What Do Black Holes Teach Us About Wigner's Friend?

This paper argues that by seriously analyzing the structural similarities between black hole paradoxes and Extended Wigner's Friend scenarios, the former supports interpretations of the latter that rely on intrinsic relationality and retrocausality rather than emergent relationality.

The Gist

The Big Picture: Two Puzzles, One Solution?

Imagine you are trying to solve a mystery. You have two very different crime scenes:

1. The "Wigner's Friend" Mystery: A quantum physics puzzle about how two people can see different realities when one is inside a lab and the other is outside.
2. The "Black Hole" Mystery: A cosmic puzzle about how information gets trapped inside a black hole and how it might escape as radiation.

Recently, physicists Hausmann and Renner noticed that these two mysteries look suspiciously similar. They are like two different locks that seem to require the same key.

In this paper, Emily Adlam asks: "If these two locks are so similar, what does the Black Hole lock tell us about how to open the Wigner's Friend lock?"

Her answer is surprising. She argues that the Black Hole puzzles suggest we need to change our minds about two big things:

1. Reality is inherently relative (not just a trick of perspective).
2. The future can influence the past (retrocausality).

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Part 1: The Wigner's Friend Paradox (The "Magic Mirror" Problem)

The Setup:
Imagine Alice is inside a sealed room measuring a coin. She sees it land on Heads.
Bob is outside the room. According to quantum mechanics, until Bob opens the door, the whole room (Alice + Coin) is in a "superposition"—a fuzzy state where the coin is both Heads and Tails, and Alice has seen both outcomes simultaneously.

The Conflict:

• Alice's view: "I definitely saw Heads."
• Bob's view: "The system is in a fuzzy mix of Heads and Tails."

The Standard Fix (The "Effective" Solution):
Most physicists (like those who believe in the "Many Worlds" theory) say: "It's just a matter of perspective."

• Analogy: Imagine a movie projector. Alice is watching the movie on the "Heads" screen. Bob is watching the "Tails" screen. They are both right, but they are in different "branches" of the movie. The underlying reality is just one big movie reel; the "relativity" is just a side effect of which screen you are looking at. This is called Effective Relationality.

Adlam's Question:
Does the Black Hole puzzle suggest this "side effect" explanation is enough? Or does it point to something deeper?

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Part 2: The Black Hole Paradox (The "Time-Traveling Mail" Problem)

The Setup:
Alice throws a piece of information (a qubit) into a Black Hole. She measures it inside.
Bob stays outside and collects all the light (Hawking radiation) the black hole emits. Eventually, Bob can reconstruct the information Alice threw in and measure it too.

The Conflict:

• Alice measured it in Basis A.
• Bob measured it in Basis B.
• Quantum mechanics says you can't measure the same thing in two incompatible ways at once.

Why it's different from the Friend:
In the "Friend" scenario, Bob erases Alice's memory to make his measurement. It feels like a local physical action.
In the "Black Hole" scenario, Alice is deep inside a trap, and Bob is far away. They can never meet to compare notes. The reason they can't compare notes isn't because Bob erased her memory; it's because of the structure of spacetime itself (the Event Horizon).

The "Teleological" Twist:
To define where the Event Horizon is, you have to know what happens in the future (will the black hole evaporate? will more stuff fall in?). The location of the "trap" depends on the future.

• Analogy: Imagine a maze where the walls move based on where you will walk tomorrow. The path you take today is determined by your future destination.

Adlam argues that if the Black Hole puzzle requires this "future-dependent" logic, then the Wigner's Friend puzzle might need it too.

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Part 3: What the Black Hole Teaches Us

Adlam compares the two puzzles and draws two major conclusions.

Conclusion 1: Reality is "Inherently" Relative, Not Just "Effectively" Relative.

• The "Effective" View (Many Worlds/Bohm): There is one true, absolute reality (the whole movie reel). We just see different parts of it.
• The "Inherent" View (Relational Quantum Mechanics): There is no single "movie reel." Reality is only the relationship between the observer and the object. There is no "God's eye view" of the universe.

The Black Hole Lesson:
In the Black Hole puzzle, you run into a problem called the "Monogamy of Entanglement."

• Analogy: Imagine a person (QR) who is married to two different people (QA and QB) at the same time. In quantum physics, you can't be "maximally entangled" (married) to two people at once.
• If you believe in an "Absolute Reality" (Effective Relationality), then the universe must contain a state where QR is married to both. This breaks the rules of physics.
• If you believe in "Inherent Relationality," then there is no single state where QR is married to both. Relative to Alice, she is married to QA. Relative to Bob, he is married to QB. Since no single observer sees both marriages at once, the rules aren't broken.

The Takeaway: The Black Hole puzzle is much easier to solve if you accept that reality is fundamentally relative, not just a matter of perspective.

Conclusion 2: The Future Might Influence the Past (Retrocausality).

• The Problem: In the Black Hole scenario, whether Alice and Bob see a contradiction depends on whether they choose to check their measurements later.
• The Solution: Adlam suggests that the state of the system now might depend on what Alice decides to do later.
• Analogy: Imagine you are writing a story. Usually, you write Chapter 1, then Chapter 2. But in this quantum story, the ending (Chapter 10) decides what Chapter 1 actually was. If you decide to end the story with a "Happy Ending," the first chapter magically rearranges itself to make sense of that happy ending.

Why this helps:
If the future measurement "reaches back" to decide the past state, then Alice and Bob never actually hold contradictory information at the same time. The universe "retroactively" ensures consistency.

Adlam notes that this sounds weird, but it avoids "causal loops" (time travel paradoxes) because the black hole acts like a one-way door that prevents you from checking the past before the future happens.

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The Final Verdict: What Should We Believe?

If we take the analogy between Black Holes and Wigner's Friend seriously, Adlam suggests we should stop looking for "Easy Fixes" (like the Many Worlds theory) and start looking at "Radical Fixes":

1. Ditch the "Absolute Reality": Don't assume there is one big, hidden truth that everyone is just seeing a slice of. Assume that facts are created by the interaction between the observer and the world.
2. Embrace "Retrocausality": Be open to the idea that future choices can influence past states, not in a sci-fi time-travel way, but as a fundamental feature of how quantum mechanics works.

In a Nutshell:
The Black Hole puzzles are like a "stress test" for our theories. Theories that work for the "Friend" scenario (like Many Worlds) fail the stress test when applied to Black Holes. The theories that pass the Black Hole test (Inherent Relationality + Retrocausality) are likely the ones that will solve the "Friend" puzzle too.

It's like realizing that the rules of chess you learned in the park don't work in the tournament; you need to learn the "Grandmaster rules" (Retrocausality and Inherent Relationality) to win at both.

Technical Summary

1. Problem Statement

The paper addresses the intersection of two major areas in modern physics: the measurement problem in quantum mechanics (specifically the Extended Wigner's Friend (EWF) paradoxes) and the information paradox in black hole physics.

• The Core Issue: Recent work by Hausmann and Renner (2025) identified structural similarities between EWF paradoxes and black hole paradoxes. Both involve scenarios where certain pairs of measurement outcomes cannot be simultaneously witnessed by a single observer, leading to apparent contradictions with standard quantum mechanics if "third-person universality" (the idea that a single global state describes all observers' outcomes) is assumed.
• The Gap: While the operational structures are similar, the physical mechanisms preventing joint access to outcomes differ. In EWF scenarios, outcomes are erased or separated by time; in black hole scenarios, they are separated by event horizons and causal structures. The paper asks: If we treat these analogies seriously, what do the solutions to black hole paradoxes imply for the resolution of the Wigner's Friend paradoxes?

2. Methodology

Adlam employs a comparative analysis based on two key assumptions:

1. Validity of the Black Hole Paradigm: She accepts the specific assumptions used by Hausmann and Renner to construct the black hole paradoxes (e.g., unitary evolution, global event horizons, Effective Field Theory outside the horizon).
2. Principle of Operational Identity: She adopts the methodological principle (citing Spekkens, Leibniz, and Einstein) that scenarios with strong operational similarities should, where possible, share similar ontological solutions.

The paper proceeds by:

• Categorizing EWF paradoxes into those based on Third-Person Universality (requiring joint predictions for all observers) and First-Person Universality (requiring only that individual observers make correct predictions for themselves).
• Analyzing two specific pairs of analogies:
  • Paradox 1: Deutsch's Wigner's Friend vs. Hayden-Preskill Black Hole experiment.
  • Paradox 2: Frauchiger-Renner Wigner's Friend vs. The Firewall Paradox (operationalized by Hausmann and Renner).
• Evaluating how different classes of quantum interpretations (Effective Relationality vs. Inherent Relationality; Retrocausality vs. Standard Causality) resolve these specific pairs.

3. Key Contributions and Arguments

A. The Distinction Between Effective and Inherent Relationality

Adlam distinguishes between two types of relational approaches:

• Effective Relationality (e.g., Everett/Many-Worlds, de Broglie-Bohm): These posit an underlying absolute quantum state. Relationality is merely a feature of how observers access branches or conditional wavefunctions.
• Inherent Relationality (e.g., Relational Quantum Mechanics - RQM): These posit that quantum states are fundamentally relative to an observer; there is no underlying absolute state.

Argument: In standard EWF paradoxes (like 1-WF and 2-WF), Effective Relationality suffices to resolve the contradiction because no single observer ever sees the conflicting outcomes. However, in the Black Hole Paradoxes (specifically Paradox 2-BH/ Firewall), the problem of Monogamy of Entanglement arises.

• In the Firewall scenario, a qubit ($Q_R$) appears to be maximally entangled with both an interior qubit ($Q_A$) and an exterior radiation qubit ($Q_B$) simultaneously.
• Effective Relationality fails here: If an underlying absolute state exists, it must violate monogamy of entanglement (a qubit cannot be maximally entangled with two systems at once).
• Inherent Relationality succeeds: If quantum states are inherently relative, there is no single absolute state where $Q_R$ is entangled with both. $Q_R$ is entangled with $Q_A$ relative to Alice, and with $Q_B$ relative to Bob, but no observer sees both relations simultaneously.
• Conclusion: The black hole analogy suggests that Inherent Relationality is a superior solution to EWF paradoxes than Effective Relationality, as it resolves the monogamy violation without requiring new physics to "fix" the underlying state.

B. The Case for Retrocausality and Teleology

Adlam argues that the mechanism preventing joint access in black holes is fundamentally different from EWF scenarios, pointing toward retrocausality or teleology.

• In EWF (Paradox 1): Bob erases Alice's outcome via a physical interaction (Hadamard gate). This is a local, forward-in-time process.
• In Black Holes (Paradox 1-BH): The separation is due to the Event Horizon, a global, teleological concept defined by the causal future (what will happen in the asymptotic future). The inability to compare outcomes depends on facts about the future history of the spacetime.
• Implication: If the solutions must be analogous, the EWF paradoxes may also require a retrocausal or teleological explanation. The "erasure" of the outcome in EWF might not be a local physical scrambling but a consequence of future boundary conditions.
• Supporting Evidence: Adlam highlights the Final State Proposal (Horowitz & Maldacena) and Kent's Lorentzian Solution (Kent, 2014). In these models, the final state of the universe (or black hole) constrains the initial state, effectively "selecting" history. In the black hole case, this prevents the violation of monogamy; in the EWF case, it implies that if an outcome is erased, it never truly "happened" in the final history, resolving the paradox without needing a global state.

C. Critique of "Fact-Based Relationality"

The paper critiques the trend in recent EWF literature (e.g., Bong et al.) to resolve paradoxes by adopting "fact-based relationality" (denying that facts are absolute).

• Adlam argues that fact-based relationality alone is insufficient for black hole paradoxes because the paradoxes often arise within a single observer's perspective (e.g., Alice inside the black hole).
• Simply denying absolute facts does not resolve the tension between Unitarity and Entanglement Monogamy in the black hole case without invoking retrocausal mechanisms.
• Therefore, the paper suggests that EWF resolutions should focus on Dynamical Relationality combined with Retrocausality, rather than purely fact-based relationalism.

4. Results

1. Preference for Inherent Relationality: The black hole paradoxes (specifically the Firewall/Monogamy issue) strongly favor Inherent Relationality over Effective Relationality (like Many-Worlds). Effective approaches struggle to explain how an underlying absolute state avoids violating entanglement monogamy in these specific scenarios.
2. Necessity of Retrocausality: The structural reliance on global, future-dependent definitions (event horizons) in black hole paradoxes suggests that retrocausal or teleological mechanisms are likely required to resolve Wigner's Friend paradoxes as well. This provides a new motivation for under-explored retrocausal interpretations of quantum mechanics.
3. Refinement of Complementarity: The analogy suggests that "Black Hole Complementarity" should be understood not just as an operational limit on observation, but potentially as a form of Inherent Relationality where no global quantum state exists to describe the interior and exterior simultaneously.

5. Significance

• Bridging Quantum Gravity and Foundations: The paper provides a novel methodological bridge between the measurement problem (quantum foundations) and quantum gravity. It suggests that insights from black hole physics can constrain the interpretation of quantum mechanics, and vice versa.
• Re-evaluating Interpretations: It challenges the dominance of "Everettian" (Many-Worlds) and "Bohmian" approaches in resolving EWF paradoxes, arguing they may be insufficient when gravitational analogies are considered.
• Reviving Retrocausality: By linking the black hole paradoxes to retrocausal solutions (like the Final State Proposal), the paper revitalizes retrocausality as a viable and perhaps necessary component of a complete theory of quantum mechanics, moving it from a fringe idea to a central candidate for resolving foundational paradoxes.
• Ontological Clarity: It clarifies that the solution to these paradoxes may not require "radical metaphysics" (like the existence of multiple worlds or the denial of all objective facts) but rather a specific type of dynamical relationality combined with retrocausality.

In summary, Adlam argues that treating the Wigner's Friend and Black Hole paradoxes as structurally identical forces us to abandon "effective" relational solutions (like Many-Worlds) in favor of inherent relationality and to seriously consider retrocausal mechanisms as the physical engine resolving these contradictions.