then “humanity is early” stops being true.”
Ah, I don’t think I was very clear either.
What I wanted to say was: keep the reference class the same, but restrict the types of observers we are we saying we are contained in(the numerator in the SSA ratio) to be only those who (amongst other things) observe the invention of the computer 80 years ago.
Yep, one can do this. We might still be atypical if we think longer-lived planets are habitable (since life has more time to appear there) but could also restrict the reference class further. Eventually we end up at minimal reference class SSA
If there are no grabby aliens, then our civilization is highly atypical. But if there are grabby aliens, then we as individuals are highly atypical, living before the space expansion which controls orders of magnitude more resources, and therefore can supports orders of magnitude more sentient observers.
A possible solution would be, if the grabby aliens have to sacrifice their sentience in return for greater expansion speed. A global race to the bottom, where those who do not reduce themselves to the most efficient replicators get outcompeted by those who do. If replicators without sentience are 1% more efficient at replication than replicators with sentience, in the long run this is all that matters.
*
(Actually, this also seems to get the math wrong. Even if grabby aliens gradually lose sentience and become pure replicators, as long as they don’t lose the sentience immediately, there should still be orders of magnitude more sentient observers in the early phase of expansion than before the expansion. So our situation before the expansion remains highly atypical.)