Why does the wavefunction 'collapse' in relational… — Plain-Language Explanation

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This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Why the "Magic" of Quantum Collapse Happens

Imagine you are trying to describe a movie to a friend. Usually, you can describe the actors, the plot, and the scenery perfectly. But what happens if you try to describe the camera that is filming the movie while you are standing inside the camera?

You can't. The camera is the tool you are using to see the movie; it isn't part of the movie itself. If you try to put the camera inside the scene it is filming, the whole description breaks down. You get a glitch.

That is exactly what this paper argues happens in Quantum Mechanics.

In "Relational Quantum Mechanics" (RQM), the universe isn't described by one big, absolute story. Instead, reality is like a series of different movies, each filmed from the perspective of a different character (or "system").

System A describes System B.
System B describes System A.

The paper argues that when two systems interact strongly (like when a scientist measures an electron), the "movie" they are watching hits a wall. Because the observer (the reference) cannot describe themselves, the description of the other system suddenly snaps or "collapses."

The Core Problem: The "Ad Hoc" Glitch

Critics of RQM have been complaining: "You say a 'quantum event' (a collapse) happens when things interact, but you can't tell us exactly when or how it happens. It feels like you just made it up to fix the math."

They are asking: "Is there a precise line where the wavefunction collapses? Is it a split-second event?"

Adlam says: No, and that's actually a good thing.

The Solution: It's Not a Snap, It's a Blur

Adlam suggests we stop thinking of a "quantum event" as a sudden, magical snap of a finger. Instead, think of it as a map breaking down.

The Analogy: The Flat Map vs. The Globe

Imagine you have a flat map of the world. It works great for driving across a city. But if you try to use that flat map to navigate the North Pole, the map starts to stretch and tear. The "North Pole" doesn't exist on the flat map in a useful way.

The Quantum State: This is your flat map. It works perfectly as long as the observer and the object are far apart or interacting very gently (weakly).
The Interaction: When the observer gets very close and interacts strongly with the object, the "flat map" (the quantum description) stops working.
The Collapse: The "collapse" isn't a magical event; it's just the moment you realize, "Hey, this map is no longer accurate." You have to stop using the map and switch to a different way of thinking.

Because the interaction is a continuous process (like getting closer and closer), the "collapse" isn't a sharp line. It's a blurry zone where the old description gets worse and worse until it's useless.

The Big Twist: The Map Isn't the Whole Territory

Here is the most important part of the paper. To make this work, Adlam argues we have to admit something scary to some physicists: Quantum Mechanics is not the whole story.

Most relational theories say, "Quantum mechanics describes everything that exists." Adlam says, "No, that's the problem."

The Old View: Quantum mechanics is the complete dictionary of reality.
Adlam's View: Quantum mechanics is just a relational approximation. It's a tool we use to describe how things look from our specific point of view.

There is likely an "Absolute Reality" underneath (like the actual 3D globe), but our quantum math is just a 2D map that works well when things are far apart. When things get close and interact, the 2D map fails, and we see a "collapse."

Why This Solves the Critics' Worries

Why can't we define the exact moment of collapse?
Because there isn't an exact moment. Just like there isn't a single second where a flat map suddenly becomes a globe, there isn't a split second where a quantum event happens. It's a gradual breakdown of the approximation. Asking for a precise time is like asking for the exact second a shadow becomes a silhouette—it's a fuzzy transition.
Why is it not "ad hoc" (made up)?
It's not made up; it's a natural consequence of the math. If you try to describe a system relative to itself, the math must break. The "collapse" is just the universe telling us, "You can't describe the camera from inside the camera."
Does this mean we need new physics?
Yes. Adlam argues that to understand exactly what happens during a collapse, we need to look beyond standard quantum mechanics into an "absolute" reality (perhaps involving gravity or deeper physics). But that's okay! It means the theory is evolving, not failing.

Summary in a Nutshell

The Problem: Critics say RQM can't explain when a quantum event happens.
The Insight: A quantum event happens when the "observer" interacts so strongly with the "object" that the observer can no longer describe the object using their usual rules.
The Metaphor: It's like trying to draw a picture of the pencil you are holding. As long as you look at it from a distance, it's easy. But if you try to draw the pencil while holding it against the paper, your hand blocks the view, and the drawing gets messy.
The Conclusion: The "collapse" is just the moment the drawing gets too messy to be useful. We don't need to find a precise line for when it happens; we just need to admit that our current "drawing" (quantum mechanics) is an approximation, and there is a deeper reality underneath that we haven't fully mapped yet.

In short: The wavefunction doesn't magically collapse; our description of it simply runs out of steam when the interaction gets too strong. And that's a feature, not a bug.

1. The Problem

Relational approaches to Quantum Mechanics (RQM) posit that quantum states are not absolute but are defined relative to a specific physical system (the reference). A central tenet of RQM is that a "quantum event" (analogous to wavefunction collapse) occurs when two systems interact.

However, recent literature (e.g., Mucino et al., 2022; Faglia, 2025; Ladyman and Thompson, 2026) has identified a critical flaw in this framework:

The Definition Problem: It is unclear how to derive precisely when, how, and under what specific circumstances a quantum event occurs solely from the standard quantum formalism.
The Ad Hoc Objection: Critics argue that postulating "quantum events" as fundamental primitives without a derivation from the relational formalism makes them an ad hoc addition to save the phenomena, rather than a natural consequence of the theory.
The Completeness Constraint: Orthodox RQM (RRQM) maintains the postulate of Completeness, asserting that quantum mechanics describes all physical facts. This prevents the theory from appealing to "absolute" facts outside the formalism to explain the breakdown of unitary evolution.

2. Methodology and Theoretical Framework

Adlam employs a conceptual analysis of relational descriptions in physics, drawing parallels with General Relativity (GR) and Quantum Reference Frames (QRF).

Relational Description as Approximation: The author argues that describing system $S$ relative to reference $R$ inherently requires treating $R$ as a fixed background (a non-dynamical degree of freedom). Consequently, $R$ cannot be assigned a quantum state relative to itself.
The Singularity Argument: When $S$ interacts significantly with $R$ , the relational description relative to $R$ encounters a "singularity." Since $R$ cannot be described relative to itself, the interaction between $S$ and $R$ cannot be modeled within the description relative to $R$ .
Formal Variants: The paper distinguishes between three versions of RQM:
1. RRQM (Orthodox RQM): Adheres to the Completeness postulate (no facts outside the formalism).
2. ARQM (Adlam-Rovelli QM): Introduces Cross-Perspective Links (CPL) to ensure a shared reality but retains Completeness.
3. ARQM (The Proposed Variant):* Retains the relational nature and CPL but abandons the Completeness postulate. It posits that quantum mechanics is an approximation of an underlying absolute reality.

3. Key Contributions

A. The Mechanism of "Collapse"

Adlam provides a naturalistic explanation for wavefunction collapse without invoking consciousness or external observers.

Collapse as Model Breakdown: A "quantum event" is not a discrete, instantaneous physical jump but a discontinuity in the description caused by the breakdown of the relational approximation.
Continuity of Interaction: Interactions are continuous. When the interaction between $S$ and $R$ is weak, the relational description (unitary evolution) is a good approximation. When the interaction becomes strong, the approximation fails because the reference system $R$ is no longer a valid fixed background. The "collapse" is the point where the observer must abandon the description relative to $R$ and "pick it up again" after the interaction.

B. Redefining Quantum Events

The paper redefines quantum events to resolve the "when" and "how" problems:

Non-Discrete Nature: Quantum events are not point-like instants but periods of time where the system and reference interact strongly.
Partial Collapse: The framework naturally accounts for weak measurements. Weak interactions cause only a "partial collapse" (a noisy, non-unitary transformation) because the approximation remains partially valid. Strong interactions cause a "full collapse" because the approximation fails completely.
Basis Selection: The specific basis in which a value becomes definite is determined by the underlying absolute reality (which is outside the quantum formalism), explaining why the formalism alone cannot predict the basis selection.

C. The Necessity of Abandoning Completeness

Adlam argues that ARQM* is the only viable path forward.

Impossibility of Internal Derivation: It is logically impossible to derive an $S-R$ quantum event strictly within the quantum formalism relative to $R$ because $R$ cannot be described relative to itself. Furthermore, deriving it from a third party $Z$ violates the core relational motivation (that the event is specific to $S$ and $R$ ) or leads to an infinite regress of relativization.
Absolute Facts: To define quantum events precisely, one must accept that there are "absolute facts" (facts about the interaction independent of any observer) that lie beyond the standard quantum formalism. The quantum formalism is a tool for describing relative facts, not absolute ones.

4. Results and Arguments

Resolution of Criticisms: The criticisms regarding the lack of a precise definition for quantum events are valid only if one insists on Completeness. If one accepts that QM is an approximation, the lack of a precise boundary for "collapse" is a feature of the approximation, not a bug of the theory.
Consistency with GR: The approach mirrors General Relativity, where coordinates are labels for reference objects, and the theory breaks down if one tries to describe the dynamics of the reference object itself within the same coordinate system.
Cross-Perspective Links (CPL): In ARQM*, CPLs are understood as connections between relative descriptions grounded in absolute facts. Since these absolute facts are outside the formalism, CPLs cannot be derived purely from quantum mechanics; they require a new formalism (potentially related to Quantum Gravity).
Response to "Ad Hoc" Claims: The occurrence of discontinuities is not ad hoc; it is a necessary consequence of the relational structure. The details of the discontinuity require physics beyond QM, but the existence of the discontinuity is explained internally.

5. Significance

Paradigm Shift: The paper challenges the dogma that a successful interpretation of QM must be "complete" (i.e., contain all physical facts within the formalism). It suggests that a relational interpretation is more coherent if it admits incompleteness.
Unification with Quantum Gravity: By framing quantum events as the breakdown of a relational approximation, the paper bridges RQM with research in Quantum Gravity and General Relativity. It suggests that the "absolute" facts governing quantum events may be found in the same sector of physics that unifies gravity and quantum mechanics (e.g., complete observables).
Practical Implications: It provides a theoretical basis for understanding "weak measurements" and "partial collapses" not as anomalies, but as the natural regime where the relational approximation is degrading but not yet broken.
Future Research Direction: The paper calls for a research program that develops a formalism capable of describing the "absolute" facts underlying relational descriptions, moving beyond the standard Hilbert space formalism to a theory that can handle the transition between relative and absolute descriptions.

In summary, Adlam argues that the "collapse" is inevitable in relational approaches because a system cannot be described relative to itself during a strong interaction. The solution to the measurement problem in RQM requires abandoning the Completeness postulate and accepting that quantum mechanics is a relational approximation of a deeper, absolute reality.

Why does the wavefunction 'collapse' in relational approaches to quantum mechanics?