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

The gist: The author argues that wavefunction collapses or 'quantum events' in relational approaches to quantum mechanics are inevitable because describing one system relative to another necessitates encountering a discontinuity when their interaction becomes significant, leading to an understanding of these events as periods where the relational description is inadequate rather than discrete instantaneous occurrences <ref:2604.12094#pg14>.

Why Wavefunction Collapses in Relational Approaches

The central motivating idea for relational views of quantum mechanics is that quantum descriptions are relativized, and it is generally agreed that this includes at least the quantum states <ref:2604.12094#pg4>. The definition of a quantum state of system S relative to system R is the correct state to predict the outcomes of measurements performed by R on S, but not necessarily the correct state to predict the outcomes of measurements performed by some other system R' <ref:2604.12094#pg4>. One natural corollary of this definition is that systems probably should not be assigned quantum states relative to themselves - such states would be lacking their central defining content as codified above, since a system cannot interact with or measure itself <ref:2604.12094#pg4>. It can be shown that assigning systems quantum states relative to themselves leads to various paradoxes (Kastner, 2024), which can easily be avoided by simply not permitting systems to be assigned states relative to themselves <ref:2604.12094#pg4>.

The Nature of Quantum Events

The author suggests that despite the terminology referring to ‘events’ and ‘collapses,’ quantum events should not in fact be understood as discrete instantaneous events - rather a ‘quantum event’ simply refers to a period of time in which it is not possible to describe the evolution of the system relative to the reference system because the system and reference are interacting strongly <ref:2604.12094#pg4>. Consequently there is no need to associate a quantum event with some specific time or fixed temporal interval, because the relevant interactions are continuous and pervasive and thus it is simply a matter of deciding when the interactions have become sufficiently strong that the relational description is no longer adequate <ref:2604.12094#pg4>. The description of S relative to R encoded in the quantum formalism should be understood as a convenient approximation which works well as long as S and R are interacting only very weakly; when the interaction becomes stronger the approximation fails <ref:2604.12094#pg4>.

Relational Quantum Mechanics (RQM) and its Variants

The author introduces relational quantum mechanics (RQM) and two variants of it, RRQM and ARQM, as well as a third variant ARQM which explicitly gives up the assumption of the completeness of quantum mechanics <ref:2604.12094#pg4>. RQM addresses this paradox by saying that when the Friend measures S, a quantum event occurs in the quantum description relative to the Friend but not in the quantum description relative to Wigner (Rovelli, 1996) <ref:2604.12094#pg4>. In RRQM, a ‘quantum event’ occurs in which the value of a variable of S becomes definite relative to R <ref:2604.12094#pg4>. The state of S then undergoes an update in accordance with the usual quantum-mechanical state update rule <ref:2604.12094#pg4>.

The Necessity of Abandoning Completeness (ARQM)

The author argues that an adequate understanding of quantum events leads naturally to the view that the relational description provided by quantum mechanics should be understood as an approximation which works only as long as the system-reference interaction is sufficiently weak <ref:2604.12094#pg4>. This suggests that once we start trying to define quantum events properly we will probably find that we must give up the idea that quantum mechanics is complete <ref:2604.12094#pg4>. ARQM, which drops Completeness, is committed to the existence of absolute facts which lie beyond what can be represented in the quantum formalism <ref:2604.12094#pg4>. This approach suggests that the quantum formalism should be understood as a special kind of description which arises specifically in the context when we are characterizing one system relative to another, and we require some other kind of formalism to characterize the absolute facts <ref:2604.12094#pg4>.

The Limits of Internal Derivation

The argument against deriving quantum events entirely within the quantum formalism is based on several points:

1. Quantum mechanics is inherently relational, so if we are using the quantum formalism to derive an S − R quantum event we must do so relative to some reference system <ref:2604.12094#pg4>.

2. Since systems are not assigned quantum states relative to themselves, we cannot describe an interaction between S and R or a joint state of S and R relative to either S or R, and thus there is no possible way of giving a precise derivation of the S − R quantum event relative to either S or R <ref:2604.12094#pg4>.

3. Since the S−R quantum event does not appear in the quantum description relative to systems other than S and R, it does not make sense to try to derive this event from the quantum description relative to some system other than S or R <ref:2604.12094#pg4>.

The regress picture suggests that every S − R quantum event eventually ends up relativized to everything except the systems S and R themselves! <ref:2604.12094#pg4>.

Conclusion and Direction for Research

The conclusion is that it is probably true that we will never be able to define quantum events in a fully satisfactory way using only the quantum formalism and nothing else <ref:2604.12094#pg4>. The approach suggests that the critics of RQM are correct in the following sense: it is probably true that we will never be able to define quantum events in a fully satisfactory way using only the quantum formalism and nothing else <ref:2604.12094#pg4>. The direction for development is toward the picture in which quantum descriptions are relational approximations to an underlying absolute reality, perhaps characterized by something like the ‘complete observables’ featuring in existing formulations of general relativity <ref:2604.12094#pg4>. This approach may be connected with ongoing research on general relativity and quantum gravity <ref:2604.12094#pg4>. The explanation for why something important is missing from quantum mechanics does not rely on stipulations that simply hide some part of nature from us in an ad hoc or conspiratorial way - rather it is a natural consequence of the very nature of scientific enquiry that some absolute facts should have been left out of quantum mechanics proper <ref:2604.12094#pg4>. The postulation of some physics beyond standard quantum mechanics to which the relative descriptions are an approximation seems quite well-motivated in this picture <ref:2604.12094#pg4>.

Specific Criticisms Addressed

The paper addresses criticisms from Ladyman and Thompson (2026) and Faglia (2025) by showing that their assumptions—that quantum events are discrete and instantaneous and involve total collapses, and that a valid formulation of RQM must derive quantum events entirely from within the standard quantum formalism—are reasonable but ultimately flawed <ref:2604.12094#pg4>. The author contends that measurements in ‘standard quantum theory’ are not really discrete and instantaneous either <ref:2604.12094#pg4>. Furthermore, the picture where a ‘quantum event’ is merely a way of describing a scenario in which a certain approximation becomes inaccurate need not lead to widespread loss of coherence, because in many cases the disturbance from the interaction is sufficiently small that the approximation remains passably good and so the coherence remains <ref:2604.12094#pg4>.

Final Summary Point

The author concludes that there is a natural and satisfying way to understand the nature of quantum events within a relational approach if we are willing to abandon Completeness and accept that the relational quantum description is an approximation to an underlying absolute story <ref:2604.12094#pg4>. This picture explains why quantum events occur and why their complete description takes us outside of the quantum formalism <ref:2604.12094#pg4>. The paper suggests that the existence of quantum events itself does not seem to follow in a particularly natural way from the underlying metaphysical commitments - as noted, they seem to be simply postulated ad hoc to save the phenomena <ref:2604.12094#pg4>. This is because within a relational approach which insists on Completeness the existence of the quantum events itself does not seem to follow in a particularly natural way from the underlying metaphysical commitments - as noted, they seem to be simply postulated ad hoc to save the phenomena <ref:2604.12094#pg4>.

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The paper is a philosophical investigation into why wavefunction collapses or 'quantum events' are inevitable in relational approaches to quantum mechanics and proposes that these events should be understood as periods where the relational description breaks down due to strong system-reference interactions, necessitating an underlying absolute reality beyond the standard quantum formalism.

1. No longer interpret measurement outcomes as discrete instantaneous events, but rather as a period of time in which it is not possible to describe the evolution of the system relative to the reference system because the system and reference are interacting strongly. This allows for modeling wavefunction collapse as a continuous spectrum of possible discontinuities ranging from no discernible effect when the interaction is very weak to a full collapse when the interaction is very strong, leading to partial collapses during intermediate cases.

2. Develop an ARQM-based framework that treats quantum events as approximations breaking down, which is a natural and satisfying way to understand the nature of the quantum events within a relational approach if we are willing to abandon Completeness. This system will model measurement not as an absolute collapse, but as a non-unitary transformation arising from the breakdown of the approximation where the description of S relative to R and leave R out of our description is still fairly good.

3. Implement cross-perspective link (CPL) heuristics that allow for partial collapses, by generalizing the CPL postulate to deal with events that are not generally instantaneous, enabling a high-level heuristic connecting quantum events involving different systems, as suggested by ARQM. This addresses the limitations of standard CPL formulations when applied to non-instantaneous processes.

4. Design an AI system capable of distinguishing between weak measurements and strong interactions by monitoring the degree of deviation from unitary evolution relative to a reference frame, allowing it to determine when the interaction between q1 and q2 has become so strong that it no longer works well to describe q1 on its own, independent of q2.

5. Create a formalism that avoids deriving quantum events entirely within the standard quantum formalism by treating them as features requiring an absolute description which lies beyond the quantum formalism, connecting the breakdown of approximation to concepts like complete observables in general relativity. This prevents the system from falling into a regress of relativization where every event is only derivable relative to an external system.

I argue that there is a straightforward way to understand the occurrence of wavefunction collapses or 'quantum events' in relational approaches to quantum mechanics: we necessarily encounter a discontinuity in our description when a system interacts with the reference relative to which we are describing it, since the reference system cannot be described relative to itself. This makes it clear how recent concerns around quantum events in relational quantum mechanics should be resolved. However, the solution requires accepting that quantum mechanics is not a complete description of all physical facts, and moreover I argue that this is most likely inevitable if we want to be able to give a precise description of quantum events.