Wondered into being by: Andrew Hamilton.... the universe... God... I don't know.
Wonderspan: 6 min
To experience this wonder at its best: Click 'full screen' - there's no sound this time.
Hello, so what's a black hole?
If you’re not talking about the government’s finances, or indeed the nation’s finances or perhaps the entire global economic system founded on debt, or being stuck for an hour in a Xenakis concert, or the few moments’ silence after one of your dad’s jokes, or even simply a hole in the road after dark, then a black hole is a very large mass compressed into an infinitely small point of infinite density. Just think of the full-stop at the end of this sentence, albeit vanishingly smaller, containing the mass of a few of our suns.
Wow. Where can I get one?
Just wait for a large star to collapse and it’s yours, but I wouldn’t go near it if I were you.
Oh?
Well, pack a few suns into an infinitely small dot and it still has the gravity of a few suns. That’s OK if you’re a few millions miles away but if you get close you’ll start to get sucked in. As you get near you’ll turn into spaghetti; immense gravitational forces will compress you into the infinitesimal infinitely dense dot and there you’ll stay forever.
Far out, man. So, stay back and watch, yeah?
Yes, and be seriously freaked out because it gets a lot weirder. Around the tiny dot is a large black sphere of nothingness of a certain size – about 20 miles across would not be unusual. In fact, this black sphere is not a sphere at all, it’s just a hole in your field of view – an absolute absence of any light at all, just blackness, so you can’t even see the tiny dot in the middle.
Woah...
Yes, woah. And that’s because no light or anything else that gets anywhere near a black hole can escape from its gravitational pull, so there’s a big black... well... hole, where you just don’t see anything in space.
No way.
Way. And freakier still: because gravity deforms space-time and the gravity near a black hole is extremely powerful, all the light travelling around the black hole (towards us from the other side, for example) gets skewed; it doesn’t travel in a straight line. In fact it gets very curvy. If we looked towards a black hole, the view of the universe around it would look as if we were gazing through a zoom lens of a camera with a black hole in the middle of it. Here’s an artist’s impression of a black hole passing in front of a galaxy, for example: http://upload.wikimedia.org/wikipedia/commons/d/d6/BlackHole_Lensing.gif
No, stop it.
I would, but it’s true. And if that’s not weird and wonderful enough for you then get this: as anything approaches a black hole – a horse, say – it appears (from our safe distance) to slow down, turn redder and dimmer, and come to a stop as it takes an infinite time to reach it.
That’s insane.
But were we riding on the horse ourselves then time would not slow down in this way at all and we’d just get turned into spaghetti, as above.
Crikey. Glue.
As far as the horse is concerned, yes. It’s because the tiny dot’s gravity is playing havoc with time itself.
So let’s say we did want to get a bit nearer. I’ll go first.
Take my horse, I’ll wait here,. You’ll know something’s up when you feel intense tidal forces pulling you apart. If you survive that, although you won’t, you’ll get some really crazy stuff to watch.
Like a light show.
Like an ever-darkening show. First, you’ll reach a boundary quite a long way outside the black hole called the photon sphere. At that point, everything will go dark in front and to the side of you; this is because all the light particles around you are getting pulled into the black hole before they can reach you. So you won’t see any stars except for those behind you. By this time you haven’t got a hope of ever coming back so you might as well enjoy the ride.
Next, you’ll reach the boundary of the black hole itself, called the event horizon. I’ll be back here watching you turn red and take an infinite time to reach it but you’ll just zoom across it without even noticing it, hurtling towards nothing you can see until you – or what’s left of you – hit the infinitely small centre of the black hole. But you’ll be dead spaghetti long before that.
Even so, you’ll add a little bit of mass to the black hole and if I’m still paying attention I’ll notice the black absence in front of me get a tiny bit bigger, although of course from my perspective, because of the effect of gravity on time, you still won’t have reached it yet.
So I just have to wait for a star to collapse to see all this?
If it’s a big enough star. Our sun is not big enough. You need something bigger – let’s say 20 times the mass of the sun – but there are plenty of those around.
Why’s that?
What an obliging questioner you are.
That’s because you’re me.
Sssh, don’t tell the others. Well, it’s complicated but I am glad you asked. During their working life, so to speak, stars are on fire and because of their heat their matter is expanded. At the same time, all the particles of matter in the star exert a gravitational force on each other, which holds the gas together and stops its heat from blowing it all into space. So, you end up with a big ball of fire held in a more-or-less steady state between the heat pushing everything out and the gravity pulling everything in. But after a few hundred million years the star runs out of fuel and the fire starts to go out, so the star’s matter starts to collapse in on itself and then it’s showtime.
Great.
Hm, you’re mocking me now. Exactly what kind of show depends on what the star is made of, how big it is, how much mass it has and so on. As most stars run out of fuel and start to collapse under their own gravity, the rapidly increasing pressure releases a huge amount of energy. A diesel engine works in the same way – when the cylinder compresses the air in the cylinder, it heats up and ignites the fuel. But let's focus on stars for now. So this energy reaches a critical point, at which the collapsing star suddenly explodes spectacularly outwards in a supernova that lasts a few weeks and emits more light in this time than our own sun can produce in its entire lifetime. From a few thousand light years away it can look quite pretty, leaving behind a nebula and what appears to us like a new, very bright star in the night sky. Here’s one that popped just last year: http://www.telegraph.co.uk/science/space/8748876/Massive-supernova-visible-from-Earth.html.
And that’s the end of that. I’m pretty blown away myself. Can I go now?
On the contrary. Even though the supernova blasts most of the star’s matter away, about a quarter remains. This matter is still subject to a large gravitational force and collapses still further, which causes more crazy stuff to happen. The matter at the star’s core becomes denser and denser as its own gravity pulls it inwards. If our star is a lucky one then the energy inside its atoms will eventually stop the matter from condensing. In this case, it will become a neutron star, which is an extremely dense ball of matter. In that case, our star, which started off with the mass of about 20 suns, would be about 36 miles across – that’s the mass of five suns crushed into a ball the size of London and spinning (in some cases) at up to a few hundred times a second. Again – stay behind the yellow line. In fact, it’s quite likely to be in a cluster of similar stars, which will be dancing around each other doing grand turns, swing outs and swing-your-partners, sometimes swapping companions to pick up others.
Dancing stars!
Behind the yellow line, please. Yep, stars actually dancing. Here’s an artist’s impression of two neutron stars spinning in decaying orbits until they collide with a very satisfying flash-bang-wollop at the end: http://upload.wikimedia.org/wikipedia/commons/4/44/Neutron_star_collision.ogv
OK, and if our star is unlucky…?
If our star doesn’t lose so much of its matter in the supernova – if it keeps, say, about 10 times its mass, then it will do something else. Under the pressure of it own gravity, its atomic particles will collapse completely and, with nothing left to hold it up, it will collapse entirely to an infinitesimal point. And there’s your black hole.
So that’s that.
Not so fast. Did you know can build your own black hole as well.
Get off!
No, really. All you need to do is gather a lot of stuff and put it in one place.
Like how much stuff? Everything from the attic?
Yes, about that much, if you have junk up there with mass equivalent to a few billion suns. Otherwise no. But once you can build up that much stuff, the gravity-to-size ratio of your pile of stuff exceeds a certain point and an event horizon (the black absence in space) builds up around the huge object. We have one of these supermassive black holes, as they’re called, at the centre of our galaxy.
Phew!
And then there are micro black holes, which spontaneously appear and disappear. This is the sort of black hole they that the Large Hadron Collider could theoretically produce, but I wouldn’t worry too much about those because...
Time to introduce this Monday’s wonder, I think
Ah, yes. So have a look at Andrew Hamilton’s simulation calculated using a supercomputer (and note his slightly manic look, as if he went into a black hole just last week and somehow escaped):
In the simulation you’re actually travelling in a decaying orbit around a black hole, like a fly in a whirlpool of water when you take the plug out of the bath. As you approach, you can clearly see the photon sphere (where most of the strange lensing is happening) and the event horizon (the black absence). The film ends at the point you reach the event horizon, which isn’t really a thing, it’s just the point at which no light can escape from the gravitational pull of the singularity.
Luckily, Andrew Hamilton has made a film of the inside, as well, complete with 'blinding flash of light'. Keep your seatbelts fastened until we come to a complete stop:
Extra…
Video of from the exterior of the space shuttle as it takes off, traverses the atmosphere and reaches space:
And finally... I should have said that last week's Imagining the Tenth Dimension video was suggested by bro Andy G - thanks Andy.
www.waysofloving.com
