BLACK HOLES Chaisson & McMillan, Ch. 22

Understanding the detailed properties of black holes requires the use of Einstein's General Theory of Relativity. However, understanding the basic concept underlying black holes is rather straightforward. Black Holes were predicted in the late 1700's by Mitchell (1783) and Laplace (1796) based on rather simple arguments.

A black hole is an object whose escape speed is equal to c, the speed of light ===> nothing, not even light cannot escape from a black hole. Black holes should appear black!

Do Black Holes Exist?

HST press release	HST press release

The above pictures show the active galaxy M87 (see Astronomy 123). We detect black holes through their effects on surrounding material. In M87, there apparently is a massive object near the center of the galaxy around which material orbits. We use the spectra of the orbiting material to infer the speed at which the material moves (see the figure on the right). This, when coupled with the size of the orbit (as can be deduced from the picture and the distance to M87) tells us the mass of the central object. Using measurements of how rapidly the light from M87 varies, we infer the size of the central object. The inferred mass and inferred size suggest that the central objects in many active galaxies must be black holes.
Stellar Mass Black Holes

Stellar mass black holes may be produced during the collapse of massive stars during supernova outbursts. Do we believe stellar mass black holes exist? Well, there are several strong candidates for stellar mass black holes.

• The first good candidate discovered was the x-ray binary source known as Cygnus X-1.
• Machos may also be candidates for stellar sized black holes (and perhaps not). Machos are based on the gravitational lensing effect: A Macho candidate:

  Ground-based and HST images of microlensing event ===> 6 MSun black hole

Time Travel
In the late 1940s, Kurt Goedel found a solution (a type of universe) to Einstein's General Theory of Relativity. For a universe filled with rotating fluid, an observer could move along interesting paths. In Goedel's solution, it was possible for a spaceship to take off and then return at an earlier time. The rotation allowed for these strange paths (closed time-like loops) to exist in the universe. This, although perhaps not quite time travel, is none the less amusing. Goedel's solution lent legitimacy to musings on the possibility of traveling through time.

Are there other ways people have considered to wander through time? Cosmic Censorship asserts that naked singularities cannot exist. Singularities must be cloaked by event horizons. This is very good for us in that the strange things which can happen near singularities are hidden from our Universe. Hmmm, suppose Cosmic Censorship wasn't true (it doesn't have to be true), what sorts of things might happen?