Quantum observers can communicate across multiverse branches

TL;DR

This paper presents a thought experiment demonstrating that, within standard quantum theory, observers in different branches of the multiverse can communicate, challenging traditional views on multiverse isolation.

Contribution

It introduces a novel Wigner's-friend scenario showing inter-branch communication is possible without violating quantum linearity, expanding understanding of multiverse interactions.

Findings

Inter-branch communication can occur within standard quantum theory.

Observers must have no memory of sent messages to preserve unitarity.

The experiment challenges conventional beliefs about multiverse isolation.

Abstract

It is commonly thought that observers in distinct branches of an Everettian multiverse cannot communicate without violating the linearity of quantum theory. Here we show a counterexample, demonstrating that inter-branch communication is in fact possible, entirely within standard quantum theory. We do this by considering a Wigner's-friend scenario, where an observer (Wigner) can have quantum control over another observer (the friend). We present a thought experiment where the friend in superposition can receive a message written by a distinct copy of themselves in the multiverse, with the aid of Wigner. To maintain the unitarity of quantum theory, the observers must have no memory of the message that they sent. Our thought experiment challenges conventional wisdom regarding the ultimate limits of what is possible in an Everettian multiverse. It has a surprising potential application which involves using knowledge-creation paradoxes for testing Everettian quantum theory against single-world theories.