Rotating black holes

A rotating black hole is often called a Kerr black hole in honor of Roy Kerr from New Zealand.

In addition to the even horizon, a Kerr black hole has an ergosphere, a region around it where no object or light can at rest with respect to distant stars: it has to rotate in the same direction as the black hole itself! This is called ``dragging of inertial frames'', i.e. all inertial frames rotate around the black hole inside the ergosphere.

In the Kerr black hole the singularity is time-like, i.e. a falling observer can bypass it and exit into a different universe from a white hole.

Two reservations:

• White holes are not observed to exist, and no mechanism is known for them to form.
• Time-like singularities are unstable. Any object falling into them will cause a formation of a space-like singularity.

Black holes have no hair!

In GR there is a special no-hair theorem that states that isolated black holes can be characterized only by three numbers: their mass, their electrical charge, and their angular momentum (i.e. how fast they rotate). All other characteristics of matter which formed a black hole are totally forgotten.

For example, if we ``construct'' two black holes with the same masses, electrical charges, and angular momenta, but will make the first black hole out of ordinary matter, and the second one out of anti-matter, they would be completely indistinguishable - none of the special quantum ``charges'' (baryonic, leptonic, ``color'', ``charm'', etc) is conserved in the black hole.

Hawking radiation

Quantum Mechanics makes a startling prediction about the black holes: if in ordinary (so called ``classic'') mechanics noting can leave a black hole, in Quantum Mechanics black holes can evaporate!

This process is very very slow: a black hole with the mass of the Sun will evaporate in 10⁶⁵ years! This is unimaginably slow, slower than the thermal death of the universe.

Black holes evaporate by emitting radiation, which is called Hawking radiation, in honor of British astrophycist Stephen Hawking, who ``discovered'' it theoretically in 1973.

Do black holes exist?

There is no direct observational proof that black holes exist. Indirect clues:

• A massive star, when it exhausts the supply of nuclear fuel, has nowhere to go (according to modern physics) as to collapse into a black hole.
• Gas around a black hole will fall onto it and form an accretion disk. This disk will shine very bright in a wide range of frequencies (radio, visible light, but especially in X-rays). These X-ray sources are observed everywhere in the universe. They thought to be powered by black holes. (And, thus, real black holes are not black at all, but very very bright!)
• Centers of most of galaxies contain many times more mass than can be counted in all stars there in very small regions of space.

Journeys into black holes

There are two web-sites where you can experience a virtual journey into a black hole. They both a linked to the course homepage.

``Virtual Trips to Black Holes and Neutron Stars'' by Robert Nemiroff

``Falling Into a Black Hole'' by Andrew Hamilton