What's inside a Black Hole?

Black holes are like gigantic vacuum cleaners -- the end point in the lives of really big stars. .Before black holes were ever discovered, people thought about the possibility of very heavy stars squashed into a relatively small volume such that their gravity is so strong not even light can escape their grip. James Bradley was an astronomer royal at the Observatory in Greenwich and one of the amazing things he did was to work out an accurate value for the speed of light. This value was needed to calculate how heavy a star must be to pull light in and swallow it. 300 years later, one of the astronomers at the Observatory would like to investigate a REAL black hole. Liz would like to know what would happen to her if she approached one and would particularly like to know what could be inside. The tricky bit is finding one to begin with. The black hole itself is invisible -- instead astronomers look for the powerful effects they exert on nearby matter. Sometimes this is seen as material swirling around a greedy black hole, but those without cosmic buffets close by are harder to spot. However, if we look towards the centre of our Milky Way we see lots of stars circling an invisible object. These stars have similar orbits to the planets in our Solar System which helps us work out that there is something in the middle -- a supermassive black hole. The one in our galaxy is nearly a quarter of a million trillion kilometres away so we are in no danger of being pulled in. So what would happen to Liz if she approached a black hole? Well near the boundary called the event horizon, she would feel the effects of the strong gravitational forces from this collapsed star. At the point of the event horizon Liz would have to travel at the speed of light to escape the gravity of the black hole... and this is impossible for her or any object to achieve! Also the strong gravitational field would slow down time. If we could see Liz's watch from our perspective here on Earth it would appear to us to tick a lot slower than our watches, although time for her would pass by normally. Eventually, after a few years sat outside the event horizon of the black hole, Liz would be shocked to know that thousands of years may have passed back home on Earth. It's as if the black hole is a time traveling machine, allowing Liz to travel into the future. Also, from our perspective we would see her become redder and redder until she disappeared -- the strong gravity would stretch the light to longer wavelength, low energy light that we can't see with our eyes. This would happen before she crossed the event horizon, but what would happen to Liz if she stepped in? Well if she fell in feet first, her feet would be pulled further along than her head so she would have to endure... spaghettification! She would be ripped apart and pulled in to the centre, the singularity - an exotic region of zero volume and infinite gravity... or is it? Liz wisely decides not to go in but thinks about the possibilities for what she would find inside. Some physicists believe if Liz jumped in, there would be no sign of her presence inside the black hole apart from a slight increase in its mass. The cosmologist Stephen Hawking believes that particles can tunnel out so that eventually over time black holes evaporate. We may not ever know if we can retrieve information from a black hole. Some physicists think that the event horizon acts as a veil blocking our view of the weird physics happening inside. Some theorise that the fabric of space is punctured at the singularity. American physicist Lee Smolin suggests that time ends inside a black hole and begins with the start of a new Universe, forever hidden from our view. A popular theory for the nature of a black hole brings in the idea of a ball of vibrating strings. These strings would keep information about the contents -- Liz's atoms would be shredded into their constituent strings and would merge with the existing orchestra inside... it's Liz but not as we know her! We might not ever know for sure what's inside a black hole, but it is fun thinking about what could be happening... and who knows, Liz may end up in a whole other Universe with its very own Greenwich Observatory!