This episode features a dialogue between David Wallace, a leading philosophical defender of the Everett (many-worlds) interpretation of quantum mechanics, and Emily Adlam, a philosopher of physics who finds Everett’s probability problem unresolved and explores relational alternatives, moderated by Curt Jaimungal; they debate wave function ontology, the Everettian probability problem, relational quantum mechanics, quantum gravity, scientific intersubjectivity, rationality in many worlds, and their differing philosophical methodologies.
Wave Function Ontology
The wave function is a representational tool encoding physical properties and relations, not a thing in itself; David treats it as encoding all properties a system has, while Emily distinguishes between representing current properties and encoding future predictions, suggesting these may come apart in quantum mechanics.
David argues asking “what is the wave function ontologically” is a category error; it is a theoretical tool for representation or inference, and the question of a universal wave function reduces to whether we can treat the universe as a quantum system — which quantum cosmology does effectively.
Emily favors relational approaches where wave functions are always attributed relative to another system, denying a universal wave function; all empirical evidence is for relational use, so positing an absolute universal wave function is an extrapolation.
Emily does not commit to relational quantum mechanics as currently formulated, which she thinks has problems, but finds its core idea promising; David is more dogmatic in defending Everett, while Emily remains uncertain which interpretation is right.
Everettian Probability Problem
Emily’s primary objection to Everett: if the probability problem cannot be resolved, believing Everett undermines the empirical confirmation of quantum mechanics itself, making it self-undermining; she wants a clear demonstration that Everettian probabilities can play the role needed for confirmation.
David agrees the probability problem is important but thinks the unitary framework is robustly scientifically successful; he is more skeptical of a priori philosophical constraints that would reject a working physical theory than of the theory itself, suggesting we may learn about epistemology from the clash.
Emily hopes for an interpretation preserving quantum mechanics’ formalism without Everett; she is skeptical of Bohmian or collapse theories that change much, and sees relational approaches as attempting to preserve the formalism.
David distinguishes “modification light” (minor tweaks) from pure interpretation; he suspects relational approaches either collapse back to Everett or require thoroughgoing modifications that face technical problems.
Emily agrees recent literature shows relational quantum mechanics either becomes Everett (as Rovelli and Di Biagio’s latest work suggests) or fails to define quantum events precisely; she remains hopeful for minor modificatory strategies but acknowledges this is a promissory note.
David presses Emily’s priors: if a no-go theorem showed only thoroughgoing modifications (collapse, hidden variables) could avoid Everett, would she accept that? Emily says yes — the epistemic concerns are serious enough to require it, despite the difficulty.
David counters that analytic philosophy lacks the track record to justify such confidence in epistemological priors over a successful physical framework; Emily replies that quantum mechanics is not the final theory, and quantum gravity may modify the formalism anyway, creating synergy.
Quantum Gravity and Subsystem Decomposition
David sees string theory, loop quantum gravity, and asymptotic safety as serious quantum gravity approaches, all unitary and unmodified; he is optimistic about string theory and thinks the measurement problem is separate.
Emily worries these approaches inherit interpretational issues (problem of time, Heisenberg cut in loop quantum gravity), suggesting a link between quantum gravity difficulties and quantum mechanics interpretation.
David distinguishes quantum gravity (isolated subsystems) from quantum cosmology (the universe as a whole); he thinks we have satisfactory quantum gravity for subsystems but no satisfactory quantum cosmology.
Emily resists a sharp division: subsystem decompositions look odd in canonical quantization, and the separation is an approximation that should not be relied on conceptually for a complete story.
David objects that approximations (like natural selection) are conceptually indispensable; he describes how string theory recovers low-energy physics from isolated systems without solving cosmology.
Emily argues that from a relational view, subsystem decompositions depend on reference frames, and a complete emergence story must account for the non-fixed nature of the decomposition itself.
David acknowledges this may depend on relational assumptions but maintains that progress on isolated systems is real and substantial even while cosmology remains opaque.
Testing Wavefunction Collapse and Alternative Proposals
David views dynamical collapse theories as good science because they parameterize testable deviations from unitary quantum mechanics; he expects them to be ruled out experimentally within 10–20 years, though he would be happy to lose that bet.
On Oppenheim’s classical gravity proposal: David says it requires a collapse mechanism to avoid rapid experimental violation; if true, macroscopic superpositions don’t exist. Emily finds the appeal in avoiding superpositions of spacetime but rates technical success as low.
David thinks technical problems of superposed geometry are overstated and that not having them is worse; he notes Penrose is a minority view but emphasizes empirical tests are near (prototyping/grant stage).
David has a friendly wager (bottle of wine) with a collapse-theory proponent on experimental outcomes.
Realism and Structural Realism
David defines realism loosely: scientific theories tell us what the world is like beyond human sensory scales (e.g., electrons exist despite being invisible).
On mathematical/ontic structural realism: David’s view aligns with some versions but the label is tangled; Emily identifies as broadly ontic structural realist, having moved from epistemic structural realism partly due to relational approaches, but tries to remain open.
Scientific Intersubjectivity Crisis
Emily argues relational quantum mechanics (and similar observer-dependent views) undermines scientific intersubjectivity: if no absolute facts exist, we cannot say in an observer-independent sense that communication succeeds or that scientists share a world; this threatens the special objectivity of science built on shared information and critique.
David disagrees: scientific practice requires effective communication and agreement, but not “in an absolute sense” — that phrase smuggles in metaphysics beyond practice. From any perspective, a good story about scientific method can be told, including cross-perspective agreement; the God’s-eye statement “all perspectives disagree” is ungrammatical in relational frameworks.
David analogizes to time: insisting scientific facts be “tenselessly true” adds metaphysics beyond practice-based analysis.
Emily identifies a tension in relational formulations: they often want to make unindexed statements about many perspectives obeying quantum mechanics (e.g., in Wigner’s friend), but by construction no perspective can access another — leading to inconsistency. A consistent relational view must deny the global statement, but many relationalists want both.
David agrees: relationists face a dilemma — commit to the bit and become Everett-like (inheriting Emily’s probability problems), or keep the hybrid and face intersubjectivity problems. Emily sees neither route as satisfying.
Many Worlds Rationality and Costanza Observers
David addresses the “what should I do/ care about” question: low-weight branches from neurological malfunctions exist but are not choices; rational agents treat extremely low-weight branches as negligible, just as in non-Everett cases. Conditional on accepting Everett, one must be unconcerned about bad outcomes in extremely low-weight branches.
Emily references “Costanza observers” (from Seinfeld’s George Costanza doing the opposite of reason and succeeding): in Everett, some low-weight branches contain observers who reason irrationally but succeed wildly. If you don’t know you’re not one, why be rational? This pressures decision-theoretic derivations of the Born rule.
David replies that theories allowing lucky irrational agents aren’t disqualified (real world has lottery winners); decision-theoretic arguments (e.g., Dutch book) establish categorical constraints, not probabilistic ones, and idealizations are needed in both Everett and non-Everett cases.
Emily argues Dutch book arguments assume high-probability convergence to idealization; without a prior probability concept, the idealization lacks justification. David thinks this objection belongs earlier — at the level of justifying decoherence and branching structure via the mod-squared amplitude metric — and that by the decision-theory stage it should be resolved.
QBism Critique
David rejects QBism: it fails to explain concrete quantum phenomena (e.g., superfluidity of helium isotopes), only addressing abstract formalism; an interpretation must explain all known physics.
Emily agrees QBism has a core insight (quantum mechanics is a user-designed formalism) but takes it to incoherence: if two observers assign orthogonal states, neither can be wrong, and measurement cannot adjudicate — either communication fails (intersubjectivity problem) or measurement results cannot constrain state assignments, cutting off empirical contact with the formalism.
David doubts the inference from “designed from our standpoint” to epistemic interpretation; general relativity was also designed from our standpoint but doesn’t demand an epistemic reading of gravitational waves. Emily clarifies she doesn’t endorse epistemic interpretation but thinks our embodied standpoint may have influenced quantum mechanics’ formulation more than we realize.
Philosophical Methodology and Mutual Respect
Emily’s style: originally a physicist; philosophy motivated by understanding and advancing physics, using philosophical tools to clarify physical ideas rather than answering traditional philosophical questions.
David’s style: conservative — tries to make sense of mainstream physics as is, at the level of high theory (mathematical rigor, approximation, tangle), rather than historical analysis, pure mathematical codification, or dissident programs; he sees defending mainstream physics as a niche activity in philosophy.
Both work in adjacent philosophical areas (decision theory, laws of nature, self-location, philosophy of mind) when they bear on physics.
David admires Emily’s intellectual honesty about the severity of difficulties in non-Everett approaches and her conviction that the work must be done anyway.
Emily values David’s serious engagement with actual physics, which clarifies the landscape and improves her own understanding, even though she is more willing to modify the formalism.
