David Deutsch’s case for the multiverse—also known as the Many-Worlds Interpretation (MWI) of quantum mechanics—is rooted in his belief that it provides the only scientifically coherent explanation of quantum phenomena, especially when taking quantum computation into account. Here’s a concise summary of his case:

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1. Quantum theory demands it (if taken literally)

• The Schrödinger equation, which governs quantum systems, predicts that particles exist in superpositions—states where multiple possibilities coexist.

• Standard interpretations (like Copenhagen) say these superpositions “collapse” into one outcome when observed.

• Deutsch rejects this collapse as an ad hoc addition. Instead, he insists the wavefunction is real and evolves deterministically.

• Thus, all outcomes actually happen—but in different, branching universes. That’s the multiverse.

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2. The power of quantum computation

• Deutsch pioneered the field of quantum computing, which leverages superpositions to perform massive parallel computations.

• In his view, when a quantum computer runs an algorithm (e.g., Shor’s algorithm), it behaves as though it is simultaneously computing in multiple universes.

• He argues this power can't be explained by a single universe—where are these parallel computations happening otherwise?

• Therefore, quantum computers imply the existence of a multiverse.

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3. The explanatory superiority of Many-Worlds

• Deutsch argues that science should seek explanatory theories, not just predictive models.

• The multiverse provides a clear, deterministic, local, and unitary explanation of quantum phenomena without arbitrary postulates like wavefunction collapse.

• It unifies quantum mechanics with epistemology and computation—everything fits better under the multiverse.

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4. Occam’s Razor favors realism about the wavefunction

• Critics say the multiverse is extravagant. Deutsch counters that dismissing real entities (like other branches of the wavefunction) is what violates Occam’s Razor.

• The “simplest” theory is the one that keeps all parts of quantum theory intact—without adding a collapse postulate or observer-based special rules.

• The multiverse is not an addition, but the literal meaning of quantum theory taken seriously.

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Summary of Deutsch's Core Claims:

Claim	Summary
Quantum theory without collapse implies many worlds	The math leads to branching universes if you don’t artificially collapse the wavefunction.
Quantum computers use other worlds	They achieve feats that can’t be explained without simultaneous computation across branches.
Other interpretations fail at explanation	They introduce arbitrary distinctions or fail to explain what the wavefunction is.
Multiverse is real, testable in principle	It’s not mystical—it’s a scientific theory with consequences, such as for quantum computation.