Crossposted to the Effective Altruism Forum

Summary

This report is the most comprehensive model to date of aliens and the Fermi paradox. In particular, it builds on Hanson et al. (2021) and Olson (2015) and focuses on the expansion of ‘grabby’ civilizations: civilizations that expand at relativistic speeds and make visible changes to the volume they control.

This report considers multiple anthropic theories: the self-indication assumption (SIA), as applied previously by Olson & Ord (2022), the self-sampling assumption (SSA), implicitly used by Hanson et al. (2021) and a decision theoretic approach, as applied previously by Finnveden (2019).

In Chapter 1, I model the appearance of intelligent civilizations (ICs) like our own. In Chapter 2, I consider how grabby civilizations (GCs) modify the number and timing of intelligent civilizations that appear.

In Chapter 3 I run Bayesian updates for each of the above anthropic theories. I update on the evidence that we are in an advanced civilization, have arrived roughly 4.5Gy into the planet’s roughly 5.5 Gy habitable duration, and do not observe any GCs.

In Chapter 4 I discuss potential implications of the results, particularly for altruists hoping to improve the far future.

Starting from a prior similar to Sandberg et al.’s (2018) literature-synthesis prior, I conclude the following:

Using SIA or applying a non-causal decision theoretic approach (such as anthropic decision theory) with total utilitarianism, one should be almost certain that there will be many GCs in our future light cone.

Using SSA^[1], or applying a non-causal decision theoretic approach with average utilitarianism, one should be confident (~85%) that GCs are not in our future light cone, thus rejecting the result of Hanson et al. (2021). However, this update is highly dependent on one’s beliefs in the habitability of planets around stars that live longer than the Sun: if one is certain that such planets can support advanced life, then one should conclude that GCs are most likely in our future light cone. Further, I explore how an average utilitarian may wager there are GCs in their future light cone if they expect significant trade with other GCs to be possible.

These results also follow when taking (log)uniform priors over all the model parameters.

All figures and results are reproducible here.

Vignettes

To set the scene, I start ...

SETI