Time is a strange thing. There is no law in physics that prohibits time from going backwards, but as we all know, it has a tendency to go always forward:)). Why is it so? We can all envisage the problems that time travel would create: people could go back in time and kill their grandparents, and give past generations modern technology or prevent some historic event from happening!
Physicists Daniel Greenberger of the City University of New York and Karl Svozil of the Vienna University of Technology in Austria have come up with a provocative idea:), but it suggests that time travel might not be possible at all:(.

Worm Hole (Courtesy: ThinkQuest)Worm-Holes are hypothetical bridges (arising out of Einstein's General Theory of Relativity) that might join distant regions of the spacetime. Imagine a 2D surface, where two points are far apart. If it were possible to bend the surface, the two points would be much closer, and a hole could be constructed to stitch the two points together. A similar construct is perhaps possible in 3D space too. However, Kip Thorne showed that such a hole could be used for time travel: a person entering from one end of the hole might reach the other side before he/she entered the hole:D.
Then how are the time paradoxes resolved? According to the new proposal, such paradoxes may be ruled out by the weirdness inherent in laws of quantum physics. The constraint arises from a quantum object's ability to behave like a wave. Quantum objects split their existence into multiple component waves, each following a distinct path through space-time. Ultimately, an object is usually most likely to end up in places where its component waves recombine, or "interfere", constructively, with the peaks and troughs of the waves lined up, say. The object is unlikely to be in places where the components interfere destructively, and cancel each other out.
When Greenberger and Svozil analysed what happens when these component waves flow into the past, they found that the paradoxes implied by Einstein's equations never arise. Waves that travel back in time interfere destructively, thus preventing anything from happening differently from that which has already taken place:)).
So, it seems that even if matter could travel back in time, it will not be able to keep its cohesion, and therefore no information can be sent back in time to influence the past!