Episode Summary
Executive Summary: Sean Carroll interviews philosopher of physics David Albert about the measurement problem in quantum mechanics, contrasting Copenhagen, collapse theories, Bohmian mechanics, and Everett/many-worlds. Albert explains why he thinks many-worlds faces serious unresolved issues—especially probability, decision theory, and self-locating uncertainty—while Carroll pushes back. The conversation is a detailed tour of foundational quantum theory and its philosophical stakes.
Main Topics: Albert’s background and entry into quantum foundations (Priority: 3/5): Albert recounts his graduate-school conflict at Rockefeller, his interest in Hume, and how Yakir Aharonov helped him persist and later collaborate on quantum problems. The measurement problem (Priority: 5/5): Albert explains how linear Schrödinger evolution appears to yield superposed measurement outcomes and brain states, conflicting with the definite outcomes we observe. Copenhagen and the collapse tradition (Priority: 4/5): They discuss the historical vagueness of Copenhagen, the collapse postulate, and why arbitrary boundaries like 'measurement' or 'macroscopic' were unsatisfactory. Competing solutions: Bohm, GRW, and Everett (Priority: 5/5): Albert situates the major options: modify the dynamics (collapse theories like GRW/Penrose), add hidden variables (Bohm), or keep the wave function complete (Everett). Many-worlds and the problem of probability (Priority: 5/5): A central exchange focuses on whether Everett can recover ordinary probability. Albert critiques decision-theoretic and rational-agent arguments for branching worlds. Self-locating uncertainty and indexical facts (Priority: 4/5): Albert examines a newer Everettian strategy: that uncertainty is about which branch 'I' will inhabit. He is uneasy about importing irreducibly indexical facts into physics. Philosophical method and historical context (Priority: 3/5): The discussion emphasizes careful argument, the role of symmetry, empirical confirmation, and how social/historical forces shaped reception of quantum interpretations.
Key Arguments: The measurement problem arises because universal application of linear quantum dynamics predicts superpositions of measurement devices and observers, yet our experience is of definite outcomes. Copenhagen-style collapse solves the problem only by introducing vague, ill-defined concepts like 'measurement,' making it an inadequate foundational theory. Bohmian mechanics and collapse theories are serious alternatives because they either add variables or alter dynamics in a precise way, unlike vague Copenhagen boundaries. Everett/many-worlds is attractive because it preserves the Schrödinger equation universally, but it still must explain why observers experience probabilistic outcomes. Albert argues that decision-theoretic derivations of Everettian probability fail because preferences over branching futures are not constrained by preferences over non-branching futures. He uses examples involving fatness, money, and branching outcomes to show that branching introduces genuinely new choice structures not captured by ordinary preferences. A later Everettian strategy based on self-locating uncertainty seems to require irreducibly indexical facts about which branch 'I' am in, which Albert finds conceptually troubling. Albert distinguishes classical statistical mechanics from Everettian branch uncertainty: in the classical case probabilities concern objective physical states and can be empirically tested; in Everett, all branches occur, complicating confirmation. The discussion suggests that if Everett is to work, it may need a coherent semantics and logic for branch-relative facts, not just an appeal to intuition or rational decision theory.
Data Points: Rockefeller thesis year: 1981 - Albert says he submitted his PhD thesis in 1981. Library call about missing thesis: 1990 - He recalls the librarian contacting him 10 years later about the missing final copy. Historical period of Copenhagen-style dominance: about 50 years - Albert describes decades of unproductive debates after von Neumann. Von Neumann book timing: early 1930s - He says von Neumann’s key book was written at the beginning of the 1930s. GRW emergence: early 1980s - Albert notes the GRW collapse theory appeared around then. Everett revival period: late 1970s through early 1990s - He says enthusiasm for Everett waned and then later revived. Probability example: 50-50 - Used in discussions of spin outcomes and principle of indifference examples. Decision-theoretic example: $100 vs. $1 - Used to illustrate how branching futures may differ from non-branching preferences. Branching confirmation example: 90% / 50-50 - Albert discusses theories assigning 90% left vs. 90% right vs. 50-50 outcomes. Heuristic quantum example: half the time up, half the time down - Used to illustrate spin measurements repeated many times.
Pivotal Quotes: "either the Schrodinger equation isn't everything or it isn't right" — David Albert: Albert cites Bell’s concise formulation of the measurement problem and the choice of response strategies. "the superposed condition ... turns out to be fantastically infectious. You touch it and you've got it too" — Sean Carroll: Carroll summarizes why superpositions spreading to macroscopic systems create the measurement problem. "it's just nuts to think that your preferences among non-branching futures could in any way constrain ... your preferences among branching futures" — David Albert: Albert’s core objection to decision-theoretic attempts to derive Everettian probabilities.
Implications: The episode clarifies why quantum foundations remain unsettled: any interpretation must explain definite outcomes and probability without handwaving. For listeners, it shows why Everett is elegant yet controversial, and why precision about chance, branching, and confirmation still matters.
About Sean Carroll MindScape
Ever wanted to know how music affects your brain, what quantum mechanics really is, or how black holes work? Do you wonder why you get emotional each time you see a certain movie, or how on earth video games are designed? Then you’ve come to the right place. Each week, Sean Carroll will host conversations with some of the most interesting thinkers in the world. From neuroscientists and engineers to authors and television producers, Sean and his guests talk about the biggest ideas in science, ...