What Is This?
In 1957, a Princeton PhD student named Hugh Everett III submitted a thesis that his advisor, John Wheeler, described as "borderline psychotic" — then helped him publish anyway. The thesis argued that the standard interpretation of quantum mechanics was wrong in a specific and troubling way, and that the correct interpretation implied something almost unthinkably strange: every time a quantum event occurs, reality branches, and every possible outcome actually happens.
This is the Many Worlds Interpretation (MWI) — also called the Everett interpretation or the universal wave function interpretation. It remains the most contested and possibly the most important idea in foundational physics.
The problem it was solving:
Quantum mechanics, as developed by Bohr, Heisenberg, Schrödinger and others in the 1920s, is extraordinarily successful at predicting experimental results. It describes the behaviour of particles as probability distributions — wave functions — that evolve deterministically according to the Schrödinger equation. When you measure a quantum system, the wave function "collapses" from a superposition of possibilities to a single definite outcome. You don't observe a particle in superposition — you observe it at a specific location, with a specific spin, with a specific energy.
The problem: the Schrödinger equation never predicts collapse. It predicts continuous, deterministic evolution of wave functions. There is no collapse in the mathematics. The Copenhagen interpretation — the standard textbook view — simply declares that collapse happens when a measurement is made, without explaining what constitutes a measurement, what causes collapse, or where collapse fits into the physics. This became known as the measurement problem, and it is a genuine foundational gap that the Copenhagen interpretation waves away with philosophical hand-waving.
Everett's solution: there is no collapse. The Schrödinger equation is always correct. Everything it predicts actually happens.
When a particle in superposition interacts with a measuring device, the measuring device also enters a superposition of states — measuring the particle in state A and measuring the particle in state B. When an observer reads the measuring device, the observer also enters a superposition — the observer who saw A and the observer who saw B. Both branches are real. Both outcomes happen. But each observer only experiences one branch — the one they inhabit.
The "many worlds" is not a metaphor. It is the literal content of taking the Schrödinger equation seriously with no additional assumptions.
Why Does It Matter?
- It may be the only interpretation of quantum mechanics that is actually consistent. The Copenhagen interpretation's collapse postulate is an addition to the Schrödinger equation — an extra rule layered on top of the physics to explain why we see definite outcomes. But what triggers collapse? What counts as a measurement? Niels Bohr never gave a satisfactory answer. Hugh Everett's approach requires no additional postulates. You take the Schrödinger equation as complete, you accept that it describes reality, and branching follows as a mathematical consequence. The price is an unimaginable proliferation of branches. But the alternative — adding an unexplained collapse mechanism — is an untestability you're accepting instead. Many physicists who have thought hardest about foundations consider MWI the least bad option.^1
- David Deutsch built the theory of quantum computing on Many Worlds. Deutsch is one of the most important figures in quantum information science. His argument for quantum computing is explicitly Everettian: a quantum computer works by performing computations in parallel across multiple branches of the wave function, and the exponential speed advantage of quantum algorithms comes from this parallel computation across what are effectively parallel universes. This isn't fringe advocacy — it's the foundation of a research programme that has produced Shor's algorithm, Grover's algorithm, and the entire field of quantum information theory.^2
- It forces a complete rethink of probability. In ordinary probability theory, probabilities describe outcomes of events that may or may not happen. In Many Worlds, all outcomes happen — deterministically. So what does "probability 30%" mean if both the 30% outcome and the 70% outcome both occur in different branches? This is the "probability problem" for MWI and it's genuinely hard. The leading answer involves the Born rule — the rule that probability is proportional to the square of the wave function amplitude — being derivable from rational decision theory (Deutsch and Wallace's decision-theoretic approach). Whether this derivation actually works is actively debated among philosophers of physics.^3
- It has radical implications for decision theory and personal identity. If you will branch at every quantum event, your future "self" is not a single individual but a branching tree. Decisions are not choices between single outcomes but choices between distributions over branches. Derek Parfit's work on personal identity and the "fission" problem — what happens to you if you split into two equally valid continuities — maps directly onto the Many Worlds picture. Every quantum interaction is a Parfit fission event. The philosophical implications for ethics, identity, and rational action under this framework are genuinely unexplored.
- Most working physicists in quantum foundations now prefer it over Copenhagen — quietly. Surveys of physicists at quantum foundations conferences show Many Worlds as the plurality choice, ahead of Copenhagen and other alternatives. The change from 20 years ago, when Copenhagen was dominant, has been significant. The shift is partly because the alternatives are worse: Copenhagen has the measurement problem, Pilot Wave / de Broglie-Bohm is non-local, Objective Collapse theories (GRW, CSL) add untested physical mechanisms. Many Worlds adds no new physics. It just takes existing physics completely seriously.
Key People & Players
Hugh Everett III (1930–1982) — Princeton PhD student whose 1957 thesis proposed the universal wave function interpretation. His advisor Wheeler initially supported him, then distanced himself under pressure from Bohr. Everett was devastated by the reception and left physics for defence consulting at the Pentagon, where he worked on game theory and nuclear strategy for the rest of his career. He never returned to foundations. His son is Mark Oliver Everett — frontman of the Eels — who made a documentary about his father, Parallel Worlds, Parallel Lives, in 2007.^4
David Deutsch (Oxford) — The most prominent contemporary defender of Many Worlds. His book The Fabric of Reality (1997) argues that Everett's interpretation is not just the correct interpretation of quantum mechanics but the key to understanding the deep structure of physical reality alongside computation theory, epistemology, and evolution. He founded the theory of quantum computation on Everettian foundations.^5
Sean Carroll (Caltech) — Physicist and public communicator who has written the most accessible contemporary defence of Many Worlds. His book Something Deeply Hidden (2019) is the best single popular treatment of why physicists should take Many Worlds seriously. His Mindscape podcast regularly engages with the philosophical implications.^6
Niels Bohr (1885–1962) — The originator and defender of the Copenhagen interpretation. His influence on quantum physics was so dominant that he effectively suppressed serious engagement with alternatives for decades. His debates with Einstein on quantum foundations are the most celebrated in 20th-century physics, and his insistence that quantum mechanics should not be interpreted realistically (that the wave function is just a calculational tool, not a description of reality) shaped physics education for generations.
David Wallace (University of Southern California) — Philosopher of physics who has done the most rigorous work on the probability problem for Many Worlds — whether the Born rule can be derived from decision theory in an Everettian framework. His book The Emergent Multiverse (2012) is the most technically serious defence of Many Worlds from a philosophical perspective.
The Current State
Many Worlds is not the consensus interpretation of quantum mechanics — there is no consensus interpretation. But it has moved from fringe to mainstream within foundations of physics over the past 30 years, driven partly by the growth of quantum information science (which benefits from Everettian reasoning) and partly by the accumulated difficulties with alternatives.
The live debates:
The probability problem: If all outcomes happen, what grounds the Born rule probabilities? Deutsch and Wallace's decision-theoretic derivation is the most technically developed answer, but critics argue it either assumes what it's trying to derive or doesn't provide probabilities in the right sense. This remains unresolved.
The preferred basis problem: When does branching happen? The Schrödinger equation doesn't single out any particular set of branches as the "real" ones. Decoherence theory (the interaction of quantum systems with their environment) provides a practical answer — environmental interaction rapidly suppresses interference between branches, making them effectively independent — but whether decoherence actually constitutes branching or merely explains its appearance is contested.
What "exists" in the theory: Many Worlds implies an enormous proliferation of branches — a new branch for every quantum event, constantly. Some physicists find this ontologically extravagant and prefer more parsimonious alternatives. Others (following Occam's razor) argue that adding no new physical postulates is simpler than adding collapse, even if the ontological count is higher.
The experimental frontier: Many Worlds makes identical experimental predictions to Copenhagen in all known cases — they differ philosophically, not empirically. The question of whether any experiment could ever distinguish them is an open question in foundations. Certain quantum gravity scenarios might in principle generate distinguishable predictions, but no practical test has been designed.
The interpretation of quantum mechanics may be the most consequential open question in physics. It doesn't affect the calculation of any measured quantity — but it completely determines what you think you're saying when you say those calculations are true. That's not a small difference.
Best Resources to Learn More
- Something Deeply Hidden by Sean Carroll (2019) — The best accessible contemporary case for Many Worlds. Clear, compelling, and honest about the difficulties.^7
- The Fabric of Reality by David Deutsch (1997) — The deeper, more philosophical case, connecting Many Worlds to computation theory, epistemology, and evolution.^8
- "Relative State Formulation of Quantum Mechanics" by Hugh Everett III (1957) — The original thesis, freely available. The first section is readable by a determined non-specialist.^9
- The Emergent Multiverse by David Wallace — The most technically rigorous contemporary defence of Many Worlds. For those who want the philosophical and mathematical details.^10
- Sean Carroll's Mindscape podcast: "The Many Worlds of Quantum Mechanics" — Multiple episodes engaging with MWI and its critics. The most accessible ongoing treatment.^11