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When it comes to interstellar travel, the biggest challenge is simply getting there. Our universe is so vast that a simple journey to the stars can take decades or centuries even at the speed of light. So science fiction often comes up with some way to shorten the journey. Hyperspace, warp drive, and such, simply to avoid the boring journey between stars. Most of these ideas simply "make it so" without worrying about the scientific details, but for those who like their science fiction a bit harder, the most popular idea is the wormhole.

The idea of a wormhole first proposed by none other than Einstein himself. In 1935 he wrote a paper with Nathan Rosen about how his theory of gravity might allow you to connect two distant points in space. Their idea wasn't trying to solve the challenges of space travel, but rather trying to connect gravity to particle physics. It's a problem that vexes us to this day. But it was noted that this "Einstein-Rosen bridge" looks rather familiar. From the outside one end looks like a black hole, where things enter and don't return. The other end looks like the opposite. A kind of white hole where things seem to appear from nowhere. This led to the idea that perhaps black holes could be wormholes to distant galaxies, or even other universes.

But when you start looking at the details, that idea doesn't really seem to work. While Einstein and Rosen were right that in theory you could make a wormhole, it turns out they aren't stable. In principle space and time can be warped and bent in lots of interesting ways, but it wouldn't stay that way. Make a wormhole between distant stars, and the tunnel will collapse and pinch off before you have a chance to traverse it. Some folks suggested lining the wormhole with some kind of strong matter to keep it open, but it turns out that the mass of that matter would just cause the wormhole to collapse even more quickly. In the 1970s Kip Thorne and others showed you could line a wormhole with "exotic matter" to keep it open, but that exotic matter couldn't be any kind of matter that existed. Basically, you could make a transversable wormhole if you also had magic pixie dust. So much for that idea.

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