The metaphysical interpretations of QM represent competing philosophical responses to the Measurement Problem, which arises from a fundamental discrepancy between the formalism of quantum theory and our empirical experience of the world. Specifically, the problem emerges from the mismatch between the mathematical structure of QM, which governs the evolution of the wave function via the Schrödinger equation (a linear, deterministic process that yields an ever-expanding superposition of possible outcomes), and the apparent collapse of these possibilities into a single, definite outcome upon measurement, as consistently observed in empirical reality. Any interpretation of QM must therefore explain how – or whether – this transition from superposition to actuality occurs. They can be classified into three broad categories, which together constitute what I term the quantum trilemma. Each of these approaches offers a distinct resolution to the Measurement Problem, yet each also encounters deep conceptual or empirical difficulties.
Physical or objective collapse theories propose that the wave function's evolution is not purely unitary, but punctuated by real, physical collapse events. Collapse occurs via a real physical process, usually stochastic, modifying Schrödinger evolution.
Copenhagen: Technically agnostic about whether collapse is physical or epistemic, but often treated as implying a physical collapse upon "measurement" (vaguely defined).
Ghirardi-Rimini-Weber (GRW): Collapse is spontaneous, with a low but non-zero rate, especially for macroscopic systems.
Continuous Spontaneous Localisation (CSL): A refinement of GRW, providing a continuous rather than discrete collapse mechanism.
Penrose Objective Reduction (OR): Collapse is induced by gravitational effects when superpositions involve significant mass/energy differences.
Flash Ontology (Tumulka et al.): Collapse is associated with spontaneous space-time events ("flashes"), a relativistic-friendly modification.
The promise of these models is empirical testability. The problem is that no such collapse mechanism has been observed. GRW/CSL are empirically testable in principle, but the parameter space is large and experiments so far have only ruled out some regions. To date, all experimental evidence remains consistent with unitary (collapse-free) evolution of the wave function. As a result, these theories rely on hypothetical processes that remain both empirically inaccessible and theoretically underdetermined, rendering them speculative add-ons rather than explanatory breakthroughs.
Von Neumann’s formalism allowed the "cut" between observer and observed to be placed arbitrarily, but concluded that only a conscious mind can complete the measurement process. This explicitly contradicts physicalism, since the observer is only being proposed in the first place because it is presumed to be external to the entire physical system being measured. Without that, it doesn't work as a solution to the problem it was created to solve. However, while CCC bypasses the need for an undefined physical mechanism, a different Pandora’s box is opened: How did measurement occur in a pre-conscious universe? In answer to this question, proponents typically invoke panpsychism or idealism, but these come with their own unresolved theoretical burdens. Most seriously, they entail that minds can exist in the absence of brains – something which is very hard to square with our incontestable knowledge that damage to brains directly causes corresponding damage to the contents of consciousness. It doesn't switch it off (as general anaesthetics do), but it degrades it, and it does so in highly predictable ways. How can this be accounted for if brains aren't necessary for consciousness? The panpsychists and idealists do offer answers to these questions, but they have not convinced the critics.
MWI denies that the wave function collapses at all. It claims that all possible outcomes of a quantum event are realised, each in a distinct and non-interacting branch of an ever-expanding multiverse. Apparent collapse is an illusion. MWI cleanly avoids the central problems of both PC and CCC, but it implies that our minds are continually splitting, and that there are an infinite number of timelines where strange versions of ourselves act in random and inexplicable ways. The role of probability, which is central to quantum predictions, is also thrown into doubt: without collapse it becomes difficult to explain why observers should expect Born-rule statistics (objective randomness) at all.
These three interpretive strategies appear to exhaust the logical space of viable responses to the Measurement Problem. Either the wave function collapses or it does not. If it collapses, the cause must be either internal to the physical system (OC) or external to it (CCC). If it does not, then all outcomes must be realised (MWI). Interpretations that attempt to evade this trilemma leave key explanatory questions unanswered. Examples include:
Copenhagen (strictly epistemic readings): Claims "collapse" is just a change in our knowledge, not in reality, which dodges the question of what the world is like ontologically.
QBism (Quantum Bayesianism): Quantum states are subjective degrees of belief. Collapse is an updating of personal knowledge, not a physical event. QBism denies the problem's existence by reinterpreting what quantum mechanics is about.
Instrumentalism / Shut Up and Calculate: Refuses to engage with ontology at all, treating QM as a predictive tool only.
Consistent Histories (Griffiths, Omnès): Provides rules for assigning probabilities to entire histories, but leaves the reality-status of unactualised histories unclear.
Decoherence: Explains apparent classicality, but does not resolve why only one outcome is experienced – merely suppresses interference terms.
Bohmian Mechanics / Pilot Wave Theory: Particles have definite positions guided by a real wave function. Collapse is replaced by particle trajectories, but the unobserved branches remain "real but empty." This blends PC (hidden variable guiding equation) and MWI (since the wavefunction itself doesn't collapse), but fails to resolve the measurement problem because of the ambiguous ontological status of the unobserved branches. What does “real but empty” mean? How is it any different to “real but not real”?
Transactional Interpretation (TI): Wave function is real but consists of offer and confirmation waves propagating forward and backward in time. Collapse is the "handshake" between them, an atemporal process. TI reframes the collapse problem but does not solve it because its handshake mechanism does not explain how or why a particular absorber is selected from among multiple candidates. It postulates (rather than derives) that the amplitude of these waves gives the probability of transaction formation, effectively inserting the Born rule by hand. The physical process of selection (the actual transition from a probabilistic field of potential transactions to a single, definite outcome) remains mysterious and unmodelled. And while retrocausality (advanced waves) adds conceptual depth, it still shifts rather than dissolves the core question of the Measurement Problem: Why this outcome, not others?
Relational Quantum Mechanics (RQM): The state of a quantum system is always relative to the observer or measuring system. There is no absolute, observer-independent state of a system. A measurement outcome is simply the establishment of a relation between two systems; there is no universal account of “what happened”, only what happened relative to a given observer. RQM rejects the need for a universal collapse mechanism, but it does not explain how and why one definite outcome arises in the experience of any given observer or what determines the particular correlation (relation) established in a measurement interaction. The Born rule and definite outcomes are assumed, not derived, and there’s also no clear mechanism for reconciling multiple observers' perspectives into a coherent global reality – even if RQM claims this is unnecessary. RQM reframes the Measurement Problem as a matter of perspective but offers no account of the mechanism by which any specific outcome is selected, even relative to a given observer. It is therefore yet another incomplete interpretive strategy that shifts the metaphysical framing but leaves the central explanatory gap unbridged.