Many-Worlds Interpretation
Overview
The Many-Worlds Interpretation (MWI) of quantum mechanics holds that there are numerous parallel worlds existing in the same space and time as our own. Every time a quantum experiment with different possible outcomes is performed, all outcomes are obtained — each in a different, newly created world — even though we are only aware of the world with the outcome we have seen. The MWI provides a solution to the measurement problem of quantum mechanics.
The MWI asserts that the universal wave function is objectively real and that there is no wave function collapse. The evolution of reality as a whole is rigidly deterministic and dynamically local — no action at a distance.
Also called the relative state formulation or the Everett interpretation, after Hugh Everett, who first proposed it in 1957. The name “many-worlds” is due to Bryce DeWitt, who popularized the formulation in the 1970s.
The Two Parts of the Theory
- The physics part: a theory yielding the time evolution of the quantum state of the single Universe, via the Schrödinger equation or its relativistic generalization. This is rigorous and mathematically unproblematic.
- The bridge to experience: a prescription setting up a correspondence between the quantum state of the Universe and our experiences. This part is approximate — “fine for all practical purposes” (FAPP) — because human language and concepts were developed before the existence of parallel worlds was suspected.
What Is a “World”?
A world is the totality of macroscopic objects — stars, cities, people, grains of sand — in a definite, classically described state.
Crucially, the MWI distinguishes “world” from “Universe”: there is only one Universe (the universal wave function), which encompasses a vast multiplicity of worlds. A world defined at any moment traces back to a unique past but branches into a multitude of futures — a strict temporal asymmetry.
In the MWI, “I” is a macroscopic object defined at a time by a complete classical description of my body and brain. It is meaningless to ask “Which future self will I be?” — I will correspond to them all. This resonates with Parfit’s (1986) critique of personal identity: when an observer divides, there is no coherent answer to “Which copy is me?”
The Core Physics
Decoherence and the Emergence of Worlds
In modern MWI, the subjective appearance of collapse is explained by quantum decoherence: when a quantum system interacts with its environment, only certain “pointer states” remain stable while others decohere into mixtures. The preferred basis of the decomposition is therefore not postulated but derived — it is the basis stable under environmental decoherence. This solves the preferred basis problem that plagued early versions of the theory. Worlds are understood as emergent (approximate, effective) entities, not fundamental ones.
Probability and the Born Rule
If all outcomes occur, where does probability come from? The MWI explains the illusion of probability:
- Measure of existence: each world i has measure μᵢ = |αᵢ|² (its squared amplitude in the universal wave function). We should care about our successive worlds in proportion to their measures.
- Probability Postulate (Born–Vaidman rule): an observer sets subjective probability of an outcome proportional to the total measure of existence of all worlds with that outcome.
- Self-location ignorance: after measurement, an observer is ignorant of which branch they are in (the “sleeping pill” argument) — this grounds an ignorance-style probability.
- Decision-theoretic derivations: Deutsch (1999) and Wallace claim to derive the Born rule from rational betting preferences; Zurek (2005) derives it from “envariance” symmetry; Sebens & Carroll (2018) from self-location uncertainty. All remain contested.
Locality
Bell’s theorem cannot get off the ground in the MWI, because it assumes a single outcome of a quantum experiment. The MWI is dynamically local — no action at a distance. Entanglement (nonseparability) remains, but is explained by the nonlocality of worlds themselves.
History
- 1952: In a Dublin lecture, Erwin Schrödinger asserted the histories described by the wave equation are “not alternatives but all really happen simultaneously” — the earliest known reference to many-worlds (per Deutsch).
- 1956–57: Hugh Everett’s Princeton PhD thesis, The Theory of the Universal Wave Function, under John Archibald Wheeler. Published 1957 as “Relative State Formulation of Quantum Mechanics.”
- 1970: H. Dieter Zeh independently reached similar conclusions and built quantum decoherence theory; also the origin of the many-minds interpretation.
- 1970s: Bryce DeWitt named and popularized the theory (including the 1973 DeWitt & Graham volume).
- 1970s–present: Decoherence developed by Wojciech Zurek (einselection, Quantum Darwinism) becomes the mechanism for world emergence; David Deutsch connects MWI to quantum computing.
Reception
Initial reception was overwhelmingly negative — the theory was ignored, and Bohr rejected it outright. Wheeler disavowed it in 1980. Today MWI is considered a mainstream interpretation alongside Copenhagen and Bohmian mechanics, but remains deeply contested:
Supporters: Deutsch, Carroll, Tegmark, Vaidman, Wallace, Saunders Critics: Penrose (gravity not accounted for), ‘t Hooft, Peres, Gell-Mann (favored decoherent histories), Ladyman & Ross, and many who find it unfalsifiable or extravagant
Polls: a pre-1991 poll of quantum cosmologists found 58% “yes, MWI is true”; a 1997 workshop poll placed MWI second; a 2011 poll found only 6/33 endorsing MWI.
Quantum Information and Quantum Computing
Researchers in quantum information often favor the MWI. In quantum computing, the parallelism of a single quantum computer mirrors the MWI picture of parallel worlds: a quantum algorithm operates by having the outcomes of parallel computations interfere to yield the desired result (Deutsch & Jozsa 1992). The usefulness of this explanatory picture is debated (Steane 2003; Duwell 2007; Cuffaro 2012), but it remains an intuitive way to conceptualize quantum algorithms. Deutsch is both the father of quantum computing and one of the strongest MWI advocates.
Speculative Implications
- Everything conceivable (consistent with physics) happens somewhere — absurd events are rare but inevitable (Tegmark).
- Quantum suicide / quantum immortality: thought experiments that would distinguish MWI from collapse theories; most experts believe they would not work because surviving branches have low measure.
- Decision-making: Carroll argues human decisions are classical (neurochemical), not quantum; the weight of outcomes concentrates in a single branch.
Key Objections
- Ockham’s razor: an extravagant multiplication of unobservable worlds. Defenders reply that MWI is most economical in laws — it eliminates the collapse postulate, the most problematic “law.”
- Preferred basis problem: which decomposition into worlds is physical? Answered via decoherence.
- The wave function is not enough: Bell and Maudlin argue the wave function (defined in configuration space, not spacetime) is the wrong kind of object. MWI defenders invoke density of particles as a primitive ontology.
- Derivation of the Born rule: many derivations are contested; critics argue the “incoherence” probability problem (how can probability exist if all outcomes occur?) is the most serious difficulty.
Why Adopt the MWI?
- Removes fundamental randomness from physics: Nature does not “play dice.”
- Removes action at a distance.
- No experimental evidence favors collapse over MWI.
- Resolves paradoxes: Schrödinger’s cat, Wigner’s friend, Elitzur–Vaidman interaction-free measurement.
- Continues the pattern of scientific revolutions that decenter humans: heliocentrism → Darwin → relativity → MWI (Berenstain 2020).
Key People
- erwin-schrodinger — Schrödinger equation, entanglement, the cat paradox, 1952 Dublin lecture precursor
- hugh-everett — originator of the relative state formulation
- bryce-dewitt — named and popularized MWI
- john-wheeler — Everett’s advisor, coined “relative state”
- h-dieter-zeh — founder of decoherence theory
- wojciech-zurek — einselection, Quantum Darwinism, envariance
- david-deutsch — quantum computing + MWI advocate; decision-theoretic Born rule
- lev-vaidman — measure of existence, self-location probability, SEP entry author
- sean-carroll — Johns Hopkins physicist; MWI advocate (Born-rule derivation with Sebens); draws on “quantum worlds” in his multiverse response to fine-tuning
References
- Vaidman, L. (2026). Many-Worlds Interpretation of Quantum Mechanics. SEP (Summer 2026). 1
- Wikipedia (2026). Many-worlds interpretation. 2