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Friday, February 6, 2009

Why Black Holes Are Astonishing

FROM J. RICHARD GOTT, COSMOLOGIST AND PROFESSOR OF ASTROPHYSICAL SCIENCES AT PRINCETON UNIVERSITY: As I wrote in my book Time Travel in Einstein’s Universe, a black hole is a hotel where you check in but you don’t check out. Once you cross the event horizon surrounding the black hole, you have crossed a point of no return, and you cannot get back out. The escape velocity at the event horizon is equal to the speed of light and, since nothing can travel faster than the speed of light, nothing that has gone in can get out. If you fall into a non-rotating black hole, tidal forces will tear you apart as you approach the singularity in the center. But your time of torture is brief. The time from when the tidal forces start to hurt you until you are completely shredded is only 0.08 of a second. That’s independent of the mass of the black hole.
The largest black hole we have found so far is one of 3 billion solar masses found in the center of the galaxy M87. From the time you cross its event horizon until you reach the center would be about 5.5 hours. For a rotating black hole—and the black hole in M87 is surely one of these—the situation is more complicated. The exact solution to Einstein’s equations of general relativity for a rotating black hole shows a ringlike singularity at the center that can be avoided and a region of time travel trapped inside the event horizon, where you could meet your future self and perhaps then pop out into another universe. But there is no coming back to visit your friends in this universe to brag about your adventures. It seems likely, however, that a singularity will develop, blocking your way to these interesting regions. If this singularity is weak, then you might be able to pass through it—like going over a speed bump—and get into the interesting time-travel region and escape to other universes. If the singularity is strong, you will be torn apart before being able to do any time travel. To understand which will occur, we may need to understand quantum gravity—how gravity behaves on microscopic scales. This is one of the reasons the problem is so interesting.
In 1974, Stephen Hawking showed that particles and anti-particles being created out of the vacuum in the vicinity of the event horizon (one falls in, whereas the other, just outside the event horizon, escapes) cause the black hole to radiate thermal radiation (now called Hawking radiation). This Hawking radiation causes the black hole to slowly lose mass and eventually evaporate completely. This is a very slow process. The black hole in M87 will take more than 1094 years (that’s a 1 with 94 zeros after it) to evaporate.

J. Richard Gott appears with Kip Thorne, Nima Arkani-Hamed, Juan Maldacena, Lee Smolin, and Leonard Susskind in "Why Black Holes Are Astonishing," the 22nd episode in the Closer to Truth: Cosmos, Consciousness, God TV series, hosted and created by Robert Lawrence Kuhn. The series airs Thursdays on the PBS HD network and many other PBS stations. Every Friday, participants will share their views on the previous day's episode.