Worm holes are probably the most loved physical phenomena by sci if writers, because they simply are so extremely funky, they could offer time travel, travel into parallel universes, as well as just a short cut to places light years away in our own universe.
Let's talk about the short cuts; when travelling through a worm hole you get from one place to another in a shorter time then it would take you to travell, with the same speed as you travelled trough the wormhole, in 'the normal way'. But when you travel through a worm hole you don't really travel faster then light, since what you really do is to "cheat" space-time, so you're not really travelling through our familiar 3 dimensions but taking a short cut in the fourth dimension called hyperspace.
Imagine a circle and that the outline is the normal three dimensions that we move in. Now if you want to get from one side of the circle to the other and only move in the outline(our three dimensions) you would get to the other side much slower then some one that travelled straight through the circle(through the fourth dimension) and the one that moved through the circle didn't even have to move faster then you! The outline of the circel reprecents space time, which is the thing on which every thing in the universe exists. And it's when you warp this spacetime into little pockets on two different places, the place were you are and the place were you want to go, and then connect those two pockets to create a tunnel between the two different places that you get a wormhole. Also note that the universe doesn't have to be circular in order for wormholes to occur. It could be flat, bent or have any shape it want.

You could say that there's two different 'kinds' of worm holes. It has been suggested that if you fall into a black hole you come out of a white hole. Then there should be a wormhole connecting the black hole to the white hole. However this worm hole would be a oneway street, since you can only fall into a black hole and out of a white hole. They there are worm hole sollutions which work both ways, these are the once you usually talk about when talking about wormholes. And it's those which I will discuss here.

As you can see this means being able to travel to other galaxies which normally would normally take thousands of years even with a space craft moving at the speed of light(note that the crew on such a ship however wouldn't age a bit, but when they returned to earth it would be a couple of thousands of years later. Which isn't very funny if you think about it).
Also travel into other universes. Think of universes as bubbles, and there's no connection between them. So then, you could make a wormhole between them, as a gate from our universe to another. Pretty neat. But the neatest thing of all is the possibility of time travel, this I will discuss later.

The problem with wormholes is that they will collapse so fast that nothing will have time to travel through them. This is because if a body which is bending spacetime is removed, spacetime wont stay that bend anymore. It will smoothen out and become flat again. And the same thing with wormhole, the severe warping which causes them to exist isn't natural and spacetime wants to become flat again, so the wormhole collapse really fast. And if that wasn't enough, as soon as the wormhole opens, radiation will start to travel through it, and the wormholes gravity will accelerate it to very high speeds. And these particles will of course bombard the throat(the throat of the wormhole is the tunnel which you travel through) of the wormhole and make it collapse even faster, not to mention that anyone trying to travel through it will get a hell of a suntan. But there is a way to hold the wormhole open. Normal matter falls downward, then would it be possible to create a type of negative matter which falls upward, matter with anti-gravity? Theoreticly there is something called exotic matter which has this ability, although it has never been seen or created. This matter could be used to hold the wormhole up by pushing outward on the sides of its troat. Thereby keeping it stable. It should be said that energy is a relative thing and it doesn't have to be negative in all reference frames(reference frames is relativity and basically means different observers), it could be positive in some. But as seen from the wormholes reference frame it has to be negative.

So how do you create a wormhole? Well there are a number of ways.
It has been suggested that wormholes occur naturally, but only on very small scales. Normally on our macroscopically scales, space time is flat(if something is not bending it). But if you zoom in on it through the particle level and down to something called the Planck length(10-33cm) spacetime will get more and more severely warped and not smooth at all any more. So at this level were space time warps naturally and in a chaotic fashion, you should be able to find tiny natural wormholes. And then it would be possible to take one of them and enlarge it so that we could travel through.

Another possibility is to bend spacetime our self, already on macroscopically scales. Then move this little pocket which we just created to the place were we want to go, tear a hole into the space time in our pocket and the space time at the place we wanted to go to, and then "sew" the two rips together to create a tunnel(the worm hole). The problem is that the ripping part would create a singularity, and singularities are very nasty things, which have a tendency to first stretch people into spaghetti and then crush them into very small things. So we don't want a singularity but is there a way to create a wormhole without a singularity? Yes there is, the problem being that at the moment it is being created, time has to be distorted in all reference frames. Which means that at that particular moment you would be able to travel both forward and backward in time. So it's not certain that this works. But at least it brings me to my next topic:


First lets see what happen to a body which is accelerated.
According to relativity, acceleration makes time slow down. But in the same theory every observer measures their own time as going at a normal rate. So if we have to observers with the extremly personal names A and B. Then A stays still while B accelerates up to close the speed of light. Now because of the acceleration, Bs time has to slow down, but at the same time he can't notice any slowing of time. So instead as seen from As point of view(reference frame) Bs time does slow down and he starts to move slower. But as seen frome Bs reference fram As time speed up instead! And that 'time speed up' around B compencates for thé fact that he sees his own time as going at a normal rate, if things speed up around him it is just as if he slowed down himself. The important thing is that both their times are "in the smae proportions" to each others, which means that in both reference frames A's time does go faster then B's.

Now we'll see the strange way in which time is connected though a wormhole.
A and B both have one mouth of the worm hole each. Then B speeds away at close the speed of light. Now he can either look back and directly observe A or he can observe A through his wormhole. If he looks back he sees A moving away from him, and so he knows that the are in different reference frames and he also notice that As time moves faster. But if he looks through the wormhole he get's one, stable view of A. A doesn't move away from him, it is just af if B himself was standing right beside him. At first glance this might look like it's just a trick, but not according to relativity. According to relativity, if two things doesn't move relative to oneanother and if non of them exist in an externally applied gravitational field, then they are in the same reference frame. So this leads to a strange observation; through the worm hole A and B is in the same reference frame but otherwise they're in different!

So now if A goes through his mouth of the wormhole he then comes out into Bs mouth, and Bs reference frame where time has been speeding up around him due to acceleration so then A has moved forward in time. But then if B goes through his mouth of the wormhole he'll come out in As reference frame, and in that frame time was moving normal (that is, slower then at B) and so B will have travelled back in time.

Now, there were some problems of course. What if a beam of radiation travelled from the still standing mouth to the moving one and into it, then it would travelled back into time and exited the still standing mouth just at the same time it originally started to move toward the moving mouth. Then it would meet up with itself and create a beam which where twice as strong and then start to move toward the moving mouth again, travel back into time, meet up with itself and the beam would get stronger, and so on. So an infinitely strong radiation beam would emerge and this beam would be enough to collapse the wormhole.

A beam normally travels in a straight path through space. But at a wormhole the space is warped which means that the straight lines bend. Therefore the wormhole would spread the beam out, making it impossible for it to add up with itself and create a infinitely strong beam.

The upper narrow part is the wormholes throat, here the beams are parallel. Then at the lower part, the wormholes throat gets connected to the spacetime at its destination. And there, space is warped in such a manner that that the straight lines the beams followed spreads out.

But this isn't the end of the story. Another kind of radiation called electromagnetic quantum vacuum fluctuations, and those beams when studied, it was found that they don't get spread out. Or more precisely, they first get spread out like the radiation above. But later they gather together all by them self to create the original beam and it goes through the wormhole and start to pile up on itself to create a beam that will collapse the wormhole. So the gravitational effect couldn't save us here, but it was also found that the intensity of the beam behaved in such a way that, first its intensity raised to very high levels but then the intensity died out. This created a radiation peak. And it was found that the time span when the energy levels got high enough to collapse a wormhole, happened within a time span of 10-43 seconds, the Planck time. And the Planck time represents such a tiny time interval that time doesn't have any specific direction, actions taking place under this time doesn't exist. Therefore the dangerous peak disappears and the energy never gets so high as to collapse the wormhole.

There was of course another problem. Time is something relative, you can't just say 10-43 seconds, you have to define in what reference frame. It had been thought that 10-43s from the reference frame of someone at rest in the wormhole . But instead it should be 10-43s as seen from the beam of the electromagnetic quantum vacuum fluctuations. So the radiation beam could only be keept weak if the dangerous peak of energy happened in the time span of 10-43s from the radiations point of view. But a time span of 10-43s in the radiations reference frame looks like a time span of 10-95s from the reference frame of someone inside the wormhole which the previous calculations where based of. So then the time span would be to short to be able to cut of the radiation energy before it became to high.

The time span is to small so the energy has time to grow large enough to colloapse a wormhole

But nothing is certain. 10-95s might very well be enough to cut of the energy in time. In order to know for sure we need better understanding of something called quantum gravity, which we don't. But right now it seems to point toward a wormhole collapse.

And also the old what-happens-if-I-travel-back-in- time-and-kill-myself-before-I-can-travel-back-in-time question.
You can create an easier example of this paradox, one which doesn't involve complex things as humans, but billiard balls. Suppose a billiard ball travels in a straight path the into a wormhole(1.) which leads back into time, and it emerges through the other mouth(2.) about the same time it began.

Then it starts to travel in a path that crosses its old one. And since it has travelled back into time it now is timed so that it collides with itself. The question is: Does it knock itself out of the way so that it can't enter the first mouth.

Here the ball travels on a path that it knocks itself so to miss the wormhole and therfore it couldn't have entered the first mouth and so it couldn't have travelled back in time and have knocked itself. And a paradox is created.

or can it still travel into the first mouth.

Here the billiard ball follows another path through space and just nudging itself so that its path change a bit but it still travels into the first mouth and no paradox arises.

Both of the balls paths it has when it exits the second mouth on the pictures above are compatible with calculations, which means that they can happen. And not just them, there are a infinite number of paths which makes the ball miss the first mouth and creating a paradox. And a infinite number which doesn't knock it so severe as to create a paradox. Both are possible, but if there where a mechanism which forbade the paths which creates a paradox from exist and only let the ones which lets the ball go into the first mouth exist. Then there wouldn't be any problem with time travel, time would be 'self consistent', meaning that time wouldn't create any paradoxes. Then of course the same thing has to apply for humans. So you can't go back and kill yourself. But for the moment there are no mechanism to forbidd certain paths.

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