Curated by UCL

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

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…

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. Later, in the cold light of day, Everett reconsidered his position—and decided to double down on it. He expanded on his arguments from that evening and turned his attack on the quantum orthodoxy into a PhD thesis. “The time has come … to treat [quantum physics] in its own right as a fundamental theory without any dependence on classical physics,” he wrote in a letter to Petersen. To solve the problem of superposition, Everett proposed something truly radical, seemingly more appropriate for the pulp sci-fi novels he read in his spare time: he said that quantum physics actually implied an infinite number of near-identical parallel universes, continually splitting off from each other whenever a quantum experiment was performed. This bizarre idea that Everett found lurking in the mathematics of quantum physics came to be known as the “many-worlds” interpretation. The many-worlds interpretation hit a roadblock almost immediately in the person of Everett’s PhD advisor at Princeton, the eminent physicist John Wheeler. Wheeler was a physicist’s physicist; he wasn’t terribly well known outside of the field, but he knew absolutely everyone important within it. He was a protégé of Bohr, and had also been close with Einstein. Fifteen years before Everett showed up at his door, Wheeler had supervised the PhD of a young Richard Feynman; he would later go on to supervise the PhDs of dozens more renowned physicists (including Kip Thorne, one of the winners of last year’s Nobel Prize in Physics). Everett’s strange ideas were initially appealing to Wheeler, because they held the promise of applying quantum theory to the entire universe itself, something that Wheeler desperately wanted to do. But Wheeler was a political animal, and he was wary of incurring the wrath of his mentor Bohr by straying from the quantum orthodoxy preached in Copenhagen. Wheeler’s attempt to square this circle was bracingly straightforward: he traveled to Copenhagen to attempt to get Bohr’s blessing on Everett’s work as an extension of the official Copenhagen line on the nature of the quantum theory. It didn’t go well. Writing to Everett from Copenhagen, Wheeler said that resolving Bohr’s criticisms of Everett’s ideas “is going to take a lot of time, a lot of heavy arguments with a practical tough-minded man like Bohr, and a lot of writing and rewriting.” Wheeler implored Everett to come to Copenhagen himself and “fight with the greatest fighter [i.e. Bohr].” Everett wasn’t particularly interested in fighting or rewriting anything. He was confident in his ideas and was not enticed by the intellectual charms of an academic career. He was more interested in money and the things it could bring: fine food and drink, material luxury, and women. He wanted a Mad Men lifestyle, not a professor’s office. Everett had already lined up a job that promised to give him just that by the time Wheeler’s letter arrived: he had taken a job as a researcher with the Pentagon, gaming out the consequences of hypothetical nuclear missile exchanges at the height of the Cold War. When Wheeler returned from Europe, he forced Everett to heavily revise his thesis and remove almost all mention of “splitting worlds.” Once that was done, Everett left Princeton and never returned to academia. In his later career working for the Pentagon, Everett considered the more grisly possible worlds of nuclear war, and went on to co-author one of the earliest and most influential reports on the subject of radioactive fallout. But Everett did eventually make it to Copenhagen. In March 1959, he went to Denmark and presented his ideas to Bohr while he was in Europe on other business. As Everett later described it, the meeting was “doomed from the beginning.” Neither Everett nor Bohr was swayed. “Bohr’s view of quantum mechanics was essentially totally accepted throughout the world by thousands of physicists doing it every day,” said Misner, who was also in Copenhagen at the time. “To expect that on the basis of a one-hour talk by a kid he was going to totally change his viewpoint would be unrealistic.” Everett’s work fell into deep obscurity. It wasn’t revived until the 1970s, and even then, it was slow to catch on. Everett did make one last foray into the academic debate over his work; Wheeler and his colleague Bryce DeWitt invited Everett to speak about his work at the University of Texas in 1977. One of the young physicists in Austin at the time was David Deutsch, who later became a staunch advocate of the many-worlds interpretation. Everett “was full of nervous energy, high-strung, extremely smart,” Deutsch recalled. “He was extremely enthusiastic about many universes, and very robust as well as subtle in its defense.” The work of DeWitt, Deutsch, and others led the many-worlds interpretation to become much more popular over the ensuing decades. But Everett didn’t live to see the many-worlds interpretation achieve its current status as the most prominent rival to the Copenhagen interpretation. He died of a massive heart attack in 1982, at the age of 51. In accordance with his wishes, his family had him cremated, and left his ashes out to be collected with the trash. But Everett’s argumentative and puckish spirit lives on in his theory, born in a drunken debate over 60 years ago, and still inspiring fiery disputes among physicists today.