Detecting Black Holes

Black holes can be detected by the effects they produce.
First of all, outside the Schwarzschild radius, black holes will obey the same rules of motion as stars, planets and galaxies do. To identify a black hole as such, we would have to see a disk of gas and matter infalling at tremendous speeds. This gas would feel a tremendous pull towards the black hole; as matter rushes in, frictional collisions would produce heat. Collisions of high energy projectiles with gas would give off X and gamma rays. Other objects in the sky could produce such fireworks; therefore, one also needs to verify that the object behind this gravitational pull has a core mass large enough to be a black hole, i.e., 3 solar masses, as we discussed earlier.

The best convincing evidence of a black hole is from an X-ray binary system, such as Cygnus X-1. In this case, one of the companions of a star is a black hole, and its mass can be deduced from the wobble in the orbit of the companion.

Matter rushing into a black hole is subjected to great tidal forces. A tidal force occurs when there is a significant difference in the force at the top vs. the bottom of an object. For example, a space ship approaching a black hole head-on, would feel a much greater force at its nose than at its bow. Light emitted by the space ship would be very much red-shifted. This is called the gravitational red-shift, and is different from the Doppler red-shift. In the gravitational red-shift, photons lose energy as they try to escape the gravitational force.

Another result of the theory of general relativity is that clocks on board of a ship approaching a black hole would slow down; indeed, the ship would seem to take, as measured from a distant observer, an infinite amount of time to reach the black hole.