Black holes

Black holes were ``discovered'' theoretically soon after Einstein published his GR by a German astronomer Karl Schwarzschild (in trenches on a German-Russian front during the WWI).

A nonrotating black hole consists of a singularity (point, where the gravitational force becomes infinite), surrounded by an event horizon.

The singularity is space-like, i.e. nothing can avoid it.

The event horizon has a property (which is essentially its definition) that light emitted from inside it cannot reach any point outside it.

The ``radius'' of horizon, which is also called
Schwarzschild radius is proportional to the mass of the black hole. If the Sun became a black hole (in reality it will not), its Schwarzschild radius will be 3 km (1.8 miles).

The Sun is 300,000 times more massive than the Earth. Thus, if the Earth became the black hole, its Schwarzschild radius would be:

A

3 km

B

1 cm

C

1.8 miles

D

9 km

E

3 cm

Properties of black holes

Let O be an observer well outside a black hole, and F an observer falling into a black hole.

• O will never see F crossing the horizon.
• F will reach singularity (and death) in a finite time.
• If F emits light at equal intervals, O will see the last flash, which will be the last flash F emitted before crossing the horizon.
• O will see F squeezing along its path.
• F will feel very strong tidal forces that will stretch him along its path and may rip him apart.
• No material object within three Schwarzschild radii can freely orbit a black hole.
• Light can orbit a black hole at 1.5 Schwarzschild radii.