Supermassive black holes

Of all the legacies of Einstein’s general theory of relativity, none is more fascinating than black holes. While we now take their existence almost for granted, black holes were viewed for much of the 20th century as mathematical curiosities with no counterparts in nature. Einstein himself never believed in black holes and wrote two papers in which he argued against their existence. Einstein’s resistance to the idea is understandable. Like most physicists of his day, he found it hard to believe that nature could permit the formation of objects as extreme as black holes. Indeed, the gravitational fields of black holes are strong enough to prevent light from escaping, and even distort space and the flow of time around them. The modern view – that black holes are the unavoidable end result of the evolution of massive stars – arose from the work of Subrahmanyan Chandrasekhar, Lev Landau, Robert Oppenheimer and others in the first half of the 20th century. However it was not until the discovery in 1963 of extremely luminous distant objects called quasars that the existence of black holes was generally acknowledged. What is more, black holes appeared to exist on a scale far larger than anyone had anticipated. Quasi-stellar objects or quasars belong to a class of galaxies known as active galactic nuclei. What makes these galaxies “active” is the emission of staggering amounts of energy from their nuclei. Moreover, the luminosities of active galactic nuclei fluctuate on very short time scales – within days or sometimes even minutes. The time variation sets an upper limit on the size of the emitting region. For this reason we know that the emitting regions of active galactic nuclei are only light-minutes or light-days across, making them less than one tenmillionth the size of the galaxy in which they sit. Astronomers were faced with a daunting task: to explain how a luminosity hundreds of times that of an entire galaxy could be emitted from a volume billions of times smaller. Of all proposed explanations, only one survived close scrutiny: the release of gravitational energy by matter falling towards a black hole. Even using an energy source as efficient as gravity, the black holes in active galactic nuclei would need to be enormous – millions or even billions of times more massive than the Sun – in order to produce the luminosities of quasars. To distinguish these black holes from the stellar-mass black holes left behind by supernova explosions, the term “supermassive black hole” was coined.