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We're not sure if wormholes can be held open, but we know they can exist, at least on quantum levels. I just watched a video where a physicist mentions that even if we could hold one open, sending matter into it should cause the wormhole to break down.

That left me wondering what exactly would happen to the matter as a wormhole collapsed.

This thought brings up some seemingly anomolous scenarios, like if some particle or even perhaps a string from string theory is perfectly symetrically placed between the two halfs of space where the wormhole collapses, could half of the particle or string be on one side of the collapse and the other half on the other side?

What happens to something inside a wormhole if it collapses? Theoretically obviously, since we dont know how to create / test them yet. What does the math say?

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This is actually happens in the most commonly studied metric in all of general relativity, the Schwarzschild metric. It's often said to describe a black hole, which is of course true. However, this black hole actually connects two universes by a wormhole. The reason you can't get from one universe to the other is because the length of the wormhole expands too quickly, so you can't actually get to the other side. As the wormhole gets longer and thinner, eventually it gets to a radius of $0$ right in the middle, and pinches off. This is the singularity. The singularity then advances, eating up the now broken wormhole on both ends, until everything inside the black hole has gone into the singularity.

enter image description here

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  • $\begingroup$ The Schwarzschild metric is static. It doesn't evolve in time. The singularity doesn't "advance". $\endgroup$
    – Paul T.
    Commented Apr 24, 2021 at 2:20
  • $\begingroup$ Sure it does. It may be static in the standard Schwarzschild $t$ coordinate, but remember that in general relativity, there's nothing particularly special about coordinates. In the global Kruskal time coordinate $T$, the singularity advances in exactly the way I have described. Note the corresponding Penrose diagrams I drew on the right. The purple line is the line of constant Kruskal time $T$ which is increasing. It starts at $T=0$, then at $T=1$ the singularity forms at the middle of the wormhole, then when $T > 1$ the wormhole is broken. $\endgroup$
    – user1379857
    Commented Apr 24, 2021 at 2:35
  • $\begingroup$ The top Penrose diagram looks like that of maximally extended Schwarzschild spacetime, including singularities (the squiggles at the top and bottom). The diagram is not explained in the answer. What do mean that the singularity forms in the center? That's not where the singularities are in diagram $\endgroup$
    – Paul T.
    Commented Apr 24, 2021 at 2:44
  • $\begingroup$ This answer could be improved by explaining the diagrams in the answer. $\endgroup$
    – Paul T.
    Commented Apr 24, 2021 at 2:44
  • $\begingroup$ For the picture on the right, the purple line is a constant Kruskal time slice. The picture on the left depicts the geometry of that time slice. Each point in the Penrose diagram is a two sphere $S^2$ (which are actually drawn on the left as 1 spheres $S^1$ because I can't draw in three dimensions). The green dot/sphere is the black hole horizon, and the red dot/degenerate-sphere is the singularity. That is how the two sides match up. $\endgroup$
    – user1379857
    Commented Apr 24, 2021 at 2:49

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