What is it like to fall into a black hole?

We put this question to Cambridge astrophysicist Zephyr Penoyre...

Zephyr - That is a very difficult question. What a black hole is, is an object that's just so heavy that light itself can't escape from it. Now, we're often told that light isn't affected by gravity. As far as things on Earth are true that is completely true. The amount of gravity, the amount of mass needed to actually affect light is huge. But black holes are remnants of massive stars that have collapsed into a very small space, and they're so dense, they're so heavy, that even light cannot escape. What actually happens when something passes, what we call the event horizon - the distance away from the actual body which light can't escape from - is very, very unclear and we may never know because it's impossible for anything that we send through, past that point, to ever come back because light - the fastest traveling thing - even that, cannot escape. Likely, it wouldn't be a particularly pleasant process. Even as you approached it, you get stretched and stretched, and stretched.

Chris - Would you feel that though? Would you actually feel yourself being pulled?

Zephyr - You'd be pulled by potentially, depending on how fast you approach the black hole. You might be pulled kind of string-thin, you might be torn.

Chris - Over what timescale would it take as you felt yourself - because literally, you're feeling a force on your legs. It would be like being put on a medieval rack then? Is that what you're saying? You would literally feel yourself being drawn out.

Zephyr - Yes. The quicker you go into the black hole, the better it is for you in the process of going in.

Chris - [Laughs] - That's one way of putting it...

Zephyr - The quicker you go in, the less stretching the time you're stretched.

Chris - So, a really agonising approach would be a really slow journey into the black hole? 

Zephyr - If you spiralled around for years and years, you will slowly get longer and longer.

Chris - And then what would happen?

Zephyr - Then that is where it gets really unclear because there are things like absence of a black hole, it doesn't seem like there's any force to stop the mass at the centre collapsing more and more. And so, you'd expect the density at the centre to go up and up, and up, faster and faster and faster, and become infinite. And that must be impossible, because if there was infinite density, it would have an infinite force and everything, everyone in universe will be sucked in. So we're very unclear on what exactly is stopping that happening in the centre of the black hole. There are theories even that time must slow down and stop at that centre, so that we can't reach this infinite density...