Time Travel is something that we all do all day, everyday, each moment of the day. You are moving forward in time by a second, each second. But that “travelling” through time follows a fixed speed and can’t go reverse. It is claimed that when the velocity of the traveler tends to the velocity of light, his clock runs slower than Earth’s Clock. And even better, if it surpasses the velocity of light, he may be able to travel back in time. But to achieve such a high velocity, a very high amount of energy would be required, making the whole process way too difficult. For a moment, consider that the energy barrier is no longer an issue. We have found out a way to travel back in time. But, theoretically and logically, there arise many paradoxes related to time travel.
The most common and widely known paradox, Grandfather Paradox gives rise to a “self-inconsistent solution”. Consider this. You travel back in time, to the period when your grandfather is a child. You kill him. This implies that your father was never born in the new timeline. Or rather, you were not born. So, if you were not born in the first place, you could have not gone back in time to kill your grandfather. This inconsistency follows throughout the pattern. This, is the paradox. What it essentially proves, is that one can’t change the course of history.
There is another theory that exists though. Some theorists like to explain this on the basis of the existence of a Parallel Universe. Now consider this. The moment you travel back in time, you enter another parallel universe, where the things don’t affect the timeline of your original universe. You killed your grandfather. Everything goes smooth. But you enter a reality where you were non-existent.
This type of paradox is a time loop paradox. It occurs when a person traveling back in time changes the course of history in such an essential way, that it needs to recur for stability to be maintained. For instance, take a person traveling to the past and preventing an occurrence. To ensure that the course of history is not changed, this will have to be done multiple times, almost as if in a loop. It basically states that things are destined to happen the same way, and whatever is meant to happen, will happen.
To explain it better, allow me to take an example. Assume that you are a time traveler, trying to stop an explosion in your city. The explosion already occurred. So, you travel back in time to stop the explosion. But in doing so, you yourself trigger the bomb by running over it. The event changes. But the outcome remains the same.
This paradox is an interesting one. It involves a person, thing or information being sent to the past, eventually resulting in no known origins of it. So say, you travel back in time with a book containing proofs to some theorems. For fame, you publish them. But now, the real origins of those theorems are in question. You did not actually come up with the proofs. But then if you published them, the original theorist to come up with them would no longer need to tinker for the proofs. So what is the origins of these proofs?
Joseph Polchinski came up with a scenario involving wormholes, billiard balls and empty space. He proposed a case, where a billiard ball enters a wormhole and emerges out the other end, which is its past, and collides with the “younger version” of itself and stops it from entering the wormhole. So does this happen? Or is this paradox forbidden from happening, as stated in Novikov’s Self-Consistency Principle.
Hitler killing Paradox
This one is similar to the Grandfather Paradox. To prevent all the damage the World War II caused to the world, you go back in time to Hitler’s childhood and kill young Adolf. This would mean that the World War II did not take place in the new timeline. Which implies that you did not have a reason to go back in time in the first place! A Paradox? Yeah! Moreover, let’s consider him dead. That would lead to a far reaching Butterfly Effect. Most of the things that resulted from World War II would not occur now. For example, No UN. No stop on Nuclear Weapons. And many such outcomes.
The bottom line is that no one can and should mess with the course of history. While Novikov’s Self-Consistency Principle does state that Time Travel is possible, but Time Paradox is forbidden, the reality can never change. Outcomes will remain the same although you change the event.