TB: Something Deeply Hidden (Carroll)
Core Thesis
Many-worlds is not a radical proposal added to quantum mechanics — it is quantum mechanics taken seriously. The Everett interpretation consists of nothing but the wave function and the Schrödinger equation. Worlds emerge automatically from decoherence; they were not added to QM but were always there. The refusal of most physicists to engage seriously with interpretation is not epistemic rigor — it is a failure of intellectual courage.
Key Takeaways
QM presents a unique interpretational challenge
- Classical mechanics is perfectly transparent; QM is not. What we see when we look at the world is fundamentally different from what actually is.
- Textbook QM has two incompatible sets of rules: smooth Schrödinger evolution when unobserved, sudden "collapse" when observed. No other theory works this way.
- QM in its current textbook form is an oracle, not a true understanding. Physicists haven't been honest with themselves about this.
The minimalist (Everett) formulation
- Take the wave function seriously as a direct representation of reality. Assume it always evolves smoothly according to the Schrödinger equation. That is all.
- The worlds are not added to QM; they are already there as possible states in Hilbert space. Ordinary Schrödinger evolution brings them to life.
- "Everett didn't introduce anything new; he removed some extraneous clunky pieces from the formalism. Every non-Everettian version of QM is a 'disappearing worlds' theory."
- WE don't choose QM; we can only choose to face up to it.
Decoherence creates the branching structure
- Decoherence: macroscopic objects becoming entangled with their environments, which we cannot track. This causes the wave function to branch into worlds that can no longer interfere.
- Being decohered means the two parts of the wave function can no longer interfere — for all intents and purposes, they are separate worlds.
- The preferred basis problem: decoherence picks out spatial states as the pointer states because interactions are local. This is why we see well-defined positions.
Probability in Everett
- In Everett, every outcome happens. Where do probabilities come in? From self-locating uncertainty: after branching you know everything about the universe except which branch you are on.
- Rational agents in an Everettian universe, by decision theory (Deutsch-Wallace), act exactly as if probabilities were given by the Born rule.
- Amplitudes squared (Born rule) are the unique assignment consistent with rational credences in a branching universe.
Emergence and quantum fields
- Worlds are emergent, like tables and chairs. Useful to divide the wave function into branches because the branches don't interact.
- QFT: "quantum fluctuations" is a misleading phrase. The vacuum state is absolutely stationary — nothing is fluctuating. What looks like fluctuations are outcomes of local measurements on a globally entangled state.
- Fields are more fundamental; particles are what we see when we observe fields under the right circumstances.
Mental Models
- The Map is Not the Territory — Copenhagen used QM as a predictive map; Everett insists it is the territory
- Complex Systems: Features from Path, Not Design — worlds are emergent from the structure of the wave function, not designed into the theory