Black hole + white hole = wormhole
When you learn general relativity - and when they invent immortality, you'll all have time - one of the tricky parts is understanding how a black hole and a white hole combine to give a wormhole.
I still don't fully understand it. But this little movie by Andrew Hamilton helps. A bit.
We're quite sure black holes are real. White holes are purely theoretical. The point is this: if you have a solution of the equation of general relativity, and you 'play the movie backwards', switching the future and the past, you get another solution. And if you do this for a black hole, you get a 'white hole'.
Let's see what a white hole would be like.
In a real-world black hole, matter collapses and forms a singularity
, where according to the theory spacetime becomes infinitely curved - but in fact, we don't know what really happens. It would be fun to look at a singularity and find out what it's really like. But unfortunately, the singularity is surrounded by an event horizon
. That's an imaginary sphere, where if you enter this sphere you can never get back out. You're doomed to fall into the singularity.
You see, when you cross the event horizon, spacetime is so curved that the singularity is not in front of you. It's in your future
- so trying to avoid it is just like trying to avoid next Tuesday!
In summary, viewed from outside: a bunch of ordinary matter collapses into a small region called a black hole. From then on, nothing ever comes out of the black hole: stuff only falls in. (This is ignoring 'Hawking radiation'.)
Now let's play this movie backwards. We start with a small region called a 'white hole'. Nothing ever goes into this white hole: stuff only comes out. Then, eventually, the white hole explodes into a bunch of ordinary matter!
Astronomers have looked for white holes. They've never seen a thing like this. It's not so surprising: the laws of physics say that theoretically, a scrambled egg could be uncooked and stuck back into the shell - and we don't see that either. Some things seem to be more probable than their time-reversed versions.
But all this was just the warmup. When you take a class in general relativity, they make you find a solution of general relativity that describes a black hole. And the simplest solution doesn't describe a star collapsing and forming a black hole - that's complicated! The simplest solution describes a black hole that has always been there and always will be
. That's a lot simpler, because it doesn't change with time: it's perfectly 'static'. You can solve the equations with pencil and paper, not a supercomputer.
But now look! On the one hand, the time-reversed version of this perfectly unchanging thing is again perfect unchanging. On the other hand, the time-reversed version of a black hole should be a white hole.
So somehow this solution describes both a black hole and a white hole!
You can actually chop this solution into two parts, a black hole part and a white hole part. But they fit together.
If we take only the black hole part, we get a picture like this: a black hole that lasts forever. Stuff can fall though the event horizon, and then it's doomed to hit the singularity. Nothing can come out.
If we take only the white hole part, we get a picture like this: a white hole that lasts forever. Stuff can come out of the singularity and come out through the 'reverse event horizon'. But nothing can go into the reverse event horizon.
If we take both parts, that's where things get funny. Now there are two
singularities, one in the past and one in the future. But event horizon and the 'reverse event horizon' are the same thing! This horizon is a sphere. Stuff can fall from our universe into this sphere, hit the future singularity and disappear. But stuff can also appear at the past singularity, shoot out of this sphere and enter our universe!
I hope you sort of understood that. It's weird but it's actually logical and symmetrical. You could have guessed it, if you just kept cool and tried to dream up the most symmetrical possibility.
But here's the part you probably couldn't have guessed: this solution also describes two separate universes, connected by a wormhole!
That's the part that freaks me out. Needless to say, this is not
something anyone has ever seen. Right now it's just a solution of the equations that describe gravity. But still, I'd like to understand it.
The movie shows how it works. In the little picture at right:
• the up-down direction is 'time': the future is up, the past is down
• the left-right direction is one dimension of 'space'
• light can only move along diagonal lines
• matter can only move slower than light, meaning more vertically than horizontally
• the blue hyperbola at top is the future singularity
• the orange diagonal lines are the event horizon: if you cross this moving more vertically than horizontally you're doomed to hit the future singularity
• the blue hyperbola at bottom is the past singularity
• the red diagonal lines are the 'reverse event horizon': you can only cross this moving more vertically than horizontally if you came out of the past singularity
• the region to the right of the diagonal lines is 'our universe'
• the region to the left of the diagonal lines is the 'other universe'.
The slice moving up through this little picture shows one way to slice spacetime. That is, it shows the passage at time. The big movie shows that as this happens, the two universes meet and become connected by a wormhole - but then this wormhole snaps and the universes separate!
Unfortunately, you can't actually go from one universe to the other universe. Because you can only go slower than light, once you cross the event horizon you are doomed to hit the future singularity. But before you do, you can meet other doomed people who came from the other universe!
Unfortunately you can never report back and tell people outside the black hole that you met people from another universe... because signals can't get out across the horizon! Bummer.
I should explain this even more, but I'm getting tired, so why don't you just read Andrew Hamilton's description:http://casa.colorado.edu/~ajsh/schwm.html
and ask questions.... or discuss it amongst yourselves. :-)
I will delete crackpot comments and other random silly comments, so we can have a reasonable conversation.
By the way, while this gif is a great idea, it's pretty small and a bit scraggly. I think someone should create a better one and put it on Wikicommons. This stuff is so cool everyone should have a chance to learn about it! Even before they invent immortality.