I’m delighted to offer some background to the compelling story that led me to write the new book Einstein’s Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics.
Browsing through the Albert Einstein Duplicate Archive at Princeton some years ago, I encountered a curious exchange between him and Austrian physicist Erwin Schrödinger. Letters, press clippings, and other materials documented the formation and breakup of a deep friendship grounded in a shared scientific perspective.
Until the clash, their correspondence depicted a warm collaboration based on a mutual feeling that probabilistic quantum mechanics was incomplete. Like Cervantes’ characters, they battled together against perceived monstrosities, such as fuzzy and indeterminate physical conditions, that their contemporaries saw as completely natural.
On at least one occasion, Einstein jokingly compared himself to Cervantes’ starry-eyed comic protagonist, Don Quixote. Schrödinger expressed an affinity for Quixote’s loyal friend Sancho Panza. Each physicist would fully admit that their quest to extend quantum mechanics and eliminate its nebulous areas might be seen as quixotic. Nevertheless, they persisted, convinced that “God does not play dice,” as Einstein was known to say.
Schooled in the writings of Schopenhauer and Spinoza, and trained in classical, deterministic physics, the two physicists spoke a common philosophical language. While, to others, their approach seemed outdated, they stuck fast to their beliefs and buoyed each other with optimism and humor.
The archive revealed the swapping of ideas that helped lead to Schrödinger’s famous cat paradox. Einstein shared his disdain for what he called spooky action at a distance: the concept that one member of a pair of particles might remain in a mixed state of a physical value until a researcher measures the properties of the other particle, even if they are widely separated. Dubbing this notion “entanglement,” Schrödinger imagined what would happen if the fate of a cat in a sealed box were linked to the state of a radioactive material. He and Einstein found the resulting situation preposterous, if interpreted through orthodox quantum principles: the cat would be in a mixed state of life and death until an observer opened the box and measured the system.
Einstein and Schrödinger’s mutual goal in their later years was to replace quantum mechanics with a broader theory that unambiguously explained the workings of the universe by unifying the forces of nature. At the time, physicists understood gravitation through Einstein’s general theory of relativity. He and Schrödinger sought to expand that theory to include electromagnetism as well — the other well-understood force at the time — and thereby produce a “theory of everything” that superseded the probabilistic approach. Isolated from the mainstream quantum community, they turned to each other for support. Each became the other’s sounding board for unification ideas.
Schrödinger tried to convince Einstein to add the nuclear force into the mix. Einstein was resistant because he wanted to solve a simpler theory first. Today we know that there are actually two nuclear forces: the weak interaction that causes many forms of radioactive decay and the strong interaction that binds quarks together into protons, neutrons, and other particles, and those, in turn, into the atomic nucleus. Einstein’s omission of nuclear interactions ultimately meant that his theory of everything could never be complete. Schrödinger was persuasive but Einstein was stubborn.
Despite differences in their approach, Einstein greatly respected Schrödinger’s viewpoint. He praised Schrödinger for his helpful analysis and steady correspondence. Relatively isolated at the Dublin Institute for Advanced Studies, a research center in Ireland, Schrödinger welcomed the chance to brainstorm with a prominent physicist. Grateful for Einstein’s kind words and insightful comments, he highly valued their collaboration. Letter after letter showed that they had become almost like brothers.
Suddenly, as the archive revealed, their friendship seemed to fall off a cliff. In late January 1947, the letters abruptly stopped. A flurry of press releases and newspaper clippings followed, reflecting a sudden new rivalry.
In essence, Schrödinger had declared victory over Einstein in the struggle for a theory of everything. At a talk at the Royal Irish Academy, attended by the prime minister and other notables, Schrödinger claimed to have found a way of uniting the natural forces through simple mathematical relationships.
The pressures to support his institution and justify his high academic standing were among the factors that led him to embellish his results. Bragging that he had found a simple solution to a problem that had cofounded Einstein for decades, his boasts were picked up by the international press. A number of media sources speculated if Schrödinger had succeeded where Einstein had failed. Without investigating the veracity of his model, newspapers reported his ambitious assertions and pressed for Einstein’s reaction.
Naturally, Einstein was furious and immediately broke off correspondence. Stunned by Schrödinger’s audacity, he scolded the press for misleading the public about the importance of such a preliminary attempt that was not backed up by solid physical evidence. Having hunted a final theory for years, he knew that Schrödinger had not found the “big game.”
Indeed Einstein was a seasoned veteran in producing a hodgepodge of unification schemes — many of which received much publicity because of his fame. From the 1920s until his death in 1955, he attempted numerous variations of the basic precepts of general relativity, hoping to open up enough room in the theory to model the electromagnetic interaction along with gravity. To create such room, he tried adding more dimensions, making the theory less symmetric, and a host of other alternatives that would make it less simple but more comprehensive. Starting in the late 1920s, most of his generalizations were dutifully reported in the media — framed as success stories, ignoring earlier failures. It was as if aged baseball giant Hank Aaron were still periodically in the news with glowing reports about his plans to reclaim his home run crown — despite having been out of the game for many decades.
The fiasco caused by Schrödinger’s premature announcement proved painful for both men. Einstein’s struggle for unification would become even lonelier. From that point on, he would share his preliminary ideas only with his research assistants. Embarrassed by the incident, Schrödinger would give up his quest for unity altogether and turn to other topics. Similarly, he would never collaborate again with a prominent physicist. Although each was in the final years of his life, the two missed an opportunity to develop their mutual ideas further and continue to offer a deterministic counterbalance to the majority view.
History has shed a positive light on Einstein and Schrödinger’s well-reasoned critiques of the orthodox interpretation of quantum mechanics. Many alternatives have emerged since that time, stimulated in part by the great physicists’ perspectives. Poisoned by the debacle, a fruitful interplay of ideas would flourish no more.
When the curtain fell on Einstein and Schrödinger’s discourse, two principled voices no longer resounded in unison. The physicists gave up any chance of working together to press forward with science’s longstanding dream for a single, unified theory of nature. An interwoven quest would end unraveled like a frayed garment.
Paul Halpern is Professor of Physics at the University of the Sciences in Philadelphia and the author of 14 books, most recently Einstein’s Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics.
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