The astrophysicist Edwin Salpeter was a seminal figure in theoretical physics. His intellectual passions ranged widely and he made pivotal contributions to the study of the universe across a variety of fields.
He showed how the element carbon was created from helium. This occurs as red-giant stars evolve to their highest luminosity. Later he demonstrated how molecular hydrogen could form despite the very low density of the interstellar gas. Molecular hydrogen is the dominant form of matter in the giant molecular clouds which form a ring around the galaxy rather closer to the centre than the Sun is. In a prescient note to the Astrophysical Journal in 1964 he was the first to suggest that quasars – objects of stellar appearance which radiate variably with a hundred times the output of a whole galaxy – are powered by the gravitational energy released as a black hole moving through a galaxy gathers interstellar material through its accretion disc.
Salpeter collaborated with Hans Bethe (winner of the Nobel Prize for Physics in 1967), and his name is commemorated in the Bethe-Salpeter equation, which describes the motion of two electrons as in a helium atom or a heavy ion. He systematised what was known about the distribution of the masses of newly formed stars – his distribution of stellar masses is called the Salpeter Function.
His astronomical work ranged widely from planets, low-mass stars and white dwarfs to pairs of galaxies and the influence of dark matter on them. In dynamics he deduced a very low limit on any conceived anisotropy of inertia, i.e. a body has the same resistance to acceleration in all directions. In his later years his research moved out of astrophysics as he interested himself in medical problems raised first by his wife's work and then by his daughter's work in epidemiology.
Ed Salpeter was born in Vienna in 1924. His father was a physicist who knew Schrödinger, but with few academic openings for those of Jewish origin in Austria, he turned to industry. After a scare on Kristallnacht, in 1938, when someone tried to arrest the 13-year-old Edwin, his father got a similar industrial position in Australia, where the family fled in June 1939.
Through hard work Salpeter completed the entrance requirements for the University of Sydney, which he entered aged 16. Physics students were expected to help with war work on radar, so he was exposed to the techniques of what became radio astronomy very early on in his career. However, the greatest challenge to physicists in the immediate post-war years was the development of a consistent quantum electrodynamics.
In 1946 he was awarded the prestigious 1851 Exhibition, a scholarship to study in the United Kingdom, and decided to join the rightly famous group of Professor Rudolf Peierls at Birmingham. There he learned the techniques that led Hans Bethe to explain, in 1947, the Lamb shift in the atomic energy levels of hydrogen, but Salpeter was applying them to the notoriously difficult problem of the internal energy of the electron. Bethe's result demonstrated the importance of the technique for atoms so on completing his PhD, Salpeter moved to the US to join Bethe at Cornell University.
Salpeter's collaboration with Bethe was highly productive and resulted in a book on the physics of one- and two-electron atoms. During this period he realised that others were more skilled than he in the highly mathematical work required in this exacting analysis, while his very wide interests made him capable of grasping what was important in a confusing mass of heterogeneous observations.
Among Bethe's many interests were the nuclear reactions that power the Sun and stars. Here the problems were less precisely defined and there was greater scope for Salpeter's powers of discernment. Thus he turned his attention to nuclear astrophysics and the mechanisms by which the heavier elements are formed by combining lighter ones within the stars.
At the time there were two rival theories of how the elements were built up from hydrogen. George Gamow suggested that they were generated in the Big Bang that started the expansion of the universe, while Fred Hoyle followed Arthur Stanley Eddington in believing that they were built in the centres of the stars and spread around via the explosions of supernovae. Both theories ran into difficulty once hydrogen had been transformed into helium because while two helium nuclei (alpha particles) could collide to form beryllium, this is unstable and decays rapidly. Thus a third alpha particle has to collide in the short interval before the beryllium breaks up.
Salpeter made the first calculation of this reaction rate using data on the unstable beryllium from W.A. Fowler's group at the California Institute of Technology. His calculations required a somewhat higher temperature than was found in the centres of the stellar models. These higher temperatures would result in most of the carbon so formed reacting with a further alpha particle to form oxygen. He noted that had there been a low-lying energy level of the carbon nucleus the reaction rate would be 1,000 times faster than he had calculated. This would allow it to operate at a slower rate at a lower temperature.
Later Hoyle, working along similar lines, demonstrated that the high abundance of carbon relative to oxygen meant that there must be such a level in carbon and even predicted its energy. Fowler was at first sceptical but was converted when the experiments of his group found the level, so verifying the truth of Hoyle's amazing prediction. Salpeter and Hoyle were awarded jointly the 1997 Crafoord prize of the Royal Swedish Academy for their work on the origin of carbon.
This triple-alpha particle process could not occur at the lower densities of the Big Bang, so it led to the acceptance of the stellar origin of the elements heavier than helium. However stars are not able to produce the high abundance of helium and deuterium that are observed and those are nicely predicted from the Big Bang.
As Salpeter's work led him into greater contact with observational astronomers he found greater scope for his powers of discernment. There must be many others who like me have (twice) been excited by a good idea and then discovered that Salpeter had already investigated it.
After Maarten Schmidt discovered quasars, there was an outpouring of theoretical ideas. At the time no galaxies were observed near quasars so they were thought not to be associated with clusters of galaxies. Salpeter ignored this unsubstantiated belief and the wilder ideas of others and relied on well established physics.
He suggested that a massive black hole might plough its way through a galaxy and accrete mass asymmetrically so that it would swirl around getting hotter as it worked its way down into the black hole. He calculated the energy output. The currently accepted theory of these objects is along the lines he advocated. Yakov Zeldovich and Igor Novikov calculated the same energy output but their model involved an object in orbit about a black hole radiating the power as gravitational waves. Such waves have not yet been detected although a projected satellite system called LISA aims to detect such events.
Apart from sabbaticals, Salpeter remained at Cornell for the rest of his life. He was approachable and enjoyed discussing ideas, giving full credit to others. He had greater analytical powers than the astronomer Tommy Gold, who was also at Cornell, and was even more prolific with ideas both good and less good. On occasion Salpeter would give a definitive discussion, attributing the original idea to Gold.
In 1950 Salpeter married Mika Mark, with whom he had two daughters. Despite the difficulties of his youth, Salpeter gave the impression of stability and ease but some of this came from Mika's ability to make decisions. Something of his character comes over in his article "A Generalist Looks Back", in the Annual Review of Astronomy and Astrophysics (2002):
Starting in the 1960s I occasionally got job offers for chairmanships and other administrative positions elsewhere. Cornell salaries were rather low and I was only a rank and file professor so these offers involved a substantial rise in salary. My wife and I once made a list of what we would do with the extra money but most ideas were a mixed blessing. The only improvement with no drawbacks at all would be to buy pure maple syrup instead of the imitation maple syrup we had used in the past. We decided we could afford maple syrup on a Cornell salary and have used it ever since.
In 2000, Mika, a professor in neurobiology and behaviour, died unexpectedly. Salpeter took over the running of her laboratory and helped those in it to find other jobs as the grant funding it wound down. Thus he spent his final years using his expertise to further medical science, sometimes collaborating on epidemiology with his daughter in California. In these years he was greatly helped by the Buddhist serenity of his second wife, Lhamo.
Edwin Ernest Salpeter, astrophysicist: born Vienna 3 December 1924; Research Fellow, Birmingham University 1948-49; Research Associate, Cornell University 1949-56, Professor of Physics and Astrophysics 1956-71, J.G. White Distinguished Professor of Physical Sciences 1971-97 (Emeritus); married firstly 1950 Mika Mark (died 2000; two daughters), secondly Lhamo Shouse; died Ithaca, New York 26 November 2008.