As a general rule physicists are very hostile to the concept of time travel. The trouble is that traveling backwards in time can be used to violate causality, which more or less demolishes the very foundation of physics.
http://www.projectrho.com/rocket/roc...html#causality
Their choices are either that time travel is impossible, or that some as-yet undiscovered law of physics magically prevents violations of causality. Using Occam's Razor, they take the simpler choice of making time travel impossible.