• Question: what exactly is a black hole

    Asked by Emstar to Alice, Bose, Christian, Emma, Steve on 4 Mar 2016. This question was also asked by anonymous.
    • Photo: Alice Harpole

      Alice Harpole answered on 4 Mar 2016:

      A black hole is a region in space where gravity is so strong that not even light can escape.

      Einstein’s theory of general relativity tells us that all massive objects bend spacetime (the ‘fabric of the universe’), but that the more compact (or dense) an object is, the more bent spacetime will become. So while you, as an object possessing mass, will bend spacetime a little bit, it is nowhere near enough for us to notice. A black hole however is much more compact, such that it bends spacetime so much that anything that comes too close will be pulled into it.

    • Photo: Steve Marsden

      Steve Marsden answered on 7 Mar 2016:

      To fully understand black holes, you need first to understand general relativity and the bending of space-time. However, there is a simpler explanation which originated prior to relativity:

      Given a body in space (a star/planet/asteroid etc), you can calculate the escape velocity. This is the lowest speed something would have to be thrown from the surface to ultimately escape the body’s gravity. As you increase the mass of the planet, the escape velocity increases. If the planet has a sufficiently large mass, the escape velocity equals the speed of light. As nothing can travel quicker than the speed of light, nothing can ever escape from it, not even light.

      Light and matter can still fall into them, but can never come out. Because no light can escape them, we see them as black discs against a starry background.