Once you're in there's no turning back

This is an artist's interpretation of a Black Hole

A black hole is basically a hole in space in which the gravitational field is so powerful that nothing can escape after having gone past the event horizon. The name came from John Wheeler who said: "Matter tells space-time how to curve, and curved space-time tells matter how to move." These holes are so dark that even light in unable to escape, making the interior invisible.

Black holes can be detected if they draw outside matter into it's funnel. An example of this would be drawing in gas from an orbiting star. The gas swirls inward, heating up to very high temperatures and emitting large amounts of radiation.

Presently black holes are understood by Einstein's theory of general relativity developed in 1916.

If an object is in a gravitational field it goes through a slowing down of time, this is called gravitational time dilation. This happens when the object approaches the event horizon and right before it is drawn inwards. To an observer, the object falling into the black hole appears to slow down, approaching but never quite reaching the event horizon. From the viewpoint of the falling object, far away objects may appear either blue or red-shifted. Light is blue-shifted by the gravity of the black hole, and is red-shifted by the velocity of the falling object.

There are three types of Black Holes:

• Stellar Black Holes- they are the end state of evolution of massive stars
• Galactic Black Holes- they are millions of solar mass and an example is the Milky Way Galaxy
  
• Primordial Black Holes- are tiny and may have been formed in the universe shortly after the Big Bang, although there is no clear evidence.
  

Black holes are hard to understand, as it is impossible to go inside one and observe.