The fact that the effects of gravity cannot be distinguished from those found in experiments performed in an accelerating rocket in space (far away from any massive object) suggests that maybe gravity isn't a real force in the sense that centrifugal and Coriolis forces are not real forces.
Einstein sought a geometric interpretation for gravity. He noted that objects on flat surface which are not subjected to forces roll in straight lines and that objects that roll on curved surfaces follow curved paths even when they are not subjected to external forces. The natural motion of an unforced object could either be a straight line (as Newton envisioned) or a curved line (as would be incomprehensible to Newton) depending upon the shape of the surface upon which it was rolling.
Einstein suggested that mass distorts the shape of space-time so that falling objects are simply rolling into the depression and orbiting bodies are simply rolling around in the depression. The objects are not undergoing forced motion; they are simply following the lay of the land.
Black holes distort space-time in a much more extreme manner (note the cusp or point the depression comes to at its bottom). An interesting discovery was made by Einstein-Rosen in the 1930's, they found that the black holes would close down but then actually re-open into another Universe forming an Einstein-Rosen bridge. Or an alternative interpretation is that the black hole would open up into another part of the space-time of our Universe. Such a passageway (wormhole) could lead to the possibility of time travel!!
There are many complaints which can be leveled at this suggestion; both physical and philosphical. Some of the physical complaints can be eased by looking at rotating black holes where you don't have to pass through the singularity, but the problems are still severe in that event horizons are one-way membranes.
There are other types of wormholes which do not suffer from some of the physical problems. These are known as Schwarzschild wormholes where you have singularities which are not cloaked by event horizons. These do not suffer from the problem of the one-way nature of event horizons but they do suffer from some related problems.
As stuff passes through the throat of the wormhole, it compresses and so its density, mass/volume, goes up ===> GM/r**2 goes up and the throat will pinch itself off. To get around this idea, Thorne, Morris, and Yurtsever postulated that some arbitrarily advanced civilization (AAC) could develop some new form of matter to hold open the throat of the wormhole. In this case, one could travel from place to place in spacetime and, under the right circumstances, could travel in time. That is one could use wormholes as time machines!!
The impossibility of time travel has not yet been demonstrated. However, there is no evidence that it has occurred and so, it is not likely to be possible. If we can understand why it is not possible then we will have done something which is quite significant. There is nothing in our current theories which tells us how time flows. There is no reason to believe, based on theory, why cause must precede effect. Since it always seems to do so probably means that there is some physical reason that escapes us (at the moment). If we could understand why time travel is not possible would help us to understand one of the more vexing problems facing us today (known as the arrow of time).
