I'd say that realistically the first time travel will involve going into the future. By travelling at high speeds and returning to find that time had travelled at a faster rate "outside". In fact that's already happened to a degree. But there'll be no way of getting back, atleast at first.
Time travel to the extent of, say, 100 years would require a ship that could travel at near lightspeed. But the travel wouldn't be immediate, the traveller would have to wait a while even then, unless he perhaps he travelled at lightspeed.
What ship can travel that fast though? How can it possibly, with the technology we have at the moment? It could not be done in the earth's atmosphere, just approaching that speed would cause the ship and any occupants to burn up. Energy makes heat, even if the ship was outside the earth wouldn't it melt?
Travelling back in time is a completely different story. At the moment it's impossible. Isn't it?
I don't like the idea of a "Multiverse". With every action of every living organism the possibile instances evolve into seperate entities? That can't be right. That's like comparing the universe(s) to bacteria. That's like saying we're something within a cell that divides as a means of reproduction. What if that were true? We're just part of a spreading disease.
If that is so then what holds the universes? Homefully not somebody's rear end... (ahem, ignore that comment).
It's an interesting concept though. Maybe we can jump between cells when they happen to bump together?
Here's another idea, what if we looked at the universe as a single big lump. Let us say... a big tree? (Whether it has branches and leaves or not is irrelevant) The tree grows, much like our expanding universe. Each layer, or ring, represents a visible capture of time. We could travel into the past by making a tunnel to the older layers. Much like a woodworm... (Okay, okay. Enough with the germs and bugs...)
But if you looked at it that way, transporting between layers, it means that you'd have no trouble with the bucket theory. The universe, past and present is one thing. You exist within it, that's what's important. Where you are within it does not matter.
That's just one theory, strange as it is.
Here's one more idea. We travel into the past pretty much every day. Every time we look through a high-power telescope (And we've all got one of them tucked away in our attic) we can see into the past. When we look at a distant planet or star we see don't see it as it is, we see it as it was maybe a thousand years ago. This is because, of course, the light from it takes time to reach us. This is also an idea that could really be expanded upon, but my post is starting to take on essay proportions so I suppose I'll leave it rest until next time.
Yes, in some cases today's science fiction is tomorrow's science. Not always, take a look at history. The major difference is that science fiction is fiction, the ideas are restricted to the imagination of the author. With science the ideas are restricted not only to imagination but also to fact, current knowledge, and technology.
Science fiction needs help, science fiction-- quite frankly-- needs a bath. No works of science fiction are free of drastic faults.
If I were to write a science fiction novel I would first explore the "travel" of light and waves. Maybe it would be more plausible to send a virtual image of yourself into the past.