Tristan Cook

I am a research analyst at the Center on Long-Term Risk.

I previously studied maths at the University of Cambridge and University of Warwick.

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Topic Contributions


What's the value of creating my own fellowship program when I can direct people to the virtual programs?

I'd add another benefit that I've not seen in the other answers: deciding on the curriculum and facilitating yourself get you to engage (critically) with a lot with EA material. Especially for the former you have to think about the EA idea-space and work out a path through it all for fellows.

I helped create a fellowship curriculum (mostly a hybrid of two existing curricula iirc) before there were virtual programs or and this definitely got me more involved with EA. Of course, there may be a trade-off in quality. 

Replicating and extending the grabby aliens model

I agree with what you say, though would note

(1) maybe doom should be disambiguated between  "the short-lived simulation that I am in is turned of"-doom (which I can't really observe) and "the basement reality Earth I am in is turned into paperclips by an unaligned AGI"-type doom.

(2) conditioning on me being in at least one short-lived simulation, if the multiverse is sufficiently large and the simulation containing me is sufficiently 'lawful' then I may also expect there to be basement reality copies of me too. In this case,  doom is implied for (what I would guess is) most exact copies of me.

Is Our Universe A Newcomb’s Paradox Simulation?

Thanks for this post! I've been meaning to write something similar, and have glad you have :-)

I agree with your claim that most observers like us (who believe they are at the hinge of history) are in (short-lived) simulations. Brian Tomasik discusses how this marginally makes one value interventions with short-term effects. 

In particular, if you think the simulations won't include other moral patients simulated to a high resolution (e.g. Tomasik suggests this may be the case for wild animals in remote places), you would instrumentally care less about their welfare (since when you act to increase their welfare, this may only have effects in basement reality as well as the more expensive simulations that do simulate such wild animals) . At the extreme is your suggestion, where you are the only person in the simulation and so you may act as a hedonist! Given some uncertainty over the distribution of "resolution of simulations", it seems likely that one should still  act altruistically.

I disagree with the claim that if we do not pursue longtermism, then no simulations of observers like us will be created. For example, I think an Earth-originating unaligned AGI would still have instrumental reasons to run simulations of 21st century Earth. Further, alien civilizations may have interest to learn about other civilizations.

Under your assumptions, I don't think this is a Newcomb-like problem.  I think CDT & EDT would agree on the decision,[1] which I think depends on the number of simulations and the degree to which  the existence of a good longterm future hinges your decisions. Supposing humanity only survives if you act as a longtermist and simulations of you are only created if humanity survives, then you can't both act hedonistically and be in a simulation. 

  1. ^

    When taking the lens of "I control my policy" as discussed here 

Fermi estimation of the impact you might have working on AI safety

This tool is impressive, thanks! I like the framing you use of safety as a race against capabilities, though think don't really know what it would look like to have "solved " AGI safety 20 years before AGI. I also appreciate all the assumptions being listed at the end of the page.

Some minor notes

  • the GitHub link in the webpage footer points to the wrong page
  • I think two of the prompts "How likely is it to work?" and "How much do you speed it up?" would be made clearer if "it" was replaced by AGI safety (if that is what it is referring to).
Bad Omens in Current Community Building

Thanks for this post! I used to do some voluntary university community building, and some of your insights definitely ring true to me, particularly the Alice example - I'm worried that I might have been the sort of facilitator to not return to the assumptions in fellowships I've facilitated.

A small note:

Well, the most obvious place to look is the most recent Leader Forum, which gives the following talent gaps (in order):

This EA Leaders Forum was nearly 3 years ago, and so talent gaps have possibly changed. There was a Meta Coordination Forum last year run by CEA, but I haven't seen any similar write-ups. This doesn't seem to be an important crux for most of your points, but thought would be worth mentioning.

Replicating and extending the grabby aliens model

This definitely sounds like a better approach than mine, thanks for sharing! This will be useful for me for any future projects

Replicating and extending the grabby aliens model

Thanks for your questions and comments! I really appreciate someone reading through in such detail :-)

  • What is the highest probability of encountering aliens in the next 1000 years according to reasonable choices once could make in your model?

SIA  (with no simulations) gives the nearest and most numerous aliens. 

My bullish prior (which has a priori has 80% credence in us not being alone) with SIA and the assumption that grabby aliens are hiding gives a median of ~ chance in a grabby civilization reaching us in the next 1000 years.

I don't condition on us not having any ICs in our past light cone. When conditioning on not being inside a GC, SIA is pretty confident (~80% certain) that we have at least one IC (origin planet) in our past light cone. When conditioning on not seeing any GCs, SIA thinks ~50% that there's at least one IC in our past light cone. Even if there origin planet is in our light cone, they may already be dead. 

  • Sometimes you just give a prior, e.g., your prior on d, where I don't really know where it comes from. If it wouldn't take too much time, it might be worth it to quickly motivate them (e.g., "I think that any interval between x and y would be reasonable because of such and such, and I fitted a lognormal". It's possible I'm just missing something obvious to those familiar with the literature.

Thanks for the suggestion, this was definitely an oversight. I'll  add in some text to motivate each prior. 

  • My prior for , the sum of delay and fuse steps: by definition it is bounded above by the time until now and bounded below by zero.
    • I set the median to ~0.5 Gy. The median is both to account for the potential delay in the Earth first becoming habitable (since the range of estimates around the first life appearing is ~600 My) and be roughly in line with estimates for the time that plants took to oxygenate the atmosphere (a potential delay/fuse step) .
    • My prior,  , roughly fits these criteria
  • My prior for  is pretty arbitrarily chosen. Here's a post-hoc  (motivated) semi-justification  for the prior. Wikipedia discusses ~8 possible factors for Rare Earths. If there are  necessary Rare-Earth like factors for life, each with fraction of planets having the property, then my prior on  isn't awfully off.
  • If one thinks that between 0.1 and 1 fraction of all planets have each of the eight factors (and they are independent) something roughly similar to my prior distribution  follows.
  • My prior for , the early universe habitability factor was mostly chosen arbitrarily. My prior implies a median time of ~10 Gy for the universe to be 50% habitable (i.e. the earliest time when   habitable planets are in fact habitable due to the absence of gamma ray bursts).   In hindsight, I'd probably choose a prior for u that implied a smaller median.
The implied distribution of the time at which the 'early' universe is  habitable for  by my prior on 
  • My prior for , the fraction of ICs that become GCs:
    • It is bounded below by 0.01, mostly to improve the Monte Carlo reliability in cases where smalleris greatly preferred
    • Has a median of ~0.5.   A Twitter poll from Robin Hanson ran gave  [I can't find the reference right now].

Lots of the priors aren't super well founded. Fortunately, if you think my bounds on each parameter is reasonable, I get the same conclusions when taking a joint prior that is uniform on  and log-uniform in all other parameters.

  • Do you think your conclusion (e.g., around likelihood of observing GCs) would change significantly if "non-terrestrial" planets were habitable?

Good question. In a hack-y and unsatisfactory way, my model does allow for this: 

If the ratio of non-terrestrial (habitable) planets to terrestrial  (habitable) planets is , they replace the product of try-once steps  with    to account for the extra planets. (My prior on  is bounded above by 1, but this could be easily changed). This approach would also suppose that non-terrestrial planets had the same distribution of habitable lifetimes as terrestrial ones.

Having said that, I don't think a better approach would change the results for the SIA and ADT updates. For SSA,  the habitability of non-terrestrial planets  makes civs like us more atypical (since we are on a terrestrial planet). If this  atypicality applies equally in worlds with many GCs and worlds with very few GCs, then I doubt it would change the results. All the anthropic theories would update strongly against the habitability of non-terrestrial planets.



Replicating and extending the grabby aliens model

Great to see this work!


 Re the SIA Doomsday argument, I think that  is self-undermining for reasons I've argued elsewhere.

I agree. When I model the existence of simulations like us, SIA does not imply doom (as seen in the marginalised posteriors for  in the appendix here). 

Further, the simulation case, SIA would prefer human civilization to be atypically likely to become a grabby civilization (this does not happen in my model as I suppose all civs have the same transition chance to become grabby).

Re the habitability of planets, I would not just model that as lifetimes, but would also consider variations in habitability/energy throughput at a given time


Smaller stars may have longer habitable windows but also smaller values for  V and M. This sort of consideration limits the plausibility of red dwarf stars being dominant, and also allows for more smearing out of ICs over stars with different lifetimes as both positive and negative factors can get taken  to the same power.

I'd definitely like to see this included in future models (I'm surprised Hanson didn't write about this in his Loud aliens paper). My intuition is that this changes little for the conclusions of SIA or anthropic decision theory with total utilitarianism, and that this weakens  the case for many aliens for SSA, since our atypicality (or earliness) is decreased if we expect habitable planets around longer lived stars to have smaller volumes and/or lower metabolisms.


I'd also add, per Snyder-Beattie, catastrophes as a factor affecting probability of the emergence of life and affecting times of IC emergence.

I hadn't seen this before, thanks for sharing! I've skimmed through and found it interesting, though I'm suspicious that at times it uses SSA -with reference class of observers on planets as habitable as long as Earth -  type reasoning.

Replicating and extending the grabby aliens model

Thanks, glad to hear it!

I wrote it in Google Docs, primarily for the ease of getting comments. I then copied it into the EA Forum editor and spent a few hours fixing the formatting - all the maths had to be rewritten, all footnote  added back in, tables fixed, image captions added - which was a bit of a hassle. 

I sadly don't have any neat tricks. I tried this Google Docs tool to convert to Markdown but it didn't work well.

The EA Forum editor now have the ability to share drafts and allow comments and collaborative editing, which I think I'll try for my next project. I'm also hoping Google Docs will add a better maths editor.

EA coworking/lounge space on

This looks great, thanks for creating it! I could see it becoming a great 'default' place for EAs to meet for coworking or social things.

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