Time travel, though interesting to think about, is one of those things that is likely to remain completely out of reach, and is probably inherently disallowed by the physics of any universe which remains in existence for any amount of time. The problem is that time travel is like a chain reaction. Once it becomes possible, and its being possible assumes that time is laid out and that we just travel along it, then almost 'immediately' (in the sense of immediate in a world where time travel can occur), all of time would be filled with travelers. Anyone who goes back and who knows the mechanism can then offer people further and further back in 'time' the ability to travel as well. It would effectively almost immediately (from any one person's real 'time' perspective) self destruct due to a massive feedback loop. Because, if time is already there in a linear path, then the entire future was there from the start, which means that time travel would have been possible from the very start as well.
Of course, its completely possible that time is not laid out in a row, and that it doesn't exist at all. i.e. that its purely a experiential side effect of the laws of thermodynamics coupled on a self aware storage mechanism. It could be perfectly likely that there is only now, and that's it. In that case, there would no such thing as time travel, period, because there's nowhere to go.
Or, as some well known physicists have said, the chair you are sitting in right now is a time travel machine. Just sit it in for an hour, and you will have traveled into the future one hour. But it doesn't have a reverse gear.
On the 'many universes' theories, which have been put forward by many people, I don't buy it. Its not just that its irrelevant to us even if it happens, but the problem is that the many universes don't just branch off on major, macrosopic events, such as Lincoln getting killed or an asteroid hitting the earth. They have to happen in response to every phsysical event, and for every possible outcome of every physical event, down to the deflection of one molecule off another in a gas. Do you have any idea how many variations that would be? I would have created so many alternative universes just typing and sending this e-mail, than there would be atoms in our universe right now. That is profligacy of gargantuan proportions, when you consider that an (at least) 15 billion light year radius universe must duplicate itself for everyone one of those possible futures, and every one of those must duplicate itself for every possible future, and so on. It is pretty inconceivable that such a thing could be possible.