Anthropic Reasoning #1: I'm Confused

I’m going to start reading a fair bit about anthropics because I suspect that understanding anthropic reasoning is key to various interesting real world problems including:

• the fermi paradox (should you buy the grabby aliens/humans are early argument?)
• X risk base rates (does CERN have a 99% chance of destroying the universe every time it’s run? What about a light switch being flipped?)

I’m writing this a bit differently. It’s less of a “here’s what I think about this problem and which solution I think is best/what I have to add” and more of a “here’s stuff I’m really confused about going into this”.

Okay. First thing first. What is anthropic reasoning? In short, it’s reasoning about situations where there are observation selection effects. Observer selection effects meaning that you as an observer will only exist to observe some subset of possible outcomes. A classic example of this is the fine tuning argument.

• theist: god exists
• atheist: what’s your evidence for that?
• theist: we live in a remarkably fine-tuned universe. If gravity was slightly stronger, the universe would have collapsed into a second big crunch shortly after the big bang. Stars, planets and intelligent life would never have formed. If gravity was weaker the expansion of the universe would have been faster and the universe would be endless sparse clouds of matter with no stars or planets. We happen to be precisely at the gravitational goldilocks zone where starts, planets and our existence is possible. This is very unlikely and should be taken as evidence of intelligent design.
• atheist: but we would only exist to observe a universe which has suitable conditions for sentient life to emerge. Even if 99.9999% of universes end up with no life, we should expect all observations to occur in the 0.0001% that have conditions that allow for life

Why is anthropic reasoning confuzzling? A few reasons:

• It seems importantly different from our usual reasoning. Usually we look at the evidence the world presents, consider our priors and shift probability weight between the different possible worlds we could be in. Walking into my building and seeing my apartment door is broken open shifts weight towards me being in the “my flat has been robbed” world and to a lesser extent to the “there was a fire in my flat and the fire brigade had to break in” worlds etc… For anthropics we also need to consider the possibility of our own existence given very uncertain counter-factuals. This is weird and requires some additional scaffolding on top of standard baysianism as I use it day to day.
• I’m not sure where it ends. Let’s say I walk up to you and tell you you shouldn’t use light switches. Why not? Because each time you do there’s a 99% chance of a FTL vacuum collapse and the universe immediately ends. You could respond to this that that’s unlikely and you’ve never observed it happening but wait, no observations here are not evidence.

What’s my plan for learning about this

• reading [[Anthropic Bias]] by Bostrom
• reading sleepingbeautyproblem.com
• annoying Andrea until she explains things to me
• writing a few blog posts along the way