Sverre Aarseth (20 July 1934 – 28 December 2024)

Sverre Johannes Aarseth was born on 20th July 1934 on a small island in the spectacular Hardangerfjord region of Norway. The island had a population of fifty souls, no shops and no electricity. As the son of the local priest, he was at the centre of the local rural community where he travelled to school by boat, sometimes rowing himself, and also became an expert potato grower. Later he acquired an excellent mathematical education as a science student at Oslo University and in 1959 his life changed dramatically when he wrote to Sir Fred Hoyle expressing his wish to uproot to Cambridge and continue his studies. It seems Hoyle did not hesitate and Sverre arrived to England for his PhD. After graduation in 1963 he became one of the first residents of Hoyle’s new Institute of Theoretical Astronomy based near the Observatories, off Madingley Road, then deep in the countryside.

Sverre pioneered the art of N-body simulations and was among the first to exploit the power of electronic computers. He travelled the country to use the most powerful machines in order to calculate the motion of many bodies under the influence of Newtonian gravity. His N-body codes continue to be used all over the world and he worked tirelessly on improvements until recent years. Sverre always supported the idea of open source software as well as the free exchange of ideas and so his N-body codes remained at the forefront of the field throughout his life. He laid the foundations for many substantial advances in dynamical astronomy. With his students he developed techniques to accelerate the calculations including regularising binary star orbits, the key to modelling close encounters. Sverre was also the first to engage with the latest hardware developments, initially with machines such as the HARPs and GRAPEs, dedicated to the rapid calculation of inverse- square forces. He later exploited much cheaper graphical processing units which he, with the help of researchers across the world, incorporated in his own custom machines. In the 1990s Sverre was the first to include in his codes the evolution of the individual bodies, now representing real stars, leading to the earliest calculations of realistic clusters. Sverre’s methods eventually led to the million body project, reaching the goal of modelling a fullsize globular cluster. His latest code included general relativity and the merging of black holes allowing him to propose that the newly found gravitational wave sources originate in star clusters. Generations of astronomers have benefited from use of his codes and for them his 2003 book “Gravitational N-Body Simulations, Tools and Algorithms”, published by the Cambridge University Press, became an essential reference.

As a brilliant scientist, Sverre always remained faithful to his beloved N-body simulations where his work has shaped the field. In recognition he earned the prestigious Brouwer Award in 1998. He remained a research fellow at the Institute of Astronomy throughout his career. He always arrived at his office around 9am and left around 5.30pm, except for Saturdays when he would go into the centre of Cambridge for a coffee and pastry. Retirement had little effect and he continued this daily working routine until the COVID–19 pandemic.

Though an obsession, Sverre’s work did not completely dominate his life. Very soon after he arrived in England he married Patricia. He enjoyed showing off his sports car. He was an expert at correspondence chess, in the days when moves were communicated on postcards, earning the formal title of International Master of Correspondence Chess in 1981. At the age of forty his recreational pursuits changed dramatically when, with a party of friends, he climbed Mt Elbert in Colorado and accepted a challenge to go back when somebody discovered, down at the tree line, that they had left their camera on the summit. Sverre realized that climbing mountains, often solo, appealed to his reckless love of adventure. Shortly after, he visited his sister in Africa and took on two more life-long obsessions, of wild animals and of walking. Thenceforward he spent his free time travelling the world mountaineering and on safari, though his safaris were typically designed by him and totally devoid of luxury. His mountain and wildlife photography presentations regularly delighted Cambridge astronomers. Sverre’s climbing exploits are recounted in his own book “Adventures of a Reckless Amateur” published in 2014 and written in the family cabin in the Norwegian mountains that he had built with his father. He went on to conquer most of the highest mountains in the western world even after losing his toes early on in his mountain adventures. In 1994, Sverre was the first to climb to a summit, later named Hombro del Pissis, at 6 430 m in the Atacama Dessert, an achievement now officially attributed to him. His love of wildlife sent him pursuing tigers in India and orangutans in Indonesia, exposed him to prowling lions in Africa and found him paddling amongst crocodiles. He became a staunch supporter of conservation efforts. An adopted rhinoceros became a prominent love of his life.

In this second phase of his life the sports car disappeared and was replaced by a bicycle on which he delighted to be the fastest on the road. He gave this up when others started to overtake him but he continued a daily run or walk until the COVID–19 pandemic lockdowns. Though he escaped the disease itself, the lack of a daily work routine took its toll and Sverre never really re-emerged from solitude. He regularly, jovially at first, pointed out that he was ready to log off. This he finally did on 28th December 2024. He is survived by his wife Patricia and close friends. His loss is very deeply felt.

- Text by Chris Tout and Anna Zytkow.