Ask Lemmy

It might be difficult.

Step 1. Suffer some unpleasantness resulting from an event in the past.

Step 2. Identify the event that was source of this problem.

Step 3. Travel back in time to just before event occurs.

Step 4. Prevent event from occurring.

Step 5. The change at Step 3 has prevented Step 1 from occurring. You have altered history to avoid unpleasantness.

Step 6. Step 2 depended upon Step 1 occurring and also does not happen. This prevents Step 3 and thus Step 4 from occurring. Now the event and the resulting unpleasantness occurs again.

https://en.wikipedia.org/wiki/Temporal_paradox

Consistency paradox

The consistency paradox or grandfather paradox occurs when the past is changed in any way that directly negates the conditions required for the time travel to occur in the first place, thus creating a contradiction. A common example given is traveling to the past and preventing the conception of one's ancestors (such as causing the death of the ancestor's parent beforehand), thus preventing the conception of oneself. If the traveler were not born, then it would not be possible to undertake such an act in the first place; therefore, the ancestor proceeds to beget the traveler's next-generation ancestor and secure the line to the traveler. There is no predicted outcome to this scenario.[8] Consistency paradoxes occur whenever changing the past is possible.[9] A possible resolution is that a time traveller can do anything that did happen, but cannot do anything that did not happen. Doing something that did not happen results in a contradiction.[8] This is referred to as the Novikov self-consistency principle.

https://en.wikipedia.org/wiki/Killing_baby_Hitler

The question of killing baby Hitler contains a version of the grandfather paradox,[20] also known as the "Hitler's Murder Paradox".[21] According to the B-theory of time,[22] if someone travelled back in time with the intention of killing baby Hitler, then their reason for travelling back in time would be eliminated. It is often concluded that as the past has already happened, alteration of the past is a logical impossibility.[23] As Hitler killed himself in 1945, it can also be inferred that no time traveler has killed baby Hitler.[24]

In contrast to the B-theory, models that adopt the A-theory of time avoid logical contradictions in the killing of baby Hitler by considering time to be two-dimensional, where the first dimension is standard time (tx) and the second dimension is known as hyper-time (Htx).[25] Theories that leave room for the past to be changed include hyper-eternalism, two-dimensional presentism and hyper-presentism, which each demonstrate the possibility of killing baby Hitler in two-dimensional time.[26] In these temporal models, both the past and the future are held to be mutable; in changing the past by killing baby Hitler, the time traveller also changes the future.[27] Although it can also be debated whether such temporal models genuinely change the past, or if killing baby Hitler simply affects the past by causing a variation in hyper-time.[28]

If time travel caused creation of a parallel universe, killing baby Hitler would only create a parallel universe without Hitler in it, and the original universe would continue existing and thus the suffering he caused in that timeline would not be alleviated by the time-traveling assassin. From this perspective, astrophysicist Brian Koberlein concluded that killing baby Hitler would be "inconsequential at best, and could be downright harmful", recommending that time travelers avoid such an activity and instead visit the 1980s.[29]