Question: is time travel possible?also what are the current theories on time travel?

Kristian Harder answered on 20 Nov 2013:

The universe we live in has at least three space dimensions, and one time dimension. At face value, space and time seem to be two very different things. But a long time ago scientists such as Lorentz and Einstein showed that this is not really true. Space and time mix up. A somewhat simplified way of seeing this is to imagine us as always moving at the speed of light through space-time. If we don’t move in space, we travel through time at the maximum speed, the speed of light. but we can “change direction” from the time dimension into the space dimensions. In other words, if we move faster in space, we slow down in time. We move through time more slowly if we move through space more quickly. This means everyone else’s time goes faster than ours, and we basically travel into the future.
As I said, this is a simplified picture, and I am worried about Tim telling me off for explaining it in this way, but I think it gives you at least a rough idea of what’s happening.
Now, for some reason we can’t seem to “turn around” in time. We can only travel in time in one direction, and this direction seems to be the same for all of us. This is a bit strange, because all laws of physics would work with either direction of time. The only one that doesn’t is the second law of thermodynamics that basically restricts the flow of time to one direction. But this law only applies to complex systems.
For individual elementary particles there does not seem to be a strict need to travel through time in one direction. In fact, there is something funny about the particles we know to exist in the universe. In addition to matter, there is antimatter. We don’t really know why it’s there, and it’s almost identical to matter, except that many properties are exactly opposite. For example the electric charge. The interesting thing about antimatter is that mathematically it can be described as matter traveling backwards in time.
We don’t know whether that’s really what it is, but the mathematical description gives us the option to see it that way.
In that sense it could be that for individual elementary particles, time travel is possible in both directions. But think of what would happen if human beings could travel in time. Traveling forward is no big deal. You leave the present, you travel into the future. Fine. Not much to complain about. Just don’t hit on your grandchildren.
But imagine you travel backwards in time. Mess and mayhem! What if you are a bad person and murder your parents before you are born. Then you don’t exist. Then how could you have murdered them?
Or, a slightly less criminal example, you use a time machine to travel into the past and tell your younger self how to build a time machine. That would mean you got the knowledge how to build a time machine from your future self. But from your point of view, where did your future self get the knowledge from? Somehow the information appears out of nowhere.
All these things just don’t work. The logic of the universe would break down if time travel into the past were possible for us. Even if we could just send information back instead of ourselves, this would be bad enough.

There is one loophole, though. There are theories that there is more than one universe. Parallel universes could exist. That doesn’t mean these parallel universes are completely disconnected and in different locations than ours. It just is a way of saying that when Schroedinger’s cat is “both dead and alive”, it is dead in some universes, and alive in some others, and they are all real. If you’d jump into a different “branch” of universes whenever you travel backwards in time, the logical problems disappear. You’d murder the parents of some other version of you, and that other version of you would then never be born, but it wouldn’t affect your own existence.
This may all seem very complicated, and, as I said, I am not even sure I am getting this right myself, because this is somewhat beyond what I usually work on in science, but I hope that it at least explains the problems a bit. 🙂