It’s basically impossible to give a short, succinct description of black holes that is also in any way even vaguely correct. They are so completely different from anything we encounter in daily life that even metaphors fail.
So the best way to think of it, for the layperson just going about life wanting to be essentially educated as to how the universe works, is to imagine a very large, very old star. This star has used up all its fusion “fuel,” if you will, and will soon collapse, exploding spectacularly in an apocalyptic cataclysm of radiation that will, briefly, outshine its whole galaxy.
Inside the very core of that star, there’s, well, more star. The end hasn’t come yet; the star is still being a star for the moment, so the interior is still star. But it’s fantastically dense. In a minute, when the star explodes, it’s going to become denser still. Because you see, the thing that explodes when a star goes supernova is the outside of the star. Imagine a bowling ball coated in cake icing … made of plastique explosive … and wired to a timer … okay this metaphor isn’t very good. But the point is, it’s the outer layer of the star that’s actually going to do the exploding here in a minute.
So let’s wait.
Okay, that was a supernova. Nice one, right? It happened kind of fast, so you might’ve missed it if you weren’t watching carefully: The interior of the star reached the point where it no longer had sufficient pressure to hold the outer layers of the star up, so it essentially collapsed. The outer layer, meanwhile, began to drop like a rock, because all the pressure that had been supporting it suddenly vanished. This caused the star’s outer layer to heat up unbelievably quickly, which caused lots of violently interesting things to happen. There was a stupendous outrushing of radiation, first, and matter shortly behind it – helium and lithium ions mostly, and some other stuff. But what you couldn’t see was that that same explosion also went inward.
A spherically symmetric shockwave propagated inward, down toward the core of the star, compressing the already hellishly dense matter that was there until … well, the world came to an end.
There is a limit to how much stuff can occupy a given volume of space. This is called the Bekenstein limit, after the boffin who figured it out, and I won’t elaborate on it here because maths. But suffice to say, there’s a limit.
When that limit is reached – and in this case, due to the simply incomprehensible pressure exerted by that inward-focused shockwave, it was – the volume in question simply goes away. Poof. It ceases to exist. If you like, you can imagine God Almighty being offended by the ambitious matter and willing it out of existence in an instant. Just pop. Gone. Forever.
What’s left, in its place, is a wee tiny … not. An isn’t. Part was, part isn’t, part won’t-ever-be, in the shape of a perfect sphere that doesn’t exist.
The boundary between where that sphere isn’t and where the rest of the universe still continues to be is called the event horizon. The event horizon is not a surface. It’s not an anything. It’s an isn’t. But it behaves like a surface in most respects. A perfect, impervious, impenetrable surface. If you threw something at it, that something would shatter into its component bits – and I don’t mean chunks, or even dust, or even atoms, or even protons and electrons. I mean individual discrete field quanta. And those field quanta would spray off into space in all directions like bits of strawberry out of a liquidizer that has been unwisely started with the lid off.
That’s what happens to all the stuff that was in the centre of that star, as well. Eventually, it’ll be sprayed out into the universe in the most fundamental form possible, as little individual quanta of energy and momentum and spin and charge.
Except due to a combination of relativity and thermodynamics, you will not actually witness that happening. Because the process takes a while. For a typical stellar black hole right now? The process will take on the order of a trillion years. So don’t wait up, is what I’m saying here.
So black holes? They have no insides. They aren’t. That’s their defining characteristic, qualitatively speaking: They aren’t. There’s nothing in them, because there’s no in, because they aren’t. ]
There’s stuff which is, even right this very moment as we sit here talking about it, in the process of scattering off black holes.
You can’t see, observe, detect or interact with any of that stuff, but we know it’s there, because it has to be. And we know eventually it’ll spray out into the universe, first and for hundreds of billions of years as photons – a few a day – with such long wavelengths that they can barely be said to exist at all.
Later, hundreds of millions of millennia after we, our species and our solar system have long since ceased to exist, black holes will start emitting radiation we’d recognize as radio waves.
Then, in an accelerating process, all the way up through the electromagnetic spectrum until finally, in the last tiny fraction of a second before the black hole evaporates entirely, the potential energy available will be in the hundreds-of-electronvolts range, and we’ll get the first electrons and antielectrons, then a few protons, and then a cataclysmic burst of short-lived exotic particles that lasts hardly longer than a single instant, then the black hole will have ceased to not exist.