gravity - Negative Energy and Wormholes

Apparently to create wormholes you need negative energy/matter. Say you had negative matter/energy, how would it be applied towards making a wormhole?

Answer

There are two related but distinct questions:

1. 
   

   how do you keep a wormhole stable?

   
   


2. 
   

   how do you make the wormhole in the first place?

   
   
   

Courtesy of Matt Visser we can give one answer to the first question. Matt's example is to make the wormhole cube shaped, and in that case all you need to do is construct a cube from string i.e. the twelve edges of the cube are made from string. However the string would have to have a negative tension, and indeed it would have to have the ridiculously high negative tension of $−1.52 \times 10^{43}$ Joules/metre. This is where your exotic matter comes in since the tension in any string made from normal matter would always be positive.

The second question is harder. Matt's analysis applies to a time independant wormhole, i.e. one that has existed for an infinite time. Constructing your cube of exotic string would warp spacetime in the manner required for a wormhole, but calculating what happens as you tie the strings into a cube is probably impossible at present.

Response to comment:

This is going to be a bit hard to explain, but the space inside the cube doesn't exist. It isn't part of the manifold on which the universe exists. If you travelled towards the cube you wouldn't hit anything - you'd just keep going without feeling anything as you past where it's wall is, but now you'd be travelling in the other region of spacetime on the other side of the wormhole.

Re your comment it doesn't sound like there's a lot of control, the wormhole Matt describes is not the same as the sort of wormhole Sci-Fi writers use to allow interstellar travel. As far as I know there is no theoretical support for the interstellar travel type wormhole. The wormhole Matt describes connects two regions of spacetime but makes no statement about the global topology, so the region of spacetime the other side of the wormhole need not be, and almost certainly isn't, some distant region of the universe around us. The wormhole does not allow FTL travel to e.g. Alpha Centauri. It just allows travel (at up to the speed of light) to the new region of spacetime on the other side of the wormhole.