TB: The Emergent Multiverse (David Wallace)
Core Thesis
The Everett interpretation is not a radical proposal — it is quantum mechanics taken literally, interpreted the same way we interpret every other scientific theory: as a model of the world. Macroscopic superpositions do not describe indefiniteness; they describe multiplicity. Worlds are emergent, local, macroscopic phenomena arising from decoherence. The Everett interpretation is the only game in town that neither modifies quantum mechanics nor abandons scientific realism.
Key Takeaways
QM taken literally
- The Everett interpretation consists of two parts: (1) the universe is faithfully represented by a unitarily evolving quantum state; (2) interpreted realistically, this state describes multiple approximately classical, approximately non-interacting regions — worlds.
- This is all. No additional physical postulates describe the division into worlds — just unitary quantum mechanics.
- It makes more sense to talk about "the Everett interpretation" than "the many-worlds interpretation" — it is just quantum mechanics, interpreted the same way we always interpret scientific theories.
Decoherence creates the branching structure
- Decoherence constantly transforms delocalized states into mixtures of localized states, preventing the system from ever ending up in states so delocalized that dynamics ceases to be approximately classical.
- Interactions with the environment give the quantum universe an approximately classical branching structure. Chaos is everywhere, and where there is chaos there is branching.
- The question "how many branches are there?" does not ultimately make sense — branches are emergent. Compare: "how many feelings did you have?"
The measurement problem is dissolved, not solved
- The Everett interpretation does not solve the measurement problem — it dissolves it. There is no measurement problem in quantum mechanics, given Everett.
- For the past 80 years, physicists made extraordinary progress with ad hoc rules that basically worked but fostered confusion. The confusion is now disappearing.
Probability in Everett
- Probability is the most serious philosophical difficulty for the Everett interpretation.
- The resolution: branch weights are objective physical quantities satisfying the probability calculus; purely by symmetry considerations, equally weighted superpositions are equally likely to give each outcome.
- A dollar bill is a physical object whose monetary character depends on how it plugs into human behavior. Similarly, branch weight is probability because of how it figures in rational action.
Intuition is not a reliable guide
- Our intuitions were formed to help ancestors on African savannahs. The universe is not obliged to conform to them.
- Physicists propose philosophically radical interpretations; philosophers propose changing the physics. This is the wrong way around.
- If you cannot argue (not just thump the table) for a philosophy of science that says "believe theories until they cross a weirdness threshold," then intuitive implausibility is not a valid objection.
Everyday life is unchanged
- Branching has no consequences for everyday beliefs and actions.
- From the perspective of any given experimenter, her experiment does have a unique, definite outcome — even in Everett.
- The reason the weather is unpredictable in a year is not computational limits — it is that there is no fact of the matter; the prediction is that it varies widely from branch to branch.
Mental Models
- The Map is Not the Territory — the Copenhagen interpretation treated the quantum formalism as a map for predictions, not a territory; Everett says it is both
- Complex Systems: Features from Path, Not Design — worlds are emergent features of the quantum state's structure, not entities added by design