Studies leading to black holes began in the 1930s, when Hindu physicist Subrahamanyan Chandrasekhar and Russian Lev Landau demonstrated that in the Newtonian theory of gravitation, a cold body with a mass superior to 1.5 times that of the Sun could not support the pressure produced by gravity. That result led scientists to ask what general relativity predicted for the same situation. In 1939, Robert Oppenheimer and two of his collaborators (Volkoff and Snyder) demonstrated that a star with that mass would collapse until it was reduced to a singularity; that is, to a point with a volume of zero and an infinite density.
Oppenheimer and his collaborators’ work received little attention or credence and it was ignored for more than two decades. British mathematician and physicist Roger Penrose and physicist Stephen Hawking applied powerful mathematic techniques to this question in the mid-nineteen sixties. They demonstrated that such singularities were inevitable when a star collapsed, providing certain conditions were met.
A couple of years after Penrose and Hawking published their first articles, the physics of space-time singularities became that of “black holes,” a felicitous term that has attracted considerable popular attention to this physical entity. The man responsible for this apparently insignificant terminological revolution was United States physicist John A. Wheeler, who was suggested that name by somebody in the audience during a conference in the fall of 1967.
While the history of black holes began with the physics work of Oppenheimer and his collaborators, for some years the field was dominated by purely mathematical studies like the previously mentioned ones by Penrose and Hawking. The underlying physical idea was that they must be very different from any other type of star, even though their origins were linked to them.
They would occur when, after exhausting its nuclear fuel, a very massive star began to contract irreversibly, due to gravitational force. A moment would thus arrive when it would form a region (called “horizon”) in which matter and radiation could only enter, without anything being able to get out, not even light (from whence the denomination, “black”). The larger such an object was, the more it would “eat”, and the more it ate, the bigger it would get. The center of a black hole is its point of collapse. According to general relativity, there, the matter that once made up the star is compressed and expulsed, apparently “out of existence.”
Clearly, “out of existence” is not an acceptable idea. However, there is a possible way out of such a paradoxical situation: the general theory of relativity is not compatible with quantum requirements, but clearly, when matter is compressed into a very reduced area, its behaviour will follow quantum rules. Thus, a true understanding of the physics of black holes calls for a quantum theory of gravitation (either by quantizing general relativity, or by constructing a new theory of gravitational interaction that can be quantized). At the present time, this has yet to be done, although some steps have been made in that direction, including one by Hawking himself, the grand guru of black holes. What is called “Hawking’s radiation”, which predicts that, due to quantum processes, black holes are not as black as we though, and are able to emit radiation.
As a result, we do not really know what those mysterious and attractive objects are. Do they, in fact, exist at all? The answer is yes. There are ever-greater indications that they do. It is now generally accepted that there are super-massive black holes at the center of those galaxies whose nucleus is more luminous that all the rest of the galaxy (about 1% of all galaxies in the Universe are that way). In over two hundred cases, it has been possible to indirectly determine the masses of those super black holes, but a direct determination has only been possible in a few cases. One of the latter is in our own Milky Way.
More information on black holes in the essay “The world after the Revolution: Physics in the Second Half of the Twentieth Century”, by José Manuel Sánchez Ron.