redhedkangaro said:
Hey guys,
This may be a silly question. I saw star trek the other day, and I know it is science fiction but I was wondering...My understanding is that black holes are a region in space-time from which nothing can escape. However it seems like in the movie they are also used to travel through space time but I thought only wormholes could do that. In general can black holes be used to travel through space time and how do black holes and worm holes differ?
Black holes come from solutions of the equations of general relativity, as developed by Schwarzschild many years ago. One feature of his space-time solution was the "wormhole" or Einstein-Rosen bridge, which seems to link two separate parts of space-time or even two separate space-times/Universes. When John Wheeler (who also invented the term "black-hole") restudied the Schwarzschild solution in the early 1960s he found that it pinched off as a singularity before anything could pass through it, even light. The work of Wheeler, and various others, showed that non-rotating stars of sufficient mass could collapse into singularities, thus (briefly) forming an Einstein-Rosen bridge.
When solutions of Einstein's equations for charged (Reiner-Nordstrom solutions) and rotating massive stars were studied further in the 1960s "wormholes" also appeared. The rotating collapsed star ("collapsar") solution of Kerr & Newmann produced a ring-shaped singularity, which at the centre of which was a large "wormhole" that didn't immediately shut-off quicker than light could travel. In the 1970s this was the main wormhole in science-fiction, notably Joe Haldeman's "The Forever War" and stories like George Zebrowski's "Fountain of Force". However by 1980 the fate of small infalling masses was studied and it was discovered that the gravitational distortions caused by such falling masses would cause the wormhole to shut-up before it could be traversed.
Carl Sagan was writing an SF novel in the mid 1980s ("Contact") and wanted a physically possible wormhole, so he asked his good friend, physicist Kip Thorne, how that might be done. Thorne, and his graduate students, developed solutions for stabilising wormholes against self-collapse and collapse by infalling masses. This eventually led to the modern study of wormholes and how to keep them open using "exotic matter" or "negative energy" and similar hard-to-make stuff.
But can the wormhole inside a black-hole be used? Igor Novikov, and colleagues, haven't given up on the wormhole inside and have written several papers that hint it might be traversed if the black-hole is big enough and old enough. Tidal forces can be extreme near black-holes that are too small, but above about 30,000 solar masses a black-hole might be traversible.
As for "white holes" they are time-reversed black-hole solutions. To avoid recollapsing into their own event horizons a white-hole must expand forever and it also must exist since the beginning of the Universe... if that sounds like the Big Bang, then you've noticed a similarity that lots of people have also noticed. The two aren't exactly the same, but some theorists - like Lee Smolin - have proposed that the collapsing matter of a black-hole actually forms a new universe that branches off from our own. One day such theories might produce observable results, but for now they're speculative - interesting, but not yet empirically supported. Like wormholes.
