TIME TRAVEL Time Travel has been in the question every since H.G. Wells wrote The Time Machine, but is was not until Einstein wrote his theory of General Relativity, which states that space and time are intertwined and thus if you effect one proportional effect the other. It was from this theory that he concluded that when you approached the speed of light time would slow, it is estimated that you have to go around 99.95 percent of light to go in to the future.
To be able to time travel as it were. Current theory requires an existence of a black hole. A black hole is infinitely dense structure, which means that it pulls the fabric of space-time to the breaking point, and containing so much gravitational force that even light cannot escape. This is the only way at present to approach the speed of light but of coarse there are problems such that you would be pulled apart by the gravitational forces.
Another is where would the substance pulled in by the force go. To answer the question where does the stuff go is a wormhole that the tiny rip at the bottom of the black hole could be connected to another black hole and by having this you joining two disparate parts of space-time by a narrow channel.
And now you have to modify the Wormhole, you would have to widen the space in-between the two black holes this could be done by using antigravity, this does not exist yet but in theory antigravity would be placed in the throat of the wormhole and opening the wormhole enough for a space traveler to pass. But you would need a lot of "antigravity" to counter act the massive gravitational forces of the wormhole. Ok now you can pass through the wormhole, now you have to displace one end of the wormhole. You would do this by grabbing one end some how and then approaching the speed of light, and over time the two black holes would be desynchronized, entering the stationary end of the wormhole and exiting from the moving end, which is in the future. And you could do this inversely creating a general time portal.
Second theory is rotating cylinders. Which is a cylinder consisting of material 10 times the mass of the sun and squeezing it together into a super dense cylinder. You would pilot a spacecraft much as like during the failed Apollo 13 mission which used the gravitational force of the moon to sling shot back to the earth. Ok getting back to the subject you would have to follow a carefully plotted spiral course around the cylinder and by doing this you would find yourself in a "Closed Time like Curve" emerging thousand even billions of years from the starting point. But there are problems with this theory are that for the mathematics to work correctly the cylinder would have to be infinitely long.
Third Theory is Cosmic Strings. A variation on the rotation cylinder, some scientists have suggested using "Cosmic Strings" to construct a time machine. At the moment, these are purely theoretical objects that might possibly be left over from the creation of the universe in the big bang. A black hole contains a one-dimensional singularity an infinitely small point in the space-time continuum. A cosmic string, if such a thing existed, would be a two dimensional singularity an infinitely thin line that has even stranger effects on the fabric of space and time. Although no one has actually found a cosmic string, astronomers have suggested that they may explain strange effects seen in distant galaxies. By maneuvering two cosmic strings close together or possibly just one string plus a black hole it is theoretically possible to create a whole array of "closed time like curves" You best bet is to fire two infinitely long cosmic strings past each other at very high speeds, then fly your ship around them in a carefully calculated figure eight. In theory, you would be able to emerge anywhere, anytime! There is of course some problems with time travel such as if you went back in time and brought along a cd consisting of a composers music now you go back in time and visit the composer before he created the music, and he listened to the music and writes the score down, and then composes the song from your cd. Question is where did the music come from. To answer this question would be that the time machine created would not let the voyager go back in time before the machine was created, so you would not have this problem because the voyager would not be able go back in time past the point of the construction of the time machine. Another answer is that there may be infinite universes other dimensions as it were and that by time traveling you are opening a portal to another universes and any changes that happen do to your time travel would not effect you dimension, but ironically there is a problem to this answer if there are infinite universes how would the travelers ever get back to there own dimension.