Stephan Mertens passed away on October 9, 2024, at the age of 62 after a seven-year battle with multiple myeloma. At the time of his passing, he was a Professor of Theoretical Physics at Otto-von-Guericke University Magdeburg in Germany and an External Professor at the Santa Fe Institute.
Mertens was an interdisciplinary physicist whose work explored the interplay of disordered systems in statistical mechanics and computation. His research ranged from phase transitions in search and optimization problems, to neural networks, to the growth of clusters in curved and high-dimensional space. At SFI, he primarily collaborated with Professor Cris Moore, co-authoring The Nature of Computation, published by Oxford University Press in 2011.
“Stephan had both a love of theory and a deep practical understanding of parallel computing, and he was able to combine theory and practice to get beautiful new results,” says Moore. “He was also deeply rooted in physics, and his enthusiasm was captivating and inspiring to others. He loved being part of a lineage that stretches back to Einstein, Newton, Kepler, and the Greeks. Stephan really believed in physics — and more broadly, in science — as a calling.”
Mertens and Moore met at a conference at the Abdus Salam International Centre for Theoretical Physics in Trieste, Italy in 2000. Their collaboration began in earnest the following year when, in the aftermath of 9/11, Mertens and his wife Doro Frederking found themselves stranded in New Mexico while visiting Los Alamos. Encouraged by a mutual colleague, David Sherrington — co-creator of the Sherrington-Kirkpatrick spin glass model — they decided to write a book together. At first, their ambitions were modest — a short introduction to computational complexity for physicists — but their book grew to cover everything from basic algorithms to quantum computation.
In 2007, Mertens joined the Santa Fe Institute as an External Professor, a role that allowed him to engage with SFI’s broad intellectual community. His yearly visits, which also included astronomical excursions, made SFI a second home. He collaborated not only with Moore but also with researchers such as Jon Machta and Simon DeDeo. “Stephan was brilliant, and also so very congenial. Such a warm person. The best kind of scientist there can be,” says SFI Professor David Wolpert.
Beyond his academic work, Mertens was committed to communicating science to a wider audience. He wrote accessible science-related articles in German and in English, with pieces appearing in Magdeburger Volkstimme, Science, and American Scientist.
Mertens’ passion for science lasted to the very end of his life. Just before his passing, Mertens wrote to Moore about a connection between Kepler’s laws of planetary motion and Ptolemy’s theory of uniform motion around a point called the equant. While known to historians of physics, Mertens was delighted by this link, and hoped to write a pedagogical article about it. He also posed several new questions, like what viewpoint in space would make Ptolemy’s theory seem the most accurate. Although Mertens was not able to complete the project, Moore, along with colleague Alex Russell, plans to publish the paper with Mertens as a posthumous co-author.
Mertens’ legacy is one of intellectual curiosity, playfulness, and joy. His passion for science and his kindness will have a lasting impact on SFI and the wider scientific community. As Moore says, “His love of physics — his childlike curiosity, his brilliance at computing, and most of all the joy he took in teaching and learning — was an inspiration to all who knew him.”