In 1957, Hugh Everett, then a graduate student at Princeton University, proposed a new interpretation of quantum mechanics. Today it is known as Everett’s many-worlds interpretation.
A commentary on Everett’s original paper is available here:
• Everett’s Many-Worlds Interpretation (2019/2/11)Imagine placing a single electron inside a box. According to quantum mechanics, the electron spreads throughout the box as a wave, called the wave function. When someone looks inside, the electron is found at a single location, and the wave function appears to collapse there.
It may be tempting to think, “The electron was always a particle; the wave only represented our uncertainty.” However, Alain Aspect’s 1982 experiment ruled out a broad class of local hidden-variable explanations. Before measurement, the electron must be treated as genuinely spread out like a wave.
Wave-function collapse is deeply puzzling. Suppose the box is divided into two chambers. The electron’s wave function extends into both chambers. If we open one chamber, we may or may not find the electron there.
If the electron is found, the wave function collapses to that chamber. If it is not found, the wave function collapses into the other chamber. The fact that the wave function changes even when the electron is not directly seen makes the situation even stranger.
Everett proposed that the wave function never collapses. If an electron is described by a wave function, then observers should also be described by wave functions. When an observer measures the electron, the observer effectively branches into
Just as the electron is a wave spread through space, the observer becomes a wave spread across many worlds.
I first encountered the many-worlds interpretation in the Japanese popular-science book “The Fate of the Universe” when I was about ten years old. The book included the following passage, translated here:
If, as some science-fiction writers suggest, the universe splits in two whenever a random event occurs, then countless universes would exist—some only slightly different from ours, others vastly different, as many as one can imagine.
(…)
This is a fascinating story. Readers may think that no scientific theory would ever accept such an idea, but one does: the “many-worlds interpretation” of quantum mechanics, proposed in 1957 by the American physicist Hugh Everett III in his Princeton doctoral thesis.
(…)
You might expect such a fantastic idea to be easy to disprove, but it is not. Everett’s theory is mathematically equivalent to the standard formulation of quantum mechanics, so no experiment can currently distinguish between them.
At first, I found the idea hard to believe. Later, while studying K-meson mixing in college, I encountered a case in which two distinct particles appear in superposition. That experience made the many-worlds interpretation feel more natural to me.
The many-worlds interpretation is discussed in books such as “The World Described by Quantum”, which also introduces David Deutsch’s work on quantum computers.
For a long time, textbooks gave the many-worlds interpretation little attention. “Introduction to Quantum Mechanics” does discuss it, although from a critical perspective. In 1994, “The Picture of the World Told by Quantum Mechanics” became one of the first popular books in Japan to strongly endorse the interpretation.
Quantum mechanics faces the measurement problem, which involves both wave-function collapse and Born’s probability rule. Since many-worlds rejects collapse, it avoids one part of the problem. However, there is still no universally accepted derivation of the Born rule within the interpretation. I believe the many-worlds interpretation will need further mathematical development before it can be considered complete.
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