Anthropic reasoning and observation selection effects: what can we really conclude from fine-tuning?

Presented by Dr Simon Friederich (University of Groningen)

The anthropic principle highlights the potential importance of "observation selection effects" in scientific inference: our observations are constrained by the fact that what we observe must be compatible with the existence of observers. Some physicists and philosophers believe that appeal to this triviality can help justify inferring from the fact that the laws of nature seem "fine-tuned for life" that there are likely multiple universes. Their idea is that the manifest existence of life-friendly conditions despite the required fine-tuning becomes non-miraculous only if there are multiple universes, all with different conditions, a few with life-friendly ones. Observation selection effects are then supposed to explain why we happen to find ourselves in one of the life-friendly ones: we simply could not have existed anywhere else.

In this talk, I will highlight various challenges that this inference faces and argue that they are serious. I also propose a novel version of the inference from fine-tuning to multiple universes: rather than considering multiple universes as a possible explanation of why we exist despite the required fine-tuning this version regards the fine-tuning as reducing, or even eliminating, a distinctive advantage that single-universe theories usually have over multiple universe theories: namely, that of offering very specific, potentially falsifiable predictions. I conclude by giving an assessment of where we stand with regard whether there likely are other universes.