Ask Glenn

What is a black hole, and what would happen if you fell into one?

It’s a super-massive object left behind after the death of some stars from which nothing – not even light – can escape. If you fell into one, you’d be ripped apart, fried or trapped forever. Possibly all three.

Loads of things stuffed into a black hole

Black holes are dead stars?

Yes, kind of. Although not all stars end up as black holes – some just burn out. Otherwise there would be black holes everywhere.

So how do you get one?

If a star is big enough – and we’re talking at least twenty to twenty-five times bigger than the Sun here – it will end its fiery life in a massive explosion called a supernova. When this happens, the outer shell of the star gets blown apart, but the inner core of it collapses in on itself. For some, the core forms a small, dense lump and the star ends its life there. For others, the core just keeps getting smaller and denser and smaller and denser. The pull of its gravity is so immense that anything within a certain distance gets sucked in and is trapped forever. Like a huge spherical, invisible whirlpool in Space.

Cool. But why are they invisible?

Because nothing within a certain distance of the black hole’s centre (a boundary line we call the event horizon), can get out of it – not even light. It was Albert Einstein who figured out, among other things, that light is actually bent round massive objects like stars by gravity. If the object is super-massive, like a black hole, then any light rays close enough to it will spiral right round into it and never escape its grasp. So it’s not just that it fails to make light (as stars do) – you can’t even see light bouncing off it (as we can with moons and planets). So it becomes invisible, and we can only perceive it as a ‘gap’ in Space. As you might imagine, these are not easy to spot!

So how do we know they’re even there?

By looking for tell-tale signs of light bent round them – light that passes close enough to be affected by the black hole’s gravity, but not so close that it gets sucked into it. Once you spot those, you can confirm the black hole is there by looking for X-rays. This is because things that are being sucked into the hole heat up as they spin round it, and when they get hot enough, they start to radiate X-rays. So if you spot a weird light-bending pattern in Space, and it’s also emitting X-rays, there’s a good chance it’s a black hole.

So why would all that nasty stuff happen if you fell in one?

Well, you’d be ripped apart by tidal forces caused by its super-strong gravity. Basically, if you jumped in feet-first, your feet and legs would be sucked in faster than your head and upper body (or vice versa if you dived in head-first). So your body would be stretched out lengthways until you snapped like a rubber band.

Yowch!

But let’s say you fired yourself into the black hole from a super-fast cannon. Maybe you could go fast enough to get to the middle before this happened. Even so, the chances are all those X-rays it chucks out would frazzle you before you made it through.

Eeeek! But what if you managed to survive that somehow? What would happen then?

Actually, no one knows for sure. We know you could never escape the black hole’s gravity, so you’d never come out of it again. So you could be trapped forever. But some scientists think it’s possible that black holes are ‘worm holes’ in Space: doorways to other points in the universe, or even other universes. But if you think about it, even if you made it in alive, it’s not clear how you’d escape the door on the other side, since this would be a black hole too.

Stretched, snapped, frazzled and possibly trapped forever, eh? Remind me not to try that...

Absolutely. There are probably safer ways to travel!