Hyperspace connections: Black holes, white holes, wormholes
When astronomer Carl Sagan decided to write a science fiction novel, he needed a fictional device that would allow his characters to travel great distances across the Universe. He knew, of course, that it is impossible to travel faster than light; and he also knew that there was a common convention in science fiction that allowed writers to use the gimmick of a shortcut through "hyperspace" as a means around this problem. But, being a scientist, Sagan wanted something that would seem to be more substantial than a conventional gimmick for his story. Was there any way to dress up the mumbo-jumbo of Sf hyperspace in a cloak of respectable sounding science? Sagan didn't know. He isn't an expert on black holes and general relativity -- his background specialty is planetary studies. But he knew just the person to turn to for some advice on how to make the obviously impossible idea of hyperspace connections through spacetime sound a bit more scientifically plausible in his book Contact.
The man Sagan turned to for advice, in the summer of 1985, was Kip Thorne, at CalTech. Thorne was sufficiently intrigued to set two of his PhD students, Michael Morris and Ulvi Yurtsever, the task of working out some details of the physical behaviour of what the relativists know as "wormholes". At that time, in the mid-1980s, relativists had long been aware that the equations of the general theory provided for the possibility of such hyperspace connections. Indeed, Einstein himself, working at Princeton with Nathan Rosen in the 1930s, had discovered that the equations of relativity -- Karl Schwarzschild's solution to Einstein's equations -- actually represent a black hole as a bridge between two regions of flat spacetime -- an "Einstein-Rosen bridge".
A black hole always has two "ends", a property ignored by everyone except a few mathematicians until the mid-1980s. Before Sagan set the ball rolling again, it had seemed that such hyperspace connections had no physical significance and could never, even in principle, be used as shortcuts to travel from one part of the Universe to another. Morris and Yurtsever found that this widely held belief was wrong. By starting out from the mathematical end of the problem, they constructed a spacetime geometry that matched Sagan's requirement of a wormhole that could be physically traversed by human beings. Then they investigated the physics, to see if there was any way in which the known laws of physics could conspire to produce the required geometry. To their own surprise, and the delight of Sagan, they found that there is.
To be sure, the physical requirements seem rather contrived and implausible. But that isn't the point. What matters is that it seems that there is nothing in the laws of physics that forbids travel through wormholes. The science fiction writers were right -- hyperspace connections do, at least in theory, provide a means to travel to far distant regions of the Universe without spending thousands of years pottering along through ordinary space at less than the speed of light. The conclusions reached by the CalTech team duly appeared as the scientifically accurate window dressing in Sagan's novel when it was published in 1986, although few readers can have appreciated that most of the "mumbo-jumbo" was soundly based on the latest discoveries made by mathematical relativists. Since then, the discovery of equations that describe physically permissible, traversable wormholes has led to a booming cottage industry of mathematicians investigating these strange phenomena. It all starts with the Einstein-Rosen bridge.