Many-Worlds Interpretation: Does Every Quantum Event Create a Parallel Reality According to Modern Physics?

The Many-Worlds Interpretation asserts that the universal wavefunction is objectively real and always evolves according to the Schrödinger equation, without ever collapsing. Each quantum measurement does not select a single outcome — instead, all possible outcomes are realized simultaneously in different “branches” of the universe.

In contrast to the Copenhagen interpretation, which requires a mysterious “collapse” postulate for the wavefunction, MWI is fully deterministic and local. The Schrödinger equation holds everywhere and at all times. No separate postulate for measurement is needed — measurement is simply an interaction between observer and system.

Hugh Everett III (1930–1982) was an American physicist born in Washington, D.C. At age 12, he wrote a letter to Einstein, who replied referring to a “very stubborn boy.” Everett studied chemical engineering at the Catholic University of America before switching to physics at Princeton, where he worked under John Archibald Wheeler.

In his doctoral dissertation “The Theory of the Universal Wave Function” (1956), Everett proposed an entirely new approach: instead of assuming the wavefunction collapses during measurement, we can apply quantum mechanics to the entire universe, including the observer. The shortened version was published in 1957 in Reviews of Modern Physics (vol. 29, pp. 454–462) under the title “Relative State Formulation of Quantum Mechanics.” Wheeler contributed the title “relative state” — Everett himself had originally called his approach the “Correlation Interpretation.”

Unfortunately, Bohr and the Copenhagen group completely rejected the theory. Léon Rosenfeld, a close collaborator of Bohr, called Everett “undescribably stupid.” Discouraged, Everett left theoretical physics for military and defense research. He died at 51 from a heart attack, never seeing the recognition that would follow.

In standard quantum mechanics, a particle can exist in a superposition of two states, e.g., spin-up and spin-down. When a measurement is made, according to the Copenhagen interpretation, the wavefunction “collapses” to one state. But this collapse isn't described by any equation — it's an ad hoc postulate.

Everett proposed that collapse never happens. Instead, measurement creates an entanglement between observer and system. After measurement, there are two relative states: one branch where the observer measured spin-up and one where they measured spin-down. Each branch evolves independently, as if the wavefunction had collapsed — but without actual collapse. As Everett wrote: the apparent collapse “emerges” from the unitary, deterministic dynamics.

Applying Occam's razor, Everett removed the collapse postulate — since its appearance is already explained mathematically, there's no need to assume it actually happens.

Quantum decoherence provides the modern foundation for MWI. The theory was developed primarily by H. Dieter Zeh (1970) and Wojciech Zurek (1981–1982) and explains why branches don't interact with each other in practice.

When a quantum system interacts with its environment (e.g., air molecules, photons), the different branches lose their coherence. The interference terms — which would allow detection of other branches — effectively vanish. The result: each branch evolves as a separate world, with no possibility of communication with the others.

Decoherence also solves the so-called preferred basis problem — there's no need to postulate a “special basis” for the splitting. The basis that is stable under environmental decoherence (pointer states) emerges naturally. The first quantitative experimental measurement of decoherence was carried out in 1996 by Serge Haroche at the École Normale Supérieure in Paris, using individual rubidium atoms in a microwave cavity.

This is the most critical question. David Deutsch proposed in 1985 a variant of the Wigner's friend thought experiment as a test. If we could “re-interfere” the two outcomes of a macroscopic observer, it would prove that superposition didn't collapse before measurement — exactly what MWI predicts.

In practice, however, this requires perfect isolation of a macroscopic observer, something technologically impossible today. Many physicists believe MWI makes exactly the same predictions as every other interpretation in every practically feasible experiment.

An even bolder thought experiment, quantum suicide, was proposed by Max Tegmark: theoretically, if MWI is correct, the experimenter would always “survive” in some branch. However, most experts believe this would not work in the real world, and it is certainly not ethically acceptable.

The renaissance of MWI began in 1970, when Bryce DeWitt published an extensive article in Physics Today titled “Quantum Mechanics and Reality,” introducing the term “many worlds.” The anthology he published in 1973 included Everett's full dissertation text and sold out completely.

Surveys among physicists show a steady rise in support. At a 2011 Austrian conference on quantum foundations, 6 out of 33 participants endorsed MWI — a proportion similar to Tegmark's 1997 survey. Prominent supporters include David Deutsch (father of quantum computing), Sean Carroll (author of “Something Deeply Hidden,” 2019), and David Wallace (author of “The Emergent Multiverse,” 2012).

The appeal of MWI lies in its mathematical simplicity: one single law (the Schrödinger equation), no ad hoc collapse, no quantum/classical divide. Everett himself argued it is “the only completely coherent approach” to explaining both the micro and macro world. Although the price — countless non-communicating branches — seems extravagant, supporters respond that nature doesn't need to be “economical” — only consistent.