The life and achievements of Stephen Hawking, the British cosmologist who theorized the black hole evaporation, are outstanding from many points of view. A nice portrait of his exceptional personal life and of his scientific endeavors is provided by the recent movie “The Theory of Everything” (2014) by James Marsh. Of course, for a larger-than-life character like Hawking, a “Dig Deeper” section is far from enough to describe his entire life. We will then focus on a very limited aspect of his scientific life: his passion for scientific wagers, or bets. Two of them are very famous and remarkable, with one being directly related to our story of black hole evaporation.

In 1975 Stephen Hawking bet with his colleague (and recent Nobel Laureate for Physics) Kip Thorne that a very peculiar source, named Cygnus X-1, would turn out not to be a black hole. Cygnus X-1 was discovered in 1964 during the first observations of the X-ray sky. It is an extremely intense emitter of X-rays located in our own Galaxy. The system turned out to be composed of two elements: a very hot, super-giant star and a massive compact object, with a mass of about 14.8 times the mass of the Sun. Because this mass is significantly larger than the so-called Tolman–Oppenheimer–Volkoff limit, which provides an approximation for the maximum mass to avoid an infinite gravitational collapse, Cygnus X-1 was identified as a black hole. In 1990, 26 years after the discovery, Hawking conceded the bet, acknowledging that improved observations led to the conclusion that Cygnus X-1 was indeed the first black hole ever discovered. Diligently, Hawking paid Thorne a subscription to the magazine Penthouse (!).

In 1997, not satisfied with the previous wager, Stephen Hawking and Kip Thorne made a bet with John Preskill on a very peculiar consequence of the evaporation of black holes, nowadays known as the information paradox. Without going into details of this paradox, which will be the focus of another TED-Ed lesson, let’s summarize it as follows. Physics requires that a quantity, which we will call “information”, is always conserved. The evaporation of a black hole, instead, seems to suggest that information is lost during the process. While Hawking and Thorne bet that information is indeed lost in a black hole, Preskill bet that it must not, under any circumstances. The prize for the winner was, quite obviously, an encyclopedia of choice, definitely a book with plenty of information. Hawking ended up convincing himself that information is not lost and conceding the bet in 2004, while Thorne has not conceded yet. The original paper presenting Hawking’s argument to show that information is not lost can be found here. The encyclopedia of choice for the winner was: “Total Baseball, The Ultimate Baseball Encyclopedia” (!). To this date, the bet is still open and the information paradox unsolved.

If you are interested in calculating properties of any black hole of your choice, including its evaporation time, try out the Black Hole Calculator of the author of this TED-Ed lesson.

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