In 1927, when Albert Einstein began his famous series of battles at the Solvay Conference in Brussels with Danish physicist Niels Bohr over the meaning of quantum mechanics, John Wheeler was just a teenager. Quantum mechanics is the physical description of how atoms behave. Unlike classical Newtonian physics, it involves instantaneous transitions, set by probabilistic rules rather than exact, mechanistic laws. Einstein objected to the jumpiness, chance elements and other indeterminate aspects of quantum theory, whereas Bohr found these acceptable. As Wheeler came of age as a physicist in the mid-to-late 1930s, he became close friends with both debaters, appreciated their well-reasoned arguments, and hoped to find a way of reconciling their clashing viewpoints. 

Wheeler shared with Einstein and Bohr a deep appreciation for the philosophical underpinnings of theoretical physics. Einstein’s readings of Baruch Spinoza and Ernst Mach had driven him toward theories that were objective, deterministic and, in principle, directly measurable for all components at all times by local observers. In contrast, Bohr, a fan of Eastern philosophy, including Taoism with its Yin-Yang union of opposites, embraced contradictions in his theory of complementarity. According to that notion, a quantum entity’s behavior, as either wavelike or  particle-like, depends on how it is measured. While many American physicists eschewed philosophy, Wheeler, the son of two librarians, embraced it. Therefore, while leaning towards Bohr’s interpretation, he found merit in both thinkers’ perspectives.

At Solvay, Einstein had argued, by means of thought experiments he presented to Bohr and others, that even if quantum mechanics matched experimental data, it was fundamentally incomplete. It fell short, Einstein believed, by including situations in which the experimenters’ choice of methods and apparatus affected whether certain parameters had definitive values or were hazy. Moreover (as highlighted by the concept of entanglement, introduced in 1935) it was non-local, meaning that a quantum state might include two things that have connected properties even though they are physically separated, perhaps even by a great distance. Each time Bohr knocked down one of his thought experiments, Einstein would propose another. The failures of quantum mechanics to provide a complete blueprint of nature, Einstein reasoned, suggested the need for a deeper, comprehensive theory that could do just that. He thereby set out to find a mechanistic, unified field theory that would supersede quantum mechanics. It was a quest that would last the rest of his life.

Wheeler first met Bohr during a 1934 research stay at his Institute for Theoretical Physics in Copenhagen. In January 1939, after Wheeler had been appointed assistant professor at Princeton University, Bohr arrived there for a research stay lasting several months. Together, Bohr and Wheeler developed a model for the activation energy of nuclear fission that predicted which isotopes of uranium and plutonium would most easily be fissile. At that point, Wheeler’s focus was more practical than philosophical.

Einstein was, by that time, Wheeler’s neighbor, both at work and at home. The German physicist-in-exile worked at the Institute for Advanced Study, which was originally located at Princeton’s Fine Hall until a dedicated facility could be built.  Wheeler’s office was similarly at Fine Hall, on the same floor as Einstein’s (and Bohr’s when he was at Princeton temporarily). Einstein’s house on Mercer Street was only several blocks away from Wheeler’s on Battle Road. Wheeler could speak German and had a pleasant, agreeable persona that exuded friendly respect—just the right ingredients to endear himself to Einstein.

As Wheeler came to know both Bohr and Einstein very well and consider them his mentors, he’d begin to think about ways of reconciling their radically different viewpoints. While Wheeler embraced Bohr’s complementarity, he agreed with Einstein that the role of a human observer—as a separate entity making a decision that triggers a quantum “roll of the dice”—was vague and perhaps even paradoxical. Weren’t humans governed on a deep level by quantum rules as well?

An opportunity came for a possible reconciliation when Wheeler’s student Richard Feynman developed a radical alternative to the standard methods of quantum mechanics. In Feynman’s so-called path integral formalism, dubbed by Wheeler “sum over histories,” quantum calculations are performed via a weighted sum of probability amplitudes for the various alternative paths in which an interaction might transpire. It is like calculating one’s overall exertion during a commute to work by reckoning with the alternatives of taking a bus, train, taxi, and walking in tandem, as if all done at once, instead of considering them separately. The classical path was simply the likeliest.   

Armed with what he thought was a superior way of looking at quantum mechanics, Wheeler stopped by Einstein’s house and engaged him in a deep discussion about Feynman’s methods. Nevertheless, the stubborn elderly physicist was not persuaded. “I can’t believe that God plays dice,” said Einstein. “But maybe I’ve earned the right to make my mistakes.”

After Einstein died in 1955, Wheeler continued to try to find common ground among the various perspectives on quantum mechanics. He was intrigued when another of his students, Hugh Everett III, eliminated the direct role of observers with his concept of a universal wave function (later dubbed by Bryce DeWitt the “many-worlds interpretation.”) Instead of “dice-rolling,” quantum physics would be fully deterministic. The only catch is that upon each quantum measurement the universe would split into myriad alternatives. Unlike “sum over histories,” with its blending of possibilities into a single reality, these would be disparate realities in their own right. An observer’s conscious existence would bifurcate as well, allowing different copies to experience different outcomes. Hence, human and particle fates would be tied together, avoiding the need for independent observers.

In the case of Everett’s hypothesis, it was Bohr who wouldn’t budge. As much as Wheeler tried to persuade Bohr that it was a more comprehensive theory, Bohr saw no need to stray from complementarity. Wheeler tried to paint Everett’s theory in its least radical light, but Bohr had little interest.

Wheeler’s final attempt at paving over the rift came in the late 1970s, more than a decade after Bohr had passed away. Wheeler sought self-consistency through a “participatory universe” in which human observation in the present could affect quantum outcomes in the past. Thus, complementarity became a closed loop in which what is observed in the past shapes the entirety of history and ultimately the observer himself. Wheeler called this a “self-excited circuit.” The only element missing, he realized, was a reason for the observers to exist at all. “How come existence?” became Wheeler’s ultimate question. It remained unanswered when he died in 2008 at the age of 96.