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Doctor Strange's multiverse: is that what physics says?

Movie · Doctor Strange in the Multiverse of Madness (2022), Everything Everywhere All at Once (2022)

Real seed. One true idea sits underneath. Everything built on it is invented.

What the scene or the claim says

Every choice spawns another universe, and with the right skill you can visit them.

What is real

Hugh Everett III's 1957 formulation of quantum mechanics drops the collapse step. The wavefunction just keeps evolving, and each outcome of a measurement exists as one branch of the whole. It is an interpretation: it makes the same predictions as textbook quantum mechanics, so no experiment separates them today.

What stops it working that way

• In Everett's picture, decoherence makes branches stop interfering with each other, so within the theory there is no route to travel between them or to send a message across.
• Nothing in the theory lets you choose which branch you end up in.
• Physicists disagree over whether it is the right reading. Copenhagen-style, Bohmian and collapse-model readings all exist.
• The cosmological multiverse of inflation and string theory is a separate speculation with a separate basis.

If you wanted it to work

Collapse models such as GRW and Penrose's do predict tiny departures from standard quantum mechanics, and experiments with ever larger objects in superposition are testing them. If one were confirmed, Everett's picture would be out. If none ever is, the branches stay unreachable, and the question stays philosophical.

Learn the physics

• Quantum mechanics vs computing

Sources

• Everett 1957 (Reviews of Modern Physics)

See it in the swipe deck, with the other cards.