Backwards time travel is definitely impossible, as well as teleportation it seems like.
I just read this article yesterday: http://www.slate.com/blogs/moneybox/...sn_t_work.html
The human body contains so much information that even in theory it would overwhelm any computer system we can imagine. Even using quantum computing and almost infinite bandwith it would take many many years to transfer a person. Also it would take some absurd amount of power.
On top of that, with any kind of communication there is interference and loss of data. Something you cant have when transporting living beings. One misplaced cell, and you're mentally retarted or handicapped or something.
I don't know why people think backwards time travel is possible anyway, its purely a sci-fi concept. Its not like the state of the past is saved somewhere. Once its gone, its gone.