Martin John Rees
1989 Balzan Prize for High Energy Astrophysics
Two of Professor Rees’s best known and most widely cited scientific contributions are the prediction of superluminal motion in quasars and the idea of powering extended radio sources via beams of low-frequency electromagnetic radiation.
The former idea arose as an explanation for rapid variability in brightness of quasars. It was noticed beginning in 1965-66 that radio (later X-ray and optical) brightnesses of quasars and active galaxies were changing in weeks or months. This seemed to require the emitting region to be so small that it could not contain the necessary energy to be radiated. Some astronomers concluded that this must mean the quasars were much closer to us than indicated by their red-shift’s. Rees recognized, however, that the variability could be understood if the emitting region consisted of gas expanding or flowing outward at dose to the speed of light. The same picture accounted for the previously-puzzling absence of X-rays from inverse Compton scattering of radio and optical light. In addition, it led Rees to predict that the variable sources should eventually show evidence of gas motions in their cores at such high speeds that, projected into the plane of the sky, the motion seemed to be faster than the speed of light. Evidence for such “superluminal” motion began to appear in Very Long Baseline Interferometry (VLBI) studies of quasars and radio-emitting galaxies in the 70s, and it has now been established in several dozen sources. Relativistic gas flows of the sort suggested by Rees continue to provide the explanation for these observations.
The second idea arose from the difficulty in understanding the emission of copious radio waves from regions far outside the visible edges of galaxies and quasars. How could energy be supplied continuously to these regions while remaining nearly invisible en route? Rees’s idea of highly-collimated beams of energy moving at or nearly at the speed of light provided a solution. Variations of the original model to which he has contributed include a model for pulsar-driven supernova remnants, the now generally-accepted “twin exhaust”, jet, or relativistic beam scenario for active galactic nuclei, and the ion-supported torus for collimating the jets.
Other topics in high energy astrophysics to which Rees and his collaborators have made important contributions include (a) the formation of X-ray-emitting binary systems by stellar capture and the dominance of accretion discs in such systems, (b) the importance of electronpositron plasmas in active galactic nuclei, (c) the possible existence of a massive black hole at the center of our galaxy, (d) production of background radiation, helium, and deuterium from pre-galactic stars, (e) limits on gravitational radiation in various contexts, and (f) the formation of galaxies in a universe dominated by cold dark matter.
Professor Rees also has a distinguished record of service to the scientific community as Director of the Institute of Astronomy in Cambridge (1977-82, 1987- ), as a member of numerous advisory committees and panels, and, especially, as a mentor to younger colleagues.