Both Scarlata and Gingras are concerned that papers by less prominent scientists have disappeared as well without anyone realizing. At a minimum, Gingras wants Planck’s papers restored. “Whoever did it, I don’t care,” he says, “just put them [back] in the database. Intellectually, it’s not acceptable.”

The retraction of the second Planck paper, published in 1940, left Gingras and Khelfaoui even more baffled. It also cited copyright violation—yet the piece had never appeared elsewhere. Then Khelfaoui noticed something that added to suspicions that an algorithm was at work. Starting in the 1920s, physicists Niels Bohr and Werner Heisenberg promoted the so-called Copenhagen interpretation of quantum mechanics, which proposes that subatomic particles exist in a strange superposition of multiple states and only cohere into definite form when observed or measured—as in the famous Schrödinger’s cat paradox. Planck opposed this notion, arguing that external reality existed beyond human measurements. In November 1940, philosopher Aloys Müller criticized Planck’s views in a Naturwissenschaften piece titled “Naturwissenschaft und reale Außenwelt” (“Natural Science and the Real External World”). A month later, Planck responded in print—and used the exact same title. This, Gingras and Khelfaoui suspect, caused Springer Nature’s copyright bot to retract the paper as plagiarism decades later, even though the contents of the two essays differ markedly.

The practice was especially common for luminaries like Planck. Albert Einstein did the same (but escaped retractions).

1942 titled “Sinn und Grenzen der exakten Wissenschaft” (“Meaning and Limits of Exact Science”), about how to achieve certainty in scientific knowledge, had also appeared in two other journals and been reprinted twice in books. Repackaging the same work multiple times is considered “self-plagiarism” and frowned upon today—the practice produces copyright conflicts and inflates scholars’ publication records. The Naturwissenschaften site gives “copyright violation” as the reason for the retraction.

The papers, both quietly retracted in 2011, originally appeared in the early 1940s in Naturwissenschaften, a German journal now owned by publishing giant Springer Nature