For more than 80 years, a manuscript written by one of the founders of quantum physics has been gathering dust in an Austrian library. Now brought to light by physics historian Flavio Del Santo of the University of Geneva, the three-page document is the very soul of brevity, but scribbled on it is a long-lost contribution to one of the most contentious debates in quantum mechanics. And it offers a surprising insight into what Erwin Schrödinger himself believed about the nature of reality. Because of his often discomfort with quantum randomness, Schrödinger believed that there were aspects of the Universe that could not be predicted, a belief he held even before the emergence of quantum theory in the 1920s. “The strongest case for indeterminism is older than quantum mechanics,” Schrödinger wrote in the unpublished manuscript. frameborder=”0″ enable=”accelerometer; autoplay; writing on clipboard; encrypted media; gyroscope; picture in picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” enablefullscreen> In classical physics, there is no such thing as true randomness. Dice can have nicks and defects that influence the way they roll. Computer random number generators are usually powered by algorithms. Even coin tosses are governed by physical forces that could, in theory, be predicted. This phenomenon is known as determinism and plays an important role in our understanding of the physical Universe, asserting that each classical behavior is determined by the conditions that precede it, even if we do not know what all those conditions are (The absoluteness of classical determinism has been questioned, but it remains a fundamental assumption of classical mechanics.) But quantum mechanics – as is its wont – marches to a very different drumbeat. On the quantum scale, the behavior of particles such as photons and electrons can only be predicted in terms of probabilities. will make an individual particle. This raises a profound question: Is the Universe fundamentally random, or are there hidden factors that determine the behavior of particles that quantum mechanics has not yet taken into account? These hypothetical hidden factors are known as hidden variables, and they are what they seem: unknown properties or conditions that, if we knew them, would allow physicists to predict the quantum behavior of individual particles. In 1935, the mathematician Grete Hermann found important gaps in von Neumann’s reasoning. the argument for hidden variables, by speaking, ironically, against them, according to von Neumann. In May 1943, the mathematician and physicist Edmund Taylor Whittaker gave a lecture arguing that quantum mechanics had finally demolished the classical notion of determinism. Then, in January, 1944, physicist Satyendra Nath Bose made a similar argument in a speech to the Indian Science Congress, declaring that “physicists have gained knowledge but have lost faith” in the deterministic laws of nature. Well, that was a spark to dry the tinder. In July 1944, mathematician Henry Piaggio published a paper in Nature titled “Collapse of Determinism,” reporting on Bose’s arguments. responses from physicists who were not so convinced that determinism was dead and buried. Over the next few months, the argument flourished in the magazine’s pages, with researchers picking apart the supposed impossibility of hidden variables, including Whittaker’s seemingly simple example involving photons and polarized light. Two polarizing filters placed over a laptop screen demonstrate how the orientation of polarized light affects its transmission. (Pieter Kuiper/Wikimedia Commons, public domain) In his 1943 lecture, Whittaker had proposed a seemingly simple demonstration of why hidden variables could not exist, using photons (individual particles of light) passing through a polarizer, a device that filters light according to its orientation. Therefore, he concluded, the result could not be predetermined by hidden variables. Enter Schrödinger. Or rather, stay behind the scenes, Schrödinger had been following the debate. 1944, he wrote his own response, also titled “Collapse of Determinism” (several papers were published as part of the debate that used the same title). But Schrödinger pointed out that there was no reason to restrict the hidden variable to only two values, one that determined whether it would pass through the polarizer and the other whether it would be reflected. A mathematical model in which the result depended on both the hidden variable of the photon and the angle of the polarizer. With the correct rules, the model reproduced the quantum mechanics predict probabilities, without requiring that each individual interaction be random. He was not defending determinism because he necessarily believed in it. He was defending the possibility of determinism against specific claims that it was impossible. “Experimental evidence + logical deduction can never prove that the deterministic or indeterministic view is impossible,” he wrote. However, why he never published it remains a mystery, although Del Santo has a possible explanation. However, Schrödinger’s model went further, providing an explicit mathematical rule for how the hidden variable of a photon would change after passing through a polarizer, allowing him to account for successive measurements. brought to light more than eight decades later. But there is another twist to the story. Although Schrödinger is often portrayed as a champion of determinism, his own philosophical position was quite different as early as the 1920s, before the emergence of modern quantum mechanics, Influenced by the physicist Franz Exner, He even described classical determinism as a “philosophical prejudice” and an “a priori belief.” And his unpublished manuscript suggests Although the “determinism collapse” wave of the 1940s is largely forgotten today, physicists remain divided over their central positions. Perhaps what we can learn from Schrödinger’s late contribution is to keep an open mind. left with a question that remains, in its own way, both living and dead: Is the Universe deterministic… or not? reconstructed here, but never as expressions of a dogmatic commitment to determinism. Their paper is available on the arXiv preprint server. This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we take pride in our process, we are human. 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