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).
Hi, coauthor of the Grabby Aliens paper here.
In my view, the correct way to calculate in many anthropic problems is along the lines of the well-explored case of Everett physics: by operationalising the problem in decision theoretic terms.
For the sleeping beauty problem, if one embeds the problem in a repeated series involving bets, and if each bet feeds into a single pot, you arrive at the Thirder position. There is then a consistency argument to make the single-shot problem match that.
Similarily, for the Grabby Aliens problem, consider that civilisations may lodge predictions about the distance to the nearest GC, which can be compared to other civilisations' guesses in the intergalactic council at a later date. Or choose a repeated game in which members of the council reset themselves to the spacetime origin point of a random other GC in the council, by simulation or other method, and make bets from there. The single-shot case, i.e. humanity's predicament, should have a matching strategy.
It is statistical prediction in this sense that I had in mind when helping with calculations+concepts for the paper.
As I've argued, anthropics reasoning, absent indistinguishable copies, is nothing special: "I observe X" gives the same update as "I exist and observe X".
So, what theory best explains the fact we exist and don't observe aliens? Apart from the various "zoo hypotheses" (there are aliens but they are hiding), the most likely theory is "evolving life is not that hard, but humans evolved unusually early". The first half makes our existence more likely, the second explains our non-observation of aliens (again, "we don't observe aliens" is the same as "we exist and don't observe aliens", which is the same as "early aliens didn't kill or conquer humanity, and we don't observe aliens").
Grabby Alien works on similar logic to well-known anthropic camps such as SSA and SIA: consider what we are as an Observation Selection Effect. As you wrote, treat ourselves as random selections from a list containing everyone. The main difference is regular anthropic camps typically apply this to individual observers, while grabby alien applies it to civilizations.
Whether this reflects good anthropic reasoning is hard to answer. If one endorses regular anthropic camps then Grabby Alien's logic is at least incomplete. It should incorporate how many observers different civilizations have. But it should be noted applying the Observation Selection Effect at the observer level is not watertight either. Maybe it should be applied to the observer-moment pair level: what I am experiencing now should be regarded as it is randomly selected from all observer-moments. Then the theory ought to be further updated reflecting the life span of all observers from different civilizations.....
I personally firmly believe the typical OSE way of anthropic reasoning is plainly wrong. What "I" am, or more preciously what the first-person perspective is, cannot be reasoned. It is a primitively axiomatic fact. I.E. "I naturally know I am this person. But there is no reason behind it, nor an explanation for why it is so. I just am." Attempting to explain it as a random sample only leads to paradoxes. A starter of my argument can be read here.
I feel like I’m missing something. I feel like there are a bunch of people who have spent a bunch of time thinking about anthropics
I have. Hi! I think the reasoning is approximately correct. The caveat is that "civilization" is not an ontologically basic element in the calculation. What you should update on is your total set of observations, and then you should prefer universes where that set of observations is more likely to be instantiated. But (without reading the grabby aliens paper) it sounds to me like this approximates the update that the paper makes.
I thought that my model aligns with UDASSA, but I've derived it independently and I'm not sure.
It's none of the three-letter-acronyms because it actually uses our knowledge that human civilization exists, and that we have a certain distribution over physical law. I think it's basically fine, though I think the paper makes some pitfalls in saying it "explains" certain things without showing that there are more "microstates" of the model where their "explanation" works versus where it doesn't.
EDIT: In response to Tristan's answer, I'd say that you can start with this distribution and recover different three-letter acronyms by ablating away different pieces of knowledge. Like Rafael says, the important thing is taking the knowledge we actually do have and thinking about different ways the rest of the universe could be.
An underlying assumption of the Grabby Aliens paper by Robin Hanson et al., if I understand it, is the following:
We should expect to find ourselves as a member of a uniformly-randomly-selected civilization out of all civilizations in the history of the universe.
In other words, if there’s a master list of every civilization in the universe’s past, present, and future, our prior should be that our human civilization should be uniformly-randomly selected from that list. If you accept that assumption, then you’re obligated to perform a Bayesian update towards hypotheses-about-the-universe that predict a master-list-of-all-civilizations with the property that human civilization looks like a “typical” civilization on the list. My impression is that this assumption (and corresponding Bayesian update) is the foundation upon which the whole Grabby Aliens paper is built. (Well, that plus the assumption that there are no other Bayesian updates that need to be taken into account, which I think is dubious, but let’s leave that aside.)
If that’s right, I’m confused where this assumption comes from. When I skim discussions of anthropic reasoning (e.g. in Nick Bostrom's book, and on lesswrong, and on Joe Carlsmith's blog, etc.), I see lots of discussion about “SIA” and “SSA” and “UDASSA” and so on. But the Grabby Aliens assumption above seems to be none of those things—in fact, it seems to require strongly rejecting all of them! (E.g., note how the Grabby Aliens assumption does not weight civilizations by their population.)
I feel like I’m missing something. I feel like there are a bunch of people who have spent a bunch of time thinking about anthropics (I’m not one of them), and who endorse some “standard” anthropic reasoning framework like SIA or SSA or UDASSA or whatever. Do all those people think that the Grabby Aliens paper is a bunch of baloney? If so, have they written about that anywhere? Or am I wrong that they’re contradictory? (Or conversely, has anyone tried to spell out in detail why the Grabby Aliens anthropic assumption above is a good assumption?)