Home > Positioning > Persons > DeWitt

Bryce DeWitt (1923–2004)

DeWitt revived Everett’s relative-state formulation of quantum mechanics from obscurity and gave it the name by which it is now known: the many-worlds interpretation. His 1970 Physics Today article “Quantum Mechanics and Reality” brought Everett’s work to a broad physics audience for the first time, and the 1973 anthology The Many-Worlds Interpretation of Quantum Mechanics (which included Everett’s full thesis, unpublished for sixteen years) established the interpretation as a serious alternative to Copenhagen. DeWitt also made foundational contributions to quantum gravity — the Wheeler-DeWitt equation, formulated in his 1967 paper, is the first attempt at a quantum equation for the geometry of spacetime — and to quantum field theory in curved spacetime, the effective-action method, and the quantisation of the gravitational field.

Life

Born 8 January 1923 in Dinuba, California. Educated at Harvard University (BA, 1943; PhD, 1950), under Julian Schwinger. The PhD was on the quantisation of the gravitational field — one of the earliest rigorous treatments of the problem that would occupy DeWitt for his career.

Worked at the Institute for Advanced Study in Princeton (1950–51), the Radiation Laboratory at the University of California, Berkeley, and the Lawrence Livermore National Laboratory. Professor of physics at the University of North Carolina at Chapel Hill (1956–72), where he founded the Institute of Field Physics — a centre for research in general relativity, quantum gravity, and quantum field theory in curved spacetime. Moved to the University of Texas at Austin (1972–2004), where he held the Jane and Roland Blumberg Centennial Professorship in Physics and where David Deutsch was among his students.

DeWitt was married to Cécile DeWitt-Morette, a mathematical physicist who founded the Les Houches Summer School of Theoretical Physics — one of the most important postgraduate institutions in theoretical physics. Died 23 September 2004 in Austin, Texas.

The many-worlds revival

Everett’s 1957 thesis was published in a shortened form and attracted almost no attention. Wheeler, who had supervised the thesis, supported the ideas but was cautious about promoting them against Bohr’s Copenhagen interpretation. Everett left physics. For thirteen years, the relative-state formulation was effectively forgotten.

DeWitt’s revival was not passive rediscovery but active advocacy. “Quantum Mechanics and Reality” (Physics Today, 1970) presented Everett’s proposal clearly and argued for it forcefully — insisting that the many-worlds interpretation is the most economical reading of the quantum formalism and that its apparent extravagance (the proliferation of branches) is a feature, not a bug: the formalism already contains the branching; the interpretation merely takes it seriously. The article provoked letters and debate in Physics Today and brought many-worlds into mainstream physics discourse for the first time.

The Many-Worlds Interpretation of Quantum Mechanics (1973, edited with Neill Graham) published Everett’s full thesis alongside interpretive essays by DeWitt and Graham, along with related papers. The volume was the canonical reference for the many-worlds programme until the 1990s, when Deutsch, David Wallace, and others developed the interpretation further.

The “many-worlds” label was DeWitt’s, not Everett’s. Everett called his proposal the “relative-state formulation” — a more austere and technically precise name. DeWitt’s label emphasised the ontological picture (many worlds) over the formal content (relative states), and the label shaped how the interpretation was received — both attracting attention (the science-fiction resonance) and inviting misunderstanding (the image of literal parallel universes splitting off at every measurement). Whether the label helped or hindered the interpretation’s reception is debated.

Quantum gravity and the Wheeler-DeWitt equation

DeWitt’s 1967 paper “Quantum Theory of Gravity. I. The Canonical Theory” (Physical Review) is the foundational paper of canonical quantum gravity. The paper applies the standard quantisation procedure (canonical quantisation) to general relativity, treating the three-dimensional geometry of space as the dynamical variable and the Hamiltonian constraint of general relativity as the quantum equation that the wave function must satisfy.

The result is the Wheeler-DeWitt equation: HΨ = 0, where H is the Hamiltonian constraint operator and Ψ is the wave function of the universe — a function on superspace (the space of all possible three-dimensional geometries). The equation is Wheeler’s programme (geometrodynamics, the wave function of the universe) given mathematical form by DeWitt.

The equation’s most striking feature: it contains no time variable. The wave function of the universe does not evolve in time; it simply is. The appearance of time — the experience of change, succession, history — must emerge from within the timeless framework, not be imposed from outside. How time emerges from a timeless equation is the “problem of time” in quantum gravity, and it remains one of the deepest open problems in theoretical physics.

DeWitt also made major contributions to quantum field theory in curved spacetime — the study of quantum fields on a classical gravitational background — including the development of the background-field method and the effective-action approach, tools that are now standard in quantum field theory and in the study of Hawking radiation and the cosmological constant.

Where DeWitt stops

The Wheeler-DeWitt equation is a formal equation without a well-defined mathematical framework. The inner product on the space of solutions — which would determine what the equation predicts — is not specified; the operator-ordering problem (how to turn the classical Hamiltonian constraint into a quantum operator) is not resolved; and the equation’s relationship to the full theory of quantum gravity (if one exists) is unclear. The equation is a starting point for canonical quantum gravity, not a completed theory. Loop quantum gravity (Rovelli, Ashtekar, Smolin) can be understood as one attempt to give the Wheeler-DeWitt equation a well-defined mathematical setting; string theory takes a different route entirely.

DeWitt’s role as the reviver and namer of many-worlds raises a question about the relationship between an idea and its populariser. Everett had the original insight; DeWitt made it visible and gave it the label under which it became famous. The relative-state formulation and the many-worlds interpretation may or may not be the same proposal — Everett’s austere formalism does not commit to the ontological picture of “many worlds” that DeWitt’s label suggests. The question of what Everett actually proposed, as distinct from what DeWitt said he proposed, is a live question in the history of quantum mechanics.

Key works

DeWitt, B. S., “Quantum Theory of Gravity. I. The Canonical Theory,” Physical Review 160 (1967) — the Wheeler-DeWitt equation
DeWitt, B. S., “Quantum Mechanics and Reality,” Physics Today 23(9) (1970) — the many-worlds revival
DeWitt, B. S., & Graham, N. (eds.), The Many-Worlds Interpretation of Quantum Mechanics (Princeton, 1973) — the anthology including Everett’s full thesis
DeWitt, B. S., The Global Approach to Quantum Field Theory (Oxford, 2003) — the mature treatment of QFT, effective action, and background-field methods