# Alice’s Adventures in Black Hole Land

There was once, according to the story, a little girl who fell down a rabbit hole and discovered a strange world where the normal physical rules of how our world works no longer applied. Today we are going to look at what might happen to her great, great granddaughter if she got bored with the more normal extreme sports like bungee jumping, white water rafting and  and free climbing, and decided to try  a new extreme sport, Accretion Disk Surfing. Here is her story.

First off, what is and accretion disk? Well contrary to SciFi films and stories, black holes do not suck everything into them. A black hole is not some kind of voracious monster, intent on destroying everything around it. Actually a black hole is nothing more than the mortal remains of a star, a star much bigger than our Sun, that blew itself apart in its spectacular death throes. I’d like to come back to supernovae at a later date as there is a lot to talk about. If the star is big enough then the core collapses down into a black hole. As it collapses it reaches a point where its gravity is so concentrated, and that’s the word to remember, not “so great” or “so colossal”, just concentrated, that its escape velocity is higher than the speed of light. Anything can become a black hole in theory, you just have to compress it enough. Here’s how to work it out. You take the mass of the object you want to turn into a black hole, the Earth, a star, your boss, and multiply it by the “Gravitational Constant”, (a constant is just a number that gets a sum to work), and multiply the whole lot by 2. You then divide by the speed of light multiplied by itself. For maths freaks it looks like this-

Quite a simple sum for something as incredible as a black hole.

So basically you’ve got the core of this dead star and this core has shrunk down to a size where its gravity, when you get close, is so concentrated that you would have to travel faster than light to get out, and as this guy

showed us, that ain’t allowed, so nothing, not even light gets out. So what happens to stuff that gets too close, you know stars, spaceships etc? Well not a lot really, until they get very close. At first all they feel is the gentle but insistent pull of gravity. No more, no less. So if Alice is a chicken, she’ll turn her spaceship around now, or just keep going straight ahead and fly past the black hole. This is kinda what that looks like-

But our Alice is no chicken, she wants the ultimate thrill and the ultimate selfie to post on her FB wall so she lets the black hole’s gravity pull her closer.

Now empty space isn’t really empty. Its full of gas and dust floating around, and there are stars and even planets wandering around and sometimes these get too close to a black hole and get pulled in. As they get closer they start to spiral in, their orbit getting closer and closer. As we know, as something spins in a smaller and smaller radius, it rotates faster and faster and this is also true of all the stuff a black hole traps. As all this stuff gets closer and moves faster, anything big gets pulled apart until there are just atoms left and these bump into each other as they hurtle round and round the black hole in ever decreasing circles. This creates friction and lots of it, and as we also know, friction creates heat and in the case of a black hole accretion disc temperatures can get extremely high. Oddly enough, it is the smaller, stellar mass black holes whose accretion discs reach the highest temperatures for the simple reason that the smaller the black hole, the smaller the radius of the accretion disk so the closer the atoms in it get to each other. So Alice’s space suit and surf board need to be highly heat resistant or it will be a very short ride! By the way, as the hot glowing matter reaches the black hole some of it gets shot out from the poles at near light speeds, forming jets that reach far out into space. In the case of a supermassive black hole at the heart of a galaxy, if a jet is pointing at us we call it a Quasar. This is short for “quasi stellar object” and when planet Earth and I were young, way back in the 1970’s, we didn’t know what these were. We could see that they were much smaller than galaxies but we could also see that they were some of the brightest yet most distant objects in the Universe, so how could something so small produce so much energy? Well now we know and it just shows how much energy there is in these jets.

So Alice parks her spaceship just outside the edge of the accretion disk, jumps on her surf board and away she goes! Well no, not really. Not at first. Unless her surfboard is rocket powered, it could take weeks or even years to reach the black hole. So let’s say her friend Bob gives her a push and she starts to ride the spiralling density waves in the disk. As she approaches the edge of the black hole, something strange happens, and this is where Einstein rears his head again. Relativity shows us that as we get closer to a source of gravity, our time seems to go slower as seen by someone further away. So Bob sees Alice getting closer and closer to the black hole, but moving slower and slower. Alice doesn’t experience this at all. For her time marches on as it always has. Finally, after an unbearably long time, Bob sees Alice grind to a complete halt and hang motionless in space, just outside the black hole.

Alice, on the other hand, is getting “Spaghettified”. This is what happens when gravity is so intense, so concentrated, that the gravity at Alice’s feet is much much stronger than at her head. This is also the case on Earth or whichever planet you live on, but the difference is so minute that it is effectively zero. With black holes it is different, painfully, nastily, messily different. Gravity at Alice’s feet is so much stronger than at her head, that it pulls her out into a long thin thread. Spaghettification!

As you can see, it’s not pretty. If she could survive, by activating her space suit’s “anti-spaghettification field”, then she would cross the black hole’s “Event Horizon”, the surface where you need to travel faster than light to get out. This is not a physical surface as such, more a kind of “Point of No Return” and from here on it is down down down, all the way to.. yeah well there is so much we don’t really know for sure about black holes. Most say that the core of the exploding star continues to collapse until it reaches infinite density in zero space. That is to say it no longer has any size dimensions. If you could fall into that then either you disappear from our Universe into maybe another Universe, somewhere else in our Universe, or just disappear completely, end of story, game over. A newer idea is that as the stellar core collapses, it reaches a size called the Planck size, a size which is the smallest size possible in the Universe. This is 1.6 * 10^-35 m, 10^20 times smaller then a proton. The idea is that nothing can be smaller than this size and when the collapsing core hits this limit, it bounces back out again in a matter of seconds, but due to the effects of relativity, this appears to the rest of the Universe to take millions or even billions of years.

So for a stellar mass black hole, Alice, if she survives the hight temperatures of the accretion disk and spaghettification. disappears into nothing, or another universe, or gets squirted out somewhere else in our Universe. If the Plank Star theory is correct then she will “meet” the Planck Star on its way out, rather like a bug “meets” the windscreen of a speeding car. SPLAT!  And that is not even including the, possible, “Firewall”, an area just inside the event horizon where, according to some researchers, all the matter and energy that falls into the black hole is torn apart and spread out and where temperatures go completely off the scale.

Things are a bit different if Alice chooses to surf into a supermassive black hole like then one at the centre of our Galaxy and, it seems,  most other galaxies. These can be billions of solar masses but because of their vast size, the transitions Alice would experience if she fell into one of these behemoths, would be much smoother and gentler than is the case with their smaller cousins. Alice could safely surf all the way to the singularity at the centre, assuming she didn’t get fried and shredded by a firewall. She probably wouldn’t even notice as she crossed the event horizon.

All this is speculation and even the experts don’t agree. What we do know about the inside of a black hole is that the laws of physics, as we understand them no longer work so we can’t just sit down and do some nice cosy equations based on our nice, familiar laws of physics. This has led to a battle between Stephen Hawking and Leonard Susskind that lasted for years. Basically, Hawking proved quite convincingly that when something fell into a black hole, its information, the “what it is” of the thing, is completely destroyed. But this contradicts one of the most important principles of physics which got Susskind and others riled. In the end the problem was, possibly, solved by the so called “Holographic Principle” but even that is the subject of discussion.

So really all we have is conjecture, until we get a breakthrough that reveals what really goes on inside a black hole. We can’t even take a picture of a black hole. We can infer the presence of one by the way stars orbit around it and from this work out the object’s mass and if the size proves to be less than the Schwarzchild radius for its mass, then we know we have a black hole. This is how we found the supermassive black hole at the centre of the Milky Way, anything that small that can deflect stars that much and can make them move at the extreme speeds we observe can only be a black hole. This is what the orbits of these stars look like

Astronomers are gathering radio telescopes to combine their images so that they can create an image of the outline of the black hole and hopefully it will be better than what we have so far

There is also a third type of black hole, the so called primordial black hole which are thought to have formed with the birth of the Universe and are tiny compared with stellar mass black holes. According to Stephen Hawking black holes radiate energy from just outside the event horizon and therefore slowly evaporate so it is not certain how many primordial black holes there should be out there now. There is also a problem with the growth of supermassive black holes. Our observations of the earliest galaxies seem to show that their central black holes grew far faster than if they had simply eaten surrounding stars and gas, too fast even if they grew through collisions with each other. We needed a missing link between stellar mass black holes and the huge monsters at the heart of galaxies and it was only recently that the existence of these was confirmed.

So we are slowly learning more and more about black holes but we still have a long way to go before we can claim to know for sure what really goes on inside a black hole, and as for accretion disc surfing, I think the Alices of this world will have to get their kicks elsewhere for a very long time to come.