Rainer “Ray” Sachs passed away in April 2024, very much missed by his family and his many colleagues and friends.
Sachs spent the majority of his career, from 1969 onwards, as a Professor of Mathematics and Physics at UC Berkeley. Unusually, he had two very distinct careers in science, both very successful, the first in general relativity and cosmology, and the second in the mathematics of radiation biology and carcinogenesis.
Sachs’s early work focused on developing general-relativity-based formalisms for the analysis of gravitational waves. And in the early 1960s he provided a general proof that positive-energy-carrying gravitational waves are indeed a consequence of general relativity, something that Einstein expected but couldn’t generally prove. 50 years later, in 2016, energy-carrying gravitational waves were directly observed.
Sachs’s second major contribution in astrophysics was the so-called Sachs-Wolfe effect: this relates to the cosmic microwave background radiation (CMB), the remnant of the Big Bang. The CMB was discovered in 1965, and it is extraordinarily uniform across the observable universe. Within a year, however, Sachs and his student Arthur Wolfe published their 1966 prediction that there would be minute angular fluctuations in the CMB temperature caused by gravitational variations – the so-called Sachs-Wolfe effect. More than two decades after their original Sachs-Wolfe paper, fluctuations in the CMB temperature were finally observed by through satellite-based imaging - and since then the fluctuations predicted by Sachs-Wolfe have been used to infer fundamental parameters of our universe, including its age (13.8 Gyr), the abundance of all matter (including dark matter), and the properties of the primordial fluctuations that gave birth to all the known structure in the universe.
By the 1980s, Sachs was looking for a new challenge. He found it inspired by the likes of Douglas Lea and John Savage, applying his extraordinary mathematical skills to the problems of radiation biology. His great passion was for understanding radiation-induced chromosome aberrations, and the repair/misrepair kinetics of radiation-induced chromosome breaks was a perfect topic for him. Just as in his astrophysics career, he was able to apply his mathematical skills to provide insights towards understanding complex experimental data. As well as “sorting out” the details of radiation-induced chromosome aberration formation, a couple of other examples are his extension of the standard linear-quadratic radiotherapy model to include redistribution and re-oxygenation, and his brilliant demonstration that the various different radiation effect models in the literature all lead at low doses to exactly the same linear-quadratic formalism, including the standard dependence on dose rate.
Sachs’s works on chromosome aberrations and repair/misrepair kinetics naturally led to modeling radiation carcinogenesis. An illustrative example of the way he worked was his analyses of chronic myeloid leukemia, which strongly pointed towards a multi-cellular basis, a result that, at the very least, challenges the central paradigm that cancer arises from a single aberrant cell. Ray took such apparently surprising results in stride, delighting in the personal challenge of better capturing reality to develop even better models from which future scientists can build.
But his papers, insightful and elegant as they are, don’t tell the full story. The scientists who collaborated with Ray invariably found it an enormous pleasure. He was genuinely – really genuinely - modest, always listening carefully to what was being said - and there would inevitably come long pauses in the conversation while he thought through what had been said, before coming out with a measured reply.
Sachs was an extremely conscientious mentor, and was extraordinarily supportive of the careers of his students and his younger colleagues; he would always take that extra step, or two steps, to help them on their way.