Leonard Susskind is deeply worried about something that in all probability will never happen, at least not until long after the last star in the universe burns out. And he isn’t the only one who’s apprehensive. Susskind, a theoretical physicist at Stanford, and many of his prominent colleagues have lately been pondering the ultimate fate of black holes, those mysterious, light-swallowing chasms in space and time that form, physicists believe, when the cores of massive stars implode. In particular, Susskind and his associates have been considering the very last moments of a black hole’s existence because the death throes of these bizarre non- objects pose an enormous challenge to science.
We’ve encountered a major paradox that nobody knows how to reconcile, says Susskind.
That paradox has its roots in work done by famed British physicist (and Trekkie) Stephen Hawking. In 1974 Hawking wrote an essay titled Black Holes Aren’t Black, in which he surprised the physics community by claiming that some radiation could escape from black holes. This was completely contrary to everything physicists then knew about black holes. After all, the very name black hole reflected the certainty that nothing--not even light--could elude the object’s immense gravitational grasp. Moreover, said Hawking, the slow bleeding of radiation meant that a black hole could eventually evaporate completely, like a snowball in the sun, and vanish from the universe.
There’s not much chance that anyone will ever actually witness the demise of a black hole. Physicists believe that for a very long time to come, all black holes in the universe will continue to take in matter--gas, dust, and even whole stars--faster than they radiate it away. I once figured out how long it would take a black hole produced by a star to evaporate, says Susskind, but it was many times larger than the age of the universe.
Nevertheless, this vanishing act set in the inconceivably distant future keeps physicists like Susskind awake at night because of its thorny theoretical implications. The evaporation is the nasty point, says Susskind. That evaporation, as well as the peculiar nature of what has come to be called the Hawking radiation that black holes emit, seems to contradict one of the most fundamental principles of physics. All present physics is based very heavily on the assumption that you can recover the past from the present--in principle, if not always in practice, says Susskind. But black holes, much to the consternation of physicists, seem to break this rule.
