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Quantum observers can communicate across multiverse branches

Maria Violaris

TL;DR

This work presents a fully unitary protocol that enables inter-branch communication in an Everettian multiverse by using a Wigner’s-friend setup with a five-subsystem Hilbert space and a partial branch-swap that transfers a locally created classical message between branches. Memory erasure is shown to be essential for the transfer, and the construction generalizes to $n$-qubit messages while remaining independent of the message content. The authors position this result as a potential knowledge-paradox test distinguishing many-worlds from single-world theories, and discuss connections to linearity, implementation feasibility, and questions of identity. Overall, the paper argues that cross-branch information transfer can occur within standard quantum theory under global control, prompting new theoretical, philosophical, and experimental investigations into the nature of branches and observers.

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.

Quantum observers can communicate across multiverse branches

TL;DR

This work presents a fully unitary protocol that enables inter-branch communication in an Everettian multiverse by using a Wigner’s-friend setup with a five-subsystem Hilbert space and a partial branch-swap that transfers a locally created classical message between branches. Memory erasure is shown to be essential for the transfer, and the construction generalizes to -qubit messages while remaining independent of the message content. The authors position this result as a potential knowledge-paradox test distinguishing many-worlds from single-world theories, and discuss connections to linearity, implementation feasibility, and questions of identity. Overall, the paper argues that cross-branch information transfer can occur within standard quantum theory under global control, prompting new theoretical, philosophical, and experimental investigations into the nature of branches and observers.

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.
Paper Structure (9 sections, 4 theorems, 14 equations, 3 figures)

This paper contains 9 sections, 4 theorems, 14 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Protocol for inter-branch communication
  3. Friends cannot remember their messages
  4. Discussion
  5. Testing many- vs single-world theories using a knowledge-paradox
  6. Does inter-branch communication violate linearity?
  7. Implementation and complexity
  8. Predictability and identity
  9. Conclusion

Key Result

Theorem 1

There exists a global quantum operation such that, starting from a superposition of two decohered branches labelled by $R=0$ and $R=1$, a classical message $\mu$ created locally by the observer in the $R=1$ branch is transferred to a subsystem in the $R=0$ branch which can be read by the observer in

Figures (3)

  • Figure 1: Hello Worlds: Wigner cannot exchange the friends' messages, but can instead switch the friends' branches such that one obtains a message from the other.
  • Figure 2: Quantum circuit implementing the inter-branch message-transfer protocol. The room-label register $R$ records the friend's location via a cnot from $F$ to $R$. The dashed box encloses the partial branch-swap operation $X_Q \otimes X_R \otimes X_F$. All operations are independent of the message value $\mu$.
  • Figure 3: Generalisation of the protocol to an $n$-qubit message. The room-label register $R$ records the friend's branch via a controlled operation from $F$ to $R$. The dashed box again applies $X_Q \otimes X_R \otimes X_F$, transporting the message across branches.

Theorems & Definitions (8)

  • Theorem 1: Inter-branch message transfer
  • proof
  • Corollary 1: Memory erasure is necessary for inter-branch communication
  • proof
  • Lemma 1: No message-independent memory-preserving branch swap
  • proof
  • Corollary 2: Message-branch amplitudes cannot be modified
  • proof