What is in a black hole ?
Answers:
Once you cross the event horizon of a black hole there is no going back. You have essentially crossed into a different universe. or a part of this universe that is permanently cut off from the rest of it.
But, crossing the event horizon is not quite the end of your journey - in a large enough black hole you can cross this boundry without doing any harm to yourself, and it can still take millions (or more) years before you reach the black hole - the only problem is that you can never stay still, and you will always be getting closer to the centre. You can also never send signals back outside the hole (because light is also dragged towards the middle of the hole.)
But what's in the centre? The answer is 'a singularity.' This is a point where spacetime has infinite curvature. Still no help? Well. it's hard to describe, because we don't have a good enough theory of quantum gravity - quantum theory breaks down when things become very big, and gravitatinoal theories (ie relativity) breaks down when things become very small. A singularity is a lot of mass squashed up into a minute volume - so neither of our two best theories of the universe are much help.
But. if you want a mental image of a singularity, think of an awful lot of stuff squashed up so small and so dense that it rips a hhole in our universe.
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