The Difficult Birth of the “Many Worlds” Interpretation of Quantum Mechanics

By Adam Becker

Over several rounds of sherry late one night in the fall of 1955, the Danish physicist Aage Petersen debated the mysteries at the heart of quantum physics with two graduate students, Charles Misner and Hugh Everett, at Princeton University. Petersen was defending the ideas of his mentor, Niels Bohr, who was the originator of the “Copenhagen interpretation,” the standard way of understanding quantum physics. The Copenhagen interpretation, named after the home of Bohr’s famous institute, stated that the quantum world of the ultra-tiny was wholly separate from the ordinary world of our everyday experiences.

Quantum physics, Petersen said, applied only to that ultra-tiny realm, where individual subatomic particles performed their strange tricks. It could never be used to describe the world of people and chairs and other objects composed of trillions of trillions of those particles—that world could only be described by the classical physics of Isaac Newton. And, Petersen claimed, this was itself dictated by quantum physics: the mathematics of quantum physics reduced to the mathematics of Newton’s physics once the number of particles involved became large.

But Everett incisively attacked the orthodox position advocated by Petersen with alcohol-fueled bravado. Quantum physics, Everett pointed out, didn’t really reduce to classical physics for large numbers of particles. According to quantum physics, even normal-sized objects like chairs could be located in two totally separate places at once—a Schrödinger’s-cat–like situation known as a “quantum superposition.” And, Everett continued, it wasn’t right to appeal to classical physics to save the day, because quantum physics was supposed to be a more fundamental theory, one that underpinned classical physics.

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