Time travel may in fact be possible, but it wouldn't work like in Back to the Future. (For one thing, you don't have worry about your parents failing to create you—you already exist.)
Einstein described our universe in four dimensions: the three dimensions of space and one of time. So traveling back in time is nothing more or less than the fourth-dimensional version of walking in a circle. All you would have to do is use an extremely strong gravitational field, like that of a black hole, to bend space-time. From this point of view, time travel seems quite difficult but not obviously impossible.
Via: Discover Magazine