If you were to ask me if time travel to the past was impossible about ten years ago, I would have agreed with you. However, quantum physicists already know that time travel is possible at the microscopic scale. In fact, the quantum eraser experiment is already proof of that. But the question is, can macroscopic objects such as you or I can time travel? However, this is not in the realm of the quantum eraser experiment. Anti-particles are already traveling into their past, and their histories are known. Basically from our view point, the moment of their creation is their destruction and their destruction from our view point is their creation. In order to really experiment with this we would need to find ancient particles whose creation/destruction occurred long in our past, but anti-matter does not last long due to the inbalance between matter and anti-matter we observe.
In the case of the "Twin Paradox" it's only a paradox due to our lack of knowledge on the mechanism of how time and space interact with the past. Quantum physics has already taught us to ignore what appears to be paradoxical and just accept it. The real solution may just be to accept a new form of logic and some theorists have already subscribed into quantum gravity. That being said, there are a few possbilities for time traveling into the past.
One possibility would probably involve the classical grandfather paradox. A time traveler travels back in time to prevent his parents from meeting and succeeds. What would actually happen? Now, before I answer that question, let me first point out about those pesky paradoxes. For those of you who are concerned about the possibility of erasing your own existence (and the entire universe), well don't be. Look at this way, if a time traveler goes back in time to prevent from humans evolving into intelligent beings and erases history, none of us would even know about it!
We would be gone and the evolution of humans would have never happened! So, what's to worry about, eh? LOL.
Okay, it's time (pun intended) now for me answer the grandfather paradox question. What would happen to the time traveler if he successfully prevents his parents from meeting? Since the conservation of energy is based on the symmetry of time, time travel would have to preserve this and the conservation of energy. Hence, whenever a time traveler succeeds in preventing his birth, his existence in the present would not literally disappear like Marty McFly in Back To The Future, but would continue to exist unaffected. Essentially, the time traveler would be the man with no past, or very likely the time traveler's matter would rearranged or reformed, or displaced to another form or item, but he would never cease to exist due to the laws of energy conservation. It would be a type of a reset button, where the universe reforms to accomodate the new timeline.
The second possibility involves the time traveler traveling back into his past and succeeds in preventing his birth from happening, then time will branch out and lead into a parallel universe, providing that the parallel universes are self looping (close casual loop) and are already there. The idea of a time traveler being responsible for creating parallel universes is a direct violation of the laws of energy conservation, therefore, they would have to have been there at the same time when our universe was created during the time of the big bang, forming many regions of separate universes, creating each universe/timeline with particles that already existed. In this case, a time traveller will just get shunted to one that already matched the future he just created. It would basically give the impression that these parallel universes were created by the time traveler, however, all it will do is allow him to exist in what appears to be the universe he came from and an altered timeline. But instead, it would be a pocket dimension where the events he altered occured.
Noted theoretical and quantum physicist Jon Cramer has discovered with his quantum eraser experiment that time traveling into the past allows us to change it, but the present always remains unchanged. The minute we look at the particle's present, it is fixed. So, there is no need for concern about erasing your own existence in the present because it has already been recorded into history when it was observed, hence, you're safe. But here is the twist: even though you can erase or prevent your own birth in the past and still exist in the present, you can erase your past simply by staying in the present without actually time traveling at all!
The reason why we're convinced that time travel into the past is not possible is because of our limited perception of time, specifically, linear time, at least that is how it appears on the surface but according to the quantum eraser experiment, time is circular not linear. So, don't be surprise if one day you receive a knock on your door from someone who claims to be your great-great-great grandchild!
