Lost for eight decades, an unpublished manuscript by Erwin Schrödinger, famous for developing the thought experiment known as Schrödinger’s cat, reveals the scientist’s previously unknown quantum musings that question a belief commonly associated with him.
In a preprint paper posted on arXiv, Flavio Del Santo, a University of Geneva researcher, traces the history of the quantum argument Schrödinger commented on and explains why the influential scientist may have abandoned the work.
In his most famous thought experiment, Schrödinger described a bizarre paradox involving a cat trapped inside a box, whose fate depends on a deadly mechanism triggered by the decay of a radioactive atom. Under a straightforward application of quantum mechanics, the atom can be described as existing in a superposition of decayed and undecayed states, leaving the cat in a corresponding superposition of being alive and dead until a measurement produces a definite outcome.
More fundamentally, Schrödinger intended the thought experiment to illustrate the difficulties that arise when applying principles of quantum superposition to everyday, macroscopic objects. Quantum mechanics describes probabilities for possible measurement outcomes, rather than necessarily assigning a single definite outcome in advance, as classical physics generally would.
Schrödinger and Quantum Mechanics
After introducing his famous cat thought experiment in 1935, Erwin Schrödinger continued pondering the apparent randomness of quantum mechanics throughout the 1940s, when he wrote the newly rediscovered manuscript. Now, more than 80 years later, Del Santo’s research offers a fresh look at the physicist’s thinking during this period.
One of the central questions was whether quantum randomness reflected a fundamental feature of nature or merely a lack of sufficiently precise information.
In classical physics, for example, a coin toss is not considered truly random. Rather, its outcome depends on physical conditions that are difficult to measure precisely. Our inability to account for every force acting on the coin makes the result effectively unpredictable.
Early quantum theorists considered whether the probabilistic nature of quantum mechanics might arise from something similar. What if quantum outcomes were not fundamentally random, but instead depended on properties that remained unobserved? If those hidden influences could be understood, perhaps quantum mechanics could be reconciled with a more deterministic description of nature.
The Debate Continued
Mathematician John von Neumann, also known for his theoretical work on self-replicating machines that later inspired the concept of the “von Neumann probe,” appeared to settle the debate for a time.
In 1932, he published a mathematical argument that was widely interpreted as ruling out hidden-variable explanations of quantum mechanics. For many physicists, this reinforced the view that quantum probabilities could not simply be explained by unknown underlying properties.
Eleven years later, in 1943, the debate resurfaced. British mathematician Edmund Taylor Whittaker argued that the probabilistic predictions of quantum mechanics were incompatible with certain deterministic explanations of physical behavior.
His example involved a light polarizer, a device that transmits or blocks light depending on its polarization. Whittaker considered whether a hidden property of the light could determine whether it passed through the filter.
He argued that if the outcome had already been determined by such a hidden instruction, rotating the polarizer after the light had begun traveling toward it should not alter the result. This appeared to conflict with quantum mechanics, which predicts that the proportion of light transmitted depends on the polarizer’s orientation.
The following year, Indian physicist Satyendra Nath Bose presented a related argument, helping revive the discussion in the scientific journal Nature.
Entering the Quantum Argument
Heinrich Pelzer eventually challenged Whittaker’s reasoning, undermining the argument against deterministic explanations.
In the meantime, Schrödinger had handwritten a three-page response to the debate, which remained largely overlooked among his papers at the University of Vienna. In the manuscript, Schrödinger constructed a hidden-variable model more elaborate than Whittaker’s, exploring whether deterministic assumptions could reproduce the predictions of quantum mechanics.
Intriguingly, when Del Santo examined the model mathematically, he found that its predictions for the behavior of polarized light agreed with those of quantum mechanics. Based on the manuscript’s dating to November 1944, Del Santo suggests that Schrödinger may have abandoned the problem after becoming aware of Pelzer’s work.
The debate over hidden variables would reemerge in subsequent decades, particularly during the 1950s and 1960s, as physicists continued investigating whether quantum events might be governed by underlying deterministic processes.
Ultimately, Del Santo considers the unpublished work especially intriguing because Schrödinger was known for his deterministic outlook. The manuscript offers a glimpse of a physicist willing to examine the foundations of his own assumptions and explore competing interpretations of quantum theory.
The paper, “Schrödinger’s Unpublished Hidden-variable Model in the 1944–45 Debate Over the ‘Collapse of Determinism’” appeared on arXiv on October 14, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.