Expo/Science & Industry/Spacetime Wrinkles

When a Black Hole Forms

What happens to a black hole after it forms? Does it vibrate? Radiate? Lose mass? Grow? Shrink?

Partial solutions of the Einstein equations point to two possible outcomes:

• A non-rotating, spherically symmetric black hole, first postulated by Schwarzschild.
• A rotating, spherical black hole, predicted in 1964 by the New Zealand mathematician Roy Kerr.

These two types of black holes have become known as Schwarzschild and Kerr black holes, respectively. Both types of black holes are "stationary" in that they do not change in time, unless they are disturbed in some way. As such, they are among the simple st objects known in General Relativity. They can be completely described in terms of just 2 numbers: their mass M and their angular momentum J.

Theoretically, black holes may also possess electric charge, Q, but it would quickly attract enough charge of the opposite sign. The net result is that any "realistic" or astrophysical black hole would tend to exhibit zero charge. This simplicity of black holes is summed up in the saying "black holes have no hair," meaning that, apart from its mass and momentum, there is no other characteristic (or "hair") that a black hole can exhibit.

(But things may not be quite so simple. Yes -- you've guessed it -- there's more to this story. To explore it, though, is beyond the scope of this exhibit.)

However, both the Schwarzschild and Kerr black holes represent end states. Their formation may result from various processes, all of them quite complicated. When a "real" black hole forms from, say, the collapse of a very mass ive star, or when a black hole is disturbed by, say, another black hole spiralling into it, the resulting dynamics cause disturbances in spacetime that should lead to the generation of gravitational waves.

By numerically solving the Einstein equations on powerful computers, scientists have been able to simulate the gravitational waves emitted by perturbed or interacting black holes. When visualized in movies generated by advan ced computer graphics, the unfolding wave patterns are not only intriguing but strikingly beautiful.

By emitting gravitational waves, non-stationary black holes lose energy, eventually become stationary and cease to radiate in this manner. In other words, they "decay" into stationary black holes, namely holes that are perfectly spherical or whose rotatio n is perfectly uniform. According to Einstein's Theory of General Relativity, such objects cannot emit gravitational waves.