Black Holes: From Speculation to Observation

In 1964, Riccardo Giacconi and his team discovered Cygnus X-1, a source in the constellation known as the Swan that is generally accepted as the first black hole detected. Cygnus X-1 has a mass nine times that of the Sun, with an implied event horizon size of 16 miles, smaller than a city. But in the early 1960s, X-ray astronomy was an emerging science,39 and a singularity was then considered purely theoretical.

Science writer Dennis Overbye recounts how physicist John Wheeler, based on his students’ prompting, eventually came to ac­cept the probabilities of the cataclysmic collapse of a star:

In 1939, J. Robert Oppenheimer, who would later be a leader in the Manhattan Project, and a student, Hartland Snyder, suggested that Einstein’s equations had made an apocalyptic prediction. A dead star of sufficient mass could collapse into a heap so dense that light could not even escape from it. The star would collapse forever while space­time wrapped around it like a dark cloak. At the center, space would be infinitely curved and matter infinitely dense, an apparent absurdity known as a singularity.

Dr. Wheeler at first resisted this conclusion, leading to a confron­tation with Dr. Oppenheimer at a conference in Belgium in 1958, in which Dr. Wheeler said that the collapse theory “does not give an ac­ceptable answer” to the fate of matter in such a star.

However, by 1967, during a presentation in New York, Wheeler had reconsidered and, “seizing on a suggestion shouted from the audience, hit on the name ‘black hole’ to dramatize this dire pos­sibility for a star and for physics.”40 Mitchell Begelman and Martin Rees likewise note that it was Wheeler who first officially used the term “black hole” in reference to a collapsed star and comment that “the name immediately caught on.” They write, “Here was something infinitely more mysterious, and perhaps much more sin­ister as well—a place where any form of matter or energy could enter, lose its identity, and be lost forever to the Universe.”41 In Black Holes and Time Warps, Caltech astrophysicist Kip Thorne similarly observes: “Of all the conceptions of the human mind, from unicorns to gargoyles to the hydrogen bomb, the most fan­tastic, perhaps, is the black hole. . . a hole with a gravitational force so strong that even light is caught and held in its grip; a hole that curves space and warps time.”42

In 1970, Stephen Hawking developed theorems that demon­strated black holes would be a standard phenomenon of Einstein’s general relativity and of the universe. By the mid-1970s, not sur­prisingly, black holes were resonating across the realms of fiction, popular music, art, and film. The Canadian rock group Rush re­leased “Cygnus X-1, Book 1: The Voyage” on their album A Fare­well to Kings (1977). It’s an extended rock composition written in homage to the X-ray emission source that Riccardo Giacconi discovered in Cygnus. The lyrics read in part:

In the constellation of Cygnus There lurks a mysterious, invisible force The Black Hole Of Cygnus X-1[. . . . ]


To telescopic eye Infinity

The star that would not die All who dare To cross her course Are swallowed by A fearsome force. . . .

I set a course just east of Lyra And northwest of Pegasus Flew into the light of Deneb Sailed across the Milky Way[. . . .]

The X-ray is her siren song My ship cannot resist her long Nearer to my deadly goal Until the Black Hole—

Gains control. . . .

Sound and fury Drowns my heart Every nerve Is torn apart. . . .43

Located approximately 6,000 light-years from Earth, Cygnus X-1 is now thought to be a high-mass X-ray binary system in which gaseous material from the blue supergiant star HDE 226868 is accreting onto a nearby black hole created by the collapse of a star nine times the mass of our Sun.44 The system emits powerful X-ray radiation as gas from the blue supergiant siphons onto, and is heated by, the black hole. Interest in Cygnus X-1 was further sparked by a public and congenial bet between Stephen Hawking and Kip Thorne regarding the physics of the object. By 1990, hav­ing argued that Cygnus X-1 was not a black hole, Hawking admit­ted to losing the wager.45