The problem with hierarchical multiverses

Lee Smolin

In an earlier post in this blog I mentioned a list of theories about multiverses, independent and often mutually contradictory, prepared by George Ellis, the cosmologist. These multiverses can be divided into two large groups:

• Non-hierarchical multiverses: such as the chaotic inflationary multiverse, where each universe is supposed to be a bubble that has stopped its inflationary growth, amid a permanent and total inflationary environment.
• Hierarchical multiverses: like Smolin's (which Ellis does not mention) and the multiverse of universe simulations (in other words: that we live in a simulation). In this post I speak exclusively about this type of multiverses, which share a property that, in my opinion, makes them implausible, if not impossible.

George Ellis and the multiverse

George Ellis

George Ellis is a South African cosmologist who rose to fame almost half a century ago when he wrote a book together with Stephen Hawking (The Large Scale Structure of Space-Time, 1973), today considered a classic.

In an earlier post in this blog, published in November 2014, I mentioned that there are six independent theories about the multiverse, almost all of them incompatible with each other. In a recent article titled Theory Confirmation and Multiverses published in the book Why Trust a Theory?, edited by Radin Dardashti, Richard David and Karim Thébault (Cambridge University Press, 2019), George Ellis updates the different multiverse theories. He does not mention six, as I did five years ago, but nine, although he has left out one of the six I mentioned in my post (Smolin’s), perhaps because this theory has been abandoned in the meantime. The nine theories are:

We are not living in a simulation: a note on Nick Bostrom’s proposal

Simulation of the collision of two galaxies

In a paper published in 2003 [1] Nick Bostrom proposed the following reasoning:

A technologically mature “posthuman” civilization would have enormous computing power. Based on this empirical fact, the simulation argument shows that at least one of the following propositions is true:

(1)   The fraction of human-level civilizations that reach a posthuman stage is very close to zero;

(2)   The fraction of posthuman civilizations that are interested in running ancestor-simulations is very close to zero;

(3)   The fraction of all people with our kind of experiences that are living in a simulation is very close to one.